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A Case Study of Comma Cloud Development in the Eastern Pacific

Richard J. ReedDepartment of Atmospheric Sciences, University of Washington, Seattle, WA 98195

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Warren BlierDepartment of Atmospheric Sciences, University of Washington, Seattle, WA 98195

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Abstract

A case study is presented of the development within a polar air stream of a comma-shaped cloud pattern (comma cloud) and associated small surface cyclone. The disturbance is traced from the time of its development over the eastern Pacific Ocean until it moves inland over California as a mature system. The first sign of the development was the appearance of a region of enhanced convection in the northwesterly flow behind an amplifying large-scale trough and ahead of an embedded short-wave trough. As the development progressed, the cloud field expanded and assumed the comma shape. The head, located on the cold, cyclonic-shear side of the jet stream, was composed of large, deep convective elements that merged sequentially. The tail, located on the warm, anticyclonic-shear side, was composed of shallower, more stratiform clouds. The surface low center formed within the comma head during the phase of rapid organization and strengthening of the convection.

Detailed surface and upper air analyses of the system over California during the mature stage revealed that the disturbance was at this stage associated with a weakly baroclinic region in the lower and middle troposphere and was capped by a strong upper-tropospheric frontal zone. Although no change of air mass accompanied the passage of the comma cloud, the disturbance did exhibit frontal characteristics at the surface as it passed stations near the southern California Coast. Precipitation within the mature cloud band tended to be stratiform, with some rather large precipitation totals reported.

The quasi-geostrophic omega equation is employed to elucidate the processes involved in the development of the comma cloud. A qualitative analysis indicates the likely importance of small static stabilities in enhancing the effect of the relatively modest positive vorticity advection. The possible importance of latent heat release on the development of the system is also discussed. Finally, the main results of this paper and a companion paper are summarized in the form of a schematic diagram.

Abstract

A case study is presented of the development within a polar air stream of a comma-shaped cloud pattern (comma cloud) and associated small surface cyclone. The disturbance is traced from the time of its development over the eastern Pacific Ocean until it moves inland over California as a mature system. The first sign of the development was the appearance of a region of enhanced convection in the northwesterly flow behind an amplifying large-scale trough and ahead of an embedded short-wave trough. As the development progressed, the cloud field expanded and assumed the comma shape. The head, located on the cold, cyclonic-shear side of the jet stream, was composed of large, deep convective elements that merged sequentially. The tail, located on the warm, anticyclonic-shear side, was composed of shallower, more stratiform clouds. The surface low center formed within the comma head during the phase of rapid organization and strengthening of the convection.

Detailed surface and upper air analyses of the system over California during the mature stage revealed that the disturbance was at this stage associated with a weakly baroclinic region in the lower and middle troposphere and was capped by a strong upper-tropospheric frontal zone. Although no change of air mass accompanied the passage of the comma cloud, the disturbance did exhibit frontal characteristics at the surface as it passed stations near the southern California Coast. Precipitation within the mature cloud band tended to be stratiform, with some rather large precipitation totals reported.

The quasi-geostrophic omega equation is employed to elucidate the processes involved in the development of the comma cloud. A qualitative analysis indicates the likely importance of small static stabilities in enhancing the effect of the relatively modest positive vorticity advection. The possible importance of latent heat release on the development of the system is also discussed. Finally, the main results of this paper and a companion paper are summarized in the form of a schematic diagram.

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