The Structure and Evolution of Cold Surges East of the Rocky Mountains

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  • 1 Department of Atmospheric Sciences, University of Washington, Seattle, Washington
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Abstract

Northerly surges of cold air often move southward along the eastern side of the Rockies from southern Canada into Mexico. The strongest surges, which generally develop in midwinter, are associated with temperature decreases and pressure rises of 20°–30°C and 15–30 mb, respectively, within 24 h. Surges are usually accompanied by a meridionally elongated pressure ridge and strong low-level ageostrophic winds that parallel the terrain. The width of the pressure ridging is approximately 1000 km over the southern plains but decreases to only a few hundred kilometers when the surge enters Mexico.

This paper provides a detailed description of a northerly surge to the east of the Rocky Mountains that occurred on 12–14 November 1986. Using both observational and model data, the structural evolution of the surge is analyzed; in addition, the dynamics of the event is explored by diagnosing the momentum, thermodynamic energy, and vorticity equations. To determine the typical synoptic-scale evolution of these cold surges, a composite study is also presented. It is concluded that these cold surges result primarily from the interaction of the evolving synoptic-scale flow with the Rocky Mountains and the sloping topography of the Great Plains, and not from the generation of rotationally trapped waves such as Kelvin, shelf, and topographic Rossby waves. When an upper-level short-wave trough moves southeastward out of western Canada, northerlies, high pressure, and cold air spread southward into the northern plains at low levels. Lee troughing occurs to the east of the central and southern Rockies and, in concert with the ridging to the north, establishes an along-barrier pressure gradient that forces ageostrophic northerly flow and the meridional advection of cold air. Blocked upslope flow at the forward portion of the surge leads to large-scale damming. As the surge enters Mexico, where the topography becomes steeper and the large-scale slope is lost, the width of the damming is greatly reduced. Consistent with damming, momentum diagnostics over both the Great Plains and coastal Mexico indicate that an antitriptic balance exists parallel to the mountains, whereas a geostrophic balance exists normal to the barrier.

Abstract

Northerly surges of cold air often move southward along the eastern side of the Rockies from southern Canada into Mexico. The strongest surges, which generally develop in midwinter, are associated with temperature decreases and pressure rises of 20°–30°C and 15–30 mb, respectively, within 24 h. Surges are usually accompanied by a meridionally elongated pressure ridge and strong low-level ageostrophic winds that parallel the terrain. The width of the pressure ridging is approximately 1000 km over the southern plains but decreases to only a few hundred kilometers when the surge enters Mexico.

This paper provides a detailed description of a northerly surge to the east of the Rocky Mountains that occurred on 12–14 November 1986. Using both observational and model data, the structural evolution of the surge is analyzed; in addition, the dynamics of the event is explored by diagnosing the momentum, thermodynamic energy, and vorticity equations. To determine the typical synoptic-scale evolution of these cold surges, a composite study is also presented. It is concluded that these cold surges result primarily from the interaction of the evolving synoptic-scale flow with the Rocky Mountains and the sloping topography of the Great Plains, and not from the generation of rotationally trapped waves such as Kelvin, shelf, and topographic Rossby waves. When an upper-level short-wave trough moves southeastward out of western Canada, northerlies, high pressure, and cold air spread southward into the northern plains at low levels. Lee troughing occurs to the east of the central and southern Rockies and, in concert with the ridging to the north, establishes an along-barrier pressure gradient that forces ageostrophic northerly flow and the meridional advection of cold air. Blocked upslope flow at the forward portion of the surge leads to large-scale damming. As the surge enters Mexico, where the topography becomes steeper and the large-scale slope is lost, the width of the damming is greatly reduced. Consistent with damming, momentum diagnostics over both the Great Plains and coastal Mexico indicate that an antitriptic balance exists parallel to the mountains, whereas a geostrophic balance exists normal to the barrier.

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