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  • 1. Abegunawardana, Sidath
    et al.
    Bodhika, J A P
    Abewardhana, Ruwan
    Fernando, Mahendra
    Cooray, Vernon
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Tekniska sektionen, Institutionen för teknikvetenskaper, Elektricitetslära.
    Sound Source Localization of Lightning Discharges2018Konferensbidrag (Refereegranskat)
  • 2. Abegunawardana, Sidath
    et al.
    Bodhika, J A P
    Nanayakkara, Sankha
    Sonnadara, Upul
    Fernando, Mahendra
    Cooray, Vernon
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Tekniska sektionen, Institutionen för teknikvetenskaper, Elektricitetslära.
    Audible Frequency Analysis of Ground Flashes2018Konferensbidrag (Refereegranskat)
    Abstract [en]

    Thunder signatures categorized into three types based on peak pressure and variation in fundamental frequency, have been studied by using acoustic spectrum of thunder. S-transformation has been used to estimate the dominant frequency variation around the peak pressure. The mean fundamental frequencies of type 3 ground and cloud flashes are 160 Hz and 98 Hz respectively. The mean frequencies of type 2 ground and cloud flashes are 108 Hz and 82 Hz respectively.

  • 3.
    Abiodun, B. and Enger, L.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper.
    The role of advection of fluxes on modelling dispersion in convective boundary2002Ingår i: Quart. J. Roy. Met. Soc., Vol. 128, s. 1589-1607Artikel i tidskrift (Refereegranskat)
  • 4.
    Addor, Nans
    et al.
    Univ Zurich, Dept Geog, Zurich, Switzerland.;Natl Ctr Atmospher Res, Appl Res Lab, POB 3000, Boulder, CO 80307 USA..
    Rohrer, Marco
    Univ Bern, Oeschger Ctr Climate Change Res, Bern, Switzerland.;Univ Bern, Inst Geog, Bern, Switzerland..
    Furrer, Reinhard
    Univ Zurich, Dept Math, Zurich, Switzerland..
    Seibert, Jan
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Luft-, vatten och landskapslära. Univ Zurich, Dept Geog, Zurich, Switzerland..
    Propagation of biases in climate models from the synoptic to the regional scale: Implications for bias adjustment2016Ingår i: Journal of Geophysical Research - Atmospheres, ISSN 2169-897X, E-ISSN 2169-8996, Vol. 121, nr 5, s. 2075-2089Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Bias adjustment methods usually do not account for the origins of biases in climate models and instead perform empirical adjustments. Biases in the synoptic circulation are for instance often overlooked when postprocessing regional climate model (RCM) simulations driven by general circulation models (GCMs). Yet considering atmospheric circulation helps to establish links between the synoptic and the regional scale, and thereby provides insights into the physical processes leading to RCM biases. Here we investigate how synoptic circulation biases impact regional climate simulations and influence our ability to mitigate biases in precipitation and temperature using quantile mapping. We considered 20 GCM-RCM combinations from the ENSEMBLES project and characterized the dominant atmospheric flow over the Alpine domain using circulation types. We report in particular a systematic overestimation of the frequency of westerly flow in winter. We show that it contributes to the generalized overestimation of winter precipitation over Switzerland, and this wet regional bias can be reduced by improving the simulation of synoptic circulation. We also demonstrate that statistical bias adjustment relying on quantile mapping is sensitive to circulation biases, which leads to residual errors in the postprocessed time series. Overall, decomposing GCM-RCM time series using circulation types reveals connections missed by analyses relying on monthly or seasonal values. Our results underscore the necessity to better diagnose process misrepresentation in climate models to progress with bias adjustment and impact modeling.

  • 5. Ahlers, R.
    et al.
    Cleaver, F
    Rusca, Maria
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Luft-, vatten och landskapslära.
    Schwartz, K.
    Unleashing Entrepreneurs or Controlling Unruly Providers?: The Formalisation of Small-scale Water Providers in Greater Maputo, Mozambique2013Ingår i: Journal of Development Studies, ISSN 0022-0388, E-ISSN 1743-9140, Vol. 49, nr 4, s. 470-482Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The existing legal and policy framework regulating water service provision in Greater Maputo, Mozambique appears fixated on the official service areas. In doing so it inadequately addresses the geographically varied service provision modalities which characterise the city. We argue that the predominant legal and policy framework does little to support development of improved services in areas unserved by the formal utility. Although ad hoc measures recognising small-scale providers as a temporary alternative to service provision by a formal utility have been implemented, these measures appear designed to increase control over these providers rather than support the service delivery capacity of small-scale providers.

  • 6.
    Ahlm, L.
    et al.
    Stockholm University.
    Krejci, R.
    Stockholm University.
    Nilsson, E. D.
    Stockholm University.
    Martensson, E. M.
    Stockholm University.
    Vogt, M.
    Stockholm University.
    Artaxo, P.
    Emission and dry deposition of accumulation mode particles in the Amazon Basin2010Ingår i: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 10, nr 21, s. 10237-10253Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Size-resolved vertical aerosol number fluxes of particles in the diameter range 0.25-2.5 mu m were measured with the eddy covariance method from a 53 m high tower over the Amazon rain forest, 60 km NNW of Manaus, Brazil. This study focuses on data measured during the relatively clean wet season, but a shorter measurement period from the more polluted dry season is used as a comparison. Size-resolved net particle fluxes of the five lowest size bins, representing 0.25-0.45 mu m in diameter, were in general dominated by deposition in more or less all wind sectors in the wet season. This is an indication that the source of primary biogenic aerosol particles may be small in this particle size range. Transfer velocities within this particle size range were observed to increase linearly with increasing friction velocity and increasing particle diameter. In the diameter range 0.5-2.5 mu m, vertical particle fluxes were highly dependent on wind direction. In wind sectors where anthropogenic influence was low, net upward fluxes were observed. However, in wind sectors associated with higher anthropogenic influence, deposition fluxes dominated. The net upward fluxes were interpreted as a result of primary biogenic aerosol emission, but deposition of anthropogenic particles seems to have masked this emission in wind sectors with higher anthropogenic influence. The net emission fluxes were at maximum in the afternoon when the mixed layer is well developed, and were best correlated with horizontal wind speed according to the equation log(10)F = 0.48.U + 2.21 where F is the net emission number flux of 0.5-2.5 mu m particles [m(-2) s(-1)] and U is the horizontal wind speed [ms(-1)] at the top of the tower.

  • 7.
    Ahlm, L.
    et al.
    Stockholm University.
    Nilsson, E. D.
    Stockholm University.
    Krejci, R.
    Stockholm University.
    Martensson, E. M.
    Stockholm University.
    Vogt, M.
    Stockholm University.
    Artaxo, P.
    A comparison of dry and wet season aerosol number fluxes over the Amazon rain forest2010Ingår i: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 10, nr 6, s. 3063-3079Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Vertical number fluxes of aerosol particles and vertical fluxes of CO(2) were measured with the eddy covariance method at the top of a 53 m high tower in the Amazon rain forest as part of the LBA (The Large Scale Biosphere Atmosphere Experiment in Amazonia) experiment. The observed aerosol number fluxes included particles with sizes down to 10 nm in diameter. The measurements were carried out during the wet and dry season in 2008. In this study focus is on the dry season aerosol fluxes, with significant influence from biomass burning, and these are compared with aerosol fluxes measured during the wet season. Net particle deposition fluxes dominated in daytime in both seasons and the deposition flux was considerably larger in the dry season due to the much higher dry season particle concentration. The particle transfer velocity increased linearly with increasing friction velocity in both seasons. The difference in transfer velocity between the two seasons was small, indicating that the seasonal change in aerosol number size distribution is not enough for causing any significant change in deposition velocity. In general, particle transfer velocities in this study are low compared to studies over boreal forests. The reasons are probably the high percentage of accumulation mode particles and the low percentage of nucleation mode particles in the Amazon boundary layer, both in the dry and wet season, and low wind speeds in the tropics compared to the midlatitudes. In the dry season, nocturnal particle fluxes behaved very similar to the nocturnal CO(2) fluxes. Throughout the night, the measured particle flux at the top of the tower was close to zero, but early in the morning there was an upward particle flux peak that is not likely a result of entrainment or local pollution. It is possible that these morning upward particle fluxes are associated with emission of primary biogenic particles from the rain forest. Emitted particles may be stored within the canopy during stable conditions at nighttime, similarly to CO(2), and being released from the canopy when conditions become more turbulent in the morning.

  • 8. Ahmad, M R
    et al.
    Esa, M R M
    Rahman, Mahbubur
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Tekniska sektionen, Institutionen för teknikvetenskaper, Elektricitetslära.
    Cooray, Vernon
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Tekniska sektionen, Institutionen för teknikvetenskaper, Elektricitetslära.
    Measurement of bit error rate at 2,4 GHz due to lightning interference2012Ingår i: Proceeding of the 31st International Conference on Lightning Protection ICLP 2012, 2012Konferensbidrag (Refereegranskat)
    Abstract [en]

    This paper analyzes the interference of lightning flashes with wireless communication systems operating in the microwave band at 2.4 GHz. A bit error rate (BER) measurement method was used to evaluate BER during 3 heavy thunderstorms on January 25, March 17 and March 20, all in year 2011. In addition, BER measurements also were done on January 21 and March 30, 2011 under fair weather (FW) conditions providing a baseline for comparison. The Transmitter-Receiver separation was fixed at 10 meter with line-of-sight (LOS) consideration. We infer that lightning interfered with the transmitted digital pulses which resulted in a higher recorded BER. The maximum recorded BER was 9.9·101 and the average recorded BER was 9.95·10 -3 during the thunderstorms with the average fair weather BER values under the influence of adjacent channel interference (ACI) and co-channel interference (CCI) being 1.75·10-5 and 7.35·10 -6 respectively. We conclude that wireless communication systems operating at 2.4 GHz microwave frequency can be significantly interfered by lightning.

  • 9.
    Ahmad, Mohd Riduan
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Tekniska sektionen, Institutionen för teknikvetenskaper, Elektricitetslära. Universiti Teknikal Malaysia Melaka.
    Interaction of Lightning Flashes with Wireless Communication Networks: Special Attention to Narrow Bipolar Pulses2014Doktorsavhandling, sammanläggning (Övrigt vetenskapligt)
    Abstract [en]

    In this thesis, the features of electric field signatures of narrow bipolar pulses (NBPs) generated by cloud flashes are investigated and their effects on wireless communication systems are studied. A handful amount of NBPs (14.5%) have been observed to occur as part of cloud-to-ground flashes in South Malaysia. Occurrence of NBPs in Sweden has been reported for the first time in this thesis. The electric field waveform characteristics of NBPs as part of cloud-to-ground flashes were similar to isolated NBPs found in Sweden and South Malaysia and also to those isolated NBPs reported by previous studies from various geographical areas. This is a strong indication that their breakdown mechanisms are similar at any latitudes regardless of geographical areas.

    A comparative study on the occurrence of NBPs and other forms of lightning flashes across various geographical areas ranging from northern regions to the tropics is presented. As the latitude decreased from Uppsala, Sweden (59.8°N) to South Malaysia (1.5°N), the percentage of NBP emissions relative to the total number of lightning flashes increased significantly from 0.13% to 12%. Occurrences of positive NBPs were more common than negative NBPs at all observed latitudes. However, as latitudes decreased, the negative NBP emissions increased significantly from 20% (Sweden) to 45% (South Malaysia). Factors involving mixed-phase region elevations and vertical extents of thundercloud tops are invoked to explain the observed results. These factors are fundamentally latitude dependent.

    In this thesis, the interaction between microwave radiations emitted by cloud-to-ground and cloud flashes events and bits transmission in wireless communication networks are also presented. To the best of our knowledge, this is the first time such effects are investigated in the literature. Narrow bipolar pulses were found to be the strongest source of interference that interfered with the bits transmission.

