Observations and Mixed-Layer Modeling of a Terrain-Induced Mesoscale Gyre: The Denver Cyclone

Abstract

In northeastern Colorado a frequently observed feature of the surface wind field is a stationary, terrain-induced mesoscale gyre, which is often associated with the formation of severe weather. Because of the gyre's proximity to the Denver metropolitan area, local weather forecasters frequently refer to it as the ?Denver Cyclone. ? The development of one such cyclone, which occurred on 1 August 1985, is documented with mesonet, radiosonde, wind-profiler, radiometer and tower data. Mixed-layer model simulations of this event closely agree with the observed gyre structure and indicate that the gyre is associated with a plume of warmer potential temperature air, which originates from a ridge of higher terrain to the south of Denver, and advects northward into the area of gyre formation. A mixed-layer vorticity budget demonstrates that the formation of the gyre results from the baroclinic and slope effects on the turbulent stress divergence profile.