The Organization of Convection in Narrow Cold-Frontal Rainbands

G. W. Kent Moore Geophysical Fluid Dynamics Program, Princeton University, Princeton, NJ 08540

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Abstract

Narrow cold-frontal rainbands are lines of intense precipitation that straddle surface cold fronts. Recent observational work has revealed that the rainfall within the band is organized into regularly spaced ellipsoidal cells called precipitation cores. The rainband is coincident with a line of intense cyclonic shear associated with a low-level jet that lies ahead of and parallel to the surface cold front. Numerous authors have suggested that the organization of cells in the rainband is the result of shear instability of the horizontal wind field. To investigate this hypothesis, a linear stability analysis was performed on an idealized frontal zone consisting of a line of convection coincident with a line of cyclonic shear. To model the convective processes, the air inside the rainband was assumed to be unstably stratified. The presence of horizontal wind shear in an unstably stratified environment resulted in the existence of a mode with a short wave cutoff. The coupling between the convective processes and the shear instability in this mode was strong; its most unstable wave had properties similar to the precipitation cores observed in the narrow cold-frontal rainband.

Abstract

Narrow cold-frontal rainbands are lines of intense precipitation that straddle surface cold fronts. Recent observational work has revealed that the rainfall within the band is organized into regularly spaced ellipsoidal cells called precipitation cores. The rainband is coincident with a line of intense cyclonic shear associated with a low-level jet that lies ahead of and parallel to the surface cold front. Numerous authors have suggested that the organization of cells in the rainband is the result of shear instability of the horizontal wind field. To investigate this hypothesis, a linear stability analysis was performed on an idealized frontal zone consisting of a line of convection coincident with a line of cyclonic shear. To model the convective processes, the air inside the rainband was assumed to be unstably stratified. The presence of horizontal wind shear in an unstably stratified environment resulted in the existence of a mode with a short wave cutoff. The coupling between the convective processes and the shear instability in this mode was strong; its most unstable wave had properties similar to the precipitation cores observed in the narrow cold-frontal rainband.

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