    Delarbeten
    1. Signatures of Narrow Bipolar Pulses as Part of Cloud-to-Ground Flashes in Tropical Thunderstorms
    Öppna denna publikation i ny flik eller fönster >>Signatures of Narrow Bipolar Pulses as Part of Cloud-to-Ground Flashes in Tropical Thunderstorms
    2014 (Engelska)Ingår i: Journal of Atmospheric and Solar-Terrestrial Physics, ISSN 1364-6826, E-ISSN 1879-1824Artikel i tidskrift (Refereegranskat) Submitted
    Nyckelord
    Narrow bipolar pulse; Return stroke; Tropical thunderstorm.
    Nationell ämneskategori
    Meteorologi och atmosfärforskning Teknik och teknologier
    Forskningsämne
    Teknisk fysik med inriktning mot atmosfäriska urladdningar
    Identifikatorer
    urn:nbn:se:uu:diva-233624 (URN)
    Tillgänglig från: 2014-10-07 Skapad: 2014-10-07 Senast uppdaterad: 2017-12-05
    2. Narrow bipolar pulses and associated microwave radiation
    Öppna denna publikation i ny flik eller fönster >>Narrow bipolar pulses and associated microwave radiation
    2013 (Engelska)Konferensbidrag, Publicerat paper (Refereegranskat)
    Ort, förlag, år, upplaga, sidor
    Stockholm: , 2013
    Nationell ämneskategori
    Teknik och teknologier
    Forskningsämne
    Teknisk fysik med inriktning mot elektricitetslära
    Identifikatorer
    urn:nbn:se:uu:diva-212898 (URN)
    Konferens
    Progress in Electromagnetics Research Symposium
    Tillgänglig från: 2013-12-16 Skapad: 2013-12-16 Senast uppdaterad: 2015-01-23
    3. Electric Field Signature of Narrow Bipolar Pulse Observed in Sweden
    Öppna denna publikation i ny flik eller fönster >>Electric Field Signature of Narrow Bipolar Pulse Observed in Sweden
    Visa övriga...
    2014 (Engelska)Ingår i: Atmospheric research, ISSN 0169-8095, E-ISSN 1873-2895Artikel i tidskrift (Refereegranskat) Submitted
    Nyckelord
    Electric field; Narrow bipolar pulse; Sweden.
    Nationell ämneskategori
    Meteorologi och atmosfärforskning Teknik och teknologier
    Forskningsämne
    Teknisk fysik med inriktning mot atmosfäriska urladdningar
    Identifikatorer
    urn:nbn:se:uu:diva-233636 (URN)
    Tillgänglig från: 2014-10-07 Skapad: 2014-10-07 Senast uppdaterad: 2017-12-05
    4. Latitude Dependence of Narrow Bipolar Pulse Emissions
    Öppna denna publikation i ny flik eller fönster >>Latitude Dependence of Narrow Bipolar Pulse Emissions
    Visa övriga...
    2015 (Engelska)Ingår i: Journal of Atmospheric and Solar-Terrestrial Physics, ISSN 1364-6826, E-ISSN 1879-1824, Vol. 128, s. 40-45Artikel i tidskrift (Refereegranskat) Published
    Nyckelord
    Latitude; Narrow bipolar pulse; Thunderstorm.
    Nationell ämneskategori
    Meteorologi och atmosfärforskning Teknik och teknologier
    Forskningsämne
    Teknisk fysik med inriktning mot atmosfäriska urladdningar
    Identifikatorer
    urn:nbn:se:uu:diva-233638 (URN)10.1016/j.jastp.2015.03.005 (DOI)000355717500005 ()
    Tillgänglig från: 2014-10-07 Skapad: 2014-10-07 Senast uppdaterad: 2017-12-05Bibliografiskt granskad
    5. Similarity between the Initial Breakdown Pulses of Negative Ground Flash and Narrow Bipolar Pulses
    Öppna denna publikation i ny flik eller fönster >>Similarity between the Initial Breakdown Pulses of Negative Ground Flash and Narrow Bipolar Pulses
    2014 (Engelska)Ingår i: 2014 INTERNATIONAL CONFERENCE ON LIGHTNING PROTECTION (ICLP), IEEE conference proceedings, 2014, s. 810-813Konferensbidrag, Publicerat paper (Refereegranskat)
    Abstract [en]

    In this paper, temporal characteristics of several initial electric field pulses of preliminary breakdown process (PBP) from very close negative cloud-to-ground (CG) flashes are compared to close narrow bipolar pulses (NBPs) to observe any similarity that may exists. Interestingly, we found that the initial PBP pulses are similar to close NBP with zero crossing time less than 5 mu s, do not preceded by any slow field change and followed by pronounce static component. As NBPs are believed to be a result of relativistic runaway electron avalanches discharge, this finding is an indication that the initial electric field pulses of PBP are perhaps the result of the same discharge mechanism.

    Ort, förlag, år, upplaga, sidor
    IEEE conference proceedings, 2014
    Nyckelord
    Narrow bipolar pulse; Preliminary breakdown pulse; Relativistic runaway electron avalanches.
    Nationell ämneskategori
    Meteorologi och atmosfärforskning Teknik och teknologier
    Forskningsämne
    Teknisk fysik med inriktning mot atmosfäriska urladdningar
    Identifikatorer
    urn:nbn:se:uu:diva-233639 (URN)000358572100153 ()978-1-4799-3544-4 (ISBN)
    Konferens
    International Conference on Lightning Protection (ICLP), OCT 11-18, 2014, Tsinghua Univ, Shanghai, PEOPLES R CHINA
    Tillgänglig från: 2014-10-07 Skapad: 2014-10-07 Senast uppdaterad: 2015-09-03Bibliografiskt granskad
    6. Lightning interference in multiple antennas wireless communication systems
    Öppna denna publikation i ny flik eller fönster >>Lightning interference in multiple antennas wireless communication systems
    Visa övriga...
    2012 (Engelska)Ingår i: Journal of Lightning Research, ISSN 1652-8034, Vol. 4, s. 155-165Artikel i tidskrift (Refereegranskat) Published
    Abstract [en]

    This paper analyzes the interference of lightning flashes with multiple antennas wireless communicationsystems operating in the microwave band at 2.4 GHz and 5.2 GHz. A bit error rate (BER) measurement method was usedto evaluate BER and packet error rate (PER) during 5 heavy thunderstorms on January 25 and March 17 to 20, 2011,respectively. In addition, BER measurements also were done on January 21 and March 30, 2011 under fair weather (FW)conditions providing a baseline for comparison. The Transmitter-Receiver separation was fixed at 10 meter with line-ofsight(LOS) consideration. We infer that lightning interfered with the transmitted digital pulses which resulted in a higherrecorded BER. The maximum recorded BER was 9.9·10-1 and the average recorded BER and PER were 2.07·10-2 and2.44·10-2 respectively during the thunderstorms with the average fair weather BER and PER values under the influence ofadjacent channel interference (ACI) and co-channel interference (CCI) being 1.75·10-5 and 7.35·10-6 respectively. Weconclude that multiple antennas wireless communication systems operating at the microwave frequency can besignificantly interfered by lightning.

    Ort, förlag, år, upplaga, sidor
    Bentham open, 2012
    Nationell ämneskategori
    Meteorologi och atmosfärforskning Teknik och teknologier
    Forskningsämne
    Teknisk fysik med inriktning mot atmosfäriska urladdningar
    Identifikatorer
    urn:nbn:se:uu:diva-190902 (URN)
    Tillgänglig från: 2013-01-09 Skapad: 2013-01-09 Senast uppdaterad: 2016-02-03
    7. Interference from cloud-to-ground and cloud flashes in wireless communication system
    Öppna denna publikation i ny flik eller fönster >>Interference from cloud-to-ground and cloud flashes in wireless communication system
    2014 (Engelska)Ingår i: Electric power systems research, ISSN 0378-7796, E-ISSN 1873-2046, Vol. 113, s. 237-246Artikel i tidskrift (Refereegranskat) Published
    Abstract [en]

    In this study, cloud-to-ground (CG) flash and intra-cloud (IC) flash events that interfere with the transmission of bits in wireless communication system operating at 2.4 GHz were analyzed. Bit error rate (BER) and consecutive lost datagram (CLD) measurement methods were used to evaluate BER and burst error from 3 tropical thunderstorms on November 27, 28, and 29 during 2012 northeastern monsoon in Malaysia. A total of 850 waveforms from the electric field change recording system were recorded and examined. Out of these, 94 waveforms of very fine structure were selected which matched perfectly with the timing information of the recorded BER. We found that both CG and IC flashes interfered significantly with the transmission of bits in wireless communication system. The severity of the interference depends mainly on two factors namely the number of pulses and the amplitude intensity of the flash. The interference level becomes worst when the number of pulses in a flash increases and the amplitude intensity of pulses in a flash intensifies. During thunderstorms, wireless communication system has experienced mostly intermittent interference due to burst error. Occasionally, in the presence of very intense NBP event, wireless communication system could experience total communication lost. In CG flash, it can be concluded that PBP is the major. source of interference that interfered with the bits transmission and caused the largest burst error. In IC flash, we found that the typical IC pulses interfered the bits transmission in the same way as PBP and mixed events in CG flash and produced comparable and in some cases higher amount of burst error. NBP has been observed to interfere the bits transmission more severely than typical IC and CG flashes and caused the most severe burst error to wireless communication system.

    Nyckelord
    Bit error rate, Cloud flash, Cloud-to-ground flash, Interference, Microwave radiation, Wireless system
    Nationell ämneskategori
    Elektroteknik och elektronik
    Identifikatorer
    urn:nbn:se:uu:diva-228679 (URN)10.1016/j.epsr.2014.03.022 (DOI)000337554200032 ()
    Tillgänglig från: 2014-07-22 Skapad: 2014-07-21 Senast uppdaterad: 2017-12-05Bibliografiskt granskad
  • 10.
    Ahmad, Mohd Riduan
    et al.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Tekniska sektionen, Institutionen för teknikvetenskaper, Elektricitetslära.
    Esa, Mona Riza Binti Mohd
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Tekniska sektionen, Institutionen för teknikvetenskaper, Elektricitetslära.
    Rahman, Mahbubur
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Tekniska sektionen, Institutionen för teknikvetenskaper, Elektricitetslära.
    Cooray, Vernon
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Tekniska sektionen, Institutionen för teknikvetenskaper, Elektricitetslära.
    Dutkiewicz, E.
    Wireless Communications and Networking Lab, Macquarie University, Sydney, Australien.
    Lightning interference in multiple antennas wireless communication systems2012Ingår i: Journal of Lightning Research, ISSN 1652-8034, Vol. 4, s. 155-165Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    This paper analyzes the interference of lightning flashes with multiple antennas wireless communicationsystems operating in the microwave band at 2.4 GHz and 5.2 GHz. A bit error rate (BER) measurement method was usedto evaluate BER and packet error rate (PER) during 5 heavy thunderstorms on January 25 and March 17 to 20, 2011,respectively. In addition, BER measurements also were done on January 21 and March 30, 2011 under fair weather (FW)conditions providing a baseline for comparison. The Transmitter-Receiver separation was fixed at 10 meter with line-ofsight(LOS) consideration. We infer that lightning interfered with the transmitted digital pulses which resulted in a higherrecorded BER. The maximum recorded BER was 9.9·10-1 and the average recorded BER and PER were 2.07·10-2 and2.44·10-2 respectively during the thunderstorms with the average fair weather BER and PER values under the influence ofadjacent channel interference (ACI) and co-channel interference (CCI) being 1.75·10-5 and 7.35·10-6 respectively. Weconclude that multiple antennas wireless communication systems operating at the microwave frequency can besignificantly interfered by lightning.

