Temporal and spatial characteristics of lightning activity versus terrain in Hong Kong

The temporal and spatial characteristics of lightning activities across Hong Kong, a mountainous metropolis surrounded by sea, has been studied. Over 0.32 million of cloud-to-ground (CG) and intra-cloud (IC) flashes reported by Hong Kong Lightning Location Network during 2005 to 2008 were analyzed. Seasonal variations show that although the lightning activity in summer is the strongest, the ratio of positive CG in summer is smaller than that in other seasons. The lightning occurrence is highly related to the underlying surface conditions. The density of negative CG increases as the surface terrain rises, with the highest density is convergent to the main mountain peak areas. Whilst the density of positive CG is inversely proportional to the surface terrain elevation, with the highest density is clustered in a broad area in valleys or mountain foots. Such characteristics of CG may be attributed to the effect of mountain terrain elevation on the air flow and hence the convection in clouds. In addition, the average current of CG flashes in urban regions is obviously stronger than that in other regions, probably due to the plenty of aerosols / pollutants in air in the urban areas. The average current of both CG and IC flashes in the sea is larger than that in the land except the city and mountain areas, probably due to the small attenuation of electric fields propagating over sea water.