Release of Potential Instability: Part II. The Mechanism of Convective/Mesoscale Interaction

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  • 1 Drexel University, Philadelphia, Pa. 19104
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Abstract

In Part I the convective processes important during the release of potential instability were described qualitatively and evaluated quantitatively in a parameterized cumulus model within a primitive equation model. Part II includes a more detailed examination of convective/mesoscale interactions through a basic simulation experiment and tests under different physical conditions and with different computational grids. The cumulus model was documented in Part I and the primitive equation model is documented herein. The example, for which detailed dynamical fields are shown, began with 6 h of convective activity that developed a saturated neutrally buoyant mesoscale updraft which produced the bulk of the precipitation by 12 h into the integration.

The potential instability process is readily understandable and verifiable in general terms by numerical simulation. Increasing moisture bandwidth or large-scale ascent results in a wider precipitation band. Permitting evaporation of convective precipitation above cloud base had surprising little effect on these rain-bands that are about 100 km wide. Decreasing cumulus updraft radius, thereby increasing entrainment effects, delays initial development of the mesoscale circulation and produces a much narrower and more intense circulation later on. Reducing the horizontal grid size from 20 to 10 km results in much narrower rainbands but does little to the area total precipitation. The rate of propagation inward of lateral boundary condition influences shows that rather large areas must be dealt with in mesoscale field projects and numerical weather prediction for phenomena with time scales of several hours.

Abstract

In Part I the convective processes important during the release of potential instability were described qualitatively and evaluated quantitatively in a parameterized cumulus model within a primitive equation model. Part II includes a more detailed examination of convective/mesoscale interactions through a basic simulation experiment and tests under different physical conditions and with different computational grids. The cumulus model was documented in Part I and the primitive equation model is documented herein. The example, for which detailed dynamical fields are shown, began with 6 h of convective activity that developed a saturated neutrally buoyant mesoscale updraft which produced the bulk of the precipitation by 12 h into the integration.

The potential instability process is readily understandable and verifiable in general terms by numerical simulation. Increasing moisture bandwidth or large-scale ascent results in a wider precipitation band. Permitting evaporation of convective precipitation above cloud base had surprising little effect on these rain-bands that are about 100 km wide. Decreasing cumulus updraft radius, thereby increasing entrainment effects, delays initial development of the mesoscale circulation and produces a much narrower and more intense circulation later on. Reducing the horizontal grid size from 20 to 10 km results in much narrower rainbands but does little to the area total precipitation. The rate of propagation inward of lateral boundary condition influences shows that rather large areas must be dealt with in mesoscale field projects and numerical weather prediction for phenomena with time scales of several hours.

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