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Pohjola, Veijo
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Publications (10 of 74) Show all publications
van Pelt, W., Pohjola, V., Pettersson, R., Ehwald, L., Reijmer, C., Boot, W. & Jakobs, S. (2018). Dynamic response of a High Arctic glacier to melt and runoff variations. Geophysical Research Letters
Open this publication in new window or tab >>Dynamic response of a High Arctic glacier to melt and runoff variations
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2018 (English)In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007Article in journal (Refereed) Accepted
Keyword
glacier, runoff, velocity, climate change
National Category
Physical Geography
Identifiers
urn:nbn:se:uu:diva-350228 (URN)10.1029/2018GL077252 (DOI)
Available from: 2018-05-08 Created: 2018-05-08 Last updated: 2018-05-12
Marchenko, S., Pohjola, V. A., Pettersson, R., van Pelt, W. J. J., Vega, C. P., Machguth, H., . . . Isaksson, E. (2017). A plot-scale study of firn stratigraphy at Lomonosovfonna, Svalbard, using ice cores, borehole video and GPR surveys in 2012–14. Journal of Glaciology, 63(237), 67-78
Open this publication in new window or tab >>A plot-scale study of firn stratigraphy at Lomonosovfonna, Svalbard, using ice cores, borehole video and GPR surveys in 2012–14
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2017 (English)In: Journal of Glaciology, ISSN 0022-1430, E-ISSN 1727-5652, Vol. 63, no 237, p. 67-78Article in journal (Refereed) Published
Abstract [en]

Spatial heterogeneity of snow and firn properties on glaciers introduces uncertainty in interpretation of point and profile observations and complicates modelling of meltwater percolation and runoff. Here we present a study of the temporal and spatial dynamics of firn density and stratigraphy at the plot-scale (approximate to 10 m x 10 m x 10 m) repeated annually during 2012-14 at the Lomonosovfonna ice-field, Svalbard. Results from cores, video inspections in boreholes and radar grid surveys are compared. Ice layers 0.1-50 cm thick comprised approximate to 8% of the borehole length. Most of them are 1-3 cm thick and could not be traced between boreholes separated by 3 m. Large lateral variability of firn structure affects representativeness of observations in single holes and calls for repeated studies in multiple points to derive a representative stratigraphy signal. Radar reflections are poorly correlated with ice layers in individual boreholes. However, the match between the high amplitude peaks in the grid-averaged radar signal and horizons of preferential ice layer formation revealed by averaging the video surveys over multiple boreholes is higher. These horizons are interpreted as buried firn layers previously exposed to melt-freeze or wind-driven densification and several of them are consistently recovered throughout three field campaigns.

Keyword
borehole video, firn core, radar, stratigraphy
National Category
Physical Geography Earth and Related Environmental Sciences
Identifiers
urn:nbn:se:uu:diva-308681 (URN)10.1017/jog.2016.118 (DOI)000394435300006 ()
Funder
Swedish Research Council, 621-2014-3735
Available from: 2016-11-29 Created: 2016-11-29 Last updated: 2018-01-13Bibliographically approved
Marchenko, S., Van Pelt, W., Carlsson, B., Pohjola, V., Pettersson, R., Machguth, H. & Reijmer, C. (2017). Parameterizing deep water percolation improves subsurface temperature simulations by a multilayer firn model. Frontiers in Earth Science, 5(16)
Open this publication in new window or tab >>Parameterizing deep water percolation improves subsurface temperature simulations by a multilayer firn model
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2017 (English)In: Frontiers in Earth Science, ISSN 1096-231X, E-ISSN 1664-8021, Vol. 5, no 16Article in journal (Refereed) Published
Abstract [en]

