Analysis on Characteristic Indices of Summer Rainfall in North China

In this paper, certain factors of the rainy season in North China are analyzed, including the start and end times, duration, amount of precipitation, and their variations, by using the daily precipitation data of more than 300 artificial meteorological stations in North China between 1960 and 2015 and the re-analysis data from National Centers for Environmental Prediction (NCEPs). The relationship between these factors and the key impact system of atmospheric circulation is also analyzed. The results show that the 5-day running average of rainfall provides a relatively feasible definition of the annual start and end times of the rainy season in North China, and these were found to be June 28 and August 23 respectively. Between 1960 and 1979, the interannual fluctuation of the start time of the rainy season was small, with an average start date of July 5 and an end date of August 20. The standard deviations of these dates were 7.5 days and 14.4 days respectively. However, between 1980 and 2015, the start date of the rainy season altered dramatically to become June 25, and the end time was delayed slightly to August 25. Furthermore, the average duration of the rainy season was significantly longer and the fluctuation of the start and end times was significantly larger, with their standard deviations being 16.0 days and 16.6 days respectively. On average, the duration of the rainy season in North China accounts for only 15% of a year, but the precipitation accounts for more than 50% of the total annual rainfall, and the correlation coefficient between the rainfall in the rainy season and that annually is 0.81. Therefore, the total annual precipitation in North China is dependent largely on the rainfall in the rainy season. Furthermore, the maintenance of the rainy season in North China is not only related to the location of the South Asia high and the westerly jet streams, but also to the interaction between the activity of the westerly system in the mid-high latitudes and the warm water vapor transport process in the low latitudes. The key region impacting the duration of the rainy season is the northern region (40-60°N), especially the northeastern region of China: when the 500 hPa geopotential height is low in this region, the rainy season in North China will last longer; and in the lower troposphere, when the south of the Baikal Lake displays a stronger anti-circulation control, the northeast airflow in the eastern part will induce the cold air southward, which is conducive to the maintenance of the rainy season in North China. The two key indices to determine the amount of precipitation in the rainy season in North China are: (1) the intensity of the interaction between the cold air in the north and the warm-wet air to the west of the subtropical high (the East Asian summer monsoon) and (2) the northward position of the East Asian summer monsoon. The year in which the amount of precipitation is significant corresponds to when the cold air in the northern region is relatively active. Meanwhile, the Western Pacific subtropical high near the continental side often displays a meridional distribution, which makes it easier to induce the west side of the southward warm-wet air flow to the north.