Evolution of a Florida Thunderstorm during the Convection and Precipitation/Electrification Experiment: The Case of 9 August 1991

Abstract

The relationships among kinematic, microphysical, and electric field properties within a multicell Florida thunderstorm are investigated using observations from three Doppler radars (one with multiple wavelength and polarization diversity capabilities), four instrumented penetrating aircraft, a surface-based electric field mill network, and other observation facilities. The storm was convectively active for about 1 h and at least five primary cells developed within the storm during this time, one of which went through three consecutive development cycles. The updrafts in this storm were 2?4 km wide, exhibited bubble-like evolution, and had lifetimes of 10?20 min. The maximum updraft determined by the multiple Doppler analysis was about 20 m s?1. A differential reflectivity (ZDR) ?column,? indicating regions containing millimeter-size raindrops, extending above the freezing level, was associated with each cell during its developing stages. This column reached altitudes exceeding 6 km (?8°C) in the stronger updrafts. As the ZDR columns reached maximum altitude, a ?cap? of enhanced linear depolarization ratio (LDR) and enhanced 3-cm wavelength attenuation (A3) formed, overlapping the upper regions of the ZDR column. These parameters indicate rapid development of mixed-phase conditions initiated by freezing of supercooled raindrops. Lightning was observed only in the central and strongest convective cell. Electric fields exceeding 10 kV m?1 were noted during aircraft penetrations in this as well as several other cells that did not produce lightning. Fields exceeding 1 kV m?1 were noted by the instrumented aircraft at midcloud levels within a few minutes of development of mixed-phase conditions at these levels or aloft. The first intracloud lightning was detected by the surface field mill network within 5 min of development of mixed-phase conditions aloft in the first cycle of development in the central cell, and the first cloud-to-ground event was noted within 9 min of this development. Lightning continued through two additional cycles of updraft growth in this central region and diminished as the convection subsided after about 30 min. Aircraft-measured electric fields and lightning retrievals from the surface field meter network are consistent with a tendency for negative charge to accumulate above the 6.5 km(?12°C) level within regions of radar reflectivity maxima and for positive charge to accumulate in the anvil region well above 9 km (?30°C).