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  • 1.
    Abiodun, B. and Enger, L.
    Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Earth Sciences, Department of Earth Sciences.
    The role of advection of fluxes on modelling dispersion in convective boundary2002In: Quart. J. Roy. Met. Soc., Vol. 128, p. 1589-1607Article in journal (Refereed)
  • 2.
    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 University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, LUVAL. Univ Zurich, Dept Geog, Zurich, Switzerland..
    Propagation of biases in climate models from the synoptic to the regional scale: Implications for bias adjustment2016In: Journal of Geophysical Research - Atmospheres, ISSN 2169-897X, E-ISSN 2169-8996, Vol. 121, no 5, p. 2075-2089Article in journal (Refereed)
    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.

  • 3.
    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 Basin2010In: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 10, no 21, p. 10237-10253Article in journal (Refereed)
    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.

  • 4.
    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 forest2010In: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 10, no 6, p. 3063-3079Article in journal (Refereed)
    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.

  • 5. Ahmad, M R
    et al.
    Esa, M R M
    Rahman, Mahbubur
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Cooray, Vernon
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Measurement of bit error rate at 2,4 GHz due to lightning interference2012In: Proceeding of the 31st International Conference on Lightning Protection ICLP 2012, 2012Conference paper (Refereed)
    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.

  • 6.
    Ahmad, Mohd Riduan
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity. Universiti Teknikal Malaysia Melaka.
    Interaction of Lightning Flashes with Wireless Communication Networks: Special Attention to Narrow Bipolar Pulses2014Doctoral thesis, comprehensive summary (Other academic)
    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.

    List of papers
    1. Signatures of Narrow Bipolar Pulses as Part of Cloud-to-Ground Flashes in Tropical Thunderstorms
    Open this publication in new window or tab >>Signatures of Narrow Bipolar Pulses as Part of Cloud-to-Ground Flashes in Tropical Thunderstorms
    2014 (English)In: Journal of Atmospheric and Solar-Terrestrial Physics, ISSN 1364-6826, E-ISSN 1879-1824Article in journal (Refereed) Submitted
    Keywords
    Narrow bipolar pulse; Return stroke; Tropical thunderstorm.
    National Category
    Meteorology and Atmospheric Sciences Engineering and Technology
    Research subject
    Engineering Science with specialization in Atmospheric Discharges
    Identifiers
    urn:nbn:se:uu:diva-233624 (URN)
    Available from: 2014-10-07 Created: 2014-10-07 Last updated: 2017-12-05
    2. Narrow bipolar pulses and associated microwave radiation
    Open this publication in new window or tab >>Narrow bipolar pulses and associated microwave radiation
    2013 (English)Conference paper, Published paper (Refereed)
    Place, publisher, year, edition, pages
    Stockholm: , 2013
    National Category
    Engineering and Technology
    Research subject
    Engineering Science with specialization in Science of Electricity
    Identifiers
    urn:nbn:se:uu:diva-212898 (URN)
    Conference
    Progress in Electromagnetics Research Symposium
    Available from: 2013-12-16 Created: 2013-12-16 Last updated: 2015-01-23
    3. Electric Field Signature of Narrow Bipolar Pulse Observed in Sweden
    Open this publication in new window or tab >>Electric Field Signature of Narrow Bipolar Pulse Observed in Sweden
    Show others...
    2014 (English)In: Atmospheric research, ISSN 0169-8095, E-ISSN 1873-2895Article in journal (Refereed) Submitted
    Keywords
    Electric field; Narrow bipolar pulse; Sweden.
    National Category
    Meteorology and Atmospheric Sciences Engineering and Technology
    Research subject
    Engineering Science with specialization in Atmospheric Discharges
    Identifiers
    urn:nbn:se:uu:diva-233636 (URN)
    Available from: 2014-10-07 Created: 2014-10-07 Last updated: 2017-12-05
    4. Latitude Dependence of Narrow Bipolar Pulse Emissions
    Open this publication in new window or tab >>Latitude Dependence of Narrow Bipolar Pulse Emissions
    Show others...
    2015 (English)In: Journal of Atmospheric and Solar-Terrestrial Physics, ISSN 1364-6826, E-ISSN 1879-1824, Vol. 128, p. 40-45Article in journal (Refereed) Published
    Keywords
    Latitude; Narrow bipolar pulse; Thunderstorm.
    National Category
    Meteorology and Atmospheric Sciences Engineering and Technology
    Research subject
    Engineering Science with specialization in Atmospheric Discharges
    Identifiers
    urn:nbn:se:uu:diva-233638 (URN)10.1016/j.jastp.2015.03.005 (DOI)000355717500005 ()
    Available from: 2014-10-07 Created: 2014-10-07 Last updated: 2017-12-05Bibliographically approved
    5. Similarity between the Initial Breakdown Pulses of Negative Ground Flash and Narrow Bipolar Pulses
    Open this publication in new window or tab >>Similarity between the Initial Breakdown Pulses of Negative Ground Flash and Narrow Bipolar Pulses
    2014 (English)In: 2014 INTERNATIONAL CONFERENCE ON LIGHTNING PROTECTION (ICLP), IEEE conference proceedings, 2014, p. 810-813Conference paper, Published paper (Refereed)
    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.

