Study of the forecasting index for short-duration heavy rainfall with different intensities based on sounding data at Hangzhou

Using hourly rainfall data from 69 national automatic benchmark stations over Zhejiang Province and 58 regional automatic weather stations over the urban area in Hangzhou between May and September from 2006 to 2015, we have calculated the cumulative probability of short-duration heavy rainfall in Zhejiang by Gamma distribution function, and integrated their frequency distributions to investigate the characteristics of hourly rainfall intensity in Hangzhou. In addition, taking Hangzhou urban area as an example, the radiosonde dataset is used to analyze the environmental indices when hourly rain intensity occurred with different levels (≥50 mm·h^-1, 30-50 mm·h^-1, 20-30 mm·h^-1and below 20 mm·h^-1), from which we extract the forecasting indices based on the kernel density estimation method. The results show that the probability of less than 10 mm·h^-1 precipitation at Hangzhou station is as high as 98.4%, and the probability that is equal to or greater than 20 mm·h^-1 is only 0.05%. Due to the effect of easterly airstream from Hangzhou Bay, the frequency of more than 20 mm·h^-1 heavy rainfall is relatively high in Hangzhou urban area, especially in the east part and northwest mountain region of Yuhang district in Hangzhou. The number of days of annual short-duration heavy rainfall ranges from 18 to 28 d in Hangzhou urban area since 2008, of which the percentage of days of short-duration heavy rainfall that is equal to or greater than 50 mm·h^-1 accounts for 10%-20% of the total ones, except being less than 10% in 2009 and 2012. The most significant factor that can be used to predict the local short-duration heavy rainfall is atmospheric precipitable water, followed by K index, best uplift index, showalter index, dew point temperature at 925 hPa and sweat index in descending order of significance. The best factor in determining of the rainfall intensity in Hangzhou is again atmospheric precipitable water, followed by vertical velocity at 850 hPa and divergence at 925 hPa.