Abstract:
A set of model runs was made with the National Center for Atmospheric Research (NCAR) Community
Climate Model, version 3 (CCM3) to investigate and help assess the relative roles of snow cover anomalies and
initial atmospheric states on the subsequent accumulation and ablation seasons. In order to elucidate the physical
mechanisms responsible for the large impact in one case but small impact in the other, two experiments with
CCM3 were made that imposed an exaggerated initial snow cover [1-m snow water equivalent (SWE)] over the
western U.S. domain. One run was started on 1 December, the other on 1 February. These runs made it clear
that the high albedo of snow was the dominant physical process. An additional set of runs with realistic yearly
snow anomalies was also made. Results suggest that for runs starting in February (late winter), the initial
prescription of snow cover is more important than the initial atmospheric state in determining the subsequent
evolution of snow cover. For runs starting in December (early winter), the results are less clear, with neither
the initial snow cover nor the initial state of the atmosphere appearing to be the dominant factor. In February,
when the sun is relatively high in the sky and days are longer, the albedo effect is a dominant factor; while in
December the sun was too low in the sky and days too short for the albedo effect to be important. As the winter
season progressed, the subsequent accumulation of snow eliminated the effects of the initial December anomalies.
