A Quantitative Approach to Evaluating the Effects of Snow Cover on Cold Airmass Temperatures across the U.S. Great Plains

Abstract

Over the past two decades a greater emphasis has been placed on the accuracy of the representation of snow cover?atmosphere interactions in weather and climate prediction models. Much of the attention centered upon snow cover is a result of concerns associated with anthropogenic and natural causes of potential changes in the global environment that may be intensified by the snow cover climatology. As a predictive tool, the importance of the interactions between snow cover and the overlying atmosphere is recognized in areas ranging from daily and seasonal surface air temperature forecasts, to anomalies in large-scale atmospheric circulation patterns. Within this study the effects of snow cover on surface air temperatures within cold air masses moving across the U.S. Great Plains in winter were investigated. Through the adaptation of a one-dimensional snowpack model, the thermal characteristics of the core of a cold air mass were derived from the equation governing the heat balance between the surface and the lower atmosphere. The methodology was based on the premise that the core of a cold air mass may be considered homogeneous and not subject to advection of air from outside, thereby isolating the exchange of energy between the surface and the atmosphere as the control on lower-atmospheric temperatures. The adapted model included the synergism of the air mass?snow cover relationship through time, incorporating the natural feedback process. Simulation of surface air temperatures within four cold air masses produced results that include 1) mean daytime temperatures 6°?10°C warmer and maximum daytime temperatures 10°?15°C warmer over bare ground compared to a snow cover, 2) mean nighttime temperatures 1°?2°C warmer over bare ground compared to a snow cover, and 3) attribution of temperature differences primarily to differences in the exchange of sensible heat between the surface and the overlying air mass. Daytime differences in the energy fluxes produced by the different surface conditions were largely the result of surface temperature differences produced by variations in the albedo and the amount of solar radiation absorbed at the surface. The authors believe that the results of this study can be used as additional guidance for more accurate forecasts of daily maximum and minimum surface air temperatures within wintertime cold air masses over, and possibly downstream from, snow cover across the North American continent. The study documents the importance of the various components of the heat balance between the lower atmosphere and surface in regard to cold airmass modification, and emphasizes the importance of an accurate representation of snow cover in forecast models.