Impacts of Increasing Model Resolutions and Shortening Forecast Lead Times on QPFs in South China During the Rainy Season.

This study investigated the impacts of increasing model resolutions and shortening forecast lead times on the quantitative precipitation forecast (QPF) for heavy-rainfall events over south China during the rainy seasons in 2013- 2020. The control experiment, where the analysis-forecast cycles run with model resolutions of about 3 km, was compared to a lower-resolution experiment with model resolutions of about 9 km, and a longer-term experiment activated 12 hours earlier. Rainfall forecasting in the presummer rainy season was significantly improved by improving model resolutions, with more improvements in cases with stronger synoptic-scale forcings. This is partially attributed to the improved initial conditions (ICs) and subsequent forecasts for low-level jets (LLJs). Forecasts of heavy rainfall induced by landfalling tropical cyclones (TCs) benefited from increasing model resolutions in the first 6 hours. Forecast improvements in rainfall due to shortening forecast lead times were more significant at earlier (1-6 h) and later (7-12 h) lead times for cases with stronger and weaker synoptic-scale forcings, respectively, due to the area- and case-dependent improvements in ICs for nonprecipitation variables. Specifically, significant improvements mainly presented over the northern South China Sea for low-level onshore wind of weak-forcing cases but over south China for LLJs of strongforcing cases during the presummer rainy season, and over south China for all the nonprecipitation variables above the surface during the TC season. However, some disadvantages of higher-resolution and shorter-term forecasts in QPFs highlight the importance of developing ensemble forecasting with proper IC perturbations, which include the complementary advantages of lower-resolution and longer-term forecasts. [ABSTRACT FROM AUTHOR]