The Front Range Blizzard of 1990. Part III: Numerical Simulations of Melting Effects

Abstract

The Front Range blizzard of 6 March 1990 resulted in heavy rain and snow along the foothills of Colorado and in southeast Wyoming. A narrow barrier jet with northerly winds behind a shallow mesoscale front developed concurrently with the heavy precipitation. It was hypothesized by Marwitz and Toth that the mesoscale front was the result of the diabatic process of melting. The CSU RAMS model was used to test the effects of melting, as well as the roles that the ice process and upslope flow played in the storm. A two-dimensional simulation was initialized with bulk microphysics and with a modified Flagler, Colorado, sounding. The results showed that the simulation was able to produce many features similar to the observations, such as surface cooling, a northerly barrier jet, and a steady. slow-moving shallow mesoscale front. It was found that these features were much less pronounced in an identical simulation but with melting turned off. Furthermore, it was found that diabatically cooled air was accumulated along the foothills and induced a direct circulation. These results supported the hypothesis that melting was a dominant process contributing to the development of the storm. A simulation without ice microphysics showed that ice altered the kinematic and thermodynamic structures. A simulation with halved upslope flow showed that upslope flow was an important factor modifying the precipitation pattern and a critical parameter in determining the kinematic, thermodynamic, and precipitation patterns.