1.Chongqing Climate Center,Chongqing;2.School of Atmospheric Science,Nanjing University,Nanjing
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 total precipitation PA in Sichuan and Chongqing is mostly distributed like less in the west and more in the east for the influence of the terrain with higher west and lower east. Specifically, there are more precipitation in the eastern Sichuan basin and mountains of southwest of Sichuan, less in plateau of the northwest while more in the mountains around the Sichuan basin and less in the Basin. The precipitation frequency PF was higher in the west and lower in the east inversely, the higher PF in the plateau. The distribution of precipitation intensity PI is likely with 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. The greater precipitation in the eastern Sichuan basin such as Leshan, Ya"an, Dazhou and Guangyuan and in the mountains of southwest of Sichuan such as Xichang and Panzhihua are mainly due to the greater precipitation intensity. The larger precipitation in a small part of the southwest mountain in the north of Xichang is mainly due to the higher precipitation frequency. The less precipitation in the western plateau of Sichuan is due to the lower precipitation intensity. 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 higher altitude areas occur in the before midnight while lower altitude areas in the after midnight. The time of the daily peaks in the areas from west to east delays gradually which indicated the precipitation system spreading from west to east in Sichuan and Chongqing. The distribution characteristics of daily peak of extreme precipitation and its occurrence time are similar to the total precipitation. The threshold of hourly extreme precipitation in summer in Sichuan and Chongqing is lower in the west and higher in the east, and gradually increasing from the west to the east. The extreme precipitation frequency accounts for almost 40% of the total precipitation frequency, with no significant difference. The proportion of extreme precipitation in the total precipitation increased from west to east, and extreme precipitation contributed more to the total precipitation in the eastern than western of Sichuan and Chongqing. The spatial and time distribution of total precipitation is similar to that of extreme precipitation. The analysis of the diurnal variation of typical precipitation at four types shows that most plateaus and mountains in the western of Sichuan are type 1, which have the least precipitation but the highest precipitation frequency. A small mountainous area near Panzhihua and Sichuan Basin including Chengdu, Meishan, Ziyang and Neijiang are type 3, which have more precipitation. The central and eastern parts of Sichuan Basin and the eastern edge of Chongqing are type 4, which also have more and obvious transitional characteristics, with complex characteristics of three precipitation variables. Finally, the rest region including plateaus, mountains and the western part of the Sichuan Basin are type 2, which including Leshan and Ya "an areas with the most precipitation and have abundant precipitation and rainstorms in summer. The higher the altitude of the region, the greater the daily peak of PF at night. The PI of the type 1 region with higher altitude is obviously weak. The distribution of the three precipitation variables is in good agreement with the spatial distribution. The occurrence time of the daily peaks of the three variables was postponed from west to east from the first night to the next morning, and the occurrence time of the lower value was postponed from west to east from noon to the evening. The daily changes of extreme precipitation PA and PF all show a single-peak structure in the early morning, and the "night rain" features obviously, the daily peak occurs around 3:00 in the morning, the lower value near 13:00 in the afternoon, the PI shows a jagged structure which is lower between 5:00 and 11:00 and higher at the rest times. The PA lower value is mainly affected by PF while the peak value is mainly affected by higher PI and PF.