The Effects of Subcloud-Layer Diabatic Processes on Cold Air Damming

J. M. Fritsch The Pennsylvania State University, Department of Meteorology, University Park, Pennsylvania

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J. Kapolka The Pennsylvania State University, Department of Meteorology, University Park, Pennsylvania

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P. A. Hirschberg The Pennsylvania State University, Department of Meteorology, University Park, Pennsylvania

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Abstract

The hypothesis that clouds and precipitation enhance cold air damming is examined. A case example of cloud/precipitation-induced enhancement of damming is presented and a conceptual model is proposed.

It is found that the subcloud-layer diabatic effects associated with areas of precipitation produce mesoscale changes in pressure, wind, and static stability. These changes tend to maintain or strengthen damming in two fundamental ways: 1) Cloud cover maintains damming by preventing or reducing the radiative destabilization in the upslope layer. Without cloud cover, the lapse rate is more likely to increase so that upslope adiabatic cooling, and therefore the potential for damming, is decreased. 2) Subcloud-layer diabatic cooling from evaporation and reduced radiation produces a hydrostatic pressure rise in the precipitation zone. The low-level wind field adjusts to the pressure rise in such a manner that it enhances advection of low-level cold air southward under progressively warmer air just above the subcloud layer. As a result, the static stability, and therefore the damming potential, are increased as the cold air advances southward. The adjustment of the wind also increases both the upslope component of the wind field and the depth of the upslope layer, thereby enhancing the adiabatic cooling along the mountains and strengthening the wedge ridge. This combination of processes can create cold surges that propagate rapidly along the mountain chain.

Abstract

The hypothesis that clouds and precipitation enhance cold air damming is examined. A case example of cloud/precipitation-induced enhancement of damming is presented and a conceptual model is proposed.

It is found that the subcloud-layer diabatic effects associated with areas of precipitation produce mesoscale changes in pressure, wind, and static stability. These changes tend to maintain or strengthen damming in two fundamental ways: 1) Cloud cover maintains damming by preventing or reducing the radiative destabilization in the upslope layer. Without cloud cover, the lapse rate is more likely to increase so that upslope adiabatic cooling, and therefore the potential for damming, is decreased. 2) Subcloud-layer diabatic cooling from evaporation and reduced radiation produces a hydrostatic pressure rise in the precipitation zone. The low-level wind field adjusts to the pressure rise in such a manner that it enhances advection of low-level cold air southward under progressively warmer air just above the subcloud layer. As a result, the static stability, and therefore the damming potential, are increased as the cold air advances southward. The adjustment of the wind also increases both the upslope component of the wind field and the depth of the upslope layer, thereby enhancing the adiabatic cooling along the mountains and strengthening the wedge ridge. This combination of processes can create cold surges that propagate rapidly along the mountain chain.

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