Observational and modeling study of mesoscale convective system electrification

Abstract

The electrical structure and development of two Mesoscale Convective Systems (MCSs) observed during the 1991 Cooperative Oklahoma Profiler Studies (COPS91) experiment are analyzed through kinematic, microphysical, and electrical data sets. Profiles of the vertical electric field structure (from which vertical profiles of charge density are derived using an approximation to Gauss' Law) were obtained from a series of balloon-borne electric field meter (EFM) flights into each MCS. Two unique electric field structures were found. In both cases, 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 observed in convective cores (s 5 nClm3). However, the data also indicate that the vertical electric field profiles were each related to unique MCS precipitation and kinematic structures, with a more complicated 5-layered charge profile (at Ts 0°C) associated with the "symmetric" MCS and a less complicated 3-layered charge profile (at Ts 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 their different 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. The residence time of ice particles in the transition zone downdraft was much larger in the asymmetric case. Second, upon entering the MCS stratiform region, ice particles in the symmetric system entered a kinematic/microphysical environment that was more conducive to in situ charging. There are also indications that gravitational separation may be responsible for some of the charge transitions in the electric field profiles. Model simulations of the symmetric MCS were conducted using a 2-D, time-dependent numerical model with bulk microphysical parameterizations. A number of charging mechanisms were considered, based on past and more recent laboratory studies. The simulations suggested that non-inductive ice-ice charge transfer in the low-liquid water content regime characteristic of MCS stratiform regions is sufficient to account for observed charge density magnitudes, and as much as 70% of the total stratiform charge (with the remaining 30% being the result of charge advection from the convective line). This result is consistent with the fraction of total stratiform precipitation generated by mesoscale ascent (in situ condensate production), thereby suggesting a linkage between stratiform electrification and stratiform precipitation processes. The model also indicates that, once these charge densities are achieved, the sink of charge resulting from fallspeed divergence becomes approximately equal to the rate of charge generation. This might lead to the quasi-steady layered structure that is commonly seen in the observations. Both noninductive charging parameterizations (from Takahashi, 1978 and Saunders et al., 1991) reproduce some of the observed stratiform charge features. The best results, however, were obtained when charge advection and non-inductive charging processes were allowed to act in unison.