A Comparison Of WSR-88D Reflectivities, SSM/I Brightness Temperatures, and Lightning for Mesoscale Convective Systems in Texas. Part I: Radar Reflectivity and Lightning

Abstract

This is the first part of a two part study. Part I compares radar data from the League City, Texas, WSR-88D and cloud-to-ground (CG) lightning data for a set of eight mesoscale convective systems (MCSs), which occur at various stages of development along the upper Texas gulf coast. Vertical profiles of radar reflectivity (VPRR) as well as plan views and vertical cross sections are constructed to characterize the structure and relative strength of each MCS. The VPRR are also compared with similar profiles from tropical oceanic MCSs. The data show that in all the majority of negative CG lightning flashes are located near high-reflectivity convective cores (>35 dBZ) in the mixed-phase region (0°C ≤T≥ ?20°C). Growing or mature MCSs typically had larger negative flash counts and higher percentages of negative lightning (≥80%) associated with convective core than MCSs at later stages of their life cycle. Comparison of the median VPRR for the various MCSs showed that although each case had high-reflectivity cores (45?55 dBZ) in the lowest 2?3 km, the more electrically active MCSs were characterized by smaller reflectivity lapse rates (decrease of reflectivity with height) in mixed-phase region than the cores in the remaining systems. Based on existing theories of charge separation, the observation of high negative flash counts coincident with convective corn having small reflectivity lapse rates in the mixed phase region is consistent with the presence of large ice particles aloft. Positive CG flashes were mostly located in low reflectivity (less than 30 dBZ near the ?10°C level) stratiform regions, independent of MCS life cycle stage or VPRR type. Several cases with reports of large hail also had high positive flash densities associated with high reflectivity cores. Part II of this study compares 85-GHz brightness temperatures from the Special Sensor Microwave/Imager to lightning data for the same set of MCSs in Part I. Results from both parts of this study strongly suggest that the presence of large ice particles aloft is the common linkage between MCSs with lightning, with high radar reflectivity aloft, and large 85-GHz temperature depressions.