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James E. Bossert
and
William R. Cotton

Abstract

This study uses observed data and a numerical simulation to examine the generation of thermally driven flows across the Colorado mountain barrier on meso-β to meso-α scales. The observations were collected from remote surface observing systems at exposed mountaintop locations throughout the state of Colorado, over the summers of 1984–88, as part of the Rocky Mountain Peaks Experiment (ROMPEX). The data show the development of a recurrent circulation system across the Colorado mountain barrier, operating on a diurnal timescale. From the observations, the basic structure of the flow system appears as a daytime inflow toward the highest terrain, and a nocturnal outflow away from it. However, when examined in detail, the flow system exhibits more unusual behavior, especially west of the barrier crest. Here, winds in the early evening are occasionally observed to onset abruptly from an easterly direction, generally counter to the upper-level winds. Observations from ROMPEX for 26 August 1985 are used to provide comparison data for a numerical simulation with the Regional Atmospheric Modeling System (RAMS). This three-dimensional case study experiment is initialized with data from the National Meteorological Center and incorporates two-way interactive grid nesting.

From the observed data and case study simulation, four distinct phases of the regional-scale circulation system have been identified. In the development phase, a deep mountain-plains solenoid is generated through terrain heating along the Front Range. This circulation system transforms in the late afternoon transition phase into a westward-propagating density current (WPDC). The third phase, called the “density-current propagation phase,” occurs as the WPDC moves westward across the mountains, leaving in its wake strong southeasterly flow at the mountaintop level. This current appears to be the cause of the peculiar easterly component winds found in the ROMPEX mountaintop observations along the western slope. In the final late-night adjustment phase, the WPDC dissipates near the western edge of the Colorado mountains and a steady southerly flow evolves over the high mountain terrain. This southerly flow is the steady response to the differential heating that develops between the low-lying plains and the intermountain region.

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James E. Bossert
and
William R. Cotton

Abstract

A series of two- and three-dimensional idealized numerical experiments are conducted to examine the effects of different physical processes upon the development of the thermally driven regional-scale circulations over mountainous terrain simulated in Part I. The goal of this paper is to understand the conditions that enhance or suppress the formation of a westward-propagating density current within the mountain boundary layer. This current evolves from the Front Range mountain-plain circulation and was found in Part I to be responsible for unusual wind phenomena observed at mountaintop locations during the Rocky Mountain Peaks Experiment over western Colorado.

The idealized experiments show that the westward-propagating density current is a robust feature under summertime conditions of weak ambient flow and is initiated by differential heating across the Colorado Front Range between the plains and the intermountain region. In addition, the longevity of the thermally driven circulation system induces a steady southerly flow component, which persists over the intermountain region at night after the density current propagates away. The unique topography of the Colorado Rocky Mountain barrier—which features low plains on the east, a high dividing range, and a high plateau on the west—enhances the development of the current. The westward-propagating disturbance also develops over a range of low-level ambient wind speed, direction, and shear but is suppressed with low-level westerly flow, which also weakens the development of its progenitor, the Front Range mountain-plains solenoid.

Low-level stratification affects the depth and strength of the Front Range mountain-plains solenoid, which is most energetic in summertime conditions of near-neutral stability below 50 kPa. High stability in the lower troposphere suppresses the vertical development of the solenoid but increases the baroclinicity across the Front Range generated by surface heating, thereby still producing a significant density-current disturbance. Wet soil over the high terrain west of the Front Range also suppresses the formation and strength of the Front Range solenoid, while wet soil along the eastern slope of the Front Range and eastern plains with drier conditions over the high mountain terrain greatly enhances the baroclinicity within the solenoid and the subsequent density-current evolution. This couplet acts as an efficient conveyer of low-level moisture into the mountain region.

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