Deep preferential percolation of melt water in snow and firn brings water lower along the vertical profile than a laterally homogeneous wetting front. This widely recognized process is an important source of uncertainty in simulations of subsurface temperature, density, and water content in seasonal snow and in firn packs on glaciers and ice sheets. However, observation and quantification of preferential flow is challenging and therefore it is not accounted for by most of the contemporary snow/firn models. Here we use temperature measurements in the accumulation zone of Lomonosovfonna, Svalbard, done in April 2012-2015 using multiple thermistor strings to describe the process of water percolation in snow and firn. Effects of water flow through the snow and firn profile are further explored using a coupled surface energy balance - firn model forced by the output of the regional climate model WRF. In situ air temperature, radiation, and surface height change measurements are used to constrain the surface energy and mass fluxes. To account for the effects of preferential water flow in snow and firn we test a set of depth-dependent functions allocating a certain fraction of the melt water available at the surface to each snow/firn layer. Experiments are performed for a range of characteristic percolation depths and results indicate a reduction in root mean square difference between the modeled and measured temperature by up to a factor of two compared to the results from the default water infiltration scheme. This illustrates the significance of accounting for preferential water percolation to simulate subsurface conditions. The suggested approach to parameterization of the preferential water flow requires low additional computational cost and can be implemented in layered snow/ firn models applied both at local and regional scales, for distributed domains with multiple mesh points.

Keyword
firn, firn modeling, preferential flow, internal accumulation, Lomonosovfonna, Svalbard, firn water content
National Category
Physical Geography
Identifiers
urn:nbn:se:uu:diva-321136 (URN)10.3389/feart.2017.00016 (DOI)000395736600001 ()
Funder
Swedish Research Council, 621-2014-3735
Available from: 2017-05-02 Created: 2017-05-02 Last updated: 2018-01-13Bibliographically approved
Vega, C. P., Pohjola, V. A., Beaudon, E., Claremar, B., van Pelt, W. J. J., Pettersson, R., . . . Bøggild, C. E. (2016). A synthetic ice core approach to estimate ion relocation in anice field site experiencing periodical melt: a case study onLomonosovfonna, Svalbard. The Cryosphere, 10, 961-976
Open this publication in new window or tab >>A synthetic ice core approach to estimate ion relocation in anice field site experiencing periodical melt: a case study onLomonosovfonna, Svalbard
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2016 (English)In: The Cryosphere, ISSN 1994-0416, E-ISSN 1994-0424, Vol. 10, p. 961-976Article in journal (Refereed) Published
Abstract [en]

Physical and chemical properties of four different ice cores (LF-97, LF-08, LF-09 and LF-11) drilled at Lomonosovfonna, Svalbard, were compared to investigate the effects of meltwater percolation on the chemical and physical stratigraphy of these records. A synthetic ice core approach was employed as reference record to estimate the ionic relocation and meltwater percolation length at this site during the period 2007-2010. Using this method, a partial ion elution sequence obtained for Lomonosovfonna was NO(3)(-)aEuro-> aEuro-SO42-, Mg2+, Cl-, K+, Na+ with nitrate being the most mobile within the snowpack. The relocation length of most of the ions was on the order of 1aEuro-m during this period. In addition, by using both a positive degree day (PDD) and a snow-energy model approaches to estimate the percentage of melt at Lomonosovfonna, we have calculated a melt percentage (MP) of the total annual accumulation within the range between 48 and 70aEuro-%, for the period between 2007 and 2010, which is above the MP range suggested by the ion relocation evidenced in the LF-syn core (i.e., MPaEuro-aEuro parts per thousand= aEuro-30aEuro-%). Using a firn-densification model to constrain the melt range, a MP of 30aEuro-% was found over the same period, which is consistent with the results of the synthetic ice core approach, and a 45aEuro-% of melt for the last 60 years. Considering the ionic relocation lengths and annual melt percentages, we estimate that the atmospheric ionic signal remains preserved in recently drilled Lomonosovfonna ice cores at an annual or bi-annual resolution when weather conditions were similar to those during the 2007-2010 period.

