The Low Frequency Characteristics of Precipitation and Its Circulation over North China in Summer 2012
The low frequency characteristics of summer rainfall over North China in 2012 and its associated low frequency circulations are studied. The results show that the summer rainfall over North China has a significant 10-13 d period. The evolution of the low frequency oscillation is well correlated to that of the severe rainfall, and such severe rain events usually take place on the peaks of the oscillation. At the same time, general circulation and impacting systems also have the low frequency characteristics. In the weak phase of the low frequency period, a low frequency anticyclone over the Bay of Bengal comes to intensify, and low frequency circulation systems over the Philippines and the South China Sea (SCS) propagate toward northwest gradually. Low frequency geopotential height anomalies over 500 hPa level propagate eastward, followed by a westerly trough at the high mid latitude. In the upper atmosphere, a low frequency cyclone over Iranian Plateau is weak, while a low frequency anticyclone over the eastern Tibetan Plateau strengthens and propagates eastward. In the strong phase of the low frequency period, low frequency south wind from the western North Pacific and the westerly trough with low frequency supply abundant water vapor to the North China. In the upper atmosphere, the low frequency anticyclone controls the east central part of China, favorable for the area from North China to the Northeast to become a divergence center. The upward motion strengthens, providing dynamic condition for precipitation. On the whole, the lower and upper low frequency circulations coordinating with each other result in severe rainfalls over North China.