A spatial analysis of snow-surface energy exchanges over the northern Great Plains of the United States in relation to synoptic scale forcing mechanisms

The northern Great Plains of the US is a region that experiences extensive winter snow covers. Meltwater provided by this snow is an important source of freshwater for agriculture, domestic uses and hydroelectric power. The g eographic location of the region, however, makes it subject to a variety of meteorological influences that can induce rapid ablation episodes and floo ding. A study of the synoptic scale weather patterns that induce large snowmelt e vents can provide a useful insight into the processes that contribute to ab lation. The first objective of this study is to identify distinct regional circulation patterns that are associated with snow depth changes of greater than 2.54 cm (1 in.). Detailed case studies representing each synoptic typ e are then examined and used to illustrate the spatial relationships betwee n synoptically induced meteorological variations and snow-surface energy fl uxes. A one-dimensional mass and energy balance model is used to compute th e surface energy fluxes and snow depth changes. The use of modeled fluxes i n lieu of measured values allows for a more spatially extensive analysis as surface fluxes over the entire study region can be analyzed in conjunction with the prevailing synoptic scale weather patterns. Three synoptic patterns are associated with large midwinter snowmelt episod es in the northern Great Plains. All three patterns involve a midlatitude c yclone moving through the region and the advection of warm air into the reg ion. The case studies reveal that subtle differences in the location and th e strength of the cyclones lead to variations in cloud cover, wind speed, t emperature and humidity among the different synoptic types. These differenc es when combined with variations in surface cover can greatly affect the ma gnitude and spatial distribution of radiative and turbulent energy exchange s. Copyright (C) 1999 Royal Meteorological Society.