Spatio-temporal variations of lightning activity over Nepal's complex terrain: Links to altitude and meteorological factors

Abstract

This study investigates the diurnal and seasonal variations of lightning activity over the complex terrain of Nepal, which spans elevations from 59 m to 8848 m above sea level. Despite lightning being a leading weather-related hazard in Nepal, causing over 100 fatalities annually, the altitudinal distribution of lightning has received limited attention. We use lightning data from the GLD360 network and meteorological parameters from the ERA5 reanalysis to analyze the influence of Convective Available Potential Energy (CAPE), wind, temperature, humidity, and cloud structure on lightning activity. Results reveal a strong altitudinal gradient in lightning flash density (FD), with maxima in the southern foothills and significant decrease toward higher elevations. Temporally, lightning exhibits two prominent peaks: one in the afternoon and the other around midnight. Afternoon lightning in the higher elevations is associated with surface heating, upslope winds, increased CAPE and moisture convergence, whereas nocturnal lightning over foothills is linked to persistent CAPE and downslope (katabatic) wind convergence. Cloud base height (CBH) further supports this spatial and temporal convection shift. Correlation analyses show strong FD–CAPE–humidity relationships in the foothills that weaken with elevation, suggesting increasing influence of orographic and microphysical processes aloft. The ratio of negative to positive CG flashes increases with elevation, likely influenced by CBH. Vertical profiles of ice water content and vertical velocity reinforce a clear transition from nocturnal convection in the southern plains to thermally and topographically driven daytime convection in the northern mountains. This study highlights how terrain modulates convective regimes and lightning variability across Nepal.