Regional-Scale Flows in Mountainous Terrain. Part II: Simplified Numerical Experiments

James E. Bossert Earth and Environmental Sciences Division, Los Alamos National Laboratory, Los Alamos, New Mexico

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William R. Cotton Department of Atmospheric Science, Colorado State University, Fort Collins, Colorado

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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.

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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