Horizontal Distribution of Electrical and Meteorological Conditions across the Stratiform Region of a Mesoscale Convective System

Abstract

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 data from the storm's mature and dissipating stages show typical MCS characteristics. The leading convective region is convex forward, and the back edge of the stratiform echo is notched inward. The maximum areal extent of the low-level echo is about 250 km × 550 km, and the radar bright band is intense (reflectivity 45–50 dBZ) through an area of at least 50 km × 100 km. The reflectivity above the bright band is horizontally stratified with decreasing intensity and echo-top height toward the rear of the system. Analyses of the velocity data reveal a convective-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 horizontal scale of 100 km across the stratiform region, suggesting that the charge structure is nearly steady state and the charge regions are horizontally extensive and layered. The basic charge structure consists of four layers: a 1–3-km-deep region of positive charge (density ρ ≈ +0.2 nC m−3) between 6 and 10 km, negative charge (ρ ≈ −1.0–2.5 nC m−3) between 5 and 6 km, positive charge (ρ ≈ +1.0–3.0 nC m−3) near 0°C, and negative charge (ρ ≈ −0.5 nC m−3) near cloud base. The upper positive and densest negative charge layers could result from advection of charge from the convective region. The negative charge layer may be augmented by noninductive collisional charging in the stratiform region. The positive charge near 0°C is probably caused by one or more in situ charging mechanisms. The negative charge near cloud base is likely the result of screening layer formation. In addition to the basic four charge layers, positive charge is found below the cloud in each sounding, and in the two soundings closest to the convection (70–100 km distant) there is a low-density negative charge region near echo top.