    Place, publisher, year, edition, pages
    IEEE conference proceedings, 2014
    Keywords
    Narrow bipolar pulse; Preliminary breakdown pulse; Relativistic runaway electron avalanches.
    National Category
    Meteorology and Atmospheric Sciences Engineering and Technology
    Research subject
    Engineering Science with specialization in Atmospheric Discharges
    Identifiers
    urn:nbn:se:uu:diva-233639 (URN)000358572100153 ()978-1-4799-3544-4 (ISBN)
    Conference
    International Conference on Lightning Protection (ICLP), OCT 11-18, 2014, Tsinghua Univ, Shanghai, PEOPLES R CHINA
    Available from: 2014-10-07 Created: 2014-10-07 Last updated: 2015-09-03Bibliographically approved
    6. Lightning interference in multiple antennas wireless communication systems
    Open this publication in new window or tab >>Lightning interference in multiple antennas wireless communication systems
    Show others...
    2012 (English)In: Journal of Lightning Research, ISSN 1652-8034, Vol. 4, p. 155-165Article in journal (Refereed) 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.

    Place, publisher, year, edition, pages
    Bentham open, 2012
    National Category
    Meteorology and Atmospheric Sciences Engineering and Technology
    Research subject
    Engineering Science with specialization in Atmospheric Discharges
    Identifiers
    urn:nbn:se:uu:diva-190902 (URN)
    Available from: 2013-01-09 Created: 2013-01-09 Last updated: 2016-02-03
    7. Interference from cloud-to-ground and cloud flashes in wireless communication system
    Open this publication in new window or tab >>Interference from cloud-to-ground and cloud flashes in wireless communication system
    2014 (English)In: Electric power systems research, ISSN 0378-7796, E-ISSN 1873-2046, Vol. 113, p. 237-246Article in journal (Refereed) 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.

    Keywords
    Bit error rate, Cloud flash, Cloud-to-ground flash, Interference, Microwave radiation, Wireless system
    National Category
    Electrical Engineering, Electronic Engineering, Information Engineering
    Identifiers
    urn:nbn:se:uu:diva-228679 (URN)10.1016/j.epsr.2014.03.022 (DOI)000337554200032 ()
    Available from: 2014-07-22 Created: 2014-07-21 Last updated: 2017-12-05Bibliographically approved
  • 7.
    Ahmad, Mohd Riduan
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Esa, Mona Riza Binti Mohd
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Rahman, Mahbubur
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Cooray, Vernon
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Dutkiewicz, E.
    Wireless Communications and Networking Lab, Macquarie University, Sydney, Australien.
    Lightning interference in multiple antennas wireless communication systems2012In: Journal of Lightning Research, ISSN 1652-8034, Vol. 4, p. 155-165Article in journal (Refereed)
    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.

  • 8.
    Ahmad, Mohd Riduan
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Mohd Esa, Mona Riza
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Cooray, Vernon
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Occurrence of Narrow Bipolar Event as Part of Cloud-to-Ground Flash in Tropical Thunderstorms2014Conference paper (Refereed)
  • 9.
    Ahmad, Mohd Riduan
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Mohd Esa, Mona Riza
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Cooray, Vernon
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Similarity between the Initial Breakdown Pulses of Negative Ground Flash and Narrow Bipolar Pulses2014In: 2014 INTERNATIONAL CONFERENCE ON LIGHTNING PROTECTION (ICLP), IEEE conference proceedings, 2014, p. 810-813Conference paper (Refereed)
    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.

  • 10.
    Ahmad, Mohd Riduan
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Mohd Esa, Mona Riza
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Cooray, Vernon
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Baharudin, Zikri Abadi
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity. Universiti Teknikal Malaysia Melaka.
    Hettiarachchi, Pasan
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Latitude Dependence of Narrow Bipolar Pulse Emissions2015In: Journal of Atmospheric and Solar-Terrestrial Physics, ISSN 1364-6826, E-ISSN 1879-1824, Vol. 128, p. 40-45Article in journal (Refereed)
  • 11.
    Ahmad, Mohd Riduan
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Mohd Esa, Mona Riza
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Cooray, Vernon
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Hettiarachchi, Pasan
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Baharudin, Zikri Abadi
    Universiti Teknikal Malaysia Melaka.
    Electric Field Signature of Narrow Bipolar Pulse Observed in Sweden2014In: Atmospheric research, ISSN 0169-8095, E-ISSN 1873-2895Article in journal (Refereed)
  • 12.
    Ahmad, Mohd Riduan
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Mohd Esa, Mona Riza
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Johari, Dalina
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Ismail, Mohd Muzafar
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Cooray, Vernon
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Chaotic Pulse Train in Cloud-to-Ground and Cloud Flashes of Tropical Thunderstorms2014Conference paper (Refereed)
    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.