  • 11.
    Ahmad, Mohd Riduan
    et al.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Tekniska sektionen, Institutionen för teknikvetenskaper, Elektricitetslära.
    Mohd Esa, Mona Riza
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Tekniska sektionen, Institutionen för teknikvetenskaper, Elektricitetslära.
    Cooray, Vernon
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Tekniska sektionen, Institutionen för teknikvetenskaper, Elektricitetslära.
    Occurrence of Narrow Bipolar Event as Part of Cloud-to-Ground Flash in Tropical Thunderstorms2014Konferensbidrag (Refereegranskat)
  • 12.
    Ahmad, Mohd Riduan
    et al.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Tekniska sektionen, Institutionen för teknikvetenskaper, Elektricitetslära.
    Mohd Esa, Mona Riza
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Tekniska sektionen, Institutionen för teknikvetenskaper, Elektricitetslära.
    Cooray, Vernon
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Tekniska sektionen, Institutionen för teknikvetenskaper, Elektricitetslära.
    Similarity between the Initial Breakdown Pulses of Negative Ground Flash and Narrow Bipolar Pulses2014Ingår i: 2014 INTERNATIONAL CONFERENCE ON LIGHTNING PROTECTION (ICLP), IEEE conference proceedings, 2014, s. 810-813Konferensbidrag (Refereegranskat)
    Abstract [en]

    In this paper, temporal characteristics of several initial electric field pulses of preliminary breakdown process (PBP) from very close negative cloud-to-ground (CG) flashes are compared to close narrow bipolar pulses (NBPs) to observe any similarity that may exists. Interestingly, we found that the initial PBP pulses are similar to close NBP with zero crossing time less than 5 mu s, do not preceded by any slow field change and followed by pronounce static component. As NBPs are believed to be a result of relativistic runaway electron avalanches discharge, this finding is an indication that the initial electric field pulses of PBP are perhaps the result of the same discharge mechanism.

  • 13.
    Ahmad, Mohd Riduan
    et al.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Tekniska sektionen, Institutionen för teknikvetenskaper, Elektricitetslära.
    Mohd Esa, Mona Riza
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Tekniska sektionen, Institutionen för teknikvetenskaper, Elektricitetslära.
    Cooray, Vernon
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Tekniska sektionen, Institutionen för teknikvetenskaper, Elektricitetslära.
    Baharudin, Zikri Abadi
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Tekniska sektionen, Institutionen för teknikvetenskaper, Elektricitetslära. Universiti Teknikal Malaysia Melaka.
    Hettiarachchi, Pasan
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Tekniska sektionen, Institutionen för teknikvetenskaper, Elektricitetslära.
    Latitude Dependence of Narrow Bipolar Pulse Emissions2015Ingår i: Journal of Atmospheric and Solar-Terrestrial Physics, ISSN 1364-6826, E-ISSN 1879-1824, Vol. 128, s. 40-45Artikel i tidskrift (Refereegranskat)
  • 14.
    Ahmad, Mohd Riduan
    et al.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Tekniska sektionen, Institutionen för teknikvetenskaper, Elektricitetslära.
    Mohd Esa, Mona Riza
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Tekniska sektionen, Institutionen för teknikvetenskaper, Elektricitetslära.
    Cooray, Vernon
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Tekniska sektionen, Institutionen för teknikvetenskaper, Elektricitetslära.
    Hettiarachchi, Pasan
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Tekniska sektionen, Institutionen för teknikvetenskaper, Elektricitetslära.
    Baharudin, Zikri Abadi
    Universiti Teknikal Malaysia Melaka.
    Electric Field Signature of Narrow Bipolar Pulse Observed in Sweden2014Ingår i: Atmospheric research, ISSN 0169-8095, E-ISSN 1873-2895Artikel i tidskrift (Refereegranskat)
  • 15.
    Ahmad, Mohd Riduan
    et al.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Tekniska sektionen, Institutionen för teknikvetenskaper, Elektricitetslära.
    Mohd Esa, Mona Riza
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Tekniska sektionen, Institutionen för teknikvetenskaper, Elektricitetslära.
    Johari, Dalina
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Tekniska sektionen, Institutionen för teknikvetenskaper, Elektricitetslära.
    Ismail, Mohd Muzafar
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Tekniska sektionen, Institutionen för teknikvetenskaper, Elektricitetslära.
    Cooray, Vernon
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Tekniska sektionen, Institutionen för teknikvetenskaper, Elektricitetslära.
    Chaotic Pulse Train in Cloud-to-Ground and Cloud Flashes of Tropical Thunderstorms2014Konferensbidrag (Refereegranskat)
    Abstract [en]

    In this paper, we report for the first time the observation of chaotic pulse train (CPT) preceding natural subsequent negative return strokes and also CPT occurrence in IC flashes from tropical thunderstorms in South Malaysia. In CG flashes, all CPTs were occurred in between return strokes with 41.1% have occurred between the first and second return strokes. The maximum number of CPT in one sequence is 3, which can be observed between the first and third return strokes only. In IC flashes, all CPTs were observed to occur in between IC flash pulses.

  • 16.
    Ahmad, Mohd Riduan
    et al.
    UTEM, Durian Tunggal, Melaka, Malaysia.
    Periannan, Dinesh
    UTEM, Durian Tunggal, Melaka, Malaysia.
    Sabri, Muhammad Haziq Mohammad
    UTEM, Durian Tunggal, Melaka, Malaysia.
    Abd Aziz, Mohamad Zoinol Abidin
    UTEM, Durian Tunggal, Melaka, Malaysia.
    Esa, Mona Riza Mohd
    UTM, Inst Voltan Tinggi & Arus Tinggi IVAT, Johor Bharu, Malaysia.
    Lu, Gaopeng
    Chinese Acad Sci, Beijing, Peoples R China.
    Zhang, Hongbo
    Chinese Acad Sci, Beijing, Peoples R China.
    Cooray, Vernon
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Tekniska sektionen, Institutionen för teknikvetenskaper, Elektricitetslära.
    Emission Heights of Narrow Bipolar Events in a Tropical Storm over the Malacca Strait2017Ingår i: 2017 International Conference On Electrical Engineering And Computer Science (Icecos), IEEE , 2017, s. 305-309Konferensbidrag (Refereegranskat)
    Abstract [en]

    Emission heights for narrow bipolar events (NBEs) have been reported mostly from observations at mid latitudes but none have been reported from tropical regions. In this paper, we are reporting for the first time the heights of NBE emissions from a tropical storm over the Malacca Strait, a narrow water passage between the Malay Peninsula and Sumatra Island. A total of 49 positive NBEs (+NBEs) were detected from the storm. The NBE activity can be divided into two stages according to the emission heights and radar reflectivity data. The first stage (or S1) lasted for only 6 minutes, started with the first detected NBE, and produced 20 NBEs (41%). The emission heights ranged between 12.0 and 16.7 km. Radar reflectivity data showed that the storm reached maximum values at 55 dBZ within the period S1. In contrast, the second stage (S2) lasted longer (32 minutes) and produced 29 NBEs (59%). The emission heights were lower and ranged from 8.5 to 13.7 km. Radar reflectivity data showed that the storm reached maximum values at 50 dBZ within the period S2.

  • 17.
    Ahmad, M.R.
    et al.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Tekniska sektionen, Institutionen för teknikvetenskaper, Elektricitetslära.
    Rashid, M.
    Aziz, M.H.A.
    Esa, M.R.M.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Tekniska sektionen, Institutionen för teknikvetenskaper, Elektricitetslära.
    Cooray, Vernon
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Tekniska sektionen, Institutionen för teknikvetenskaper, Elektricitetslära.
    Rahman, Mahbubur
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Tekniska sektionen, Institutionen för teknikvetenskaper, Elektricitetslära.
    Dutkiewicz, E.
    Analysis of Lightning-induced Transient in 2.4 GHz Wireless Communication System2011Ingår i: Proceeding of IEEE International Conference on Space Science and Communication (IconSpace), Penang, Malaysia, pp225-230, 2011Konferensbidrag (Refereegranskat)
  • 18. Ahmad, Noor Azlinda
    et al.
    Baharudin, Zikri A
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Tekniska sektionen, Institutionen för teknikvetenskaper, Elektricitetslära.
    Fernando, M.
    Cooray, Vernon
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Tekniska sektionen, Institutionen för teknikvetenskaper, Elektricitetslära.
    Radiation field spectra of long-duration cloud flashes2015Ingår i: Atmospheric Science Letters, ISSN 1530-261X, E-ISSN 1530-261X, Vol. 16, nr 2, s. 91-95Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The preliminary results of radiation electric fields produced by long-duration cloud flashes have been Fourier analyzed to determine the frequency spectrum in the range of 10kHz-10MHz. The flashes were recorded within a distance of less than 20km. The spectrum was normalized to 50km distance and it shows a f(-1) dependence within the entire frequency range.

  • 19.
    Ahmad, Noor Azlinda
    et al.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Tekniska sektionen, Institutionen för teknikvetenskaper, Elektricitetslära.
    Fernando, Mahendra
    Baharudin, Z. A.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Tekniska sektionen, Institutionen för teknikvetenskaper, Elektricitetslära.
    Rahman, Mahbubur
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Tekniska sektionen, Institutionen för teknikvetenskaper, Elektricitetslära.
    Cooray, Vernon
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Tekniska sektionen, Institutionen för teknikvetenskaper, Elektricitetslära.
    Saleh, Ziad
    Dwyer, Joseph R.
    Rassoul, Hamid K.
    The first electric field pulse of cloud and cloud-to-ground lightning discharges2010Ingår i: Journal of Atmospheric and Solar-Terrestrial Physics, ISSN 1364-6826, E-ISSN 1879-1824, Vol. 72, nr 2-3, s. 143-150Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    In this study, the first electric field pulse of cloud and cloud-to-ground discharges were analyzed and compared with other pulses of cloud discharges. Thirty eight cloud discharges and 101 cloud-to-ground discharges have been studied in this analysis. Pulses in cloud discharges were classified as [`]small', [`]medium' and [`]large', depending upon the value of their relative amplitude with respect to that of the average amplitude of the five largest pulses in the flash. We found that parameters, such as pulse duration, rise time, zero crossing time and full-width at half-maximum (FWHMs) of the first pulse of cloud and cloud-to-ground discharges are similar to small pulses that appear in the later stage of cloud discharges. Hence, we suggest that the mechanism of the first pulse of cloud and cloud-to-ground discharges and the mechanism of pulses at the later stage of cloud discharges could be the same.

  • 20.
    Akyuz, Mose
    et al.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Tekniska sektionen, Institutionen för teknikvetenskaper, Elektricitetslära och åskforskning.
    Rahman, Mahbubur
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Tekniska sektionen, Institutionen för teknikvetenskaper, Elektricitetslära och åskforskning.
    Larsson, Anders
    Cooray, Vernon
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Tekniska sektionen, Institutionen för teknikvetenskaper, Elektricitetslära och åskforskning.
    Franke, Axel
    Characteristics of Laser-triggered Electric Discharges in Air2005Ingår i: IEEE transactions on dielectrics and electrical insulation, ISSN 1070-9878, E-ISSN 1558-4135, Vol. 12, nr 5, s. 1060-1070Artikel i tidskrift (Refereegranskat)
  • 21. Alda-Vidal, C.
    et al.
    Kooy, M.
    Rusca, Maria
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Luft-, vatten och landskapslära.
    Mapping operation and maintenance: an everyday urbanism analysis of inequalities within piped water supply in Lilongwe, Malawi2018Ingår i: Urban Geography, Vol. 39, nr 1, s. 104-121Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    In this article, we analyze the production of inequalities within the centralized water supply network of Lilongwe. We use a process-based analysis to understand how urban infrastructure is made to work and explain the disparity in levels of service by tracing the everyday practices of those who operate the infrastructure. This extends existing analyses of everyday practices in relation to urban water inequalities in African cities by focusing on formal operators, rather than water users, and looking within the networked system, rather than outside it. Our findings show that these practices work to exacerbate existing water stress in poor areas of the city. We conclude with a reflection on how understanding these practices as the product of the perceptions, rationalizations, and interpretations of utility staff who seek to manage the city’s (limited) water as best they can offers insight into what is required for a more progressive urban water politics.

