Comparing lightning polarity and cloud microphysical properties over regions of high ground flash density in South Africa

Abstract

Positive lightning flashes are known to be more intense and cause more damage than negative flashes, although positive flashes only occur about 10% of the time. This study expounds on cloud microphysical aspects of thunderstorms and investigates the occurrence, timing and location of ice particles within thunderstorms and correlates this to the occurrence of positive cloud-to-ground lightning events. Satellite data obtained from the Meteosat Second Generation (MSG) satellite, were used to: 1) depict Cloud Top Temperatures (CTT) by considering Infra Red (IR) radiation with a wave length of 10.8μm, 2) compare results from the CTT with the Brightness Temperature Difference (BTD) calculated by subtracting IR10.8μm from IR8.7μm, 3) after the cloud particle phase was determined from the abovementioned comparison, the sum of cloud-to-ground lightning strokes over a 1-hour period around the time of a reported lightning fatality was compared with cloud microphysical properties and then 4) these results were further compared with the lightning polarity obtained from the South African Weather Service (SAWS) Lightning Detection Network (LDN) data set. Four case studies were identified to investigate from the many available case study dates. These four cases occurred on 3 separate days namely: 1)22 November 2007, iii 2) 10 February 2009 and 3) 29 October 2009. There were two fatal events reported on 22 November 2007 and therefore two cases studies were compiled for this one date. On 10 February 2009 over 250 insurance damage claims were honoured but no fatality was reported. The three case studies mentioned above were classified as primary case studies. The 29 October 2009 case was classified as a secondary, more generalized case which was chosen in order to test whether the results gained from the first three cases were indeed noteworthy. Results gathered from two of the three primary case studies showed that the fatalities occurred when the most intense part of the thunderstorm was to the east of the location where the lightning struck the victims, although actual storm properties were not considered as being particularly severe. The lightning data for the primary case studies showed that the percentage of cloud-to-ground lightning was within 10% of the total number of strokes recorded for 22 November 2007, and above 10% for 10 February 2009. In the one secondary case study of 29 October 2009 the percentage of positive lightning was only between 2% and 4% of the total number of strokes recorded, which was significantly lower than in the three primary cases. A significant difference in cloud microphysics between the primary and secondary cases was the possible occurrence of super-cooled liquid water found in Cumulonimbus (CB) clouds in the secondary case. This could have been a determining factor for the difference in percentage of positive lightning between the primary and secondary case studies.