  • 13.
    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 University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Emission Heights of Narrow Bipolar Events in a Tropical Storm over the Malacca Strait2017In: 2017 International Conference On Electrical Engineering And Computer Science (Icecos), IEEE , 2017, p. 305-309Conference paper (Refereed)
    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.

  • 14.
    Ahmad, M.R.
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Rashid, M.
    Aziz, M.H.A.
    Esa, M.R.M.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Cooray, Vernon
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Rahman, Mahbubur
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Dutkiewicz, E.
    Analysis of Lightning-induced Transient in 2.4 GHz Wireless Communication System2011In: Proceeding of IEEE International Conference on Space Science and Communication (IconSpace), Penang, Malaysia, pp225-230, 2011Conference paper (Refereed)
  • 15. Ahmad, Noor Azlinda
    et al.
    Baharudin, Zikri A
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Fernando, M.
    Cooray, Vernon
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Radiation field spectra of long-duration cloud flashes2015In: Atmospheric Science Letters, ISSN 1530-261X, E-ISSN 1530-261X, Vol. 16, no 2, p. 91-95Article in journal (Refereed)
    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.

  • 16.
    Ahmad, Noor Azlinda
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Fernando, Mahendra
    Baharudin, Z. A.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Rahman, Mahbubur
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Cooray, Vernon
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Saleh, Ziad
    Dwyer, Joseph R.
    Rassoul, Hamid K.
    The first electric field pulse of cloud and cloud-to-ground lightning discharges2010In: Journal of Atmospheric and Solar-Terrestrial Physics, ISSN 1364-6826, E-ISSN 1879-1824, Vol. 72, no 2-3, p. 143-150Article in journal (Refereed)
    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.

  • 17.
    Akyuz, Mose
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Division for Electricity and Lightning Research.
    Rahman, Mahbubur
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Division for Electricity and Lightning Research.
    Larsson, Anders
    Cooray, Vernon
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Division for Electricity and Lightning Research.
    Franke, Axel
    Characteristics of Laser-triggered Electric Discharges in Air2005In: IEEE transactions on dielectrics and electrical insulation, ISSN 1070-9878, E-ISSN 1558-4135, Vol. 12, no 5, p. 1060-1070Article in journal (Refereed)
  • 18.
    Andersson, Ambjörn
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, LUVAL.
    Utvärdering av svavel- och kvävedepositioner från sjöfart: en modellstudie2010Independent thesis Advanced level (degree of Master (One Year)), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    The shipping sector is a significant source of sulfur- and nitrogen-emissions. Depositions of oxidized sulfur and nitrogen i.e. sulfate and nitrate, in the form of wet and dry deposition contribute to acidification and deterioration of air quality. Political instruments to reduce sulfur emissions during the last years have contributed to the reduction of sulfur emissions, i.e. new instruments have been implemented in form of restrictions in sulfur content in ships fuel.

    The shipping sector’s emissions of sulfur and nitrogen have been studied by using “the Unified EMEP Model”, a chemical atmospheric transport model, with data collected during the year 2005. The deposition of sulfur and nitrogen from shipping in different countries has been compared to the total deposition from all sources in Europe. Deposition of sulfur and nitrogen in the Baltic Sea and the countries in the in the draining basin from individual sea has also been studied.

    The results have shown that deposition of sulfur and nitrogen is intense near the source. But it also spread far away from the source, however with decreasing concentrations in relation with distance to the source. Depositions in the north of Europe from shipping are highest near big ports and   Clear tracks from shipping in the depositions field are obtained near big ports and also in the Atlantic and the Mediterranean Sea. Countries with longer coastline have higher concentrations than those with less or none sea traffic in the near region.

    To study how roughness length over sea impacts the spread of sulfur the Charnock-parameter has been varied from 0,00072 to 0,085. The study doesn’t provide any clear findings of influence of the parameter in spread of sulfur. A theory to this result is the low resolution in the model. It is probably too small differences in the depositions field due to the roughness length.

    Monthly variations of sulfur and nitrogen deposits in the Baltic Sea have been made in order to study how variations in such precipitation affect the distribution of sulfur and nitrogen.

     

  • 19.
    Andersson, Andreas
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, LUVAL.
    Air-sea exchange of O2 and CO2: Processes controlling the transfer efficiency2017Doctoral thesis, comprehensive summary (Other academic)
    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.

    List of papers
    1. Using a High-Frequency Fluorescent Oxygen Probe in Atmospheric Eddy Covariance Applications
    Open this publication in new window or tab >>Using a High-Frequency Fluorescent Oxygen Probe in Atmospheric Eddy Covariance Applications
    2014 (English)In: Journal of Atmospheric and Oceanic Technology, ISSN 0739-0572, E-ISSN 1520-0426, Vol. 31, no 11, p. 2498-2511Article in journal (Refereed) 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.