National Category
Geosciences, Multidisciplinary
Research subject
Earth Science with specialization in Environmental Analysis
Identifiers
urn:nbn:se:uu:diva-296180 (URN)10.5194/tc-10-961-2016 (DOI)000379415500002 ()
Funder
EU, European Research Council, 3.2.0801.12-0044
Available from: 2016-06-14 Created: 2016-06-14 Last updated: 2017-11-28Bibliographically approved
van Pelt, W. J. J., Kohler, J., Liston, G., Hagen, J. O., Luks, B., Reijmer, C. & Pohjola, V. A. (2016). Multi-decadal climate and seasonal snow conditions in Svalbard. Journal of Geophysical Research - Earth Surface, 121(11), 2100-2117
Open this publication in new window or tab >>Multi-decadal climate and seasonal snow conditions in Svalbard
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2016 (English)In: Journal of Geophysical Research - Earth Surface, ISSN 2169-9003, E-ISSN 2169-9011, Vol. 121, no 11, p. 2100-2117Article in journal (Refereed) Published
Abstract [en]

Svalbard climate is undergoing amplified change with respect to the global mean. Changing climate conditions directly affect the evolution of the seasonal snowpack, through its impact on accumulation, melt, and moisture exchange. We analyze long-term trends and spatial patterns of seasonal snow conditions in Svalbard between 1961 and 2012. Downscaled regional climate model output is used to drive a snow modeling system (SnowModel), with coupled modules simulating the surface energy balance and snowpack evolution. The precipitation forcing is calibrated and validated against snow depth data on a set of glaciers around Svalbard. Climate trends reveal seasonally inhomogeneous warming and a weakly positive precipitation trend, with strongest changes in the north. In response to autumn warming the date of snow onset increased (2days decade(-1)), whereas in spring/summer opposing effects cause a nonsignificant trend in the snow disappearance date. Maximum snow water equivalent (SWE) in winter/spring shows a modest increase (+0.01 meters water equivalent (mwe)decade(-1)), while the end-of-summer minimum snow area fraction declined strongly (from 48% to 36%). The equilibrium line altitude is highest in relatively dry inland regions, and time series show a clear positive trend (25mdecade(-1)) as a result of summer warming. Finally, rain-on-snow in the core winter season, affecting ground ice formation and limiting access of grazing animals to food supplies, peaks during specific years (1994, 1996, 2000, and 2012) and is found to be concentrated in the lower lying coastal regions in southwestern Svalbard.

Keyword
seasonal snow, modeling, climate, Svalbard
National Category
Earth and Related Environmental Sciences Physical Geography
Identifiers
urn:nbn:se:uu:diva-307251 (URN)10.1002/2016JF003999 (DOI)000392825900008 ()
Funder
Swedish Research Council
Available from: 2016-11-11 Created: 2016-11-11 Last updated: 2018-01-13Bibliographically approved
van Pelt, W. J. J., Pohjola, V. & Reijmer, C. (2016). The changing impact of snow conditions and refreezing on the mass balance of an idealized Svalbard glacier. Frontiers in Earth Science, 4(102)
Open this publication in new window or tab >>The changing impact of snow conditions and refreezing on the mass balance of an idealized Svalbard glacier
2016 (English)In: Frontiers in Earth Science, ISSN 1096-231X, E-ISSN 1664-8021, Vol. 4, no 102Article in journal (Refereed) Published
National Category
Natural Sciences Earth and Related Environmental Sciences Physical Geography
Identifiers
urn:nbn:se:uu:diva-307252 (URN)10.3389/feart.2016.00102 (DOI)000393607800001 ()
Available from: 2016-11-11 Created: 2016-11-11 Last updated: 2018-01-13Bibliographically approved
Vega, C. P., Pohjola, V. A., Samyn, D., Pettersson, R., Isaksson, E., Björkman, M. P., . . . Kaiser, J. (2015). First Ice Core Records of NO3- Stable Isotopes from Lomonosovfonna, Svalbard. Journal of Geophysical Research - Atmospheres, 120(1), 313-330
Open this publication in new window or tab >>First Ice Core Records of NO3- Stable Isotopes from Lomonosovfonna, Svalbard
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2015 (English)In: Journal of Geophysical Research - Atmospheres, ISSN 2169-897X, E-ISSN 2169-8996, Vol. 120, no 1, p. 313-330Article in journal (Refereed) Published
Abstract [en]

