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Träfflista för sökning "hsv:(NATURVETENSKAP) hsv:(Geovetenskap och miljövetenskap) hsv:(Oceanografi hydrologi och vattenresurser) ;pers:(Gustafsson David)"

Sökning: hsv:(NATURVETENSKAP) hsv:(Geovetenskap och miljövetenskap) hsv:(Oceanografi hydrologi och vattenresurser) > Gustafsson David

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1.
  • Granlund, Nils, et al. (författare)
  • Laboratory study of the influence of salinity on the relationship between electrical conductivity and wetness of snow
  • 2010
  • Ingår i: Hydrological Processes. - : John Wiley & Sons. - 0885-6087 .- 1099-1085. ; 24:14, s. 1981-1984
  • Tidskriftsartikel (refereegranskat)abstract
    • Snow water equivalent of a snowpack can be estimated using ground-penetrating radar from the radar wave two-way travel time. However, such estimates often have low accuracy when the snowpack contains liquid water. If snow wetness is known, it is possible to take it into account in the estimates; it is therefore desirable to be able to determine snow wetness from already available radar data. Our approach is based on using radar wave attenuation, and it requires that the relationship between electrical conductivity and wetness of snow should be known. This relationship has been tentatively established in previous laboratory experiments, but only for a specific liquid water salinity and radar frequency. This article presents the results of new laboratory experiments conducted to investigate if and how this relationship is influenced by salinity. In each experiment, a certain amount of snow was melted and a known amount of salt (different for different experiments) was added to the water. Water salinity was measured, and the water was added step-wise to a one-meter thick snowpack, with radar measurements taken between additions of water. Our experiments have confirmed the earlier established linear relationship between electrical conductivity and wetness of snow, and they allow us to suggest that the influence of liquid water salinity on electrical conductivity is negligible when compared to the influence of liquid water content in snow.
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  • Feiccabrino, James, et al. (författare)
  • Improving surface based precipitation phase determination through air mass boundary identification
  • 2012
  • Ingår i: Nordic Hydrology. - : IWA Publishing. - 0029-1277 .- 1996-9694 .- 1998-9563 .- 2224-7955. ; 43:3, s. 179-191
  • Tidskriftsartikel (refereegranskat)abstract
    • Most hydrological models apply one empirical formula based on surface air temperature for precipitation phase determination. This approach is flawed as surface precipitation phase results from energy exchanges between falling precipitation and air in the lower atmosphere. Different lower atmospheric conditions cause different precipitation phase probabilities for near-freezing temperatures. Often directly measured lower atmospheric conditions are not available for remote areas. However, meteorological observations occurring before/after similar air mass boundaries have similar atmospheric conditions that vary from most other observations. Therefore, hydrological models can indirectly account for lower atmospheric conditions. Twenty years of manual observations from eight United States weather stations were used to compare misclassified precipitation proportions when analyzing (a) all precipitation observations together and (b) identified cold air mass boundary observations (CAB) and non-CAB observations separately. The CAB observations were identified by a rapid surface air temperature decrease. A two-surface air temperature threshold method with one threshold all snow (T-S degrees C) and one all rain (T-R degrees C) having a linear snow fraction decrease between the thresholds was used. The T-S (0 degrees C), and T-R (4 degrees C) values for CAB were 1 degrees C warmer than for non-CAB (-1 degrees C, 3 degrees C). Analyzing CAB and non-CAB separately reduced misclassified precipitation 23%, from 7.0 to 5.4%.
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4.
  • Feiccabrino, James, et al. (författare)
  • Surface-based precipitation phase determination methods in hydrological models
  • 2013
  • Ingår i: Hydrology Research. - : IWA Publishing. - 1998-9563 .- 0029-1277 .- 2224-7955. ; 44:1, s. 44-57
  • Tidskriftsartikel (refereegranskat)abstract
    • We compared solid and liquid precipitation mass output from three categories of common model precipitation phase determination schemes (PPDS) to the recorded precipitation phase in a set of 45 years of 3-hour manual meteorological observations from 19 Swedish meteorological stations. In the first category of rain/snow thresholds, it was found that rain/snow air temperature threshold (ATT) is a better precipitation phase indicator than a rain/snow dew point temperature threshold. When a rain/snow ATT of 0.0 degrees C (a default value used in some recent models) was replaced by 1.0 degrees C, misclassified precipitation was reduced by almost one half. A second category of PPDS use two ATTs, one snow and one rain, with a linear decrease in snow fraction between. This category identified precipitation phase better than a rain/snow ATT at 17 stations. Using all observations from all the meteorological stations, a final category using an air-temperature-dependent snow probability curve resulted in slightly lower misclassified precipitation mass at 13 of the 19 stations. However, schemes from the linear decrease in snow fraction category had the lowest misclassified precipitation mass at four meteorological stations.
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6.
