Case Study of an Orographically Induced Mesoscale Vortex (Denver Cyclone)

Abstract

Observations taken during the Convection Initiation and Downburst Experiment (CINDE) are used to describe the formation and structure of an orographically induced mesoscale vortex that frequently occurs in northeastern Colorado. This vortex, known locally as the Denver Cyclone due to its proximity to the Denver metropolitan area, is frequently associated with severe weather. We present a case study of the Denver Cyclone of 25 June 1987, that formed during the late morning hours and remained nearly stationary for over 24 hours. Interesting features of the case study vortex are: low-level convergence into the center of the cyclone during nighttime hours but divergence at the center when daytime heating becomes significant; a very shallow initial vertical extent at night, growing to nearly 1500 m during the daytime hours; a cold pool of air on the west side of the vortex, with highest surface potential temperatures present in a warm plume on the east side; a perturbation low pressure of ?150 Pa in the region of warmest potential temperatures; a sloping zone of low-level convergence, in the region of lower pressure, that triggers intense convective activity, and an upwind tilt of the center axis of the vortex.