A Statistical Analysis of Snow Depth Variability in Norway and Evaluation of Norwegian Snow Maps

DYRRDAL, ANITA VERPE. A Statistical Analysis of Snow Depth Variability in Norway and Evaluation of Norwegian Snow Maps. (Under the direction of Dr. Frederick H.M. Semazzi). This study examines snow depth variability in Norway by applying a number of statistical tools. A time series analysis of snow depth and length of snow season is carried out at eleven meteorological stations situated in three different parts of the country, named Region 1, Region 2, and Region 3. In addition, empirical orthogonal function (EOF) analysis and simple correlation analysis is performed in order to determine the spatial patterns and dominant modes of snow depth variability in Norway. The snow map service introduced by the Norwegian Meteorological Institute and Norwegian Water Resources and Energy Directorate in 2004 is evaluated at the same stations mentioned above. The focus is on the start and end of the snow season and the total number of snow days per hydrological year (Sep 1st – Aug 31st). A decreasing trend in snow depth is evident at nine of eleven stations. Stations in Region 1 and Region 2 reveal a later start of snow season, while stations in Region 3 reveal a slightly earlier start of snow season. An earlier end of snow season and a decrease in number of snow days is seen at all eleven stations. Region 3 shows the strongest decrease in the number of snow days and daily snow depth, probably due to low elevation and proximity to the coast, leaving this region more sensitive to global warming. Two leading eigenmodes (EOF1 and EOF2) accounting for 41.4% and 18.6% of the variability in snow depth, respectively, are identified and attempted defined. Existing Northern Hemisphere teleconnection indices are mainly established to explain variability in temperature and precipitation. Snow accumulation is dependent on both previous mentioned variables, and so the EOFs cannot be defined by one single teleconnection index. EOF1 appears to be related to East Atlantic/Western Russia (EA/WR) pattern, Arctic Oscillation (AO) and possibly to enhanced onshore flow, where the prevailing wind perpendicular to the southern coast of Norway helps push precipitation inland. The time series reveal a decadal variability which might be attributable to the Gulf Stream or to other unknown internal oceanic processes. EOF2 has an inter-annual variability and an obvious increasing trend. This leads us to believe that it is connected to climate change, and previous studies have shown that the North Atlantic Oscillation (NAO) exhibits the same increasing trend. The correlation analysis, however, shows strong correlations to the East Atlantic pattern (EA) and the Scandinavia pattern (SCAND). In evaluating the snow maps, simulations by a precipitation / degree-day snow model were compared to observations. The approach of calculating snow depth used in the model tends to compact the snow too much, resulting in underestimation of snow depth at most stations. In Region 1 the snow model simulates a shorter snow season than what is observed. In Region 2 the results are ambiguous and correlations between simulations and observations are low. In Region 3, on the other hand, the model performs really well, and there are no significant differences between simulated and observed snow season. A Statistical Analysis of Snow Depth Variability in Norway and Evaluation of Norwegian Snow Maps by Anita Verpe Dyrrdal A thesis submitted to the Graduate Faculty of North Carolina State University In partial fulfillment of the Requirements for the degree of Master of Science Marine, Earth and Atmospheric Sciences Raleigh, North Carolina 2008