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Analysis of a Microburst in the FACE Meteorological Mesonetwork in Southern Florida

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  • 1 National Oceanic and Atmospheric Administration, Weather Research Program, Boulder, CO 80303
  • | 2 Lightning Location and Protection, Inc., Tucson, AZ 85719
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Abstract

A microburst embedded in heavy rain in a humid environment struck very near the Field Observing Site (FOS) of the Florida Area Cumulus Experiment (FACE), producing a diverging pattern of wind damage in sugar cane.

While the dry, virga-type microburst is now beginning to be understood as a result of the SAWS project, the wet, or heavy-rain-embedded, microburst still remains a mystery. The fortuitous occurrence of a wet microburst in a humid environment, with a well-marked wind damage pattern and a well-instrumented site (including upper-air soundings), furnishes a means of gleaning some understanding of the larger-scale processes that are conducive to strong downdrafts in wet environments. In this case several features were present: 1) an elevated dry layer (above 500 mb), 2) overlying a nearly moist adiabatic lower tropospheric layer (below 500 mb), 3) a short-wave trough approaching the area from the north-northeast along the western side of a synoptic-scale trough with 4) increased shear in the lower troposphere, and 5) strong boundary-layer forcing, first by a lake breeze front off Lake Okeechobee, then by convective gust fronts. The site of the microburst itself was in the portion of the storm where a new cell was initiated by a strong gust front in an area where rain was still failing from an older, dissipating cell. The strong boundary-layer forcing may have generated an impulsive updraft surge in a very wet environment with lingering precipitation, which was followed by an impulsive collapse in a water-loaded downdraft. In this case, however, the negative buoyancy due to water loading was an order of magnitude less than that due to evaporation.

Abstract

A microburst embedded in heavy rain in a humid environment struck very near the Field Observing Site (FOS) of the Florida Area Cumulus Experiment (FACE), producing a diverging pattern of wind damage in sugar cane.

While the dry, virga-type microburst is now beginning to be understood as a result of the SAWS project, the wet, or heavy-rain-embedded, microburst still remains a mystery. The fortuitous occurrence of a wet microburst in a humid environment, with a well-marked wind damage pattern and a well-instrumented site (including upper-air soundings), furnishes a means of gleaning some understanding of the larger-scale processes that are conducive to strong downdrafts in wet environments. In this case several features were present: 1) an elevated dry layer (above 500 mb), 2) overlying a nearly moist adiabatic lower tropospheric layer (below 500 mb), 3) a short-wave trough approaching the area from the north-northeast along the western side of a synoptic-scale trough with 4) increased shear in the lower troposphere, and 5) strong boundary-layer forcing, first by a lake breeze front off Lake Okeechobee, then by convective gust fronts. The site of the microburst itself was in the portion of the storm where a new cell was initiated by a strong gust front in an area where rain was still failing from an older, dissipating cell. The strong boundary-layer forcing may have generated an impulsive updraft surge in a very wet environment with lingering precipitation, which was followed by an impulsive collapse in a water-loaded downdraft. In this case, however, the negative buoyancy due to water loading was an order of magnitude less than that due to evaporation.

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