Lightning flash density 1995-2010 in Brisbane, Australia

Abstract

Observations of nearby lightning have been made in Brisbane, Australia (27.3ºS, 153.0ºE) from 1995 to 2010 using CGR3 and CGR4 lightning sensors and a 500 Hz CIGRE lightning flash counter, with the objective of measuring long-term lightning flash densities and their diurnal and annual variations. The observations with the CGR4 sensor covered a circular area around the sensor of about 400 km2 . The 16-year average flash densities based on direct measurements were: ground flash density = 2.29 km-2 yr-1, cloud flash density = 2.81 km-2 yr-1 and total flash density = 5.1 km-2 yr-1 and average of 16 annual measurements of the cloud flash-to-ground flash ratio (denoted Z) = 1.56. The most probable long-term ratio of positive to all ground flashes is about 0.04 based on the period 2006 to 2010 when the more reliable measurements were made. The range of values was about 0.02 to 0.17 with a 16-year average of about 0.06. The average annual variation of total flash density shows that about 35 per cent of all lightning occurs from January to June and about 65 per cent from July to December. The diurnal variation of total flash density has the expected peak between 1700 and 1800 hours when 16 per cent of all lightning occurs and a secondary peak between 2000 and 2100 hours when 13 per cent of all lightning occurs. The inter-annual variation in total flash density is large with about a 9:1 range from 1.4 to 12.2 km-2 yr-1. The average annual thunder day level was 25 with a range from 14 to 31 per year, about a 2:1 range. There is a weak correlation between annual flash densities and thunder days, but attempting to predict a flash density from the thunder day level is subject to large uncertainty. It was found that, averaged over the 16-year study period, 50 per cent of all local lightning occurs on about two days per year.