    National Category
    Meteorology and Atmospheric Sciences
    Identifiers
    urn:nbn:se:uu:diva-239999 (URN)10.1175/JTECH-D-13-00249.1 (DOI)000345008300009 ()
    Available from: 2015-01-05 Created: 2015-01-05 Last updated: 2017-12-05Bibliographically approved
    2. Using eddy covariance to estimate air-sea gas transfer velocity for oxygen
    Open this publication in new window or tab >>Using eddy covariance to estimate air-sea gas transfer velocity for oxygen
    2016 (English)In: Journal of Marine Systems, ISSN 0924-7963, E-ISSN 1879-1573, Vol. 159, p. 67-75Article in journal (Refereed) 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.

    National Category
    Meteorology and Atmospheric Sciences
    Research subject
    Meteorology
    Identifiers
    urn:nbn:se:uu:diva-287887 (URN)10.1016/j.jmarsys.2016.02.008 (DOI)000375506200006 ()
    Available from: 2016-04-26 Created: 2016-04-26 Last updated: 2017-11-30
    3. Air-sea gas transfer in high Arctic fjords
    Open this publication in new window or tab >>Air-sea gas transfer in high Arctic fjords
    Show others...
    2017 (English)In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 44, no 5, p. 2519-2526Article in journal (Refereed) 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.

    Keywords
    air-sea gas exchange, transfer velocity, surface cooling, water-side convection, CO2 flux, Arctic
    National Category
    Geosciences, Multidisciplinary
    Research subject
    Meteorology
    Identifiers
    urn:nbn:se:uu:diva-314161 (URN)10.1002/2016GL072373 (DOI)000398183700053 ()
    Available from: 2017-01-29 Created: 2017-01-29 Last updated: 2017-05-04Bibliographically approved
    4. Enhanced air-sea exchange of CO2 over a high Arctic fjord during unstable very close to neutral conditions
    Open this publication in new window or tab >>Enhanced air-sea exchange of CO2 over a high Arctic fjord during unstable very close to neutral conditions
    Show others...
    (English)Manuscript (preprint) (Other academic)
    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.

    Keywords
    Transfer velocity, Arctic, UVCN, air-sea exchange
    National Category
    Natural Sciences
    Research subject
    Meteorology
    Identifiers
    urn:nbn:se:uu:diva-314162 (URN)
    Available from: 2017-01-29 Created: 2017-01-29 Last updated: 2017-01-29
  • 20.
    Andersson, Andreas
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, LUVAL.
    En statistisk kartläggning av dimma för Arlanda flygplats2010Independent thesis Basic level (degree of Bachelor), 10 credits / 15 HE creditsStudent thesis
    Abstract [en]

    Since the beginning of weather forecasting fog has caused many meteorologists severe problems, and in aviation it has been one of the main reasons for several crashes. Even today with the latest numerical models, predicting fog is a difficult task. New studies have shown that to predict fog the model must be able to resolve inversion layers down to 10 m. Even then the precision in determination of the visibility inside the fog is down to 50 % (Gultepe, 2007). In those cases a statistical mapping of fog at a certain place could be a good complement to the numerical models in determining time of formation and visibility. In this study I have used METAR observations made at Arlanda airport from 1993-07-01 to 2009-11-03, which are made every half an hour. In the mapping Radiation fog, Advection fog, Precipitation fog and fog caused by lowering of clouds (cbl. fog) was sorted out. I also looked into the behavior of ice fog vs. water fog.The results showed that radiation fog was the most common fog type (43,6 %) and had the lowest rate of visibility followed by cbl. fog. Radiation fog also had its highest frequency on summer and autumn, while advection fog and precipitation fog was concentrated into wintertime. Additionally the study showed that the frequency of the times when radiation fog covers the whole airport started to decrease at the beginning of the 21st century. Coinciding with when the expansion of the airport was finished including a new runway. In the comparison between water fog and ice fog it was clear that the distribution of visibility was similar. The lowest visibilities in water fog was found at the highest temperatures and highest specific liquid water content while in ice fog the lowest visibilities where found for temperatures in the neighborhood of - 20 °C. For temperatures below - 20 °C there were no observations of fog.

  • 21.
    Andersson, Andreas
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, LUVAL.
    Rutgersson, Anna
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, LUVAL.
    Sahlee, Erik
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, LUVAL.
    Using eddy covariance to estimate air-sea gas transfer velocity for oxygen2016In: Journal of Marine Systems, ISSN 0924-7963, E-ISSN 1879-1573, Vol. 159, p. 67-75Article in journal (Refereed)
    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.

  • 22.
    Andersson, Andreas
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, LUVAL.
    Rutgersson, Anna
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, LUVAL.
    Sahlée, Erik
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, LUVAL.
    Using a High-Frequency Fluorescent Oxygen Probe in Atmospheric Eddy Covariance Applications2014In: Journal of Atmospheric and Oceanic Technology, ISSN 0739-0572, E-ISSN 1520-0426, Vol. 31, no 11, p. 2498-2511Article in journal (Refereed)
    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.