  • 22. Alda-Vidal, C.
    et al.
    Rusca, Maria
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Luft-, vatten och landskapslära.
    Zwarteveen, M.
    Schwartz, K.
    Pouw, N.
    Occupational genders and gendered occupations: the case of water provisioning in Maputo, Mozambique2017Ingår i: Gender, Place and Culture: A Journal of Feminist Geography, ISSN 0966-369X, E-ISSN 1360-0524, Vol. 24, nr 7, s. 974-990Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Taking issue with how associations between technical prowess or entrepreneurship and masculinity tend to be taken for granted or are seen as stemming from natural or intrinsic gender differences, over the last two decades feminist scholars have developed theoretical approaches to understand the gendering of professions and abilities as the performative outcome of particular cultures and histories. We build on these insights to explore how associations between masculinities, technology and entrepreneurship shape ideas and practices of small-scale water provision in Maputo. Our findings show how activities (i.e. technical craftsmanship, hard physical work) or abilities (i.e. risk-taking, innovativeness) regarded as masculine tend to be considered the defining features of the profession. This shapes how men and women make sense of and talk about their work, each of them tactically emphasizing and performing those aspects best fitting their gender. Our detailed documentation of men’s and women’s everyday involvements in water provisioning challenges the existence of sharp boundaries and distinctions between genders and professional responsibilities. It shows that water provisioning requires many other types of work and skills and male and female household members collaborate and share their work. The strong normative-cultural associations between gender and water provisioning lead to a distinct under-recognition of women’s importance as water providers. We conclude that strategies to effectively support small-scale water businesses while creating more space and power for women involved in the business require the explicit recognition and re-conceptualization of water provisioning as a household business.

  • 23.
    Aldén, Liselotte
    et al.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper. Nätverket för vindbruk.
    Ridbäck, Ulrika
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper. Nätverket för vindbruk.
    Ny och pågående vindkraftsforskning i Sverige 2018: En sammanställning av svenska forskningsprogram och publikationer inom vindkraftsområdet2019Rapport (Övrig (populärvetenskap, debatt, mm))
    Abstract [sv]

    Uppsala universitet Campus Gotland producerar årligen en sammanställning av svensk forskning om vindkraft i serien Ny och pågående forskning om vindkraft i Sverige. Syftet med dessa publikationer är att intresserade ska få en lättillgänglig överblick av vad som skett under året inom vindkraftsforskningen i landet. Sammanställningen är indelad i två avsnitt. Det första är en presentation av de forskningsprogram och forskningscentra som är verksamma inom vindkraftsforskning i Sverige med fokus på deras pågående projekt. Därefter följer en ämnesindelad förteckning över publicerad forskning om vindkraft under aktuellt år vid svenska universitet, högskolor och institutioner. Där ingår vetenskapliga artiklar och rapporter, doktors- och licentiatavhandlingar samt uppsatser på kandidat-, magister- och mastersnivå samt sammanfattningar med statistik och jämförelser med tidigare år.

  • 24.
    Alm, Jens
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Meteorologiska institutionen.
    Fördelningen av blixtnedslag i Sverige1995Självständigt arbete på grundnivå (kandidatexamen), 10 poäng / 15 hpStudentuppsats (Examensarbete)
    Abstract [sv]

    Detta examensarbete har utförts för att bestämma fördelningen av blixtnedslag i Sverige. En databas med blixtdata för åren 1987-1994 har använts. Antalet blixtar per 100 km2 och månad har beräknats, de högsta medelvärdena är 10 blixtnedslag och dessa erhålles under juli månad. Andelen positiva blixtar har även undersökts och funnits variera beroende av latitud och temperatur. Under vintermånaderna är andelen positiva blixtnedslag större än under sommarmånaderna.

  • 25.
    Andersson, Ambjörn
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Luft-, vatten och landskapslära.
    Utvärdering av svavel- och kvävedepositioner från sjöfart: en modellstudie2010Självständigt arbete på avancerad nivå (magisterexamen), 20 poäng / 30 hpStudentuppsats (Examensarbete)
    Abstract [sv]

    Sjöfartsektorn är en betydande källa för svavel- och kväveutsläpp. Depositioner av oxiderat svavel, dvs. sulfat, och oxiderat kväve dvs. nitrat, i form av våt och torrdeposition bidrar till försurning och försämrad luftkvalitet. Styrmedel för att minska svavelutsläppen har under de senaste åren bidragit till minskade svavelutsläpp. Det har t.ex. införts begränsningar i hur mycket svavel som fartygsbränslen får innehålla.

    För att studera sjöfartens utsläpp av svavel- och kväveföroreningar har vi i denna studie använt en kemisk transportmodell för atmosfären ”the Unified EMEP Model”, för att simulera utsläppsspridningen under 2005. Deposition av svavel- och kväveföreningar från sjöfart har på landsnivå jämförts med den totala depositionen från alla utsläppskällor i Europa. Studier har även gjorts av hur mycket svavel- och kväveföreningar som har deponerats i Östersjöbassängen och dess omgivande länder från sjöfart i olika havsområden.

    Resultaten visar att svavel- och kväveföroreningar deponeras med hög koncentration nära utsläppskällan. Dock sprids de även långt från källan, koncentrationerna minskar med ökade avstånd. Deposition i norra Europa från sjöfart är som störst nära stora hamnar, spår i depositionsfälten syns även ut över Atlanten mot Amerika. Länder omgärdade av farvatten får högre koncentrationer relativt länder med mindre eller ingen sjötrafik i närheten.

    För att studera hur skrovlighetslängden över hav påverkar spridningen av svavel har Charnock-parametern varierats mellan 0,00072 och 0,085. Studien visar ingen tydlig påverkan på spridningen pga. variationer i skrovlighetslängden. En förklaring till detta resultat kan vara, att upplösningen i modellen är för grov för att kunna studera de små variationer som uppstår i depositionsfältet på grund av olika skrovlighetslängder.

    För att studera hur variationer i bl.a. nederbörd påverkar spridningen av svavel och kväve har månadsvis variation av svavel- och kvävedepositioner i Östersjön gjorts.

  • 26.
    Andersson, Andreas
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Luft-, vatten och landskapslära.
    Air-sea exchange of O2 and CO2: Processes controlling the transfer efficiency2017Doktorsavhandling, sammanläggning (Övrigt vetenskapligt)
    Abstract [en]

    World oceans cover more than 70% of the earth surface and constitutes a major sink of atmospheric CO2. Two of the most important gases in the marine carbon cycling are O2 and CO2 and hence accurate descriptions of the air-sea gas exchange of these gases are crucial. Still there is a lack of knowledge of the relative importance of processes controlling the efficiency of the air-sea gas transfer. This is especially true for Arctic and high latitude seas were studies on air-sea gas exchange are few. By studying processes causing water-side turbulence, using gases of different solubility and various measurement techniques, more knowledge on the governing processes can be obtained.

    Here we present the very first air-sea fluxes of O2 using atmospheric eddy covariance measurements and investigate the dependence between the gas transfer velocity of O2 and turbulence generated by the mean wind. The instrument was found to suffer from the limited precision and time response, causing significant corrections on the O2 flux. After correcting for this, the O2 fluxes displays an anti-correlation with the air-sea fluxes of CO2 in agreement with the measured air-sea gradient of O2. The transfer velocities for O2 indicates a stronger wind dependence than other commonly used parameterizations of the transfer velocity for CO2 and O2, this especially for wind speeds > 5 m s-1 where the typical onset of wave breaking occur.

    During two winter months eddy covariance measurements were taken over a high Arctic fjord. The data revealed a significant enhancement of the gas transfer velocity for CO2 from water-side convection, generated by cooling of surface waters. The dependence between water-side convection and gas transfer velocity were found for winds as high as 9 m s-1, but were strongest for wind speeds< 7  m s-1.  The data also showed on enhanced air-sea gas transfer of CO2 when conditions were unstable very close to neutral. This enhanced transfer were associated to increased contribution to the CO2 flux from downdraft of air with higher concentrations of CO2.  The combined effect of water-side convection and turbulence generated by wind results in a very effective transfer, thus the air-sea gas exchange at these latitudes may be significantly underestimated.

    Delarbeten
    1. Using a High-Frequency Fluorescent Oxygen Probe in Atmospheric Eddy Covariance Applications
    Öppna denna publikation i ny flik eller fönster >>Using a High-Frequency Fluorescent Oxygen Probe in Atmospheric Eddy Covariance Applications
    2014 (Engelska)Ingår i: Journal of Atmospheric and Oceanic Technology, ISSN 0739-0572, E-ISSN 1520-0426, Vol. 31, nr 11, s. 2498-2511Artikel i tidskrift (Refereegranskat) Published
    Abstract [en]

    During the years 2010-13, atmospheric eddy covariance measurement of oxygen was performed at the marine site Ostergarnsholm in the Baltic Sea. The fast response optode Microx TX3 was used with two different types of tapered sensors. In spite of the increased lifetime, the optical isolated sensor is limited by the slower response time and is unsuitable for ground-based eddy covariance measurements. The sensor without optical isolation shows a -2/3 slope within the inertial subrange and attains sufficient response time and precision to be used in air-sea applications during continuous periods of 1-4 days. Spectral and cospectral analysis shows oxygen measured with the nonoptical isolated sensor to follow the same shape as for CO2 and water vapor when normalized. The sampling rate of the Microx TX3 is 2Hz; however, the sensor was found to have a limited response and resolution, yielding a flux loss in the frequency range f > 0.3Hz. This can be corrected for by applying cospectral similarity simultaneously using measurements of latent heat as the reference signal. On average the magnitude of the cospectral correction added 20% to the uncorrected oxygen flux during neutral atmospheric stratification.

    Nationell ämneskategori
    Meteorologi och atmosfärforskning
    Identifikatorer
    urn:nbn:se:uu:diva-239999 (URN)10.1175/JTECH-D-13-00249.1 (DOI)000345008300009 ()
    Tillgänglig från: 2015-01-05 Skapad: 2015-01-05 Senast uppdaterad: 2017-12-05Bibliografiskt granskad
    2. Using eddy covariance to estimate air-sea gas transfer velocity for oxygen
    Öppna denna publikation i ny flik eller fönster >>Using eddy covariance to estimate air-sea gas transfer velocity for oxygen
    2016 (Engelska)Ingår i: Journal of Marine Systems, ISSN 0924-7963, E-ISSN 1879-1573, Vol. 159, s. 67-75Artikel i tidskrift (Refereegranskat) Published
    Abstract [en]

    Air-sea gas transfer velocity for O2 is calculated using directly measured fluxes with the eddy covariance technique. It is a direct method and is frequently used to determine fluxes of heat, humidity, and CO2, but has not previously been used to estimate transfer velocities for O2, using atmospheric eddy covariance data. The measured O2 fluxes are upward directed, in agreement with the measured air-sea gradient of the O-2 concentration, and opposite to the direction of the simultaneously measured CO2 fluxes. The transfer velocities estimated from measurements are compared with prominent wind speed parameterizations of the transfer velocity for CO2 and O2, previously established from various measurement techniques. Our result indicates stronger wind speed dependence for the transfer velocity of O2 compared to CO2 starting at intermediate wind speeds. This stronger wind speed dependence appears to coincide with the onset of whitecap formation in the flux footprint and the strong curvature of a cubic wind -dependent function for the transfer velocity provides the best fit to the data. Additional data using the measured O2 flux and an indirect method (based on the Photosynthetic Quotient) to estimate oxygen concentration in water, support the stronger wind dependence for the transfer velocity of O2 O-2 to CO2.

    Nationell ämneskategori
    Meteorologi och atmosfärforskning
    Forskningsämne
    Meteorologi
    Identifikatorer
    urn:nbn:se:uu:diva-287887 (URN)10.1016/j.jmarsys.2016.02.008 (DOI)000375506200006 ()
    Tillgänglig från: 2016-04-26 Skapad: 2016-04-26 Senast uppdaterad: 2017-11-30
    3. Air-sea gas transfer in high Arctic fjords
    Öppna denna publikation i ny flik eller fönster >>Air-sea gas transfer in high Arctic fjords
    Visa övriga...
    2017 (Engelska)Ingår i: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 44, nr 5, s. 2519-2526Artikel i tidskrift (Refereegranskat) Published
    Abstract [en]

    In Arctic fjords and high-latitude seas, strong surface cooling dominates during a large part of the year, generating water-side convection (w*w) and enhanced turbulence in the water. These regions are key areas for the global carbon cycle; thus, a correct description of their air-sea gas exchange is crucial. CO2-data were measured via the eddy covariance technique in marine Arctic conditions and reveal that water-side convection has a major impact on the gas transfer velocity. This is observed even at wind speeds as high as 9 m s-1, where convective motions are generally thought to be suppressed by wind-driven turbulence. The enhanced air-sea transfer of CO2 caused by water-side convection nearly doubled the CO2uptake, after scaled to open sea conditions the contribution from  to the CO2 flux remained as high as 34%; this phenomenon is expected to be highly important for the total carbon uptake in marine Arctic areas.

