四川省暖季降水时空分布特征及机理

CHARACTERISTICS AND MECHANISMS OF THE SPATIOTEMPORAL RAINFALL DISTRIBUTION DURING THE WARM SEASON OVER SICHUAN PROVINCE

关键词:  四川省  日变化  夜雨  复杂地形

Key words:  Sichuan province  diurnal cycle  nocturnal precipitation  complex topography

摘要:

特殊的地理位置和地形分布造成四川地区降水具有鲜明特色，存在明显的区域差异。利用1998—2017年共20年的高时空分辨率卫星反演CMORPH降水资料，研究四川省暖季（4—9月）降水的时空分布特征，并结合ERA5再分析资料探究其降水的内在机制。研究发现：（1） 4—5月降水量偏低，6—8月最多，9月也相对较少。暖季的逐月降水在四川盆地西南部均有一个强降水中心，而6—9月在四川省最南部存在一个强度相当的降水中心，这种分布特征是由高低层大尺度环境场的配置所引起；（2） 四川暖季降水峰值出现在夜间至凌晨，但不同月份以及不同区域的降水存在较大的差异，两个夜间降水峰值中心的形成均与地形造成的低层风场辐合以及水汽累积密切相关；（3） 位于四川省西部（青藏高原的东坡）有一个相对较弱的午后降水峰值，这是受到两侧的山谷风辐合所致，且受到高低层环境场作用影响其分布区域和强度。

Abstract:

The special geographical position and complex topography of Sichuan lead to distinct features of the precipitation with significantly regional differences over the province. In this study, the characteristics and the underlying mechanisms of the warm season (from April to September) precipitation are investigated through the satellite-retrieved CMORPH rainfall data with high spatiotemporal resolution and the ERA5 reanalysis data for the 1998-2017 period. The results are shown as follows. (1) Large rainfall amount differences exist over the months from April to September, with the smallest rainfall in April and May, largest in June, July and August, and relatively small in September, due to the great impact by the allocation between the upper-level and low-level environmental fields. Two monthly heavy rainfall centers are found, located over the southwestern Sichuan basin for the whole warm season and the southernmost area of the province June through September, respectively. (2) Diurnal precipitation cycles are with nocturnal peaks in all months over the province, though spatial distributions differ much among individual months and regions. It is demonstrated that the rainfall is closely associated with the convergence of terrain-related low-level wind fields and accumulated moisture content. (3) A secondary relatively weak and movable rainfall peak appears over western Sichuan (i.e. the eastern slope of the Tibet Plateau) in the afternoon, caused by the wind convergence from the east and west sides of mountains. The distribution area and intensity of the peak are much subject to upper- and low-level wind fields and topographic effects.