Analysis of Triggering Mechanisms for Localized Nocturnal Convection Over a Complex Mountainous Region in South China and Sources of Forecast Errors in the Convection-Permitting Model

The concave mountainous area of Pearl River Delta is a summer rainfall hotspot along the South China coast due to the presence of warm-moist monsoon flow and complex orography. This study evaluated the performance of a convection-permitting Weather Research and Forecasting (WRF) model in forecasting nocturnal rainfall in this area, focusing on days with low level southwesterly winds during the summers of 2013–2015. Results showed that the nocturnal rainfall exhibited two centers, one located along the large-scale northern mountains and the other along the small-scale Huadu Hill. WRF demonstrated superior performance in predicting rainfall over the northern mountainous region. In contrast, WRF significantly underestimated nocturnal rainfall both near local Huadu Hill and in the foothill area of northern mountains, which were strongly influenced by local forcings. Using high-resolution analyses from Variational Doppler Radar Analysis System (VDRAS), which assimilated both Doppler radar and Automatic Weather Stations observations, we firstly investigated the mesoscale mechanism governing the convection initiation (CI) of a typical localized nocturnal convection. Results showed that the enhanced prevailing low-level southerly winds, combined with local circulation induced by the urban heat island effect and orographic forcings, led to the formation of low-level convergence and strong updrafts before CI. Subsequently, we identified the sources of forecast errors in triggering CI. Results revealed that WRF severely underestimated thermal contrast between the Guangdong-Hong Kong-Macao Greater Bay Area urban agglomeration and the concave mountains, leading to the absence of the northeastern/northern inflows toward the cities. Consequently, low level convergence and updrafts near the CI position were too weak to lift air parcels above the severely overestimated level of free convection, thereby failing to trigger the convection.VDRAS-based sensitivity experiments, with a specific focus on assimilating surface temperature, validated the crucial role of urban-mountain thermal contrast on local winds that triggered the nocturnal convection.

This study underscores the significance of urban-mountain thermal contrast and local circulations in determining nighttime precipitation formation and prediction, particularly in geographically complex regions characterized by concave mountains and urban agglomerations. The findings highlight the need for improved representation of these local forcing mechanisms in numerical weather prediction models to enhance their accuracy in forecasting nocturnal rainfall events.