    Nyckelord
    air-sea gas exchange, transfer velocity, surface cooling, water-side convection, CO2 flux, Arctic
    Nationell ämneskategori
    Multidisciplinär geovetenskap
    Forskningsämne
    Meteorologi
    Identifikatorer
    urn:nbn:se:uu:diva-314161 (URN)10.1002/2016GL072373 (DOI)000398183700053 ()
    Tillgänglig från: 2017-01-29 Skapad: 2017-01-29 Senast uppdaterad: 2017-05-04Bibliografiskt granskad
    4. Enhanced air-sea exchange of CO2 over a high Arctic fjord during unstable very close to neutral conditions
    Öppna denna publikation i ny flik eller fönster >>Enhanced air-sea exchange of CO2 over a high Arctic fjord during unstable very close to neutral conditions
    Visa övriga...
    (Engelska)Manuskript (preprint) (Övrigt vetenskapligt)
    Abstract [en]

    Eddy covariance measurements over a high Arctic fjord reveals higher turbulent levels than normally found from classical surface layer theory. When conditions become unstable close to neutral i.e. -0.15<z/L< 0 the exchange coefficient for sensible heat, CH, is significantly enhanced compared to traditional parameterizations. Inspection of co-spectra of vertical wind (w) and temperature (T), wT show how a high frequency peak starts to develop around f≈1 Hz as z/L>-0.15, simultaneously quadrant analysis displays how the contribution from downdrafts to the vertical flux of temperature and CO2 increases. These findings are the signature of the evolving UVCN (Unstable Very Close to Neutral) regime, previously shown to enhance the vertical fluxes of temperature and humidity. In this study we show that the additional small scale turbulence related to these conditions also has the potential to enhance the vertical flux of CO2. Different to the vertical flux of temperature and humidity wq, the enhancement are not solely explained by the different properties of the air from aloft. We suggest that a part of the observed increase in CO2 flux and gas transfer velocity of CO2 when z/L> -0.1, also is generated by the increased levels of , causing higher levels of water-side turbulence. In winter the Arctic marine boundary layer is characterized by unstable stratification and during the nearly two months of measurements presented here as much as 36% of all data where associated to conditions with z/L in the range -0.15<z/L< 0.

    Nyckelord
    Transfer velocity, Arctic, UVCN, air-sea exchange
    Nationell ämneskategori
    Naturvetenskap
    Forskningsämne
    Meteorologi
    Identifikatorer
    urn:nbn:se:uu:diva-314162 (URN)
    Tillgänglig från: 2017-01-29 Skapad: 2017-01-29 Senast uppdaterad: 2017-01-29
  • 27.
    Andersson, Andreas
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Luft-, vatten och landskapslära.
    En statistisk kartläggning av dimma för Arlanda flygplats2010Självständigt arbete på grundnivå (kandidatexamen), 10 poäng / 15 hpStudentuppsats (Examensarbete)
    Abstract [sv]

    Dimma är ett komplext småskaligt väderfenomen som gäckat meteorologerna ända sedan väderprognosernas start och varit en bidragande orsak till att många mist livet inte minst inom flyget. Än idag är prognostisering av dimma mycket svårt i synnerhet strålningsdimma. Studier (COST, 2007) har visat att för att en modell ska kunna fånga formation av dimma krävs det i vissa fall att den skall kunna lösa upp inversionsskikt på 10m.Därtill uppgår osäkerheten i många fall för numeriska modeller vid fastställande av sikten i dimma till 50 %. Då kan en statistisk kartläggning specifikt för varje dimtyp utgöra ett bra komplement till modellerna för att få en uppfattning om exempelvis sikten i dimman. I den här studien användes observationer gjorda med en halvtimmes mellanrum från Arlanda under perioden 1993-07-01 – 2009-11-03.I denna kartläggning identifierades de fyra dimtyperna: strålningsdimma, nederbördsdimma, advektionsdimma samt dimma orsakad av en sänkning av molnbasen till marknivå (molndimma). Här framkom att strålningsdimman var den vanligaste dimtypen (43,6%) och mest frekvent under sommaren och hösten, till skillnad från nederbördsdimma och advektionsdimma som var vanligast under vinterhalvåret. För strålningsdimman fann man också de sämsta siktvärdena följt av molndimma. Det framkom också att frekvensen av de fall där strålningsdimman täcker hela flygplatsen avsevärt minskat sedan början på 2000-talet, detta samtidigt som utbyggnaden av flygplatsen stod klar.Vid studierna av isdimma och vattendimma visade det sig att den lägsta sikten för vattendimmor uppkom då temperaturen och den specifika fuktigheten var som högst. Medan för isdimma fann man de lägsta siktvärdena vid temperaturer runt -20 °C

  • 28.
    Andersson, Andreas
    et al.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Luft-, vatten och landskapslära.
    Rutgersson, Anna
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Luft-, vatten och landskapslära.
    Sahlee, Erik
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Luft-, vatten och landskapslära.
    Using eddy covariance to estimate air-sea gas transfer velocity for oxygen2016Ingår i: Journal of Marine Systems, ISSN 0924-7963, E-ISSN 1879-1573, Vol. 159, s. 67-75Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Air-sea gas transfer velocity for O2 is calculated using directly measured fluxes with the eddy covariance technique. It is a direct method and is frequently used to determine fluxes of heat, humidity, and CO2, but has not previously been used to estimate transfer velocities for O2, using atmospheric eddy covariance data. The measured O2 fluxes are upward directed, in agreement with the measured air-sea gradient of the O-2 concentration, and opposite to the direction of the simultaneously measured CO2 fluxes. The transfer velocities estimated from measurements are compared with prominent wind speed parameterizations of the transfer velocity for CO2 and O2, previously established from various measurement techniques. Our result indicates stronger wind speed dependence for the transfer velocity of O2 compared to CO2 starting at intermediate wind speeds. This stronger wind speed dependence appears to coincide with the onset of whitecap formation in the flux footprint and the strong curvature of a cubic wind -dependent function for the transfer velocity provides the best fit to the data. Additional data using the measured O2 flux and an indirect method (based on the Photosynthetic Quotient) to estimate oxygen concentration in water, support the stronger wind dependence for the transfer velocity of O2 O-2 to CO2.

  • 29.
    Andersson, Andreas
    et al.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Luft-, vatten och landskapslära.
    Rutgersson, Anna
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Luft-, vatten och landskapslära.
    Sahlée, Erik
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Luft-, vatten och landskapslära.
    Using a High-Frequency Fluorescent Oxygen Probe in Atmospheric Eddy Covariance Applications2014Ingår i: Journal of Atmospheric and Oceanic Technology, ISSN 0739-0572, E-ISSN 1520-0426, Vol. 31, nr 11, s. 2498-2511Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    During the years 2010-13, atmospheric eddy covariance measurement of oxygen was performed at the marine site Ostergarnsholm in the Baltic Sea. The fast response optode Microx TX3 was used with two different types of tapered sensors. In spite of the increased lifetime, the optical isolated sensor is limited by the slower response time and is unsuitable for ground-based eddy covariance measurements. The sensor without optical isolation shows a -2/3 slope within the inertial subrange and attains sufficient response time and precision to be used in air-sea applications during continuous periods of 1-4 days. Spectral and cospectral analysis shows oxygen measured with the nonoptical isolated sensor to follow the same shape as for CO2 and water vapor when normalized. The sampling rate of the Microx TX3 is 2Hz; however, the sensor was found to have a limited response and resolution, yielding a flux loss in the frequency range f > 0.3Hz. This can be corrected for by applying cospectral similarity simultaneously using measurements of latent heat as the reference signal. On average the magnitude of the cospectral correction added 20% to the uncorrected oxygen flux during neutral atmospheric stratification.

  • 30.
    Andersson, Andreas
    et al.
    Mittuniversitetet.
    Sjöblom, Anna
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Luft-, vatten och landskapslära.
    Sahlée, Erik
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Luft-, vatten och landskapslära.
    Falck, Eva
    UNIS.
    Rutgersson, Anna
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Luft-, vatten och landskapslära.
    Enhanced Air–Sea Exchange of Heat and Carbon Dioxide Over a High Arctic Fjord During Unstable Very-Close-to-Neutral Conditions2019Ingår i: Boundary-layer Meteorology, ISSN 0006-8314, E-ISSN 1573-1472, Vol. 170, nr 3, s. 471-488Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Eddy-covariance measurements made in the marine atmospheric boundary layer above a high Arctic fjord (Adventfjorden, Svalbard) are analyzed. When conditions are unstable, but close to neutral −0.1 < z/L < 0, where z is the height, and L is the Obukhov length, the exchange coefficient for sensible heat CH is significantly enhanced compared with that expected from classical surface-layer theory. Cospectra of the vertical velocity component (w) and temperature (T) reveal that a high-frequency peak develops at f ≈ 1 Hz for z/L > − 0.15. A quadrant analysis reveals that the contribution from downdrafts to the vertical heat flux increases as conditions become close to neutral. These findings are the signature of the evolving unstable very-close-to-neutral (UVCN) regime previously shown to enhance the magnitude of sensible and latent heat fluxes in the marine surface layer over the Baltic Sea. Our data reveal the significance of the UVCN regime for the vertical flux of the carbon dioxide (CO2) concentration (C). The cospectrum of w and C clearly shows how the high-frequency peak grows in magnitude for z/L > − 0.15, while the high-frequency peak dominates for z/L > − 0.02. As found for the heat flux, the quadrant analysis of the CO2 flux shows a connection between the additional small-scale turbulence and downdrafts from above. In contrast to the vertical fluxes of sensible and latent heat, which are primarily enhanced by the very different properties of the air from aloft (colder and drier) during UVCN conditions, the increase in the air–sea transfer of CO2 is possibly a result of the additional small-scale turbulence causing an increase in the water-side turbulence. The data indicate an increase in the gas-transfer velocity for CO2for z/L > − 0.15 but with a large scatter. During the nearly 2 months of continuous measurements (March–April 2013), as much as 36% of all data are associated with the stability range −0.15 < z/L < 0, suggesting that the UVCN regime is of significance in the wintertime Arctic for the air–sea transfer of heat and possibly also CO2.

  • 31.
    Andersson, Lovisa
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Luft-, vatten och landskapslära.
    Studie av MIUU-modellen och implementering av urbana parametrar i modellens energibalansrutin2009Självständigt arbete på avancerad nivå (magisterexamen), 20 poäng / 30 hpStudentuppsats (Examensarbete)
    Abstract [sv]

    Denna studie undersöker MIUU-modellen med dess befintliga parameterisering av den nedre begränsningsytan, vilken beskrivs med hjälp av en energibalansrutin. I energibalansrutinen, som presenterades av Deardorff (1978), beräknas yttemperaturen prognostiskt. Innan denna studie fanns inga parametervärden för urbana områden presenterade i MIUU-modellen. I varje gridpunkt beskrevs markytans egenskaper med hjälp av parametervärden för lerjord, endast med undantag för vattenytor. Här undersöks om MIUU-modellen klarar av att modellera den urbana miljöns påverkningar på de meteorologiska parametrarna genom att parametervärden i energiekvationen ändras till värden mer representativa för urbana områden. Parametrar som ändras i energibalansekvationen är albedot, emissiviteten, skrovlighetslängden, fuktighetsparametrar samt temperaturen nere i marken. Även effekter av ändringar av den geostrofiska vinden samt molnigheten undersöks.

