Spatial and Temporal Variations of Nocturnal Precipitation in Sichuan over the Eastern Tibetan Plateau

Utilizing hourly precipitation data from the 29 weather stations in Sichuan Province from 1971 to 2012, preliminary regional average and trend analyses are conducted on the spatial and temporal variation characteristics of the nocturnal precipitation rate, intensity, and frequency during Sichuan's rainy season (from May to September). The main results are as follows: (1) There are prominent regional and vertical differences in the nocturnal precipitation. Specifically, the nocturnal precipitation rate reaches its maximum in the southwestern basin, whereas the northeastern Sichuan Plateau and northeastern basin are two areas with low precipitation rates. (2) In northeastern Sichuan, the nocturnal precipitation rate and frequency are low, but its intensity is greater. However, the western Sichuan Plateau presents very differently. In the southwest of the basin, nocturnal rain occurs infrequently, but with greater intensity and proportion. (3) The average daily nocturnal precipitation rate over 42 years first declined, and then increased. In contrast, the intensity of nocturnal precipitation showed a declining trend after rising, and the daily variation of the nocturnal rain frequency is clearly bimodal. (4) The annual variability of the rate, intensity, and frequency of nocturnal precipitation in Sichuan show some differences. Nocturnal precipitation frequency and rate are both are at a high level, but show a reducing trend in the 1970s and 1980s. The nocturnal precipitation intensity, in contrast, is low in the 1970s and high in the 1980s, showing an increasing trend. In the 1990s, the nighttime rainfall intensity, frequency, and proportion are all lower in value. In the 21st century, the intensity and proportion of the nocturnal rainfall are significantly increased, with the frequency increases lagging behind. The frequency and proportion of the nocturnal precipitation fluctuate prominently during this period.