Analysis of Diurnal Variation Characteristics of Summer Extreme Precipitation in Sichuan and Chongqing

Hourly precipitation data in the summer of 1991–2012 in Sichuan and Chongqing were used to analyze the spatial and temporal distribution characteristics of total precipitation and extreme precipitation, especially the diurnal variation characteristics of extreme precipitation. The results show that the area’s precipitation amount (PA) is low in the west and high in the east influenced by the terrain, which is high in the west and low in the east. Specifically, there is more precipitation in the eastern Sichuan Basin and southwestern mountains than in the northwest plateau and more precipitation in the mountains around the basin than in the basin. The precipitation frequency (PF) was higher in the west and the plateau but lower in the east. The precipitation intensity (PI) distribution is similar to that of PA, which increases gradually from west to east. The spatial distribution characteristics of PA, PF, and PI of extreme precipitation are similar to those of total precipitation. Greater precipitation in the eastern Sichuan basin, such as Leshan, Ya’an, Dazhou, and Guangyuan, and in the southwestern mountains, such as Xichang and Panzhihua, is mainly because of greater PI. The larger precipitation in a small part of the southwest mountain in the north of Xichang is mainly because of high PF. Less precipitation in the western plateau of Sichuan is due to lower PI. The daily peak of PA increased from west to east, while the daily peak of PF showed an opposite trend, decreasing from west to east. Almost all of them occur at night, and the “night rain” feature is very prominent. Most of the daily peaks in the high-altitude areas occur in the early midnight, whereas the peaks in the low-altitude areas occur in the later midnight. The time of the daily peaks from west to east delays gradually, which is indicative of the precipitation system spreading from west to east in Sichuan and Chongqing. Four regions are obtained by clustering of the typical form of precipitation area site daily change curve analysis. The first region is the plateau, the second is the southern mountains transitioning from the plateau to the basin, the third is the basin, and the fourth is the transition area from the basin to the eastern hills. The diurnal variation of the PA, PF, and PI curves has the same characteristics as “one peak and one valley.” The peak values of PA from regions one to four are from midnight to early morning, and the valley ones are from afternoon to evening, which gradually delays from west to east. The peak and valley time of PF is close to those of PA, and the diurnal fluctuation of PI is larger than that of PA and PF. The small PA of the first region is mainly due to its weaker PI. The PA of the second region is high mainly because of its higher PF. The higher PA of the third and fourth regions is mainly due to their stronger PI. The distribution characteristics of the daily peak of extreme precipitation and its occurrence time are similar to that of total precipitation. The threshold value of extreme hourly precipitation in summers in Sichuan and Chongqing was low in the west and high in the east and gradually increased from west to east. The proportion of extreme precipitation to total precipitation increased from west to east. The extreme precipitation contributed more to the total precipitation in the eastern parts of Sichuan and Chongqing but less to the western part.