Analysis on spatial distribution characteristics of afternoon flash heavy rain from June to September in Zhejiang

By using several kinds of data, including the hourly data from the automatic weather stations in Zhejiang Province and the National Centers for Environmental Prediction (NCEP/NCAR) global reanalysis data with resolution of 1°×1° from June to September during 2012 to 2016, the temporal and spatial distributions of afternoon flash heavy rainfalls in relation to weather patterns and physical parameter characteristics were studied. Results show:(1) Probability of afternoon flash heavy rainfalls was relatively high in the downtown areas in cities such as Hangzhou and Ningbo in northern Zhejiang, as well as the hilly areas in middle and southern parts of Zhejiang, while the probability of flash heavy rainfall was relatively low in coastal areas, Jinhua-Quzhou basin and the Qiandao lake. (2) Meteorological factors in triggering afternoon flash heavy rainfalls varied greatly by temperatures, subtropical high, boundary line etc. from June to September. Severe convective rainstorms are more likely to happen in July and August than in June and September. Focus should be paid on factors, such as unstable energy at the edge of subtropical high, the function of boundary convergence, the urban-heat island effects and the forced uplift mechanisms over mountain terrain during July and August. (3) Requirement for instability and water vapor were higher for triggering afternoon flash heavy rainfall in the plain areas than in hilly areas, and vertical wind shears between 1 000 and 700 hPa was important in rainstorm maintaining over the plain areas. (4) CAPE tended to surge six hours before most afternoon flash heavy rainfall occurred. Although factors of instability and vapors displayed little change, most meteorological physical parameters developed to be more favorable for afternoon flash heavy rainfalls.