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  • Author or Editor: David H. Levinson x
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David H. Levinson
and
Robert M. Banta

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.

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Paul J. Neiman
,
F. Martin Ralph
,
Robert L. Weber
,
Taneil Uttal
,
Louisa B. Nance
, and
David H. Levinson

Abstract

Through the integrated analysis of remote sensing and in situ data taken along the Front Range of Colorado, this study describes the interactions that occurred between a leeside arctic front and topographically modulated flows. These interactions resulted in nonclassical frontal behavior and structure across northeastern Colorado. The shallow arctic front initially advanced southwestward toward the Front Range foothills, before retreating eastward. Then, a secondary surge of arctic air migrated westward into the foothills. During its initial southwestward advance, the front exhibited obstacle-like, density-current characteristics. Its initial advance was interrupted by strong downslope northwesterly flow associated with a high-amplitude mountain wave downstream of the Continental Divide, and by a temporal decrease in the density contrast across the front due to diurnal heating in the cold air and weak cold advection in the warm air. The direction and depth of flow within the arctic air also influenced the frontal propagation.

The shallow, obstacle-like front actively generated both vertically propagating and vertically trapped gravity waves as it advanced into the downslope northwesterly flow, resulting in midtropospheric lenticular wave clouds aloft that tracked with the front. Because the front entered a region where strong downslope winds and mountain waves extended downstream over the high plains, the wave field in northeastern Colorado included both frontally forced and true mountain-forced gravity waves. A sequence of Scorer parameter profiles calculated from hourly observations reveals a sharp contrast between the prefrontal and postfrontal wave environments. Consequently, analytic resonant wave mode calculations based on the Scorer parameter profiles reveal that the waves supported in the postfrontal regime differed markedly from those supported in the prefrontal environment. This result is consistent with wind profiler observations that showed the amplitude of vertical motions decreasing substantially through 16 km above mean sea level (MSL) after the shallow frontal passage.

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