The Trend and Distribution of Thunderstorm Activity and Influencing Factors in Nagqu Prefecture

Using synoptic meteorology, linear statistical methods and wavelet analysis, thunderstorm data from seven meteorological observatories in Nagqu Prefecture from 1966 to 2011 are analyzed to find the trend and space distribution of the number of thunderstorm days and influencing factors. A significant decreasing trend is found in the number of annual thunderstorm days, and the decreasing rate is around five days per decade. The cause for this trend might be that stronger latitudinal atmospheric circulation and weaker longitudinal circulation in the context of global warming reduces the intensity and frequency of cold air, and the mechanical lifting of moist air, which is not conductive to the formation of thunder clouds. Spatially, there is a deceasing number of thunderstorm days from the north to the south, from high mountainous valleys in the east to plains in the west with the highest number of thunderstorm days in the northeast and the lowest in the southeast. This feature is closely associated with differences in topography, thermal conditions, moisture conditions and climate across the prefecture. Due to high temperatures and strong convective activities in summer, there are significantly more thunderstorm days in spring and fall, and there are less thunderstorm days in winter with cold drying weather and relatively stable atmospheric stratification. As the weather becomes drier in winter, thunderstorms start later and end earlier with a decreasing trend in the period of thunderstorm activity within a year. The inter-annual difference in the number of thunderstorm days mainly depends on the westward move of the subtropical high over the West Pacific Ocean, the position and strength of the ridge line between the east Lake Balkhash high and the Tibetan Plateau high, and the shortwave trough off the Tibetan Plateau. When the west Pacific subtropical high weakens or moves eastward, and the warm moist airflow is prevalent in Nagqu, the number of thunderstorm days is higher. When the west Pacific subtropical high strengthens or moves westward, and the northwest airflow is prevalent in Nagqu, the number of thunderstorm days is lower. Two variation periods of 5-10 years and 20 years are found in the number of mean annual thunderstorm days between May and September. These variations indicate that Nagqu Prefecture will experience a period of a higher number of thunderstorm days.