Abstract:

Based on the daily surface thunderstorm observation data from three stations in Dalian city from 1961 to 2013 and the monthly average NCEP/NCAR (the National Centers for Environmental Prediction/National Center for Atmospheric Research) reanalysis data from 1948 to 2016,the temporal and spatial distribution of the number of thunderstorm days in Dalian city was analyzed using the linear trend estimation and synthetic analysis methods.The background characteristics of the average atmospheric circulation from May to September in the years with severe thunderstorms were discussed.The results show that the thunderstorm events in Dalian have obvious regional characteristics.There are a higher number of thunderstorm days in the northern inland area and a lower one in the southern coastal area.Except in February,thunderstorms can occur in any month,and the number of thunderstorm days reach the peak value in July and August.Thunderstorms mainly occur from May to September.They have a strong seasonality.There is a higher number of thunderstorm days in summer (from June to August),and a lower number of thunderstorm days in winter.The annual average number of thunderstorm days shows a decreasing trend,especially in the northern area.The starting time of thunderstorm mostly appears in April,and the ending day mostly appears in October.The starting day of thunderstorm is more stable than the ending day.Both the starting and ending dates of thunderstorm in the northern inland area are more stable than those in the southern coastal area.The starting dates of thunderstorm are ahead notably,whereas the delay trend in the ending dates of thunderstorm is insignificant.The obvious delay trend of ending date is only in Dalian station.The thresholds of five-day average temperature corresponding to the starting and ending days of thunderstorm in the northern inland area are -1 ℃ and 10 ℃,respectively,those in the southern area (and eastern area) are 6 ℃ (-1 ℃) and 3 ℃ (8 ℃),respectively.In the active thunderstorm years,the Mongolian low vortex at 500 hPa is anomaly strong,the subtropical high is more westward and northward,the southerly wind moisture transport at 850 hPa and the ascending motion of the whole atmosphere layer over Dalian are both abnormally strong.All of these are beneficial to the occurrence of more thunderstorm events.While in the inactive thunderstorm years,the characteristics of weather situation are basically contrary to those in active thunderstorm years.

Key words: Thunderstorm day, Synthetic analysis methods, Atmospheric circulation background