The Diurnal Interaction Between Convection and Peninsular-Scale Forcing Over South Florida

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  • 1 Department of Environmental Sciences, University of Virginia, Charlottesville 22903
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Abstract

Surface wind measurements taken during the summers of 1973 and 1975 in the Florida Area Cumulus Experiments (FACE) fine-mesh networks are used to calculate surface divergence on the convective scale and mesoscale.

Examination of the daily time series of divergence averaged over a 30 km × 25 km grid showed that on days with deep convective activity over and around the network, a definable sequence in the network-averaged surface wind divergence is observed. The sequence consists of five stages: persistent convergence, peak convergence, non-divergence, peak divergence and the return to a non-divergent state.

Use is made of observations of individual cases and of radar composites built around the above sequence in the network-averaged divergence fields to demonstrate that a convective-scale feedback mechanism consisting of outflows from previous storms triggering new growth is frequently observed during both experimental periods.

It is shown that the observations taken over the FACE 1975 network are consistent with the model results of Pielke (1974) and estimates of peninsular-scale convergence. Comparison of the convective-scale vertical transports over the network at the near surface (4 m) level with the peninsular-scale forcing reveals a relationship between the two. After the initiation of convective processes by the peninsular-scale forcing, downdraft-induced convergence maintains and intensifies the convective-scale activity long after the peninsular-scale forcing has passed its peak. The daily cycle of initiation, intensification, and the eventual decline of convective activity is related to the rate of change of the peninsular-scale divergence.

The links established between the various scales are of fundamental importance to the understanding of the initiation, maintenance, and decay of deep precipitating convection and to its theoretical parameterization.

Abstract

Surface wind measurements taken during the summers of 1973 and 1975 in the Florida Area Cumulus Experiments (FACE) fine-mesh networks are used to calculate surface divergence on the convective scale and mesoscale.

Examination of the daily time series of divergence averaged over a 30 km × 25 km grid showed that on days with deep convective activity over and around the network, a definable sequence in the network-averaged surface wind divergence is observed. The sequence consists of five stages: persistent convergence, peak convergence, non-divergence, peak divergence and the return to a non-divergent state.

Use is made of observations of individual cases and of radar composites built around the above sequence in the network-averaged divergence fields to demonstrate that a convective-scale feedback mechanism consisting of outflows from previous storms triggering new growth is frequently observed during both experimental periods.

It is shown that the observations taken over the FACE 1975 network are consistent with the model results of Pielke (1974) and estimates of peninsular-scale convergence. Comparison of the convective-scale vertical transports over the network at the near surface (4 m) level with the peninsular-scale forcing reveals a relationship between the two. After the initiation of convective processes by the peninsular-scale forcing, downdraft-induced convergence maintains and intensifies the convective-scale activity long after the peninsular-scale forcing has passed its peak. The daily cycle of initiation, intensification, and the eventual decline of convective activity is related to the rate of change of the peninsular-scale divergence.

The links established between the various scales are of fundamental importance to the understanding of the initiation, maintenance, and decay of deep precipitating convection and to its theoretical parameterization.

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