Climatic characteristics of thunderstorm days and the influence of atmospheric environment in Northwestern China

The annual trend in thunderstorm activity during a 42-year period from 1971 to 2012 over the transition region of the Tibetan Plateau and the Loess Plateau in China was documented. The delimiting factor was the number of days with thunderstorms collected from 64 stations over the study area. Reanalysis data from JRA-55, NCEP and ERA-Interim were used to understand the underlying reasons for the trend in the number of thunderstorm days. The results show that thunderstorms in this area were most active in the summer, followed by spring and autumn, while no thunderstorms were reported during the winter. Two centers with high number of thunderstorm days were revealed, one of which is located in the Gannan area close to the Tibetan Plateau and the other is in the Yongdeng and Wushaoling area east of the Qilian Mountains. The Loess Plateau in comparison has a lower frequency of thunderstorm days. There was no trend in the mean annual thunderstorm days from 1971 to the early 1990s, but a significant decreasing trend occurred afterward (until 2012), with the most important trend occurring during the summer when thunderstorms were the most numerous. The decrease in thunderstorm days was associated with a nonsignificant decrease in the convective available potential energy. Circulation patterns and moisture fluxes were analyzed to understand the observed difference between the two periods. Years with relatively high thunderstorm days (1971–1990) were characterized by relatively high water vapor flux to the study region, as well as having the western ridge of the western Pacific subtropical high located further east than during the years 1991–2012 with fewer thunderstorms. There were also relatively low 500 hPa geopotential heights in the north of China and the Mongolia in the earlier period. It is suggested that the relative changes in the position of the subtropical high and the 500 hPa trough are related to changes in the intensity of the Asian summer monsoon.