A Study of Convection Initiation over Steep Terrain under Weak Synoptic Forcing: From Kilometer-Scale to Hectometer-Scale Simulations

Abstract Accurate forecasting of orographic convection has been a major challenge due to the limited capability of numerical models in effectively reproducing the effects of steep terrain forcing. Based on kilometer-scale (3- and 1-km grid spacings) and hectometer-scale (333-m grid spacing) convection-permitting simulations, this study investigates the initiation process of nocturnal convection over Cang Mountain, a representative steep terrain and convection hotspot in the Hengduan Mountain region. In the absence of strong synoptic forcing, a typical convection event around the midnight of 24 July 2022 is initiated over the western slope of Cang Mountain and subsequently propagates northeastward. As the model’s horizontal resolution is refined from 3 km to 333 m, the initiation position of convection shifts southward, with a slightly advanced timing and increased intensity. Quantitative evaluations demonstrate that the hectometer-scale simulation substantially outperforms in capturing the convection-associated processes influenced by the steep terrain features. The hectometer-scale simulating results reveal that the nocturnal acceleration of low-level southwesterly airflows intensifies moisture transport through the gorge passage on the southwest flank of Cang Mountain, leading to humidified atmosphere over the western slope. Within the 6-min period prior to convection initiation, the low-level southwesterly winds transition from divergence to convergence over the western slope, enhancing local instability and favoring the formation of convective clouds. Additional sensitivity experiments using coarser topography manifest that the underlying steep topography of Cang Mountain plays a vital role in intensifying updrafts to overcome convective inhibition to the level of free convection, as well as elevating the moist absolutely unstable layer, thus triggering convection over the western slope. These findings underpin our conviction that employing hectometer-scale numerical models provides a great added value for advancing the understanding of convection initiation in complex terrain areas. Significance Statement The mountain-triggered convection can impact downstream areas, frequently causing severe weather events. It is therefore of great significance to gain an in-depth understanding of initiation mechanisms of orographic convection by utilizing high-resolution numerical modeling. The kilometer-scale and hectometer-scale simulations are employed to investigate the initiation process of nocturnal convection over Cang Mountain, a steeply sloping terrain typical of the Hengduan Mountain region. Compared to kilometer-scale simulations, the hectometer-scale simulation more effectively reproduces the spatiotemporal pattern of nocturnal mountain-triggered convection, aligning well with observations. Results further reveal that the local airflow patterns and thermal conditions, influenced by the fine-scale terrain features, modulate the initiation location and concrete convection structure over Cang Mountain. This study highlights the added value of hectometer-scale numerical models in simulating isolated convection in complex terrain areas, which may help improve the forecasting of convection-related severe weather.