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Idealized Numerical Simulations of the Interactions between Buoyant Plumes and Density Currents

Philip CunninghamDepartment of Meteorology, and Geophysical Fluid Dynamics Institute, The Florida State University, Tallahassee, Florida

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Abstract

Idealized numerical experiments using a large-eddy simulation (LES) model are performed to examine the fundamental dynamical processes associated with the interactions between buoyant plumes and density currents. The aim of these simulations is to provide insight into the rapid changes in the structure of plumes that may be observed during the passage of density current phenomena such as thunderstorm outflows, sea-breeze fronts, or intense cold fronts. The LES model results indicate that when the ambient winds are calm the vertical velocity in the plume decreases with the passage of the density current, but that when the ambient winds oppose the motion of the density current a significant increase in vertical velocity in the plume may occur temporarily. In the latter case, the pressure perturbation and the associated region of horizontal convergence that lead the head of the density current interact with the tilted plume, causing the base of the plume to become vertical and resulting in a dramatic increase in vertical velocity within the plume. This basic dynamical behavior occurs over a relatively broad range of parameters, provided the characteristic velocity in the density current (taken as the densimetric speed) exceeds the ambient wind speed. When this is the case, the interaction is dominated by the effect of the density current on the buoyant plume such that the plume is essentially advected as a passive tracer by the flow due to the density current, and the increase in vertical velocity depends on the inverse of the convective Froude number of the buoyant plume.

Corresponding author address: Philip Cunningham, Department of Meteorology, and Geophysical Fluid Dynamics Institute, The Florida State University, Tallahassee, FL 32306-4520. Email: cunningham@met.fsu.edu

Abstract

Idealized numerical experiments using a large-eddy simulation (LES) model are performed to examine the fundamental dynamical processes associated with the interactions between buoyant plumes and density currents. The aim of these simulations is to provide insight into the rapid changes in the structure of plumes that may be observed during the passage of density current phenomena such as thunderstorm outflows, sea-breeze fronts, or intense cold fronts. The LES model results indicate that when the ambient winds are calm the vertical velocity in the plume decreases with the passage of the density current, but that when the ambient winds oppose the motion of the density current a significant increase in vertical velocity in the plume may occur temporarily. In the latter case, the pressure perturbation and the associated region of horizontal convergence that lead the head of the density current interact with the tilted plume, causing the base of the plume to become vertical and resulting in a dramatic increase in vertical velocity within the plume. This basic dynamical behavior occurs over a relatively broad range of parameters, provided the characteristic velocity in the density current (taken as the densimetric speed) exceeds the ambient wind speed. When this is the case, the interaction is dominated by the effect of the density current on the buoyant plume such that the plume is essentially advected as a passive tracer by the flow due to the density current, and the increase in vertical velocity depends on the inverse of the convective Froude number of the buoyant plume.

Corresponding author address: Philip Cunningham, Department of Meteorology, and Geophysical Fluid Dynamics Institute, The Florida State University, Tallahassee, FL 32306-4520. Email: cunningham@met.fsu.edu

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