MICROPHYSICAL AND ELECTRICAL EVOLUTION OF A FLORIDA THUNDERSTORM .1. OBSERVATIONS

This study deals with the microphysical and electrical evolution of a thunderstorm that occurred on August 9, 1991, during the Convection an d Precipitation/Electrification (CaPE) Experiment in eastern Florida. During its approximately 1-hour lifetime, the storm was penetrated sev eral times by the Institute of Atmospheric Sciences' T-28 aircraft at midlevels. It was also penetrated at low and middle-levels by a Nation al Oceanographic and Atmospheric Administration (NOAA) P-3 and scanned by three radars, one of which had multiparameter capabilities, operat ed by the National Center for Atmospheric Research. Two stages of the storm's evolution are analyzed herein during which the storm grew to p roduce precipitation and lightning. The first stage, sampled during th e first T-28 penetration at 5.25 km (-3 degrees C) and the P-3 at 6.4 km (-10 degrees C), nas characterized by a 2- to 3-km wide updraft (ma ximum 14 m s(-1)) with cloud li;quid water contents up to 4 g m(-3), l ow concentrations of graupel at -10 degrees C, and small to medium rai ndrops in concentrations of less than 200 m(-3) at -3 degrees C. A dow ndraft region also existed that was devoid of cloud liquid water, but contained graupel up to 2 mm. Radar data (Z(DR)) are consistent with a coalescence-dominated precipitation generation mechanism followed by transport of. drops in the updraft to heights with temperatures colder than -7 degrees C, where freezing formed graupel that continued to gr ow by riming. Electrification during this stage remained weak. The sec ond stage, sampled during the second and third T-28 penetrations and t he second P-3 penetration, was characterized at midlevels by a narrowe r updraft and a more diffuse, broad downdraft separated by a 1- to 2-k m wide transition zone. The updraft continued to show significant clou d liquid water (similar to 2 g m(-3)) with few precipitation particles , while the downdraft had very little cloud liquid with graupel in con centrations >1 l(-1). The transition zone shared both updraft and down draft characteristics. The increase in ice concentration tvas accompan ied by a rapid increase in the electrification of the cloud with peak electric fields reaching -20 kV m(-1) at T-28 altitude and the detecti on of lightning by ground-based sensors and pilot report. As time prog ressed, precipitation particle concentrations reached several per lite r at midlevels in both updrafts and downdrafts. The observations are c onsistent with electrification through a precipitation-based mechanism involving the development of the ice phase.