Regional trends of lightning activity in the tropics and subtropics

Based on 18 years of lightning data from 1996 to 2013 detected by optical transient detector (OTD) and lightning imaging sensor (LIS), the variation trend of lightning flash density (LFD) in the tropics and subtropics are investigated. Meteorological parameters from observational and reanalysis datasets are analyzed to understand the causes of LFD trends. The averaged LFD shows no significant trend during the period of analysis in the tropics and subtropics, while the regional LFD trends are significant. LFDs show significant negative linear trends in southeast North America (SENA), central South America (CSA) and eastern Australia (EA), with trend values of -0.26 fl km(-2) year(-1), -0.17 fl km(-2) year(-1), and - 0.14 fl km(-2) year(-1), respectively. At the same time, LFDs increased significantly in northwest South Asia (NWSA), with a trend of 0.24 fl km(-2) year(-1). Using a multiple linear regression method, the possible causes of LFD trends over these regions are investigated. The negative linear trends in SENA, CSA and EA are more likely due to the decreases of the convective available potential energy (CAPE), which may contribute -0.13 fl k( -2) year(-1), -0.14 fl km(-2) year(-1) and - 0.11 fl km(-2) year(-1) to the LFD trends in these regions, respectively. Increase of aerosol optical depth (AOD) exerts more influence on the increasing of LFD in NWSA, and could explain 0.08 fl km(-2) year(-1) of the LFD trends.