Samples from two ice cores drilled at Lomonosovfonna, Svalbard, covering the period 1957-2009, and 1650-1995, respectively, were analyzed for NO(3)(-)concentrations, and NO3- stable isotopes (N-15 and O-18). Post-1950 N-15 has an average of (-6.91.9), which is lower than the isotopic signal known for Summit, Greenland but agrees with values observed in recent Svalbard snow and aerosol. Pre-1900 N-15 has an average of (4.21.6)parts per thousand suggesting that natural sources, enriched in the N-15 isotope, dominated before industrialization. The post-1950 O-18 average of (75.1 +/- 4.1)parts per thousand agrees with data from low and polar latitudes, suggesting similar atmospheric NOy (NOy=NO+NO2+HNO3) processing pathways. The combination of anthropogenic source N-15 and transport isotope effect was estimated as -29.1 parts per thousand for the last 60years. This value is below the usual range of NOx (NOx=NO+NO2) anthropogenic sources which is likely the result of a transport isotope effect of -32 parts per thousand. We suggest that the N-15 recorded at Lomonosovfonna is influenced mainly by fossil fuel combustion, soil emissions, and forest fires; the first and second being responsible for the marked decrease in N-15 observed in the post-1950s record with soil emissions being associated to the decreasing trend in N-15 observed up to present time, and the third being responsible for the sharp increase of N-15 around 2000.

National Category
Earth and Related Environmental Sciences
Research subject
Earth Science with specialization in Physical Geography
Identifiers
urn:nbn:se:uu:diva-221776 (URN)10.1002/2013JD020930 (DOI)000349104700020 ()
Available from: 2014-04-03 Created: 2014-04-03 Last updated: 2017-12-05Bibliographically approved
Vega, C. P., Björkman, M. P., Pohjola, V. A., Isaksson, E., Pettersson, R., Martma, T., . . . Kaiser, J. (2015). Nitrate stable isotopes and major ions in snow and ice samples from four Svalbard sites. Polar Research, 34, Article ID 23246.
Open this publication in new window or tab >>Nitrate stable isotopes and major ions in snow and ice samples from four Svalbard sites
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2015 (English)In: Polar Research, ISSN 0800-0395, E-ISSN 1751-8369, Vol. 34, article id 23246Article, review/survey (Refereed) Published
Abstract [en]

Increasing reactive nitrogen (N-r) deposition in the Arctic may adversely impact N-limited ecosystems. To investigate atmospheric transport of N-r to Svalbard, Norwegian Arctic, snow and firn samples were collected from glaciers and analysed to define spatial and temporal variations (1 10 years) in major ion concentrations and the stable isotope composition (delta N-15 and delta O-18) of nitrate (NO3-) across the archipelago. The delta N-15(NO3-) and delta O-18(NO3-) averaged -4 parts per thousand and 67 parts per thousand in seasonal snow (2010-11) and -9 parts per thousand and 74 parts per thousand in firn accumulated over the decade 2001-2011. East-west zonal gradients were observed across the archipelago for some major ions (non-sea salt sulphate and magnesium) and also for delta N-15(NO3-) and delta O-18(NO3-) in snow, which suggests a different origin for air masses arriving in different sectors of Svalbard. We propose that snowfall associated with long-distance air mass transport over the Arctic Ocean inherits relatively low delta N-15(NO3-) due to in-transport N isotope fractionation. In contrast, faster air mass transport from the north-west Atlantic or northern Europe results in snowfall with higher delta N-15(NO3-) because in-transport fractionation of N is then time-limited.

National Category
Earth and Related Environmental Sciences
Identifiers
urn:nbn:se:uu:diva-221777 (URN)10.3402/polar.v34.23246 (DOI)000353050700001 ()
Available from: 2014-04-03 Created: 2014-04-03 Last updated: 2017-12-05Bibliographically approved
Soto Gómez, A. J., di Baldassarre, G., Rodhe, A. & Pohjola, V. A. (2015). Remotely Sensed Nightlights to Map Societal Exposure to Hydrometeorological Hazards. Remote Sensing, 7(9), 12380-12399
Open this publication in new window or tab >>Remotely Sensed Nightlights to Map Societal Exposure to Hydrometeorological Hazards
2015 (English)In: Remote Sensing, ISSN 2072-4292, E-ISSN 2072-4292, Vol. 7, no 9, p. 12380-12399Article in journal (Refereed) Published
Abstract [en]

