Electrical and Kinematic Structure of the Stratiform Precipitation Region Trailing an Oklahoma Squall Line

Abstract

An electric field sounding through the transition zone precipitation minimum that trailed an Oklahoma squall line on 18 June 1987 provides information about the electrical structure within a midlatitude trailing stratiform cloud. A single-Doppler radar analysis concurrent with the flight depicts a kinematic structure dominated by two mesoscale flow regimes previously identified in squall-line systems: a strong midlevel, front-to-rear flow coinciding with the stratiform cloud layer and a descending rear inflow that sloped from 6.5 km AGL at the stratiform cloud's trailing edge to 1.5 km AGL at the convective line. Electric field magnitudes as high as 113 kV m?1 were observed by the electric field sounding, which reveals an electric field structure comparable in magnitude and complexity to structures reported for convective cells of thunderstorms. The charge regions inferred with an approximation to Gauss' law have charge density magnitudes of 0.2?4.1 nC m?3 and vertical thicknesses of 130?1160 m; these values, too, are comparable to those reported for thunderstorm cells. In agreement with previous studies, an analysis of the lightning data revealed a ?bipolar? cloud-to-ground lightning pattern with positive flashes being relatively more common in the stratiform region. From the analysis, we conclude that the stratiform region electrical structure may have been advected from the squall line convective cells as the in-cloud charge regions were primarily found within the front-to-rear flow. Screening layers were found at the lower and upper cloud boundaries. In situ microphysical charging also seems to be a possible source of charge in the stratiform region. We hypothesize that the radar-derived similarities of this system to those previously documented suggests that the newly-documented stratiform electrical structure might also be representative of this type of mesoscale convective system.