Convection Initiation over Mountain Slopes in North China: Roles of Upslope Winds and Orographic Waves

Using high-resolution observations, mesoscale simulations, and idealized experiments, this study investigates the mechanisms governing an episode of orographic convection initiation (CI) during the North China Heavy Rainfall Experiment. On 4 August 2024, repeated CI occurred over the eastern slopes of Taihang Mountains in the late afternoon, subsequently enhancing an upstream downhill convective storm. Wind profiler radar data and dense automatic weather stations reveal that CI was supported by strengthening southeasterly upslope winds. These winds primarily resulted from the migration of the mountain-plain solenoid and the mountainward-propagating outflow from a convective cold pool over the plain, with sensitivity experiments showing the latter contributed roughly 22% of the wind strength. The upslope flows gradually transported unstable air from the plain to the slope, fostering CI. Mesoscale simulations further highlight the key role of orographic waves near the mountain ridge, which generated strong downslope winds. The near-surface convergence between downslope and upslope flows, combined with wave-induced divergence aloft, produced deep ascent over the slope. Removing mountain ridges weakened wave strength and reduced downslope wind speeds by ~8 m s⁻¹.Without orographic heating in the idealized simulation (i.e., no mountain-plain solenoid), only strong wave descent occurred below 2 km, inhibiting CI. These findings underscore the critical interplay among plain convection, orographic waves, and the mountain-plain solenoid, offering new insight into the processes controlling orographic CI in North China.