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Wendy Schreiber-Abshire and Alfred R. Rodi

Abstract

In July 1987 during the CINDE project, three similar mesoscale planetary boundary layer convergence zones were observed to form in northeastern Colorado near Denver under synoptic-scale southwesterly flow. A number of recent studies have documented the importance of such convergence zones on the local weather in the Denver area. The three case studies presented in this paper are the boundary type previously classified by Wilson and Schreiber (1986) to be of unknown origin.

The analysis of mesonet, radar, and sounding data indicates that during periods of southwesterly flow at mountaintop levels over Colorado, the ridgetop winds may intrude into the Denver basin once the nocturnal temperature inversion has been eroded, provided that no other dominant synoptic-scale surface feature is affecting northeastern Colorado. When such an intrusion occurs, the southwest flow progresses northeastward until it reaches the frequently observed cold pool of air over the Platte River valley, which forms as the result of the nighttime drainage flow from the surrounding elevated terrain. It is at the leading edge of this cold pool that a surface-based convergence zone forms and remains until the cold pool is dissipated by insolation and mixing.

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Thomas R. Parish, Alfred R. Rodi, and Richard D. Clark

Abstract

A case study of the kinematical and dynamical evolution of the summertime Great Plains low level jet (LLJ) is presented. Airborne radar altimetry was used to discern the x and y components of the geostrophic wind at three levels in the lower atmosphere throughout the LLJ episode. Results appear to confirm previous theoretical and numerical studies regarding the importance of the diurnal cycle of heating over sloping terrain in producing an oscillating horizontal pressure gradient force. Inertial turning of the LLJ as a result of frictional decoupling was also documented. It is concluded that the inertial oscillation resulting from the sudden decrease in friction in the lower atmosphere during the early evening is the dominant mechanism in forcing this example of a summertime Great Plains LLJ.

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