Electrification of Stratiform Regions in Mesoscale Convective Systems. Part I: An Observational Comparison of Symmetric and Asymmetric MCSs

Abstract

The kinematic, microphysical, and electrical structures of two mesoscale convective systems (MCSs) observed during the 1991 Cooperative Oklahoma Profiler Studies (COPS91) experiment are analyzed. Profiles of the vertical electric field structure and charge density were obtained from a series of balloon-borne electric field meter (EFM) flights into each MCS. Contrasting electric field structures were found in the stratiform regions of these MCSs. In both systems, the EFM data indicate that the MCS charge structure was characterized by horizontally extensive regions of charge and charge density magnitudes on the order of what is typically observed in convective cores (?5 nC m?3). However, the vertical electric field profiles were each related to unique MCS precipitation and kinematic structures, with a five-layer charge profile (at T ? 0°C) associated with the ?symmetric? MCS and a simpler three-layer charge profile (at T ? 0°C) associated with the ?asymmetric? MCSs. The observational analysis identified several kinematic, thermodynamic, and microphysical differences between the two systems that offer at least some explanation for the observed electrical structures. First, ice particles detrained from the convective line of the symmetric MCS had much shorter ?residence times? in the unfavorable growth/charging region associated with the transition zone downdraft compared to the asymmetric case. Second, upon entering the trailing stratiform region, ice particles in the symmetric system were immersed in an environment that was more conducive to in situ charging via noninductive charging mechanisms. Strong mesoscale ascent in the stratiform region of the symmetric MCS led to the presence of supercooled cloud water, and hence significant electrification. There are also indications that fallspeed differences between particle types may be responsible for producing some of the charge transitions in the electric field profiles. In contrast, strong mesoscale ascent was not present in the asymmetric case, and hence conditions were less favorable for noninductive charging.