Analysis of Abnormal Signals in the Upper Troposphere and Stratosphere before the Persistent Heavy Rainfall Event in South China in June 2005

Using the ERA-Interim reanalysis data, the upper tropospheric and stratospheric signals before the "05·6" heavy rainfall event in South China are analyzed. The results show that before the heavy rain, the tropopause over the rainfall region exhibited a lower-lift-lower trend that was consistent with the evolution of potential vorticity (PV) anomalies over southern Japan. Specifically, a relatively high PV belt over Japan extended southward and led to higher PV (lower tropopause) over southern China. Compared with the mean values between 1991 and 2010, easterly winds in the upper troposphere and lower stratosphere (UTLS) over the subtropical region were stronger and located more northward before the heavy rain. There also existed positive easterly wind anomalies over South China, which propagated downward on the 9th day before the heavy rain. The center of the South Asian High was located more eastward and southward than its mean position before the heavy rainfall. The relatively high geopotential height that enhanced twice with time could be found in the upper troposphere and lower stratosphere over South China. The shift of the center of the South Asia high, the early downward propagation of lower stratospheric easterly winds and the airmass of high PV all were favorable for the onset of the heavy precipitation.