HORIZONTAL DISTRIBUTION OF ELECTRICAL AND METEOROLOGICAL CONDITIONS ACROSS THE STRATIFORM REGION OF A MESOSCALE CONVECTIVE SYSTEM

Five soundings of the electric field and thermodynamic properties were made in a mesoscale convective system (MCS) that occurred in Oklahoma and Texas on 2-3 June 1991. Airborne Doppler radar data were obtained from three passes through the stratiform echo. From these electrical, kinematical, and reflectivity measurements, a conceptual model of the electrical structure of an MCS is developed. Low-level reflectivity d ata from the storm's mature and dissipating stages show typical MCS ch aracteristics. The leading convective region is convex forward, and th e back edge of the stratiform echo is notched inward. The maximum area l extent of the low-level echo is about 250 km x 550 km, and the radar bright band is intense (reflectivity 45-50 dBZ) through an area of at least 50 km x 100 km. The reflectivity above the bright band is horiz ontally stratified with decreasing intensity and echo-top height towar d the rear of the system. Analyses of the velocity data reveal a conve ctive-line-relative flow structure of front-to-rear flow and mesoscale ascent aloft, and weak rear inflow and descent below about 5 km. The electric field soundings are similar over a period of 3 h and a horizo ntal, scale of 100 km across the stratiform region, suggesting that th e charge structure is nearly steady state and the charge regions are h orizontally extensive and layered. The basic charge structure consists of four layers: a 1-3-km-deep region of positive charge (density rho almost-equal-to +0.2 nC m-3) between 6 and 10 km, negative charge (rho almost-equal-to -1.0-2.5 nC m-3) between 5 and 6 km, positive charge (rho almost-equal-to +1.0-3.0 nC m-3) near 0-degrees-C, and negative c harge (rho almost-equal-to -0.5 nC m-3) near cloud base. The upper pos itive and densest negative charge layers could result from advection o f charge from the convective region. The negative charge layer may be augmented by noninductive collisional charging in the stratiform regio n. The positive charge near 0-degrees-C is probably caused by one or m ore in situ charging mechanisms. The negative charge near cloud base i s likely the result of screening layer formation. In addition to the b asic four charge layers, positive charge is found below the cloud in e ach sounding, and in the two soundings closest to the convection (70-1 00 km distant) there is a low-density negative charge region near echo top.