The unevenness in observed daily precipitation in mainland China

Precipitation plays a key role in terrestrial modeling and meteorological studies, which has large intermittency and fluctuation. Precipitation unevenness is used to quantify its lack of uniformity over different scales. This study investigated the daily precipitation unevenness from meteorological stations during 1961 to 2018 over mainland China, and further on six special regions to illustrate the spatial differences. The precipitation amount, number of wet days, and precipitation intensity were calculated to characterize the spatial distribution at an annual time scale, and during the warm and cold seasons. All indices were larger in southern areas than in northern areas, and were also larger during the warm season than during the cold season in most regions. Then the daily precipitation of each wet day is arranged in descending order to define the wettest n days, with n being any number of interests. The precipitation unevenness was evaluated through the cumulative fraction of the precipitation in wettest n days to the total, and was also evaluated using the number of days that account for half of the total precipitation. The cumulative fractions in the southern areas were generally smaller than in the northern areas for given number of wet days. At an annual time scale, approximately 12%, 45%, and 65% of the total precipitation fell on the wettest 1, 5, and 10 days in the three northern areas, respectively, contrasted to 8%, 25%, and 40% in the three southern areas. The smallest mean number of days to account for half of the total precipitation was 7.52 in northwest China, which is the largest value of 15.59 in southwest China. Northwest China was the region with the largest unevenness, while southwest China was the region with the largest seasonal discrepancy and the Yangtze River Basin had the smallest seasonal discrepancy.