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An Investigation of the Development of Cumulonimbus Systems over South Florida. Part I: Boundary Layer Interactions

John B. CunningNOAA, Environmental Sciences Group, Weather Research Program, Boulder, CO 80303

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Mark DemariaNational Center for Atmospheric Research, Boulder, CO 80307

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Abstract

This paper investigates the interactions between two developing cumulonimbus systems and the boundary layer (surface winds, pressure, divergence and temperature fields) on a case study day, 25 August 1975. These two systems formed within 45 min of each other in similar locations and environments. Analysts indicate that in the case of rapid convective development, a surface pressure low develops below the convection and in the environment surrounding the convective system. The pressure low beneath the convection is hypothesized to be formed by hydrostatic and nonhydrostatic (dynamic) effects, and the extension of the low in the environment surrounding the system is hypothesized to be induced by hydrostatic adjustment resulting from subsidence warming. The environmental flow responds to the induced pressure field, causing the convergence to increase in both the convective region and the mesoscale region surrounding the convection. Once rainfall occurs at the surface, the pressure begins to rise in the convective area, reducing or reversing the pressure gradient, causing the convergence in the mesoscale and convective-sale areas to decrease. Once the precipitation stage is reached, the interaction of the outflows from the convection with the surrounding flow appears to become the dominant mechanism for enhancing convergence in the boundary layer.

Abstract

This paper investigates the interactions between two developing cumulonimbus systems and the boundary layer (surface winds, pressure, divergence and temperature fields) on a case study day, 25 August 1975. These two systems formed within 45 min of each other in similar locations and environments. Analysts indicate that in the case of rapid convective development, a surface pressure low develops below the convection and in the environment surrounding the convective system. The pressure low beneath the convection is hypothesized to be formed by hydrostatic and nonhydrostatic (dynamic) effects, and the extension of the low in the environment surrounding the system is hypothesized to be induced by hydrostatic adjustment resulting from subsidence warming. The environmental flow responds to the induced pressure field, causing the convergence to increase in both the convective region and the mesoscale region surrounding the convection. Once rainfall occurs at the surface, the pressure begins to rise in the convective area, reducing or reversing the pressure gradient, causing the convergence in the mesoscale and convective-sale areas to decrease. Once the precipitation stage is reached, the interaction of the outflows from the convection with the surrounding flow appears to become the dominant mechanism for enhancing convergence in the boundary layer.

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