The Spatial Characteristics of Afternoon Lightning in Weak-Synoptic Forcing Weathers over a Subtropical Monsoon Island

Taiwan, located at western Pacific coast region, is a mountainous island bisected by the Tropic of Cancer and has 300-km north-south stretch of the Central Mountain Range (CMR) with more than 50 peaks above 3,000 m. The hot and humid weather condition from April to October provides plentiful lightning strikes over this island. In this study, the lightning data (2007 to 2009) from Total Lightning Detection System (TLDS) operated by Tai-Power Company was analyzed in the afternoon period (12 to 18 LST) under weak synoptic forcing weather conditions. TLDS is formed by seven Vaisala VHF/LF detection sites surrounding Taiwan started from 2004. The 30-m resolution of land digital topography is used in this study to diagnose the detailed geographic characteristics of different lightning types. The result showed that there are four hot spots of inter/intra cloud (IC) lightning along the western slope of CMR. These hot spots locate at the downstream of urban cites and have wider land area. Relatively, rare lightning happened on the eastern slope of CMR where faces to the open Pacific Ocean and has smaller valley land area. The ratio between CG (cloud-ground) and IC in these hot spots are less than 12 %. Specially, the spots in middle Taiwan have rare CG strikes. We found the lowland area (ground level below 500m) at these rare-CG regions are less than 50%. It implies that the deep convection needs favorable terrain boundary condition for developing. Meanwhile, this lightning-location information is also useful for cloud-seeding planning on summer storms in Taiwan.