The Influence of Topography and Ambient Stability on the Characteristics of Cold-Air Pools: A Numerical Investigation

Marwan Katurji Department of Geography, Michigan State University, East Lansing, Michigan

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Shiyuan Zhong Department of Geography, Michigan State University, East Lansing, Michigan

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Abstract

A high-resolution numerical investigation of a cold-air pooling process (under quiescent conditions) is carried out that systematically highlights the relations between the characteristics of the cold-air pools (e.g., slope winds, vertical temperature and wind structure, and cooling rate) and the characteristics of the topography (e.g., basin size and slope angle) under different ambient stabilities. The Advanced Regional Prediction System model is used to simulate 40 different scenarios at 100-m (10 m) horizontal (vertical) resolution. Results are within the range of similar observed phenomena. The main physical process governing the cooling process near the basin floor (<200 m in height) was found to be longwave radiative flux divergence, whereas vertical advection of temperature dominated the cooling process for the upper-basin areas. The maximum downslope wind speed is linearly correlated with both basin size and slope angle, with stronger wind corresponding to larger basin and lower slope angle. As the basin size increases, the influence of slope angle on maximum downslope wind decreases and the maximum is located farther down the slope. These relationships do not appear to be sensitive to stability, but weaker stability produces more cooling in the basin atmosphere by allowing stronger rising motion and adiabatic cooling. Insight gained from this study helps to improve the understanding of the cold-air pooling process within the investigated settings.

Corresponding author address: Marwan Katurji, Dept. of Geography, Michigan State University, East Lansing, MI 48824. E-mail: katurji@msu.edu

Abstract

A high-resolution numerical investigation of a cold-air pooling process (under quiescent conditions) is carried out that systematically highlights the relations between the characteristics of the cold-air pools (e.g., slope winds, vertical temperature and wind structure, and cooling rate) and the characteristics of the topography (e.g., basin size and slope angle) under different ambient stabilities. The Advanced Regional Prediction System model is used to simulate 40 different scenarios at 100-m (10 m) horizontal (vertical) resolution. Results are within the range of similar observed phenomena. The main physical process governing the cooling process near the basin floor (<200 m in height) was found to be longwave radiative flux divergence, whereas vertical advection of temperature dominated the cooling process for the upper-basin areas. The maximum downslope wind speed is linearly correlated with both basin size and slope angle, with stronger wind corresponding to larger basin and lower slope angle. As the basin size increases, the influence of slope angle on maximum downslope wind decreases and the maximum is located farther down the slope. These relationships do not appear to be sensitive to stability, but weaker stability produces more cooling in the basin atmosphere by allowing stronger rising motion and adiabatic cooling. Insight gained from this study helps to improve the understanding of the cold-air pooling process within the investigated settings.

Corresponding author address: Marwan Katurji, Dept. of Geography, Michigan State University, East Lansing, MI 48824. E-mail: katurji@msu.edu
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