The Climatic Background and Medium-range Circulation Features of Continuous Torrential Rain from April to June in Guangdong

To better understand the cause of medium-range continuous torrential rain and forecast its inducing flood from April to June in Guangdong, the climatic background and medium-range circulation characteristics of continuous torrential rain happens in the target season during 1961—2001 in the Guangdong Province are analyzed using statistic and composite analysis. It shows that the continuous torrential rain has the highest probability (47%) in June and lowest probability (13%) in April. The wavelet transform and maximum entropy spectral demonstrate that the continuous torrential rain in Guangdong exhibits periodic oscillation of about 2—5 and 10 years, and the number of the continuous torrential rain will increase in the coming years. The medium-range circulation characteristics and evolving circulation features of high-and low-latitudes prone to Guangdong continuous torrential rain are analyzed with the pentad mean 500 hPa geopotential height. The two indexes of mediumrange circulation characteristics favorable for Guangdong continuous torrential rain are summarized. Their common feature is that the very stable blocking high in the west and blocking high in the east happen in the midhigh latitudes. The superposition between the east blocking high in the mid-high latitude and the positive anomalies east of 120°E in the low-latitudes make the pattern of east high and west low very steady. At 850 hPa, the significant composite pentad southwest wind belt in 10°—25°N, 80°—115°E provides water vapor for continuous torrential rain, from which the water vapor of the Bay of Bengal is continually transported to South China. The positive vertical velocity center is located over Guangdong Province in composite pentad vertical velocity field at 850 hPa. All these conditions are in favor of continuous torrential rain.Based on the above results and pentad mean 500 hPa geopotential height from daily prediction products of the European Center for Medium-Range Weather Forecasts (ECMWF), medium-range forecasting of continuous torrential rain in the Guangdong Province is made by dynamic-similarity diagnostic weather analysis and interpretation of K_i, S_i, depression of the dew point in 850 hPa and other physical factors from numerical simulation according to forecasting experiments for precipitation. The accuracy is 12/17 (71%) for 17 forecasted continuous torrential rains process from April through June during 2003—2005. This demonstrates that the method has good reference for operational forecasting of continuous torrential rain.