Diurnal variations of summer rainfall response to large-scale circulations and low-level winds over the Sichuan Basin

Abstract

The evident nocturnal peak dominates the summer rainfall over the Sichuan Basin (SCB), which is closely related to the nocturnal intensification of low-level winds. Based on 21-year IMERG rainfall product and reanalysis data during summertime (June–August) from 2000 to 2020, the low-level winds are classified into four groups, with strong or weak daily mean wind accompanied by a large or small diurnal amplitude, to clarify their influences on rainfall over the SCB. The results show that under strong daily mean wind conditions, the westward extension of the Western Pacific Subtropical High (WPSH) determines the southwesterly monsoon airflow to be tuned to the southerly over the eastern Yunnan-Guizhou Plateau, which provides abundant warm and moist air resources for rainfall within the basin through the southeastern side of the SCB. Strong mean winds, coupled with a large diurnal amplitude due to the acceleration of easterly, strengthen the moisture convergence at night, and contribute to the rainfall increasing remarkable over the SCB with a peak at midnight. Meanwhile, there is an apparent anomalous low-level warming over the SCB, creating more unstable atmospheric conditions. In addition, the weak upward motion associated with the afternoon heating over the eastern slope of the Tibetan Plateau (TP) enhances the easterly and facilitates the development of upward motion over the western SCB in the early night, which is responsible for the larger diurnal amplitude of rainfall. On the contrary, rainfall is suppressed and the diurnal amplitude of rainfall is gentle under the condition of weak daily wind with small diurnal amplitude, due to the weak moisture transport and cooling planetary boundary layer related to the deepened midlatitude trough, as well as more active convection over the eastern slope of TP during the day. The results imply that atmospheric conditions associated with diurnal variation of low-level winds should be considered as a key component in regulating the rainfall and moisture budget over the SCB, the strength of low-level winds in the early evening may provide a predictive signal for the development of nocturnal rainfall over the SCB.