Thermodynamic Effects of Terrain on the Southerly Monsoon Surge and Extreme Rainfall Over North China

The southerly monsoon surge (SMS) produces a large amount of extreme rainfall (ER) over the North China Plain (NCP). This study proposes a new perspective in which the thermal contrast between the NCP and Mt. Taihang to its west can enhance the southerlies in planetary boundary layer (PBL) during SMS episodes and thus plays an important role in the production of ER. Based on reanalysis data, this study finds 145 SMS episodes in 30 warm seasons from 1988 to 2017, during which the NCP encountered 66% of the ER. These episodes feature strong poleward transport of moisture in the PBL. The diabatic heating over Mt. Taihang produces strong low-level baroclinicity, which together with synoptic forcing in the free atmosphere drives the strong PBL southerlies. The south wind components in PBL and in low troposphere are only moderately correlated, whereas their difference is closely related to the zonal temperature gradient. Based on convection-permitting numerical simulations, this study further investigates the impacts of mountain-plain thermal contrast on the production of ER during a 20-day SMS episode in the summer of 2024. A sensitivity experiment reveals significantly weakened PBL southerlies and moisture transport over NCP when the PBL over Mt. Taihang is cooled by ∼1 K. The simulated ER amount is 38.7% less than that in the control experiment. Additional experiments indicate that the low-level temperature over mountain modulates the diurnal variation of the PBL wind over plain via the Holton mechanism.