This study used remotely sensed maps of nightlights to investigate the etiology of increasing disaster losses from hydrometeorological hazards in a data-scarce area. We explored trends in the probability of occurrence of hazardous events (extreme rainfall) and exposure of the local population as components of risk. The temporal variation of the spatial distribution of exposure to hydrometeorological hazards was studied using nightlight satellite imagery as a proxy. Temporal (yearly) and spatial (1 km) resolution make them more useful than official census data. Additionally, satellite nightlights can track informal (unofficial) human settlements. The study focused on the Samala River catchment in Guatemala. The analyses of disasters, using DesInventar Disaster Information Management System data, showed that fatalities caused by hydrometeorological events have increased. Such an increase in disaster losses can be explained by trends in both: (i) catchment conditions that tend to lead to more frequent hydrometeorological extremes (more frequent occurrence of days with wet conditions); and (ii) increasing human exposure to hazardous events (as observed by amount and intensity of nightlights in areas close to rivers). Our study shows the value of remote sensing data and provides a framework to explore the dynamics of disaster risk when ground data are spatially and temporally limited.

Keyword
Natural disasters trends, nighttime satellite images, nightlights, exposure mapping, local scale, Guatemala
National Category
Geosciences, Multidisciplinary
Identifiers
urn:nbn:se:uu:diva-260706 (URN)10.3390/rs70912380 (DOI)000362511400065 ()
Funder
Sida - Swedish International Development Cooperation Agency, 54100006
Available from: 2015-08-24 Created: 2015-08-24 Last updated: 2017-12-04Bibliographically approved
Soto Gómez, A. J., Rodhe, A., Pohjola, V. & Boelhouwers, J. (2015). Spatial Distribution of Disasters Caused by Natural Hazards in the Samala River Catchment, Guatemala. Geografiska Annaler. Series A, Physical Geography, 97(1), 181-196
Open this publication in new window or tab >>Spatial Distribution of Disasters Caused by Natural Hazards in the Samala River Catchment, Guatemala
2015 (English)In: Geografiska Annaler. Series A, Physical Geography, ISSN 0435-3676, E-ISSN 1468-0459, Vol. 97, no 1, p. 181-196Article in journal (Refereed) Published
Abstract [en]

Understanding the conditions of the locations where disasters are reported and applying that knowledge to disaster risk reduction actions is essential and especially needed on the local scale. This study assessed how important the physical configuration is as cause of disasters by studying the links between the locations where disasters have been reported and the physical attributes of those locations. The Samala River catchment in Guatemala was used as a case study because it is a relatively small but complex area exposed to multiple natural hazards. Disasters of hydro-meteorological origin were addressed for the study because they are the most frequent type of disasters reported in the area. The method proposed in this work classified the study area into geomorphological units that were used to analyze where disasters were reported, the physical conditions of the particular locations of disaster reports, and to what degree disasters are spatially linked to slope and temporally to precipitation. We found that analyzing the study area based on the geomorphological configuration was useful and allowed analyses on comparatively homogeneous zones and to hypothesize on the particular geomorphological processes related to the occurrences of disasters. For steep geomorphological units we found a clear spatial relationship between the number of disasters reported and the slope of the locations, with higher frequency of disasters in the less sloping areas of the unit. The need to consider social factors for understanding this relationship was emphasized. As expected there was a very strong temporal relationship between disaster occurrence and wetness, as estimated from the antecedent rainfall, with high risk for disaster at high wetness. The applied methodological approach provides a tool for disaster research on physically complex areas, which is common to active tectonic and volcanic regions.

Keyword
spatial distribution, natural disasters, physical causes of natural disasters, Guatemala
National Category
Physical Geography
Identifiers
urn:nbn:se:uu:diva-251500 (URN)10.1111/geoa.12097 (DOI)000350500400012 ()
Available from: 2015-04-23 Created: 2015-04-20 Last updated: 2018-01-11Bibliographically approved
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