  • 23.
    Andersson, Lovisa
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, LUVAL.
    Studie av MIUU-modellen och implementering av urbana parametrar i modellens energibalansrutin2009Independent thesis Advanced level (degree of Master (One Year)), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    This is a study of the MIUU-model with its existing parameterization of the lower boundary. In this model the lower boundary conditions are determined by an energy balance routine presented by Deardorff (1978), which has a prognostic equation for the surface temperature. Prior to this study there were no urban parameters presented in the MIUU-model. The different ground properties were all, with the sole exception for water, described by the values for clay loam. This study investigates if the urban impacts on the meteorological parameters can be modeled by the MIUU-model. The values of the parameters in the energy balance equation are changed to values more representative of urban areas. The parameters altered in the energy equation are the albedot, emissivity, roughness length, soil moisture and the temperature in the ground. The effects of changing the geostrophic wind and the cloudiness have also been studied.

     

    Implementing the urban parameters in the MIUU-model results in an increased surface temperature over the urban areas during the day. It also leads to a higher turbulent kinetic energy due to the urban areas roughness. The MIUU-model can however, with its present parameterisation of the energy balance routine and the used urban parameters, not model an urban heat island after sundown. In spite of that, this should be when the differences between urban and rural areas are most prominent. Possibly this could depend on the changes made to the soil moisture parameters, or on the models definitions of the ground heat flux. Therefore, consistent with the fact that implementation of the urban parameters results in decreased ground heat flux over the urban areas at both day and night. A reduced ground heat flux is also found even though the shielding factor of vegetation decreases and the shortwave radiation to the ground increases.

  • 24.
    Andersson, Magnus
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Uppsala Centre for Sustainable Development, 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 University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, Uppsala Centre for Sustainable Development, CSD Uppsala, The Baltic University Programme.
    Azar, Christian
    University of Gothenburg.
    10. Impacts on the Global Atmosphere: Climate Change and Ozone Depletion2003In: 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, p. 294-323Chapter in book (Other (popular science, discussion, etc.))
  • 25.
    Andersson, Stefan
    Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Earth Sciences, Department of Earth Sciences.
    Undersökning av luftkvaliteten vid småskalig biobränsleförbränning i två kommuner med modellsystemet VEDAIR2007Independent thesis Basic level (professional degree), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    An Internet application, VEDAIR, for estimation of air quality in regions with small-scale combustion of bio fuel has been developed by Swedish Meteorological and Hydrological Institute with financing from Swedish Energy Agency and Swedish Environmental Protection Agency. The model contains user interface and a coupled model concept where concentrations of atmospheric pollutions are calculated for regional, urban and local contributions. In this report the model is described and a first study of air quality considering PM10 (particles with a diameter less than 10 µm) in two Swedish municipalities, Vänersborg and Gnosjö, is introduced.

    According to the result, there are areas with poor air quality even in smaller Swedish municipalities, mostly due to high emission from buildings with older wood heating furnace without hydraulic accumulator.

    In Vänersborg three areas were analysed in VEDAIR for the year 2003: Vargön, Mariedal and the downtown district. In the whole municipality the upper evaluation threshold for PM10, which is defined as 14 µg/m3 for annual average, was exceeded due to background contribution of particles. In all areas, however, the concentrations of PM10 were less than the environmental quality norms. In Vargön, where wood heating furnaces are commonly used, in some areas the concentration of particles was as big as or bigger than the downtown district (annual average of 20 µg/m3). However, in downtown the pollutants were more spread. Biggest concentrations of PM10 were estimated in Mariedal, due to emissions from older wood heating furnaces and streets, mostly in areas with unfavourable meteorological conditions. Measurements of PM10 in the area are therefore suggested. Biggest concentrations of PM10 were prevailed during winter, due to great heating need in combination with cooler air temperature and more stable stratification. The local contribution of PM10 was negative correlated with temperature. In the downtown district, however, the biggest concentrations of PM10 were obtained during spring due to usage of studded tyres.

    In Gnosjö the air quality considering PM10 was analysed in the areas Götarp, downtown and Gårö. Calculations in VEDAIR showed that the emissions from traffic were much less than wood heating furnace. The background annual average of PM10 in Gnosjö was 1 µg/m3 higher than Vänersborg. The air quality was quite equal as in Vänersborg, but there was an area in Gårö where the environmental quality norms considering PM10 almost were exceeded, this mostly due to emission from one building with high heating need.

    In this study a comparison between calculated concentrations of PM10 with measured concentrations in Gnosjö for the period 3 November to 31 December 2003 was also made. The validation showed that VEDAIR underestimated the concentrations, but the result was anyway reasonable. Before, validations have only been performed for the northern part of Sweden, and this study indicates that VEDAIR also estimates concentrations of PM10 sufficiently well in southern part of Sweden.

    The study also showed that if a house owner would replace an old wood heating furnace by a modern furnace with hydraulic accumulator, the annual average of the concentration of PM10 could decrease by 4 µg/m3 which is a reduction of 25%.

  • 26.
    Andersson, Ulrika
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, LUVAL.
    Åska längs Sveriges kuster2002Independent thesis Advanced level (degree of Master (One Year)), 20 credits / 30 HE creditsStudent thesis
    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.

  • 27. Andin, Caroline
    et al.
    Zdanowicz, Christian M.
    Copland, Luke
    Synoptic variability of extreme snowfall in the St. Elias Mountains,Yukon, Canada2015Conference paper (Refereed)
    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.

