Lightning Climatology over the Northwest Pacific Region Based on Data from the World Wide Lightning Location Network

The climatology of spatial and temporal variability of lightning activity over the northwest Pacific region (0°–55°N, 100°–180°E) is investigated by analyzing data of 2005-2015 from the World Wide Lightning Location Network (WWLLN). The contribution of tropical cyclone (TC) to the climatological lightning and impacts of El Nino and La Nina events on lightning distribution over this region are also investigated. The spatial distribution shows that the mean lightning density (LD) over the northwest Pacific region is 3.0 fl km^-2 yr^-1. The highest densities are located along the coastal areas of southeast Asia and the Maritime Continent (MC) islands, while the lowest LDs are over the deep ocean and high-latitude continent. Land areas show much higher LDs than oceanic areas, except the region of the South China Sea (SCS). The distribution of areas with high LD is significantly different from areas with high precipitation rate. Tropical ocean is characterized by high precipitation rate but low lightning flash density, which is consistent with previous studies.

Significant seasonal variations of lightning discharge are observed over the northwest Pacific region. The average LD in the region reaches a minimum in the winter season (from December to February), especially in the deep ocean. However, in the oceanic area (30°-40°N, 140°-155°E) east of Japan, LD presents the highest value of the year in winter time. A sharp increase of LD in the Indo-China Peninsula and the SCS, the initial onset areas of Asian monsoon, is observed during the pre-monsoon season (from March to May), with the wind directions in these areas changing from northeasterly in winter season to westerly/southwesterly in the pre-monsoon season. In the monsoon season, the enhancement regions of lightning and convective activity are mainly located in the central and southern SCS and the oceanic area east of the Philippines. The monthly variation in lightning activity shows an obvious thunderstorm season during July, August and September, consistent with the peak months of precipitation rate. The weakest months of lightning activity and precipitation rate are both observed in February.

The diurnal variations of lightning activity in four designated areas are compared. LDs in the open sea remain the lowest with flat diurnal variations. The diurnal cycles of lightning in inland and MC differ remarkably from the ocean. On land areas, lightning activity presents an evident peak in the late afternoon (1700-1800 LT). Unlike the flat variations over the open sea, lightning activity in the SCS is much stronger and shows an obvious peak at morning hours (0900 LT). The maximum LD over the SCS is ahead of 3 hours of the maximum in the open sea. This may be due to the geographic effects, local weather patterns, coastline effects, and gravity wave of the nearby continent.

TC lightning over the northwest Pacific region mainly occurs in the area of 10°-25°N and 107°-103°E, and shows a unimodal latitude distribution peaking at 14-22°N. Two maximums of TC LD are observed, locating east of the Philippines and coastline and offshore of south China, indicating that strong lightning activity is produced when storms are approving the land during the landfall. Two areas with high contribution rate of TC lightning are found, namely, the oceanic area east of the Philippines and the offshore southwest of Taiwan.

Lightning variations associated with 2005-2015 El Nino and La Nina events are investigated. Compared to the average LD in all periods, the average LD during the El Nino event increases by 10.3%, while during the La Nina event decreases by 4.8%. Two distinct regions have been found to have more lighting during the El Nino: 1) the coastal areas of MC islands, Strait of Malacca, southern SCS, and oceanic areas east of the Philippines in the 0-10° N tropical region; 2) the East China Sea from Taiwan island to Japan in the 25-40° N extratropical region. The increased LDs in these areas are due to the enhanced synoptic-scale forcing, strong land-sea breeze circulations, and the increased number and intensity of tropical storm developed in the northwest Pacific and moved toward the Philippines. While during the La Nina periods, large positive anomalies of LD are presented in the western coastal areas of the Philippines and the central and northern SCS. A northward shift of enhanced lightning activity in the SCS is observed during the La Nina periods. The positive anomalies are associated with the convergence of wind vector in the central and northern SCS.