A Mesoscale Analysis over South Florida for a High Rainfall Event

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  • 1 Center for Advanced Studies, Department of Environmental Sciences, University of Virginia, Charlottesville 22903
  • | 2 Department of Atmospheric Sciences, Colorado State University, Fort Collins 80521
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Abstract

The meteorological data collected during the 1973 NOAA-EML Florida Area Cumulus Experiment was used to describe the mesoscale weather patterns on 4 August 1973. It was found that the high rainfall on this date was due to the superposition of a synoptic-scale disturbance and the normal shallow sea breeze convergence field. The synoptic disturbance was not resolved from the conventional synoptic analyses. On this date, the thunderstorm activity was highly correlated with the diurnal heating and apparently developed in favored regions related to both the sea breeze convergence zone along the west coast and to the larger scale disturbance. The surface wind and temperature patterns Were found to be strongly controlled by the diurnal heating cycle, and by the occurrence or non-occurrence of showers. It is concluded that the reduction in lower level wind speed after a rain occurrence was a result of surface cooling causing a decoupling of the surface from the larger scale pressure gradient.

The analysis of cloud base vertical velocity and its variance illustrates a strong coupling between the mesoscale and the thermal scale in spite of the fact that spectral analysis indicated a marked scale separation between the thermal convective scale and its larger scale. The strong excursion of the slope of the vertical velocity energy spectrum from a −5/3 slope over the scale range of 0.2–0.8 km indicates that eddies on this scale range are a strong source of kinetic energy generation.

Abstract

The meteorological data collected during the 1973 NOAA-EML Florida Area Cumulus Experiment was used to describe the mesoscale weather patterns on 4 August 1973. It was found that the high rainfall on this date was due to the superposition of a synoptic-scale disturbance and the normal shallow sea breeze convergence field. The synoptic disturbance was not resolved from the conventional synoptic analyses. On this date, the thunderstorm activity was highly correlated with the diurnal heating and apparently developed in favored regions related to both the sea breeze convergence zone along the west coast and to the larger scale disturbance. The surface wind and temperature patterns Were found to be strongly controlled by the diurnal heating cycle, and by the occurrence or non-occurrence of showers. It is concluded that the reduction in lower level wind speed after a rain occurrence was a result of surface cooling causing a decoupling of the surface from the larger scale pressure gradient.

The analysis of cloud base vertical velocity and its variance illustrates a strong coupling between the mesoscale and the thermal scale in spite of the fact that spectral analysis indicated a marked scale separation between the thermal convective scale and its larger scale. The strong excursion of the slope of the vertical velocity energy spectrum from a −5/3 slope over the scale range of 0.2–0.8 km indicates that eddies on this scale range are a strong source of kinetic energy generation.

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