  • 28.
    ANDREN, A
    Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Earth Sciences, Department of Earth Sciences.
    THE STRUCTURE OF STABLY STRATIFIED ATMOSPHERIC BOUNDARY-LAYERS - A LARGE-EDDY SIMULATION STUDY1995In: QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, ISSN 0035-9009, Vol. 121, no 525, p. 961-985Article in journal (Refereed)
    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

  • 29.
    Ann-Sofi Smedman, Hans. Bergström and Ulf Högstrmöm
    Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Earth Sciences, Department of Earth Sciences.
    A study of low level jets over the Baltic Sea east of the island of Gotland1996In: Wind Energy Report, no WE 96:2Report (Other scientific)
  • 30.
    Ann-Sofi Smedman, Hans. Bergström and Ulf Högstrmöm
    Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Earth Sciences, Department of Earth Sciences.
    Measured and modelled local wind field over a frozen lake in a mountainous area1995In: Wind Energy Report, no WE 95:4Report (Other scientific)
  • 31.
    Anton, Nygren
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, LUVAL.
    Black Carbons Effects on Climate: Can We Even Say Something about Them?2016Independent thesis Basic level (degree of Bachelor), 10 credits / 15 HE creditsStudent thesis
    Abstract [en]

    Black carbon affects the atmosphere, clouds, and the albedo of snow. These effects of black carbon are a fact but exactly what they entail is not well known. The atmosphere, the albedo in ice crystals and snow (due to warming of the snows’ surface), clouds’ life time and cloud cover are all influenced because black carbon absorbs radiation, thus altering the radiative forcing (RF).Many models and measurements have been done to evaluate the effects of black carbon but they show very different results. This literature study examines papers and articles to see what and how much we know about black carbon and its climate effects. I conclude that there have been great differences in results that are evident due to the span that black carbon is thought to change the RF in the atmosphere (results ranging from +0.4 to +1.2 Wm-2), and snow (+0.007 to +0.054 Wm-2), as well as the albedo in snow (results ranging from -0.02 to -0.17, only including the results from studies with BC concentration of 1000 ng/g for increased comparability). I as well as many others suggest that models tend to get very different results because they use different starting conditions and equations and because models handle aerosols and clouds in different ways. This is largely because of a lack of information, especially concerning aerosols and clouds.

  • 32. Arevalo, Liliana
    et al.
    Cooray, Vernon
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Streamer to leader transition criteria for propagation of long sparks and lightning leaders2014In: 2014 INTERNATIONAL CONFERENCE ON LIGHTNING PROTECTION (ICLP), IEEE conference proceedings, 2014, p. 480-483Conference paper (Refereed)
    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.

  • 33.
    Arevalo, Liliana
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity. Ludvika ULHC, ABB Power Grids Grid Integrat HVDC, Dept Res & Dev, Lyviksvagen 3, S-77180 Lyviksvagen, Sweden..
    Cooray, Vernon
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Unstable Leader Inception Criteria of Atmospheric Discharges2017In: Atmosphere, ISSN 2073-4433, E-ISSN 2073-4433, Vol. 8, no 9, article id 156Article in journal (Refereed)
    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.

  • 34. Arevalo, Liliana
    et al.
    Cooray, Vernon
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Upward leader inception caused by a sudden change of cloud electric field2014In: 2014 INTERNATIONAL CONFERENCE ON LIGHTNING PROTECTION (ICLP), IEEE conference proceedings, 2014, p. 484-487Conference paper (Refereed)
    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.

  • 35.
    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 University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, LUVAL. 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?2018In: Hydrology and Earth System Sciences, ISSN 1027-5606, E-ISSN 1607-7938, Vol. 22, no 4, p. 2057-2072Article in journal (Refereed)
    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.

  • 36.
    Arnqvist, Johan
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, LUVAL.
    Mean Wind and Turbulence Conditions in the Boundary Layer above Forests2015Doctoral thesis, comprehensive summary (Other academic)
    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.

    List of papers
    1. Flux-profile relation with roughness sublayer correction
    Open this publication in new window or tab >>Flux-profile relation with roughness sublayer correction
    2015 (English)In: Quarterly Journal of the Royal Meteorological Society, ISSN 0035-9009, E-ISSN 1477-870X, Vol. 141, no 689, p. 1191-1197Article in journal (Refereed) 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.

    Keywords
    roughness sublayer; wind profile; dimensionless gradient; stability expressions
    National Category
    Meteorology and Atmospheric Sciences
    Identifiers
    urn:nbn:se:uu:diva-237757 (URN)10.1002/qj.2426 (DOI)000356805700016 ()
    Available from: 2014-12-04 Created: 2014-12-04 Last updated: 2017-12-05Bibliographically approved
    2. Wind statistics from a forested landscape
    Open this publication in new window or tab >>Wind statistics from a forested landscape
    2015 (English)In: Boundary-layer Meteorology, ISSN 0006-8314, E-ISSN 1573-1472, Vol. 156, no 1, p. 53-71Article in journal (Refereed) 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.