    Resultat från studien visar att effekter av en urbanisering i MIUU-modellen kan ses under dagen då dessa påverkningar uttrycks genom en förhöjd marktemperatur över staden samt en större turbulent kinetisk energi p.g.a. städernas högre definierade skrovlighet. MIUU-modellen klarar, med sin nuvarande parameterisering av energibalansrutinen och med de i studien använda parametervärdena, dock inte av att modellera en urban värmeö efter solnedgång. Detta trots att det är då skillnader mellan rurala och urbana områden borde framträda som mest. Möjligen kan detta bero på de ändringar som gjorts av fuktighetsparametrarna eller på hur markvärmeflödet definierats i MIUU-modellen. Implementering av de urbana parametervärdena i MIUU-modellen resulterar nämligen i ett minskat markvärmeflöde över staden under såväl natt som dag. Ett minskat markvärmeflöde fås även vid en minskad avskärmningsfaktor, då blockeringen av den inkommande kortvågsstrålningen hos det definierade vegetationslagret är som minst.

  • 32.
    Andersson, Magnus
    et al.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Uppsala centrum för hållbar utveckling, CSD Uppsala, The Baltic University Programme.
    Tol, Richard S.J.
    Max Planck Institute for Meteorology in Hamburg.
    Graham, L. Phil
    Swedish Meteorological and Hydrological Institute.
    Bergström, Sten
    Swedish Meteorological and Hydrological Institute.
    Rydén, Lars
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Uppsala centrum för hållbar utveckling, CSD Uppsala, The Baltic University Programme.
    Azar, Christian
    University of Gothenburg.
    10. Impacts on the Global Atmosphere: Climate Change and Ozone Depletion2003Ingår i: Environmental Science: Understanding, protecting and managing the environment in the Baltic Sea Region / [ed] Lars Rydén, Pawel Migula and Magnus Andersson, Uppsala: Baltic University Press , 2003, 1, s. 294-323Kapitel i bok, del av antologi (Övrig (populärvetenskap, debatt, mm))
  • 33.
    Andersson, Stefan
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper.
    Undersökning av luftkvaliteten vid småskalig biobränsleförbränning i två kommuner med modellsystemet VEDAIR2007Självständigt arbete på grundnivå (yrkesexamen), 20 poäng / 30 hpStudentuppsats
    Abstract [sv]

    Ett internetverktyg, VEDAIR, för bedömning av luftkvalitet vid småskalig förbränning av biobränsle har utvecklats av SMHI med finansiering från bland andra Energimyndigheten och Naturvårdsverket. Modellen består av användargränssnitt och ett kopplat modellsystem där halterna av luftföroreningar beräknas för såväl regionala (internationellt som nationellt), urbana och lokala bidrag. I denna rapport presenteras modellen tillsammans med en första studie av luftkvaliteten med avseende på PM10 (partiklar med en diameter mindre än 10 µm) inom ett par testkommuner, nämligen Vänersborg i Västergötlands län och Gnosjö i Jönköpings län.

    Resultatet visade att det finns områden även i små kommuner där sämre luftkvalitet kan utgöra ett problem. Till största delen berodde detta på hög emission från fastigheter med äldre vedeldningspannor utan ackumulatortank.

    I Vänersborg undersöktes luftkvaliteten med avseende på PM10 i stadsdelarna Vargön, Mariedal och centrum för år 2003. För hela kommunen gällde att övre utvärderingströskel (gränsvärde för vilka mätningar är obligatoriska), som för årsmedelvärdet PM10 är 14 μg/m3 luft, överskrids pga bakgrundsbidrag. Samtliga av de undersökta områdena uppvisade dock halter som är lägre än miljökvalitetsnormerna. Beräkningar i VEDAIR för Vargön, inom vilket vedeldningen är huvudsakliga källan till emissionen av PM10, visade att koncentrationen partiklar inom vissa områden var lika höga eller högre än områden runt de mest trafikerade vägarna i centrala Vänersborg (årsmedelvärde på ca 20 μg/m3). I centrum var dock föroreningarna mer utbredda. Allra högst halter PM10 erhölls vid beräkningar i Mariedal, till följd av utbredd vedeldning med äldre vedpannor, trafikerade vägar och ogynnsamma meteorologiska förutsättningar. Enligt detta kan mätning av partiklar inom området vara befogat. Tidsserier av totalhalten PM10 visade att högst halter erhölls på vintern i Vargön, till följd av större uppvärmningsbehov i kombination med lägre utomhustemperatur och mer stabil skiktning. Det lokala bidraget uppvisade en negativ korrelation mot temperaturen. I centrum var halterna dock högst på våren pga slitagepartiklar från dubbdäck samt vägdamm.

    För Gnosjö undersöktes luftkvaliteten i tre potentiella problemområden; Gårö, centrala Gnosjö och Götarp. Beräkningar i VEDAIR för Gnosjö visade att trafikens bidrag var betydligt mindre, men vedeldning var vanligt förekommande. Samtidigt var årsmedelvärdet för det regionala bakgrundsbidraget ca 1 μg/m3 högre än i Vänersborg. Luftkvaliteten var generellt sett likvärdig den för Vänersborg, men inom stadsdelen Gårö fanns ett område där koncentrationen PM10 var mycket nära miljökvalitetsnormerna. Huvudkällan till emissionen var en enskild fastighet med stort energibehov som värmdes upp uteslutande med ved.

    I studien gjordes även jämförelse av beräknade PM10-halter från VEDAIR med uppmätta värden i Gnosjö för perioden 3 november till 31 december 2003. Valideringen visade att VEDAIR gav något underskattade, men ändå hyggliga resultat. Tidigare valideringar har enbart utförts för Norrland, och de nya resultaten tyder på att VEDAIR även ger tillförlitliga beräkningar för södra Sverige.

    Studien visade också att om en enskild fastighetsägare skulle byta ut en äldre vedeldningspanna mot en modern med ackumulatortank skulle årsmedelvärdet PM10 lokalt i fastighetens närområde kunna sjukna med uppåt 4 μg/m3, vilket är en minskning med 25%.

  • 34.
    Andersson, Ulrika
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Luft-, vatten och landskapslära.
    Åska längs Sveriges kuster2002Självständigt arbete på avancerad nivå (magisterexamen), 20 poäng / 30 hpStudentuppsats (Examensarbete)
    Abstract [sv]

    Åskan har i alla tider påverkat livet på jorden med sin enorma kraft. Det har skapats och berättats många skrönor och historier angående olika budskap och bestraffningar i samband med åska. Inte förrän i modern tid har människan börjat att förstå uppkomsten och fysiken bakom åska, som inte än är helt förklarat i detalj. Sedan år 1987, då blixtpejlsystemet LLP (Lightning Location System) installerades i Sverige, har blixtnedslagen och deras egenskaper registrerats och sparats i stora databaser i Sverige. Detta kan ses som en milstolpe i åskforskningen, då mer information om ett blixtnedslag går att erhållas. Utbyggnader och effektivisering av LLP-systemet (blixtpejlsystemet) sker nu i samarbete med andra nordiska länder.

    Examensarbetet börjar med en introduktion om hur åska uppkommer samt dess elektriska uppladdningsprocesser för att sedan vidare gå in på blixtens fysik. Därefter följer en presentation av LLP-systemet, dess uppbyggnad och funktion.

    Fördelningen av blixtnedslag längs södra Sveriges kuster under olika förutsättningar, som tid på året, blixtens amplitud och nedslagsplats har studerats. Olika uppkomna effekter av blixtdensiteten längs kusterna till följd av meteorologiska faktorer, som land- och sjöbris, hävningseffekter samt konvektion har studerats. Studier visar att blixtnedslagens densitet är beroende av underlaget och dess egenskaper, som varierar under årstiderna. Sommartid observeras fler blixtnedslag in över land, medan för höstperioden förflyttas detta maximum ut över hav. Land- och sjöbris kan på västkusten ha en påverkande effekt, då ett intensitets-minimum observeras strax utanför kustbandet.

    En fallstudie av en åskfrontpassage, som passerade över södra och mellersta Sverige den 16 augusti 2001, har studerats med avseende på den uppkomna tråglinjen, så kallad ”squall line”, i samband med frontpassagen. Den genererade kraftig och intensiv blixtaktivitet innan själva frontens ankomst västerifrån.

    Här ovan har enbart meteorologiska effekter på uppkomsten av åska behandlats. Även den lokala mark- och havsytan kan ge upphov till elektriska fenomen, som har betydelse för uppkomsten av blixtar. Därför studerades de elektriska rymdladdningarna i atmosfären och olika effekter beroende på underlagets skrovlighet. De teoretiska studierna visar på en ökning av rymdladdningsdensiteten över områden med skrovligare underlag, som in över land, vid kraftiga konvektiva moln med höga elektriska fält. Lägre rymdladdningsdensitet över havsytor medför att över hav kan det förekomma mycket högre elektriska fältstyrkor än över ojämn mark. Däremot kan höga föremål ute till havs, som master och vindkraftverk, lätt utlösa blixturladdningar.

  • 35. Andin, Caroline
    et al.
    Zdanowicz, Christian M.
    Copland, Luke
    Synoptic variability of extreme snowfall in the St. Elias Mountains,Yukon, Canada2015Konferensbidrag (Refereegranskat)
    Abstract [en]

    Glaciers in the Wrangell and St. Elias Mountains (Alaska and Yukon) are presently experiencing some of thehighest regional wastage rates worldwide. While the effect of regional temperatures on glacier melt rates in thisregion has been investigated, comparatively little is known about how synoptic climate variations, for example inthe position and strength of the Aleutian Low, modulate snow accumulation on these glaciers. Such informationis needed to accurately forecast future wastage rates, glacier-water resource availability, and contributions tosea-level rise. Starting in 2000, automated weather stations (AWS) were established in the central St-EliasMountains (Yukon) at altitudes ranging from 1190 to 5400 m asl, to collect climatological data in support ofglaciological research. These data are the longest continuous year-round observations of surface climate everobtained from this vast glaciated region. Here we present an analysis of snowfall events in the icefields of theSt-Elias Mountains based on a decade-long series of AWS observations of snow accumulation. Specifically,we investigated the synoptic patterns and air mass trajectories associated with the largest snowfall events (> 25cm/12 hours) that occurred between 2002 and 2012. Nearly 80% of these events occurred during the cold season(October-March), and in 74 % of cases the precipitating air masses originated from the North Pacific south of50N. Zonal air mass advection over Alaska, or from the Bering Sea or the Arctic Ocean, was comparativelyrare (20%). Somewhat counter-intuitively, dominant surface winds in the St. Elias Mountains during highsnowfall events were predominantly easterly, probably due to boundary-layer frictional drag and topographicfunneling effects. Composite maps of sea-level pressure and 700 mb winds reveal that intense snowfall eventsbetween 2002 and 2012 were associated with synoptic situations characterized by a split, eastwardly-shifted orlongitudinally-stretched Aleutian Low (AL) having an easternmost node near the Kenai Peninsula, conditionsthat drove a strong southwesterly upper airstream across the Gulf of Alaska towards the coast. Situations with asingle-node, westerly-shifted AL were comparatively rare. The spatial configuration of the synoptic AL pressurepattern appears to play a greater role in determining snowfall amount in the central St. Elias Mountains than dopressure anomalies within the AL. The estimated snowfall gradient from coastal Alaska to the central St. EliasMountains during intense snowfall events averaged +2.0 0.7 mm/km (SWE), while the continental-side gradientfrom the mountains towards the Yukon plateau averaged -3.3 0.9 mm/km (SWE). The findings presented herecan better constrain the climatic interpretation of long proxy records of snow accumulation variations developedfrom glacier cores drilled in the St. Elias Mountains or nearby regions.

  • 36.
    Andrae, Ulf
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Meteorologiska institutionen.
    Turbulence structures in a non-stationary marine atmospheric boundary layer1996Självständigt arbete på grundnivå (kandidatexamen), 10 poäng / 15 hpStudentuppsats (Examensarbete)
    Abstract [en]

    The vertical structure in the coastal marine atmosphere has been investigated using data from aircraft measurements performed along the Blekinge coast. The present data are from the third of October 1990. The main feature is fairly homogeneous horizontal conditions and a subceeing boundary layer which lowers from 600 meters down to about 50 meters during the day. The turbulence were found to be in a decreasing state. The turbulence parameters were normalized using normal stationary scaling, in order to compare with other results.

