A case study of the synoptic patterns influencing midwinter snowmelt across the northern Great Plains

Snow cover is found across extensive areas of the northern hemisphere durin g the winter and early spring seasons. Meltwater provided by this snow cove r can be an important source of freshwater for agriculture, domestic uses a nd hydroelectric power. Rapid ablation of the snowpack, however, can also p ose environmental hazards such as flooding. The ability to forecast meltwater quantities is dependent upon a knowledge of the factors influencing the snowmelt process. This paper employs a hybri d modelling and synoptic climatological approach to investigate the relatio nships between synoptic weather patterns, surface energy fluxes and midwint er snowmelt in the northern Great Plains. The first objective of this study is to identify distinct synoptic patterns that are associated with days wh ere significant snow cover ablation occurred. The second objective is to ev aluate the relationships between synoptic-scale weather patterns, snow surf ace energy transfers and snowmelt. A case study of 21 February 1975 is used to illustrate these relationships. Unlike the other synoptic-type studies, which rely on empirically derived energy flux data from single index sires , this study employs a physically based snowpack model to generate estimate s of energy fluxes, The use of modelled fluxes instead of measured values a llows for a more spatially extensive analysis as surface fluxes over the en tire study region can be analysed in conjunction with the prevailing synopt ic-scale weather patterns. Three major synoptic types, characterized by the presence of a midlatitude cyclone, are associated with large midwinter snowmelt episodes in the north ern Great Plains. The ease study illustrates how variations in temperature, humidity, cloud cover and wind speeds associated with such cyclonic storms can play a major role in affecting snow surface-atmosphere energy exchange s, As expected, elevated wind speeds and stronger temperature and humidity gradients significantly increased the transfers of sensible and latent heat between the snow surface and the atmosphere. Increased cloud cover near th e low pressure centre reduced incoming solar radiation but through counter radiation also reduced the loss of long-wave radiation. (C) 1998 John Wiley & Sons, Ltd.