Northeastward propagation of nocturnal precipitation over the Sichuan Basin

The Sichuan Basin (SCB) is located to the east of the Tibetan Plateau (TP) and features nocturnal precipitation. This study explores spatial variation of nocturnal precipitation during the warm season over the SCB using high spatiotemporal resolution merged precipitation products from 2008 to 2017. It is found that there is a prominent northeastward timing delay of precipitation peak over the SCB, that is, the diurnal peak of precipitation occurs in the midnight over the southwestern SCB whereas in the early morning over the northeastern SCB. This time delay is partly ascribed to a northeastward propagation of precipitation over the SCB, account for about 40% and 25% of the diurnal variation of precipitation over the central and northeastern SCB, respectively. Precipitation first occurs over the southwestern edge of the SCB at about 1900-2000 LST, and then propagates northeastward steered by the large-scale mid-tropospheric southwesterly airflow, which is consistent with the northeastward propagation of the large moisture convergence. The precipitation reaches the northeastern SCB in the early morning. The diurnal variation of 850-hPa wind has strong easterly wind deviations in the early evening over the southwestern SCB, which favours for an initiation of precipitation by orographic lift of air. The diurnal variation of difference in equivalent potential temperature between 500 hPa and 850 hPa has a maximum negative deviation in the early evening over the southwestern SCB, which enhances the instability and therefore favours an initiation of precipitation. The Yunnan-Guizhou Plateau (YGP) may play an important role in the initiation of precipitation over the southwestern SCB in the early evening.