Factors Governing Cellular Convection in Orographic Precipitation

Daniel J. Kirshbaum Department of Atmospheric Sciences, University of Washington, Seattle, Washington

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Dale R. Durran Department of Atmospheric Sciences, University of Washington, Seattle, Washington

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Abstract

The development of shallow cellular convection in warm orographic clouds is investigated through idealized numerical simulations of moist flow over topography using a cloud-resolving numerical model. Buoyant instability, a necessary element for moist convection, is found to be diagnosed most accurately through analysis of the moist Brunt–Väisälä frequency (Nm) rather than the vertical profile of θe. In statically unstable orographic clouds (N2m < 0), additional environmental and terrain-related factors are shown to have major effects on the amount of cellularity that occurs in 2D simulations. One of these factors, the basic-state wind shear, may suppress convection in 2D yet allow for longitudinal convective roll circulations in 3D. The presence of convective structures within an orographic cloud substantially enhanced the maximum rainfall rates, precipitation efficiencies, and precipitation accumulations in all simulations.

Corresponding author address: Daniel J. Kirshbaum, Department of Atmospheric Sciences, University of Washington, Box 351640, Seattle, WA 98195-1640. Email: dank@atmos.washington.edu

Abstract

The development of shallow cellular convection in warm orographic clouds is investigated through idealized numerical simulations of moist flow over topography using a cloud-resolving numerical model. Buoyant instability, a necessary element for moist convection, is found to be diagnosed most accurately through analysis of the moist Brunt–Väisälä frequency (Nm) rather than the vertical profile of θe. In statically unstable orographic clouds (N2m < 0), additional environmental and terrain-related factors are shown to have major effects on the amount of cellularity that occurs in 2D simulations. One of these factors, the basic-state wind shear, may suppress convection in 2D yet allow for longitudinal convective roll circulations in 3D. The presence of convective structures within an orographic cloud substantially enhanced the maximum rainfall rates, precipitation efficiencies, and precipitation accumulations in all simulations.

Corresponding author address: Daniel J. Kirshbaum, Department of Atmospheric Sciences, University of Washington, Box 351640, Seattle, WA 98195-1640. Email: dank@atmos.washington.edu

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