Relationship Between Areal Rainfall and Circulation Characteristics in Xijiang River Basins

The reanalysis grid data of daily average 850 hPa and 500 hPa geopotential height field during 1971-2015 provided by NCEP/NCAR are used for computing six daily circulation indices of Xijiang River Basins using Lamb-Jenkinson atmospheric circulation classification method. The daily circulation is divided into straight wind type, rotary type and mixed type according to the relationship between the geostrophic wind speed, wind direction and the geostrophic vorticity. The circulation frequency of precipitation weather circulation in Xijiang River Basins is researched, showing that the probability of precipitation is the greatest when the circulation is southwest wind type at 850 hPa and west wind type at 500 hPa, which is the dominant circulation for precipitation in Xijiang River Basins.The contribution rate of different types of circulation to the total areal rainfall is calculated, and types with top 3 contribution rates are called the dominant circulation pattern. It's found that for the annual total rainfall and sub basin rainfall, the cyclone type at 850 hPa and westerly type at 500 hPa has the maximum contribution in each layer. The contribution of areal rainfall is larger in the east part and smaller in the west part of Xijiang River Basins. The main configuration of south wind type at 850 hPa and anticyclone type at 500 hPa causes rainy autumn over the western sub basin.By the influence situation of the first dominant circulation configuration of season total areal rainfall contribution for strong precipitation, it shows that the probability of strong precipitation are 21.1%, 18.7% and 2.0% while the configuration are cyclone at 850 hPa and west wind type at 500 hPa in summer or spring, south wind at 850 hPa and anticyclone type at 500 hPa in autumn, southwest wind at 850 hPa and west wind type at 500 hPa in winter, respectively.Analysis on the dominant circulation configuration for season rainfall indicate that, in spring and summer it's cyclone at 850 hPa and westerly at 500 hPa, in autumn it's southerly at 850 hPa and anti-cyclone at 500 hPa, and in winter it's southwest wind at 850 hPa and westerly at 500 hPa. The corresponding probability of heavy rainfall are 21.1%, 18.7%, 4% and 2%.In the past 45 years, areal rainfall contribution rates of cyclone at 850 hPa and the west wind at 500 hPa are increasing, which has a significant positive correlation (correlation coefficients are 0.63 and 0.69), indicating that it is the dominant circulation pattern with the trend of rainy in Xijiang River Basins.