  • 37.
    ANDREN, A
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper.
    THE STRUCTURE OF STABLY STRATIFIED ATMOSPHERIC BOUNDARY-LAYERS - A LARGE-EDDY SIMULATION STUDY1995Ingår i: QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, ISSN 0035-9009, Vol. 121, nr 525, s. 961-985Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Dry stably-stratified planetary boundary layers have been studied by large-eddy simulation. Results from simulations using two different subgrid-scale models are compared. A recently suggested improved subgrid-scale model version is shown to give improve

  • 38.
    Ann-Sofi Smedman, Hans. Bergström and Ulf Högstrmöm
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper.
    A study of low level jets over the Baltic Sea east of the island of Gotland1996Ingår i: Wind Energy Report, nr WE 96:2Rapport (Övrigt vetenskapligt)
  • 39.
    Ann-Sofi Smedman, Hans. Bergström and Ulf Högstrmöm
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper.
    Measured and modelled local wind field over a frozen lake in a mountainous area1995Ingår i: Wind Energy Report, nr WE 95:4Rapport (Övrigt vetenskapligt)
  • 40.
    Anton, Nygren
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Luft-, vatten och landskapslära.
    Black Carbons Effects on Climate: Can We Even Say Something about Them?2016Självständigt arbete på grundnivå (kandidatexamen), 10 poäng / 15 hpStudentuppsats (Examensarbete)
    Abstract [sv]

    Sot påverkar atmosfären, moln och albedot i snö. Dessa effekter av sot är ett faktum, men exakt vad de innebär är inte väl känt. Atmosfären, albedo i iskristaller och snö, uppvärmningen av snö och isytor, molns livstid och molntäckets utbredning påverkas av sot eftersom sot absorberar strålning, vilket förändra strålningsdrivningen och kan förånga moln, och att sot kan vara nuklider som kan skapa moln.Många modeller och mätningar har gjorts för att utvärdera effekterna av sot men de visar mycket olika resultat. Denna litteraturstudie undersöker artiklar för att se vad och hur mycket vi vet om sot och dess klimatpåverkan. Jag drar slutsatsen att det har finns stora skillnader i resultat vilket återspeglas i det stora intervall som sot tros ändra strålningsdrivningen i atmosfären (resultat som sträcker sig från 0,4 till 1,2 Wm-2) och snö (0,007 till 0.054 Wm-2), liksom albedo i snö (resultat som sträcker sig från -0,02 till -0,17 Wm-2). Jag samt mång andra föreslår att modeller tenderar att få väldigt olika resultat eftersom de använder olika startvillkor, ekvationer och eftersom modellerna hanterar aerosoler och moln på olika sätt. Detta är till stor del på grund av bristande information, särskilt när det gäller aerosoler och moln.

  • 41. Arevalo, Liliana
    et al.
    Cooray, Vernon
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Tekniska sektionen, Institutionen för teknikvetenskaper, Elektricitetslära.
    Streamer to leader transition criteria for propagation of long sparks and lightning leaders2014Ingår i: 2014 INTERNATIONAL CONFERENCE ON LIGHTNING PROTECTION (ICLP), IEEE conference proceedings, 2014, s. 480-483Konferensbidrag (Refereegranskat)
    Abstract [en]

    Certain models have been dedicated to analyze the breakdown of long spark gaps and the lightning attachment process based on the mechanism of leader propagation. One of the most important processes on the mechanism of leader is the transition between streamers to leader. The streamer to leader transition is characterized by a rapid increase in the electron density and gas temperature, which is a consequence of the onset of thermal-ionization instability. To simplify the complexity of the physical process lightning attachment and long spark gaps models assumed that a minimum charge of 1 mu C is necessary to thermalize a leader channel, independently of the electric field and atmospheric conditions as temperature, pressure and humidity. In this paper an approach that takes into account the continuity equations and the gas temperature balance equation is used to investigate the minimum charge required to start the streamer to leader transition. The obtained results are compared with the minimum charge criteria used for long spark gaps and lightning attachment modeling. Simulation shows that the required charge to thermalize a leader depends on the vibrational energy relaxation. Results also indicate that only a small part of the energy input, transferred by electrons to gas molecules in the stem, contributes immediately to the temperature rise.

  • 42.
    Arevalo, Liliana
    et al.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Tekniska sektionen, Institutionen för teknikvetenskaper, Elektricitetslära. Ludvika ULHC, ABB Power Grids Grid Integrat HVDC, Dept Res & Dev, Lyviksvagen 3, S-77180 Lyviksvagen, Sweden..
    Cooray, Vernon
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Tekniska sektionen, Institutionen för teknikvetenskaper, Elektricitetslära.
    Unstable Leader Inception Criteria of Atmospheric Discharges2017Ingår i: Atmosphere, ISSN 2073-4433, E-ISSN 2073-4433, Vol. 8, nr 9, artikel-id 156Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    In the literature, there are different criteria to represent the formation of a leader channel in short and long gap discharges. Due to the complexity of the physics of the heating phenomena, and the limitations of the computational resources, a simplified criterion for the minimum amount of electrical charge required to incept an unstable leader has recently been used for modeling long gap discharges and lightning attachments. The criterion is based on the assumption that the total energy of the streamer is used to heat up the gas, among other principles. However, from a physics point of view, energy can also be transferred to other molecular processes, such as rotation, translation, and vibrational excitation. In this paper, the leader inception mechanism was studied based on fundamental particle physics and the energy balance of the gas media. The heating process of the plasma is evaluated with a detailed two-dimensional self-consistent model. The model is able to represent the streamer propagation, dark period, and unsuccessful leaders that may occur prior to the heating of the channel. The main processes that participate in heating the gas are identified within the model, indicating that impact ionization and detachment are the leading sources of energy injection, and that recombination is responsible for loss of electrons and limiting the energy. The model was applied to a well-known experiment for long air gaps under positive switching impulses reported in the literature, and used to validate models for lightning attachments and long gap discharges. Results indicate that the streamer-leader transition depends on the amount of energy transferred to the heating process. The minimum electric charge required for leader inception varies with the gap geometry, the background electric field, the reduction of electric field due to the space charge, the energy expended on the vibrational relation, and the environmental conditions, among others.

  • 43. Arevalo, Liliana
    et al.
    Cooray, Vernon
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Tekniska sektionen, Institutionen för teknikvetenskaper, Elektricitetslära.
    Upward leader inception caused by a sudden change of cloud electric field2014Ingår i: 2014 INTERNATIONAL CONFERENCE ON LIGHTNING PROTECTION (ICLP), IEEE conference proceedings, 2014, s. 484-487Konferensbidrag (Refereegranskat)
    Abstract [en]

    Discharge processes such as glow, streamer, and leader inception among others take place before an upward leader can be launched from a grounded structure during thunderstorms. Electrostatic fields below the thundercloud could lead to the formation of glow charge from grounded objects. If the electric field is high enough and ionization keeps expanding into the gap, streamers can be incepted. Depending on the available charge and the thermodynamic properties of the gas, there is a possibility to incept or not a positive upward leader towards the cloud. Usually, the inception of positive upward leaders is directly related with the appearance of a downward coming leader from cloud towards the grounded object. Such a downward leader will intensify the electric field in such a way that the streamer discharges could thermalize and produce an unstable upward leader channel. However, experimental observations have indicated the inception of upward leaders from grounded structures without registering connecting downward leaders towards the structure. The present paper intends to explain the inception of positive upward leaders from the top of a rod, whenever the electric field produced by the cloud suddenly changes e.g. due to intra-cloud discharges or distance cloud to ground flash. A two dimensional model based on the gas-dynamic equations, the main processes responsible for gas heating such as vibrational excitation and transfer of energy into electronic, rotational and translational excitation, coupled with Poisson equation is presented in this paper. Rods of different lengths under thundercloud electric field were studied. Simulation results indicate that positive upward leaders can be incepted from long rods under certain conditions of thundercloud electric field without the need of a coming downward leader. However, for rods of tenths of meters the thundercloud electric field is not enough to incept positive upward leaders and an intensification of the electric field is required in order to incept a positive upward leader from the structure, e.g., a coming downward leader.

  • 44. Arevalo, Liliana
    et al.
    Wu, Dong
    Hettiarachchi, Pasan
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Tekniska sektionen, Institutionen för teknikvetenskaper, Elektricitetslära.
    Cooray, Vernon
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Tekniska sektionen, Institutionen för teknikvetenskaper, Elektricitetslära.
    Lobato, André
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Tekniska sektionen, Institutionen för teknikvetenskaper, Elektricitetslära.
    Rahman, Mahbubur
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Tekniska sektionen, Institutionen för teknikvetenskaper, Elektricitetslära.
    Wooi, Chin-Leong
    The leader propagation velocity in long air gaps2018Konferensbidrag (Refereegranskat)
  • 45.
    Arnal, Louise
    et al.
    Univ Reading, Dept Geog & Environm Sci, Reading RG6 6AB, Berks, England.;European Ctr Medium Range Weather Forecasts, Shinfield Pk, Reading RG6 9AX, Berks, England..
    Cloke, Hannah L.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Luft-, vatten och landskapslära. Univ Reading, Dept Geog & Environm Sci, Reading RG6 6AB, Berks, England.;Univ Reading, Dept Meteorol, Reading RG6 6BB, Berks, England.;CNDS, Ctr Nat Hazards & Disaster Sci, S-75236 Uppsala, Sweden..
    Stephens, Elisabeth
    Univ Reading, Dept Geog & Environm Sci, Reading RG6 6AB, Berks, England..
    Wetterhall, Fredrik
    European Ctr Medium Range Weather Forecasts, Shinfield Pk, Reading RG6 9AX, Berks, England..
    Prudhomme, Christel
    European Ctr Medium Range Weather Forecasts, Shinfield Pk, Reading RG6 9AX, Berks, England.;Loughborough Univ Technol, Dept Geog, Loughborough LE11 3TU, Leics, England.;NERC Ctr Ecol & Hydrol, Wallingford OX10 8BB, Oxon, England..
    Neumann, Jessica
    Univ Reading, Dept Geog & Environm Sci, Reading RG6 6AB, Berks, England..
    Krzeminski, Blazej
    European Ctr Medium Range Weather Forecasts, Shinfield Pk, Reading RG6 9AX, Berks, England..
    Pappenberger, Florian
    European Ctr Medium Range Weather Forecasts, Shinfield Pk, Reading RG6 9AX, Berks, England..
    Skilful seasonal forecasts of streamflow over Europe?2018Ingår i: Hydrology and Earth System Sciences, ISSN 1027-5606, E-ISSN 1607-7938, Vol. 22, nr 4, s. 2057-2072Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    This paper considers whether there is any added value in using seasonal climate forecasts instead of historical meteorological observations for forecasting streamflow on seasonal timescales over Europe. A Europe-wide analysis of the skill of the newly operational EFAS (European Flood Awareness System) seasonal streamflow forecasts (produced by forcing the Lisflood model with the ECMWF System 4 seasonal climate forecasts), benchmarked against the ensemble streamflow prediction (ESP) forecasting approach (produced by forcing the Lisflood model with historical meteorological observations), is undertaken. The results suggest that, on average, the System 4 seasonal climate forecasts improve the streamflow predictability over historical meteorological observations for the first month of lead time only (in terms of hindcast accuracy, sharpness and overall performance). However, the predictability varies in space and time and is greater in winter and autumn. Parts of Europe additionally exhibit a longer predictability, up to 7 months of lead time, for certain months within a season. In terms of hindcast reliability, the EFAS seasonal streamflow hindcasts are on average less skilful than the ESP for all lead times. The results also highlight the potential usefulness of the EFAS seasonal streamflow forecasts for decision-making (measured in terms of the hindcast discrimination for the lower and upper terciles of the simulated streamflow). Although the ESP is the most potentially useful forecasting approach in Europe, the EFAS seasonal streamflow forecasts appear more potentially useful than the ESP in some regions and for certain seasons, especially in winter for almost 40 % of Europe. Patterns in the EFAS seasonal streamflow hindcast skill are however not mirrored in the System 4 seasonal climate hindcasts, hinting at the need for a better understanding of the link between hydrological and meteorological variables on seasonal timescales, with the aim of improving climate-model-based seasonal streamflow forecasting.

