Occurrence of positive and negative polarity cloud-to-ground lightning flashes: Case study of CGR4 and GPATS data for Brisbane, Australia

The occurrences of positive and negative polarity cloud-to-ground lightning flashes were detected by ground-based lightning flash counters CGR4 in Brisbane, Australia. Positive (negative) flashes constituted 2-5 per cent (95-98 per cent) of total cloud-to-ground flashes. These results were compared with data obtained by a GPATS lightning location system over the period 2005-2008. It was found that prior to January 2007 the GPATS lightning location system was reporting an excessive proportion of ground flashes as being positive, with a correspondingly low proportion as negative. Over the same period, the CGR4 lightning flash counter reported positive to negative ground flash ratios consistent with those obtained by other researchers both in Australia and elsewhere using a range of instrumentation. The high proportion of positive ground flashes (up to 50 per cent) recorded by the GPATS lightning location system in Australia prior to 2007 may be attributed to one or more possible factors, including (i) changes in the processing of raw GPATS sensor data implemented in January 2007, (ii) upgrades to sensor firmware, and (iii) the installation of more GPATS sensors.