OBSERVATIONS OF A TERRAIN-FORCED MESOSCALE VORTEX AND CANYON DRAINAGEFLOWS ALONG THE FRONT RANGE OF COLORADO

Observations taken during the February 1991 Atmospheric Studies in Com plex Terrain (ASCOT) Winter Validation Study are used to describe the wind field associated with a terrain-forced mesoscale vortex and therm ally forced canyon drainage flows along the Front Range of northeaster n Colorado. A case study is presented of the night of 6/7 February 199 1 when a weak vortex formed and propagated through the ASCOT domain. T he NOAA/ERL Environmental Technology Laboratory Doppler lidar, one of an ensemble of instruments participating in the ASCOT field experiment , obtained high-resolution measurements of the structure of both the v ortex and the canyon drainage flows. The lidar observations documented the kinematic and structural changes in the cyclone and their relatio nship to a drainage jet exiting a nearby canyon. Lidar analyses clearl y show the layering and stratification present during this case, speci fically the drainage jet flowing under the cyclone. A period of strong intensification of the drainage hows occurred, following the apparent inhibition of the exit jet by southeasterly flow and the subsequent r elease of the exit jet, as north-northwesterly flow developed along th e foothills. Additional analyses of the mesoscale surface wind field r eveal the movement and spatial variations of the cyclone from initiati on to dissipation. The ambient dow remained weak and the cyclone propa gated from north to south, which is opposite to previous modeled and o bservational studies, and on several occasions the cyclone split into two separate vortices. A tracer diffusion test performed during this c ase shows that the vortex changed the trajectories of the test release cloud from northerly to southerly due both to the movement of the cyc lone and to the presence of northerly flow associated with the vortex. Estimates of Froude number are consistent with previous studies that showed Denver cyclones are associated with periods of low-Froude numbe r how.