Lightning and SSM/I-Ice-Scattering Mesoscale Convective Systems in the Global Tropics

Abstract

This study presents a systematic comparison of the distributions of mesoscale convective systems (MCSs) and lightning for 19 geographical regions classified as land, ocean, or a mixture of land and ocean between 35°N and 35°S over four 3-month periods beginning in June of 1995. The 85-GHz brightness temperatures and the lightning data are from the Special Sensor Microwave Imager (SSM/I) and the Optical Transient Detector, respectively. The MCSs are defined and classified according to their 85-GHz polarization-corrected brightness temperature (PCT), and the lightning flashes are grouped into lightning clusters. In each of the four periods, the land bias among the lightning clusters is much stronger than among the MCSs. For instance, ocean regions contain only 15%?21% of the total lightning cluster population in a given period (compared with 56%?66% over land), and the majority (>80%) of the oceanic lightning clusters are weak, with few flashes. In contrast, MCSs are more evenly distributed between land and ocean regions with 37%?41% and 40%?45% occurring over the land and oceans, respectively, in a given period. In land regions, MCSs with moderate to strong ice-scattering signatures (minimum 85-GHz PCT ≤ 190 K) and lightning clusters with moderate to high flash rates (four or more flashes) are both relatively numerous, with tropical Africa typically dominating all regions in terms of ice-scattering intensity and lightning flash rates. However, the lightning?ice-scattering relationship is less clear over the oceans. Moderate to strong ice-scattering MCSs occur with far greater frequency over ocean regions than do the moderate-to-high-flash-rate clusters. In addition, the lightning flash densities and flash-to-MCS ratios computed for each region show order-of-magnitude or larger differences between land and ocean. This result suggests that, even when normalized for the intensity of 85-GHz ice scattering, a land MCS is more likely to produce lightning than is an MCS over the ocean. This fact implies differences in the ice microphysics processes between land and ocean convective storms. These differences are under active investigation.