Numerical simulation and analysis of the modulation effect of sub-grid turbulent orographic form drag on warm-sector heavy rainfall in South China

Abstract

Numerical models frequently cannot accurately predict warm-sector heavy rainfall (WSHR) in South China, which presents a challenge in forecasting severe weather events in the region. Considering the substantial impact of complex orography on the forecasting of WSHR in South China, to improve the accuracy of numerical models in predicting WSHR, this study utilized the non-hydrostatic mesoscale numerical model Weather Research Forecast (WRF) to simulate a WSHR event in the Pearl River Delta from 12:00 UTC on May 9, 2022, to 12:00 UTC on May 11, 2022. The modulation effect of Turbulent Orographic Form Drag (TOFD) on the prediction accuracy of the WSHR was investigated through sensitivity tests. The simulations suggest that TOFD improved the forecasting accuracy for WSHR in South China. TOFD significantly impacted the intensity and location of WSHR in the Pearl River Delta region. After incorporating TOFD, the forecast accuracy of WSHR improved in some regions (such as Guangzhou). Specifically, in the Pearl River Delta region, the TS score for 6-h heavy precipitation (>100 mm) increases by 91.12 %. The precipitation center shifts eastward, and the area affected by WSHR expands. Furthermore, the incorporation of TOFD in the simulations resulted in a delay of the WSHR onset time by 1–2 h and an extension of its duration by 1 h. Both these improvements brought the model results closer to actual observations. Additionally, with the inclusion of TOFD, the weakening of southerly winds has led to enhanced wind field convergence and stronger moisture convergence, resulting in increased moisture. In warm and moist atmospheric environments, there was an extended period of energy accumulation, resulting in a thicker mixed layer, increased negative buoyancy, and intensified upward airflow. As the system continued to move eastward, incorporating TOFD resulted in a further eastward positioning of the WSHR. Additionally, the intensity of the WSHR was stronger and the duration of intense precipitation was longer. The study highlights the critical role of TOFD in the realistic representation of WSHR by numerical models for South China.