Abstract
Observations taken during the February 1991 Atmospheric Studies in Complex Terrain (ASCOT) Winter Validation Study are used to describe the wind field associated with a terrain-forced mesoscale vortex and thermally forced canyon drainage flows along the Front Range of northeastern Colorado. A case study is presented of the night of 6/7 February 1991 when a weak vortex formed and propagated through the ASCOT domain.
The 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 vortex and the canyon drainage flows. The lidar observations documented the kinematic and structural changes in the cyclone and their relationship to a drainage jet exiting a nearby canyon. Lidar analyses clearly show the layering and stratification present during this case, specifically the drainage jet flowing under the cyclone. A period of strong intensification of the drainage flows occurred, following the apparent inhibition of the exit jet by southeasterly flow and the subsequent release of the exit jet, as north-northwesterly flow developed along the foothills.
Additional analyses of the mesoscale surface wind field reveal the movement and spatial variations of the cyclone from initiation to dissipation. The ambient flow remained weak and the cyclone propagated from north to south, which is opposite to previous modeled and observational studies, and on several occasions the cyclone split into two separate vortices. A tracer diffusion test performed during this case shows that the vortex changed the trajectories of the test release cloud from northerly to southerly due both to the movement of the cyclone 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 number flow.
