Onset and End of the First Rainy Season in South China

Abstract

AbstractThe onset and the end of the First Rainy Season (FRS) in South China are defined in this paper using daily rainfall data and NCEP/NCAR reanalysis daily data in the period 1957~2004. After choosing a reasonable area and representative stations in South China, the evolution of the variables including rainfall, precipitable water (PW), water vapor flux (WVF), vertical velocity, and potential pseudo‐equivalent temperature (PPET) are analyzed. Results show that all these variables have experienced obviously stagewise changes around the onset and the end. The averaged onset date of the FRS occurs in pentad 19 (the 1st pentad in April), as the precipitation increases outstandingly, in accordance with dramatic increments in PW, WVF and PPET. Meanwhile the WVF changes from divergence to convergence, accompanied by gradually enhanced vertical activities. Nevertheless, the rainfall in April is moderate which depicts the frontal precipitation. After the East Asian summer monsoon outbreaks in May, South China enters into a period with much more intensed precipitation and vigorous convections. As the peak of the rainy season is in June, the season ends at pentad 34 (the 4th pentad of June) when all these variables exhibit sudden decreases. According to the above analyses, the timetable of the annual onset and end of the FRS in South China is given by choosing threshold criteria. Investigations on the potential height and wind fields before and after the beginning (ending) pentads show that the climatological atmospheric circulations are in favor of the increase in temperature and humidity centered at the beginning so as to cause remarkable rainfall increase. As for the ending of the FRS, it is due to the seasonal transition of the general circulation in East Asia, especially due to the first northward shift of the Western Pacific Subtropical High (WPSH).