Observations and Modeling of a Mesoscale Cold Surge during WISPIT

Abstract

A transient feature marked primarily by a sudden increase in windspeed and brief period of heavy snow was observed over northeasternColorado on 11-12 March 1993. Little, if any, abrupt changes in surfacepressure or temperature were noted, such as might accompany a gravitycurrent. Gradual temperature falls and pressure rises were observedbehind the velocity surge; however, there was no cyclonic wind shiftnor did the feature last for more than a few hours, suggesting thatit was not likely a classical front. Data indicate that the surgeoriginated near the peak of the Cheyenne Ridge, a 400-m, east-westelongated hill near the border between Colorado and Wyoming.Simulations using an idealized, two-dimensional isentropic model showthat a transient resembling observed surge results from impulsivelystarted flow over heated terrain. Heating creates convergence thatis in phase with the velocity surge at the leading edge of the topographicwave. The result is a coherent disturbance in velocity, which movesdownstream at nearly the mean flow speed.Simulations with The Pennsylvania State University-National Centerfor Atmospheric Research nonhydrostatic model (MM5) using a horizontalresolution of 6.7 km capture the evolution of the observed surge.These simulations support the identification of the surge as a topographicwave modified by heating, but they also indicate an important roleplayed by latent heating in the updraft at the leading edge of thesurge. Because the latent heating occurs mainly downstream from theCheyenne Ridge, the formation of the topographic wave is unalteredby it, however, the updraft at the leading edge of the surge it greatlyintensified, leading to a narrow, propagating band of heavy snow.