Abstract: To monitor the rainy season in North China in a better way, new monitoring indexes are defined in this paper based on daily precipitation data from 37 stations of North China and day-by-day general atmospheric circulation data reanalyzed by NCEP/NCAR. By using the indexes, the characteristic line of 10 g·kg^-1 specific humidity at 850 hPa is used for monitoring the rainy season in North China, with the changing characteristics at 500 hPa height field, sea-level pressure field and 850hPa wind field as the reference indexes. Results indicate that the 10 g·kg^-1 specific humidity line at 850hPa crossed over Line 35°N and reached North China from the end of June to early July but returned southward to Line 35°N in late August, which well corresponded to the precipitation concentration period during summer in North China. The rainy season in North China was monitored and analyzed based on the rainy season monitoring indexes including the starting date of rainy season, the ending date of rainy season, the period of the rainy season, the total precipitation in the rainy season and the average precipitation intensity defined in this paper. It shows that the average starting date of the rainy season in North China was around July 4th and the average ending date was around September 2nd during 1961-2014, with an average period of 60 days. The average annual accumulative precipitation during the rainy season was 278 mm and the average daily precipitation intensity was 4.5 mm·d^-1. 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 1960s, relatively strong and normal in 1970s, normal and relatively weak in 1980s, relatively weak in 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 showed an increase trend but was not as great as that during the 1960s-1970s;after a successive 3-year relatively strong intensity, the intensity was significantly weakened during 2013-2014;so it is uncertain whether North China was subject to a "shift of rainy season". The results of practical application indicate that the rainy season monitoring indexes defined in the paper are significantly improved when compared with the monitoring method based on daily precipitation. The monitored changes of rainy season in many years conform to the actual conditions. Therefore, it is a good technical method to conduct monitoring with these indexes, and can be referred to and applied in the monitoring work.

Key words: North China, Rainy season, Monitoring, Index