  • 46.
    Arnqvist, Johan
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Luft-, vatten och landskapslära.
    Mean Wind and Turbulence Conditions in the Boundary Layer above Forests2015Doktorsavhandling, sammanläggning (Övrigt vetenskapligt)
    Abstract [en]

    As wind turbines have grown, new installation areas become possible. Placing wind turbines in forested landscapes introduce uncertainties to the wind resource estimation. Even though close-to-canopy processes have been studied intensively during the last thirty years, the focus has mostly been on exchange processes and the height span of the studies has been below the rotor of a modern wind turbine.

    This thesis contains analysis of new measurements from a 138 m high tower in a forested landscape. The previous knowledge of near-canopy processes is extended to the region above the roughness sublayer. It is shown that above the roughness sublayer, the surface layer behaves as over low vegetation, and Monin-Obukhov similarity is shown to hold for several variables. However, in stable stratification, effects that could be linked to the boundary layer depth are shown to be present in the measurements. These include wind turning with height, the behaviour of the turbulence length scale and the curvature of the wind profile.

    Two new analytical models are presented in the thesis. One is a flux-profile expression in the roughness sublayer, which allows for analytical integration of the wind gradient. The model suggests that the roughness-sublayer effect depends on stratification and that the aerodynamic roughness length changes with stability. A decrease of roughness length in stable stratification is confirmed with a new method to determine the roughness length using measurements from the 138 m tower.

    The other model determines the spectral tensor in stable stratification using analytical solution to the rapid distortion equations for stratified shear flow, with homogeneous stratification and shear. By using a formulation for the integration time of the distortions of an isotropic spectrum, a model is derived which provides the cross spectra of velocity and temperature at any two given points in space.

    Finally the existence of waves in the wind over forests is investigated and it is concluded that the Kelvin-Helmholtz instability can create waves which are coherent in time and exist over the entire height span of wind turbine rotors. Linear wave theory is shown to be able to explain certain features of the waves.

    Delarbeten
    1. Flux-profile relation with roughness sublayer correction
    Öppna denna publikation i ny flik eller fönster >>Flux-profile relation with roughness sublayer correction
    2015 (Engelska)Ingår i: Quarterly Journal of the Royal Meteorological Society, ISSN 0035-9009, E-ISSN 1477-870X, Vol. 141, nr 689, s. 1191-1197Artikel i tidskrift (Refereegranskat) Published
    Abstract [en]

    Calculation of momentum flux using Monin–Obukhov similarity theory over forested areas is well known to underestimate the flux. Several suggestions of corrections to the standard flux-profile expression have been proposed in order to increase the magnitude of turbulent flux. The aim of this article is to find a simple, analytical representation for the characteristics of the flow within the canopy layer and the surface layer, including the roughness sublayer. A new form of the roughness sublayer correction is proposed, based on the desire to connect the shape of the roughness sublayer correction to forest characteristics. The new flux-profile relation can be used to find the flux or the wind profile whenever simple and fast estimations are needed, as for mesoscale modelling, scalar transport models, or sound propagation models.

    Nyckelord
    roughness sublayer; wind profile; dimensionless gradient; stability expressions
    Nationell ämneskategori
    Meteorologi och atmosfärforskning
    Identifikatorer
    urn:nbn:se:uu:diva-237757 (URN)10.1002/qj.2426 (DOI)000356805700016 ()
    Tillgänglig från: 2014-12-04 Skapad: 2014-12-04 Senast uppdaterad: 2017-12-05Bibliografiskt granskad
    2. Wind statistics from a forested landscape
    Öppna denna publikation i ny flik eller fönster >>Wind statistics from a forested landscape
    2015 (Engelska)Ingår i: Boundary-layer Meteorology, ISSN 0006-8314, E-ISSN 1573-1472, Vol. 156, nr 1, s. 53-71Artikel i tidskrift (Refereegranskat) Published
    Abstract [en]

    An analysis and interpretation of measurements from a 138-m tall tower located in a forested landscape is presented. Measurement errors and statistical uncertainties are carefully evaluated to ensure high data quality. A 40∘ wide wind-direction sector is selected as the most representative for large-scale forest conditions, and from that sector first-, second- and third-order statistics, as well as analyses regarding the characteristic length scale, the flux-profile relationship and surface roughness are presented for a wide range of stability conditions. The results are discussed with focus on the validity of different scaling regimes. Significant wind veer, decay of momentum fluxes and reduction in shear length scales with height are observed for all stability classes, indicating the influence of the limited depth of the boundary layer on the measured profiles. Roughness sublayer characteristics are however not detected in the presented analysis. Dimensionless gradients are shown to follow theoretical curves up to 100 m in stable conditions despite surface-layer approximations being invalid. This is attributed to a balance of momentum decay and reduced shear length scale growth with height. The wind profile shows a strong stability dependence of the aerodynamic roughness length, with a 50 % decrease from neutral to stable conditions.

    Nyckelord
    Above canopy turbulence statistics Atmospheric boundary layers, Decreasing roughness, Forest Canopy Flows, Wind power
    Nationell ämneskategori
    Meteorologi och atmosfärforskning
    Identifikatorer
    urn:nbn:se:uu:diva-237762 (URN)10.1007/s10546-015-0016-x (DOI)000355153200004 ()
    Tillgänglig från: 2014-12-04 Skapad: 2014-12-04 Senast uppdaterad: 2017-12-05Bibliografiskt granskad
    3. A spectral model for stably stratied turbulence
    Öppna denna publikation i ny flik eller fönster >>A spectral model for stably stratied turbulence
    (Engelska)Ingår i: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645Artikel i tidskrift (Refereegranskat) Submitted
    Abstract [en]

    A solution of the inviscid rapid distortion equations of a stratied flow with homogeneous shear is proposed, extending the work of Hanazaki and Hunt (J. Fluid Mech., 2004,vol. 507, pp. 1-42) to the two horizontal velocity components. The analytical solution allowed the determination of the spectral tensor evolution at any given time starting from a known initial condition. By following the same approach adopted by Mann (J.Fluid Mech., 1994, vol. 273, pp. 141-168), a model for the velocity spectral tensor in the atmospheric boundary layer is obtained where the spectral tensor, assumed to be isotropic at the initial time, evolves until the break-up time where the spectral tensor is supposed to achieve its final state observed in the boundary layer. The model predictions are compared with atmospheric measurements obtained over a forested area, giving the opportunity to calibrate the model parameters and further validation is provided by lowroughness data. Characteristic values of the model coffecients and their dependence on the Richardson number are proposed and discussed.

    Nationell ämneskategori
    Meteorologi och atmosfärforskning
    Identifikatorer
    urn:nbn:se:uu:diva-237761 (URN)
    Tillgänglig från: 2014-12-04 Skapad: 2014-12-04 Senast uppdaterad: 2017-12-05
    4. Canopy waves, observations and predictions from lineartheory
    Öppna denna publikation i ny flik eller fönster >>Canopy waves, observations and predictions from lineartheory
    (Engelska)Manuskript (preprint) (Övrigt vetenskapligt)
    Abstract [en]

    In this paper the existence of canopy waves is examined using measurements from a 138 m high tower placed in a forest. Characteristics of the waves are examined in relation to wind energy. Using wavelet analysis it is shown that when the wave signal is clear, the phase lag between horizontal and vertical velocity is close to 90 degrees, which limits the contribution of the waves to themomentum flux. Results from numerical solution of linear wave equations is shown to agree with measurements in terms of wave period and the vertical shape of the wave amplitude. Linear analysis and measurements suggests that Kelvin-Helmholtz instability causes unstable wave growth and that the most unstable wave number normally has a period of 10-100 s. In addition to the Kelvin-Helmholtz instability, the linear analysis predicts that instabilities of the Holmboe kind, with higher frequency, can develop over forests in certain conditions.

    Nyckelord
    Forest, Holmboe instability, Kelvin-Helmholtz instability, Linear wave theory, Shear instability, Wind power
    Nationell ämneskategori
    Meteorologi och atmosfärforskning
    Identifikatorer
    urn:nbn:se:uu:diva-237763 (URN)
    Tillgänglig från: 2014-12-04 Skapad: 2014-12-04 Senast uppdaterad: 2015-03-09
  • 47.
    Arnqvist, Johan
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper.
    Mean wind and turbulence conditions over forests2013Licentiatavhandling, sammanläggning (Övrigt vetenskapligt)
  • 48.
    Arnqvist, Johan
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Luft-, vatten och landskapslära.
    Strömningen i och över en skog: utvärdering av en 'mixing-layer' hypotes2009Självständigt arbete på avancerad nivå (magisterexamen), 20 poäng / 30 hpStudentuppsats/Examensarbete
    Abstract [en]

    A new theory for predicting the windprofile over a canopy has been evaluated. The theory was first presented by Harman and Finnigan (2007). The theory relies on the forming of a mixing-layer above the canopy, due to different mean wind in and above the canopy. Characteristics from both mixing-layer and Monin Obukhov similarity theory have been used to develop the governingequations that give the wind profile. The theory has been used to calculate wind profiles for sixdifferent atmospheric stabilities. In order to evaluate the theory, profiles from the theory have beencompared to measurements from Jädraås forest, Sweden. Profiles from Monin Obukhov similarity theory were also used for comparison.In general the mixing-layer theory gives better results than Monin Obukhov similarity theory. Agreement with measurements is good in neutral conditions, but fails when the atmospheric stability is altered, especially in convective conditions. This is believed to be due to the canopy lacking in thickness. The mean wind speed is systematically underestimated and this is also believed to be caused by insufficient thickness of the canopy. A correction for this behaviour is proposed. The theory gives higher values of the mean wind speed in convective conditions with the correction and the calculated values of mean wind speed are closer to the measurements.

  • 49.
    Arnqvist, Johan
    et al.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Luft-, vatten och landskapslära.
    Bergrström, Hans
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Luft-, vatten och landskapslära.
    Nappo, Carmen
    CJN Res Meteorol, Knoxville, TN USA..
    Examination of the mechanism behind observed canopy waves2016Ingår i: Agricultural and Forest Meteorology, ISSN 0168-1923, E-ISSN 1873-2240, Vol. 218, s. 196-203Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    In this paper, we document the existence of wave-like motions above a forest canopy using data taken from a 138 m high tower placed within a forest Characteristics of the waves are examined in relation to their possible effects on wind energy. It is shown that when the wave signal is relatively clean, the phase lag between horizontal and vertical velocity is close to 90, which limits the contribution of the waves to the downward momentum flux. Numerical solutions of the linear wave equations agree with measurements in terms of wave period and the vertical shape of the wave amplitude. Linear analysis show that shear instability is the cause of unstable wave growth, and that the fastest growing unstable wave number typically has a period of 10-100 s. In addition to the shear instability, the linear analysis predicts that under certain conditions instabilities of the Holmboe kind can develop over forests.

  • 50.
    Arnqvist, Johan
    et al.
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Luft-, vatten och landskapslära.
    Bergström, Hans
    Uppsala universitet, Teknisk-naturvetenskapliga vetenskapsområdet, Geovetenskapliga sektionen, Institutionen för geovetenskaper, Luft-, vatten och landskapslära.
    Flux-profile relation with roughness sublayer correction2015Ingår i: Quarterly Journal of the Royal Meteorological Society, ISSN 0035-9009, E-ISSN 1477-870X, Vol. 141, nr 689, s. 1191-1197Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Calculation of momentum flux using Monin–Obukhov similarity theory over forested areas is well known to underestimate the flux. Several suggestions of corrections to the standard flux-profile expression have been proposed in order to increase the magnitude of turbulent flux. The aim of this article is to find a simple, analytical representation for the characteristics of the flow within the canopy layer and the surface layer, including the roughness sublayer. A new form of the roughness sublayer correction is proposed, based on the desire to connect the shape of the roughness sublayer correction to forest characteristics. The new flux-profile relation can be used to find the flux or the wind profile whenever simple and fast estimations are needed, as for mesoscale modelling, scalar transport models, or sound propagation models.

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