Lightning activity and its association with surface thermodynamics over the Tibetan Plateau

Lightning activity over the Tibetan Plateau, which has an average altitude of higher than 4000 m above sea level, is investigated by using 20-year (1995-2014) LIS/OTD data. The effect of surface thermodynamics on lightning activity is examined for four sub-regions across the plateau based on the NCEP reanalysis data. The average lightning flash density over the entire Tibetan Plateau is 2.5 fl.km(-2) yr(-1), and the maximum flash density is 7.7 fl.km(-2) yr(-1) at eastern plateau. The geographical distribution of lightning flash density shows a gradual decrease from east to west across the Tibetan Plateau, from 4.3 fl.km(-2) yr(-1) on average over the Eastern Tibetan Plateau to 1.7 fl.km(-2) yr(-1) over the Western Tibetan Plateau. Over the plateau's main highland (> 4500 m), there is a local maximum value of 5.7 fl.km(-2) yr(-1) in the center area of Nagqu. Lightning activity over the Tibetan Plateau mainly occurs in boreal summer, and 96.8% of the annual flashes occur during May-September. There is more lightning in the relatively dry spring, even in summer, over the main highland of the Tibetan Plateau. The results show that humidity and sensible heat fluxes play a vital role in the occurrence of lightning over the Tibetan Plateau. A new parameter, product of rainfall, Bowen ratio (defined as the ratio of the sensible heat fluxes to latent heat fluxes) and surface specific humidity, is proposed and shows a better correlation with lightning activity, generally displaying a larger correlation coefficient than rainfall alone or other parameters over the study sub-regions.