The impact of mountain-plain thermal contrast on precipitation distributions during the "23.7" record-breaking heavy rainfall over North China

A record-breaking heavy rainfall event struck North China from 29 July to 1 August 2023, during which the southwest mountain region of Beijing city recorded a rainfall amount over 2 times that of its eastern plain region. Based on multisource observations and convection-permitting numerical simulations, this study analyzes the thermal structure in these regions and its impact on precipitation. The rainfall was caused by a deep extratropical cyclone, which produced strong low-level southeast winds that transported abundant warm moist air toward North China. The light rainfall on 29 July caused extensive evaporation and cooling, and the increase in low-level easterly wind led to uneven heating in the planetary boundary layer (PBL) during the following two days. The PBL over mountains was significantly colder than over plains. The WRF model reasonably reproduces the evolution of rainfall, but with a dry bias. The simulated differences in rainfall amount and temperature between two regions are similar to the observations. The diagnosis of the vertical momentum budget attributes the enhancement of updrafts to horizontal convergence within the PBL, which mainly resulted from the deceleration of easterly wind over the windward slope. The simulated maximum temperature gradient occurred in the strongest convergence zone. Furthermore, the force analysis indicates the important role played by the thermal contrast in the deceleration of PBL easterly wind. The thermal contrast forced a mountain breeze effect, which together with the topographic blocking significantly decelerated the easterly wind and further enhanced updrafts and rainfall over mountainous regions.