    Keywords
    Above canopy turbulence statistics Atmospheric boundary layers, Decreasing roughness, Forest Canopy Flows, Wind power
    National Category
    Meteorology and Atmospheric Sciences
    Identifiers
    urn:nbn:se:uu:diva-237762 (URN)10.1007/s10546-015-0016-x (DOI)000355153200004 ()
    Available from: 2014-12-04 Created: 2014-12-04 Last updated: 2017-12-05Bibliographically approved
    3. A spectral model for stably stratied turbulence
    Open this publication in new window or tab >>A spectral model for stably stratied turbulence
    (English)In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645Article in journal (Refereed) 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.

    National Category
    Meteorology and Atmospheric Sciences
    Identifiers
    urn:nbn:se:uu:diva-237761 (URN)
    Available from: 2014-12-04 Created: 2014-12-04 Last updated: 2017-12-05
    4. Canopy waves, observations and predictions from lineartheory
    Open this publication in new window or tab >>Canopy waves, observations and predictions from lineartheory
    (English)Manuscript (preprint) (Other academic)
    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.

    Keywords
    Forest, Holmboe instability, Kelvin-Helmholtz instability, Linear wave theory, Shear instability, Wind power
    National Category
    Meteorology and Atmospheric Sciences
    Identifiers
    urn:nbn:se:uu:diva-237763 (URN)
    Available from: 2014-12-04 Created: 2014-12-04 Last updated: 2015-03-09
  • 37.
    Arnqvist, Johan
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences.
    Mean wind and turbulence conditions over forests2013Licentiate thesis, comprehensive summary (Other academic)
  • 38.
    Arnqvist, Johan
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, LUVAL.
    Strömningen i och över en skog: utvärdering av en 'mixing-layer' hypotes2009Independent thesis Advanced level (degree of Master (One Year)), 20 credits / 30 HE creditsStudent thesis
    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.

  • 39.
    Arnqvist, Johan
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, LUVAL.
    Bergrström, Hans
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, LUVAL.
    Nappo, Carmen
    CJN Res Meteorol, Knoxville, TN USA..
    Examination of the mechanism behind observed canopy waves2016In: Agricultural and Forest Meteorology, ISSN 0168-1923, E-ISSN 1873-2240, Vol. 218, p. 196-203Article in journal (Refereed)
    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.

  • 40.
    Arnqvist, Johan
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, LUVAL.
    Bergström, Hans
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, LUVAL.
    Flux-profile relation with roughness sublayer correction2015In: Quarterly Journal of the Royal Meteorological Society, ISSN 0035-9009, E-ISSN 1477-870X, Vol. 141, no 689, p. 1191-1197Article in journal (Refereed)
    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.

  • 41.
    Arnqvist, Johan
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, LUVAL.
    Bergström, Hans
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, LUVAL.
    Nappo, Carmen
    CJN Research Meteorology.
    Canopy waves, observations and predictions from lineartheoryManuscript (preprint) (Other academic)
    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.

  • 42.
    Arnqvist, Johan
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, LUVAL.
    Dellwik, Ebba
    DTU.
    Segalini, Antonio
    KTH.
    Bergström, Hans
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, LUVAL.
    Wind statistics from a forested landscape2015In: Boundary-layer Meteorology, ISSN 0006-8314, E-ISSN 1573-1472, Vol. 156, no 1, p. 53-71Article in journal (Refereed)
    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.

  • 43.
    Aronsson, Inga-Lill
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of High Voltage Research.
    Human Response to the Lightning Hazard in a Cultural Context1986Report (Other academic)
    Abstract [en]

    This paper explores the human responses to lightning hazards from a cultural anthropological perspective. By definition, a natural hazard involves human action, initiative, and choices. There is always an interaction between man and nature, because humans respond and adapt themselves to the natural phenomena. These adjustments and responses differ from one society to another. The author was, at the time of writing, affiliated to both the Institute of High Voltage Research, and, the Department of Cultural Anthropology, Uppsala University.

  • 44.
    Baharudin, Zikri A.
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Fernando, M.
    Dept of Physics, University of Colombo, Sri Lanka.
    Ahmad, Noor Azlinda
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Mäkelä, J. S.
    Nokia OY, Salo, Finland.
    Rahman, Mahbubur
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Cooray, Vernon
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Electric field changes generated by the preliminary breakdown for the negative cloud-to-ground lightning flashes in Malaysia and Sweden2012In: Journal of Atmospheric and Solar-Terrestrial Physics, ISSN 1364-6826, E-ISSN 1879-1824, Vol. 84-85, p. 15-24Article in journal (Refereed)
    Abstract [en]

    We present the study of the electric field changes generated by the preliminary breakdown for negative cloud-to-ground lightning flashes in Malaysia and Sweden concerning the association of slow field changes associated in preliminary breakdown process. We examined the total of 1685 negative cloud-to-ground lightning flashes from the total of 39 thunderstorms by recording the slow electric field, fast electric field and narrowband radiation field at 3 and 30 MHz signals simultaneously. Our results show that there is a pre-starting time, i.e. the duration between the first preliminary breakdown pulse and slow field changes starting point, which is found to be after the first preliminary breakdown pulse. The pre-starting time has the arithmetic and geometric mean range from 1.4-6.47 and 1-3.36 ms, respectively. The mean values of pre-starting time in Malaysia are greater than the values observed in Sweden by more than a factor of 3. From the two data sets it shows that the slow field changes never start before the preliminary breakdown. Furthermore, the use of single-station electric fields measurement with high resolutions of 12 bits transient recorder with several nanosecond accuracy allow one to distinguish the slow field changes generated by preliminary breakdown, which preceded the negative first return stroke, between tens to hundreds of milliseconds of pre-return stroke duration.

