Analysis Of The Characteristics Of The Rainy Season Variation In North China

To find out the causes of abnormal summer rainfall in North China and improve climate prediction technologies, and to monitor the rainy season in North China in a better way, a comprehensive analysis on the relationship between summer monsoon in East Asia and summer precipitation in North China is carried out based on such data as summer precipitation in North China and general atmospheric circulation reanalyzed by NCEP/NCAR. The results of the analysis indicate that:1) Irrespective of strong or weak summer monsoon years in East Asia, the circulation conditions contributing to more summer rainfall in North China are sufficient water vapor sources (strong summer monsoon years:abnormal water vapor carried by southwest wind; weak summer monsoon years:abnormal water vapor carried by southeast wind) and good dynamic conditions (with convergence circulation at 850hPa in North China, prominent zonal circulation at 500hPa at middle latitude, and more low-pressure trough transit in North China). The circulation pattern related to less summer rainfall in North China is significantly different:No effective dynamic conditions (with divergence circulation at 850hPa in North China, prominent meridional circulation at 500hPa at middle latitude, and less low-pressure trough transit in North China) are created both in strong summer monsoon years with sufficient water vapor sources (abnormal southwest wind) and weak summer monsoon years with insufficient water vapor sources (abnormal northerly wind).2) Although there are sufficient water vapor sources in strong summer monsoon years in East Asia, the summer rainfall differs a lot in different years in North China due to diverse dynamic conditions. If the dynamic conditions are good, North China will have abnormally more summer rainfall; otherwise, it will also have abnormally less summer rainfall. In weak monsoon years in East Asia, water vapor carried by southwest wind is reduced significantly, so North China will have abnormally less summer rainfall if there is lack of water vapor supplement from other paths and the dynamic conditions are poor. Although water vapor carried by southwest wind is reduced significantly in weak summer monsoon years, there are also sufficient water vapor sources in North China if more water vapor is carried by southeast wind. If the dynamic conditions are good in that case, North China may also have abnormally more summer rainfall.3) The rainy season in North China is monitored and analyzed based on the rainy season monitoring indexes defined in this paper. It is discovered that the average starting date of the rainy season in North China was around July 4 and the average ending date was around September 2 during 1961-2014, with an average period of 60 days. The average annual accumulative precipitation during the rainy season was 278mm and the average daily precipitation intensity was 4.5mm/d. The precipitation intensity in the rainy season in North China during 1961-2014 showed a reduction trend. Specifically, it was mainly strong and relatively strong in the rainy season in the 1960s, relatively strong and normal in 1970s, normal and relatively weak in the 1980s, relatively weak in the 1990s, relatively weak and normal during 2000-2009, relatively strong in 2010 and 2012, normal in 2011 and relatively weak during 2013-2014. In recent years, the precipitation intensity in the rainy season shows an increase trend but is not as great as that during the 1960s-1970s. After relatively strong intensity for successive 3 years, the intensity was significantly weakened during 2013-2014. It is uncertain whether North China was subject to a "shift of rainy season".