The Trends of Warm-Season Thunderstorm and Lightning Days in China and the Influence of Environmental Factors

Previous studies indicated that numbers of meso-micro scale severe convective weather exhibited a significant decrease over China's mainland in the past decades. However, the possible mechanism of the decrease has not been well understood. This paper analyzes the changes in frequency of thunderstorm and lightning days using an updated data set of denser national observational stations and theirs links to the atmospheric circulation and regional environmental factors. The results confirm the highly significant decreasing trend of frequency of thunderstorm and lightning days during 1961-2013, with the decreasing rates of -2.6 days/10 years and -6.5 days/10 years in the warm season, respectively. The decrease of geopotential height difference between south and north, the weakening of westerly jet strength at 200 hPa and the decline of zonal wind speed of high level are found to be the direct reasons for the wide-spread decrease of thunderstorm and lightning days across the country. Meanwhile, there is a significant positive correlation between thunderstorm days and convective available potential energy (CAPE), indicating a reduction of potential atmospheric instability accompanying the decline of thunderstorm days especially in the south of China. The decrease in the relative humidity at the lower troposphere, the weakening of 0-6 km vertical wind shear and the accompanying slackening of actual convective activity, may have also contributed to the downward trends of the severe convective weather. Our analysis shows a strong link of the decreasing thunderstorm and lightning days to the long-term changes of the atmospheric circulation and environmental factors. Further investigation is needed to examine what have driven the changes of the environmental factors over the time period.