  • 45.
    Baharudin, Zikri Abadi
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Cooray, Vernon
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Rahman, Mahbubur
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Hettiarachchi, Pasan
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Ahmad, Noor Azlinda
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    On the characteristics of positive lightning ground flashes in Sweden2016In: Journal of Atmospheric and Solar-Terrestrial Physics, ISSN 1364-6826, E-ISSN 1879-1824, Vol. 138, p. 106-111Article in journal (Refereed)
    Abstract [en]

    In this study the stroke characteristics of positive cloud-to-ground flashes in Sweden were obtained from the electric field records measured from 14 thunderstorms. The electric fields were measured with nanosecond resolution. Together with the fast and the slow electric field records, the narrowband radiation field at 3 and 30 MHz signals were also measured simultaneously. Out of a total of 107 flashes, 30 flashes had two strokes, 7 had three strokes and 3 flashes had four strokes. The arithmetic and geometric means of the interstroke intervals were found to be 116 and 70 ms, respectively. The arithmetic and geometric mean ratio between the peak electric field of the Subsequent Return Stroke (SRS) and the first Return Stroke (RS) were 0.48 and 0.36, respectively. Of the 40 positive multiple-stroke ground flashes, 5% have at least one SRS with field peak higher than the first RS. The percentage of SRS with field peaks greater than the first RS was 6%. In our best of our knowledge, this is the first time a large sample of positive return strokes in Sweden was analysed. It was found to be statistically more significant than the previous studies.

  • 46.
    Baltscheffsky, Magnus
    Uppsala University, Disciplinary Domain of Science and Technology, Earth Sciences, Department of Earth Sciences, LUVAL.
    Simulation and Analysis of Wind Characteristics in the Region Hosting the Sailing Competitions of the 2012 Olympic Games2011Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
  • 47.
    Barthelmie, R., Larsen, G., Bergström, H., Magnusson, M., Schlez,W., Rados, K., Lange, B., Vølund, P., Neckelmann, S., Christensen, L., Schepers, G., Hegberg, T., Folkerts, L.
    Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Earth Sciences, Department of Earth Sciences.
    ENDOW:Efficient Development of Offshore Windfarms.2002In: Wind Engineering, Vol. 25, no 5, p. 263-270Article in journal (Refereed)
  • 48. Barthelmie, Rebecca
    et al.
    Larsen, G.
    Bergström, Hans
    Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Earth Sciences, Department of Earth Sciences. Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Earth Sciences, Department of Earth Sciences, Air and Water Science.
    Magnusson, Mikael
    Hassan, U.
    Schlez, W.
    Rados, K.
    Lange, B.
    Waldl, I.
    Vølund, P.
    Neckelmann, S.
    Christensen, T.
    Nielsen, T.G.
    Højstrup, J.
    Schepers, G.
    Hegberg, T.
    Coelingh, J.
    Folkerts, L.
    Efficient Development of Offshore Windfarms: A New Project for Investigating Wake and Boundary-Layer Interactions2001In: EWEC2001 2-6 July 2001, Copenhagen, Denmark, 2001Conference paper (Other scientific)
  • 49. Barthelmie, Rebecca
    et al.
    Larsen, Gunner
    Bergström, Hans
    Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Earth Sciences, Department of Earth Sciences. Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Earth Sciences, Department of Earth Sciences, Air and Water Science.
    Magnusson, Mikael
    Rados, Kostas
    Lange, Bernhard
    Vølund, Per
    Neckelmann, S.
    Christensen, L.
    Schepers, G.
    Hegberg, T.
    Folkerts, L.
    ENDOW:Efficient Development of Offshore Windfarms2001In: Wind Engineering, Vol. 25, no 5, p. 263-270Article in journal (Refereed)
  • 50.
    Barthelmie, RJ
    et al.
    Uppsala University, Teknisk-naturvetenskapliga vetenskapsområdet, Earth Sciences, Department of Earth Sciences.
    Grisogono, B
    Pryor, SC
    Observations and simulations of diurnal cycles of near-surface wind speeds over land and sea1996In: JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES, ISSN 0148-0227, Vol. 101, no D16, p. 21327-21337Article in journal (Refereed)
    Abstract [en]

    The role of stability and roughness changes in producing diurnal cycles of wind speed offshore is examined using experimental data and numerical simulations. Overnight, the transition from stable conditions over land to less stable conditions over sea com

1234567 1 - 50 of 662
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