Roles of terrain in the convection initiation of an extreme rainfall event along the eastern slope of Wushan Mountain, China under weak synoptic forcing

Abstract The roles of terrain in the convection initiations (CIs) under weak synoptic forcing remain unclear. Take as an example an extreme rainfall event on the eastern slope of Wushan mountain and nearby regions on 25 July 2019 that resulted from a linear convective system originating from the emergence of several convective cells triggered along the eastern slope of Wushan mountain. The influence of the topographic thermodynamic and dynamic forcing on the afternoon (1500–1630 Beijing Time [ UTC + 8 h]) CIs associated with this event has been systematically studied in this study. Results show that the CIs occur in highly unstable and moderate wind shear environments and are closely correlated with the acceleration of upstream low‐level southeasterly winds, which are induced by the superposition effect of the large‐scale pressure field and mesoscale thermally forced mountain–plains solenoid (MPS) circulation. The large‐scale pressure field accelerates the low‐level southerly winds by increasing the northward pressure gradient force, while the mesoscale MPS circulation in the afternoon strengthens the low‐level easterly winds by increasing the westward pressure gradient force. As the upstream airflows are accelerated, the strong convergence uplift due to the topographic dynamic blocking triggers new convections in the transition region from blocked to unblocked flows. The numerical sensitivity experiments further highlight the key role of the thermodynamic forcing of Wushan mountain in the low‐level airflow acceleration and CIs. Overall, the topographic thermodynamic effect plays a relatively more important role than the dynamic forcing under a weak synoptic forcing situation given the low‐level airflow acceleration is a preceding factor.