Convection-permitting ensemble forecasts of a double-rainbelt event in South China during the pre-summer rainy season

Abstract

Heavy rainfall occurred at the inland frontal zone and coastal warm sector in South China on 12–13 June 2019. Three convection-permitting ensemble forecast (CPEF) experiments with perturbed initial conditions (ICs) and lateral boundary conditions (BCs) as well as model physical schemes have been conducted to predict the double-rainbelt process. This study evaluates the predictability of different CPEF experiments and analyzes the causes of precipitation forecast errors by identifying the sensitive factors for frontal rainfall (FR) and warm-sector rainfall (WR). In this double-rainbelt event, the forecast skill of FR is significantly better than that of WR. The experiment with perturbed ICs/BCs combining suitable model physical schemes performs best. Based on the optimal ensemble experiment, the ensemble sensitivity analysis has been conducted for the two rainbelts. FR is sensitive to the synoptic forcing and its sensitive area mainly locates near the frontal system, while the coastal boundary layer jet and water vapor from the South China Sea play an important role in the WR process. The ICs and BCs greatly influence the location of FR through varying the movement direction and velocity of the fronts. The physical schemes have a great impact on convection triggering along the coasts, thus affecting the occurrence of the whole WR process, of which the choice of microphysical scheme is critical.