Snow cover is found across extensive areas of the northern hemisphere during the winter and early spring seasons. Meltwater provided by this snow cover can be an important source of freshwater for agriculture, domestic uses and hydroelectric power. Rapid ablation of the snowpack, however, can also pose environmental hazards such as ¯ooding.
The ability to forecast meltwater quantities is dependent upon a knowledge of the factors in¯uencing the snowmelt process. This paper employs a hybrid modelling and synoptic climatological approach to investigate the relationships between synoptic weather patterns, surface energy ¯uxes and midwinter snowmelt in the northern Great Plains. The ®rst objective of this study is to identify distinct synoptic patterns that are associated with days where signi®cant snow cover ablation occurred. The second objective is to evaluate the relationships between synoptic-scale weather patterns, snow surface 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 ¯ux data from single index sites, this study employs a physically based snowpack model to generate estimates of energy ¯uxes. The use of modelled ¯uxes instead of measured values allows for a more spatially extensive analysis as surface ¯uxes over the entire study region can be analysed in conjunction with the prevailing synoptic-scale weather patterns.
Three major synoptic types, characterized by the presence of a midlatitude cyclone, are associated with large midwinter snowmelt episodes in the northern Great Plains. The case study illustrates how variations in temperature, humidity, cloud cover and wind speeds associated with such cyclonic storms can play a major role in aecting snow surface±atmosphere energy exchanges. As expected, elevated wind speeds and stronger temperature and humidity gradients signi®cantly increased the transfers of sensible and latent heat between the snow surface and the atmosphere. Increased cloud cover near the low pressure centre reduced incoming solar radiation but through counter radiation also reduced the loss of long-wave radiation.