Some Dynamical Aspects of Precipitating Convection

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  • 1 Center for Meteorology and Physical Oceanography, Massachusetts Institute of Technology, Cambridge, MA 02139
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Abstract

A simple linear model is developed with the idea of demonstrating the basic physical processes that serve to distinguish the dynamics of precipitating convection from those of the nonprecipitating variety. In particular, it is shown that the hypothesis advanced by Seitter and Kuo to explain the slope and propagation of squall lines in the context of a fully nonlinear numerical model operates also within a linear model. With a hierarchy of linear models, it is demonstrated that 1) precipitating convection in a basic state consisting of a resting, uniform, unstable cloud can propagate and exhibit sloping up- and down-drafts; 2) subcloud evaporation of falling precipitation leads to modifications of the aforementioned instabilities and the formation of a new mode that travels rapidly and has peak amplitude in the subcloud layer; and 3) the introduction of a shear layer at the cloud base serves to couple the subcloud layer mode mentioned here with the cloud layer and yields a deep, rapidly growing, down-shear propagating mode which, while it has no critical level, nevertheless extracts kinetic energy from the mean shear. These models predict that small vertical shear favors slow-moving shear-parallel squall lines, somewhat larger shear leads to fast-moving shear-perpendicular lines, and very large shear favors three-dimensional convection.

Abstract

A simple linear model is developed with the idea of demonstrating the basic physical processes that serve to distinguish the dynamics of precipitating convection from those of the nonprecipitating variety. In particular, it is shown that the hypothesis advanced by Seitter and Kuo to explain the slope and propagation of squall lines in the context of a fully nonlinear numerical model operates also within a linear model. With a hierarchy of linear models, it is demonstrated that 1) precipitating convection in a basic state consisting of a resting, uniform, unstable cloud can propagate and exhibit sloping up- and down-drafts; 2) subcloud evaporation of falling precipitation leads to modifications of the aforementioned instabilities and the formation of a new mode that travels rapidly and has peak amplitude in the subcloud layer; and 3) the introduction of a shear layer at the cloud base serves to couple the subcloud layer mode mentioned here with the cloud layer and yields a deep, rapidly growing, down-shear propagating mode which, while it has no critical level, nevertheless extracts kinetic energy from the mean shear. These models predict that small vertical shear favors slow-moving shear-parallel squall lines, somewhat larger shear leads to fast-moving shear-perpendicular lines, and very large shear favors three-dimensional convection.

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