  • Gustafsson, David, et al. (författare)
  • Boreal forest surface parameterization in the ECMWF model - 1D test with NOPEX long-term data
  • 2003
  • Ingår i: Journal of applied meteorology (1988). - 0894-8763 .- 1520-0450. ; 42:1, s. 95-112
  • Tidskriftsartikel (refereegranskat)abstract
    • The objective of the present study was to assess the performance and recent improvements of the land surface scheme used operationally in the European Centre for Medium-Range Weather Forecasts (ECMWF) in a Scandinavian boreal forest climate/ecosystem. The previous (the 1999 scheme of P. Viterbo and A. K. Betts) and the new (Tiled ECMWF Surface Scheme for Exchange Processes over Land, TESSEL) surface schemes were validated by single-column runs against data from NOPEX (Northern Hemisphere Climate-Processes Land-Surface Experiment). Driving and validation datasets were prepared for a 3-yr period (1994-96). The new surface scheme, with separate surface energy balances for subgrid fractions (tiling), improved predictions of seasonal as well as diurnal variation in surface energy fluxes in comparison with the old scheme. Simulated wintertime evaporation improved significantly as a consequence of the introduced additional aerodynamic resistance for evaporation from snow lying under high vegetation. Simulated springtime evaporation also improved because the limitation of transpiration in frozen soils was now accounted for. However, downward sensible heat flux was still underestimated during winter, especially at nighttime, whereas soil temperatures were underestimated in winter and overestimated in summer. The new scheme also underestimated evaporation during dry periods in summer, whereas soil moisture was overestimated. Sensitivity tests showed that further improvements of simulated surface heat fluxes and soil temperatures could be obtained by calibration of parameters governing the coupling between the surface and the atmosphere and the ground heat flux, and parameters governing the water uptake by the vegetation. Model performance also improved when the seasonal variation in vegetation properties was included.
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9.
  • Gustafsson, David, 1973-, et al. (författare)
  • Impulse radar measurements of snow interception : laboratory tests and field application to a forest stand in northern Sweden
  • 2009
  • Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
    • Snow melt runoff is a dominating water resource in many alpine and high latitude regions. Therefore, hydrological model predictions and information on the amount of snow are important for efficient management of for instance hydropower production. Snow interception and evaporation of snow from forest canopies is known to reduce the snow cover accumulation in forest areas compared to open areas, but there is a need for better process understanding in order to improve the model predictions. The exchange of heat and water between snow cover, canopy, and atmosphere involve many processes that can be difficult to observe at the relevant scales. Particularly, the snow interception storage is difficult to observe on a forest stand level compared with snow cover development and forest evaporation. In this study, a new application of ground penetrating impulse radar (GPR) to measure the total amount, spatial distribution and phase of the interception storage in the forest canopy is presented. The propagation velocity and frequency dependent attenuation of a broadband impulse radar wave can be used to give a direct estimate of the complex effective dielectric permittivity. The real part of the effective dielectric permittivity, estimated from the propagation velocity alone, can be used to estimate either the mass of liquid water or the mass of dry snow on the canopy if a suitable mixing formula for the effective permittivity is known. However, to separate between liquid and frozen interception additional information that we intend to get from the attenuation is needed. It has been shown for snow that the liquid water content can be estimated from the imaginary component of the effective permittivity alone. Thus, the contribution from liquid water to the real component can be subtracted, and the remaining fraction depended only on the amount of frozen snow. Laboratory experiments were performed with a GPR system, measuring the propagation velocity and frequency dependent attenuation through a sample of Norway spruce branches loaded with different amount of liquid water and snow. The results were used to establish empirical mixing formulas relating imaginary and real components of the effective dielectric permittivity to the volumetric fraction of liquid and frozen water. The obtained formulas were tested in a field application in northern Sweden, in a homogeneous stand dominated by Norway spruce. The mass of snow stored in the tree canopies were measured in two ways: firstly by measuring the weight of a single tree scaled to a forest stand average and secondly using impulse radar measurements through a small section of the forest. The transmitting and receiving antennas were placed in two small towers, separated horizontally by 15 m. The amount of intercepted snow determined from the radar measurements compared well with the measurements from the single tree weighing lysimeter, especially during cold conditions. Systematic differences were observed in situations with melting snow on the trees, when the estimation of liquid water content was overestimated by the frequency attenuation method. However, this might be due to a combination of uncertainties in the mixing models and inadequate corrections for drift in the measurement system. Overall, the results were promising and showed that impulse radar can be used to study snow interception.
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10.
  • Gustafsson, David, 1973- (författare)
  • Measurements of snow water equivalent using multiple-offset ground-penetrating radar
  • 2006
  • Ingår i: <em></em>.
  • Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
    • Ground-penetrating radar (GPR) measurements of snow water equivalent depend onaccurate information of the radar wave propagation velocity in the snow cover, aswell as on the relationship between the dielectric constant and snow density. This paperevaluates a simplified procedure to derive snow water equivalent from GPR traveltime data only, using multiple-offset GPR antennas arranged in an array. The dielectricconstant and the depth of the snow cover are determined by multi-channel measurementsusing several combinations of transmitter and receiver antennas in the array,based on basic common-mid-point principles. This set-up enables continuous determinationof both snow cover depth and snow density along the measurement profile,which reduces the uncertainties due to spatial variation of snow density. The measurementsystem is evaluated using manual snow depth and snow density measurements attwo sites in northern Sweden during winter 2004/2005. Results show that the spatialvariation of both snow depth and snow density were improved with the multi-offsetmeasurements compared to the use of a single channel measurement.
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