The Behavior of Dry Cold Fronts Traveling Along a Coastline

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  • 1 Department of Atmospheric Science, Colorado State University, Fort Collins, Colorado 80523
  • | 2 CSIRO Division of Atmospheric Research, Melbourne, Australia
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Abstract

Observations of intense, dry summertime cold fronts in southeast Australia provide evidence for frontal deformation (cold air surges) in the coastal region well to the west of the main mountain range. This compares with the severe deformation that occurs when such cold fronts, known as Southerly Busters, interact with the mountains to the east. Three-dimensional numerical simulations of unsteady gravity currents, in the absence of orography, provide some insight into the factors affecting the evolution of cold air surges, including differential friction between land and sea; boundary-layer heating over land and the magnitude of the large-scale offshore wind. The latter affects, in turn, the presence of an internal boundary layer of cool air in the prefrontal region over the sea and a sea breeze over land. Depending upon the magnitude of the offshore wind a preferred region of enhanced vertical motion occurs just ahead of the gravity-current front, and within about 200 km of the coastline. This carries implications for the prefrontal development of squall lines that is commonly observed in summer.

Abstract

Observations of intense, dry summertime cold fronts in southeast Australia provide evidence for frontal deformation (cold air surges) in the coastal region well to the west of the main mountain range. This compares with the severe deformation that occurs when such cold fronts, known as Southerly Busters, interact with the mountains to the east. Three-dimensional numerical simulations of unsteady gravity currents, in the absence of orography, provide some insight into the factors affecting the evolution of cold air surges, including differential friction between land and sea; boundary-layer heating over land and the magnitude of the large-scale offshore wind. The latter affects, in turn, the presence of an internal boundary layer of cool air in the prefrontal region over the sea and a sea breeze over land. Depending upon the magnitude of the offshore wind a preferred region of enhanced vertical motion occurs just ahead of the gravity-current front, and within about 200 km of the coastline. This carries implications for the prefrontal development of squall lines that is commonly observed in summer.

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