Using Convective-Scale Ensemble Simulation to Understanding the Forecast Uncertainty of Two Heavy Rainfall Cases over the Jianghuai areas

A systematic frontal rainfall (FR) and a localized warm-sector rainfall (WR) event during the warm season in the Yangtze-Huai River Basin (YHRB) in east China are selected as research subjects in this paper. Seven convective-scale ensemble forecast experiments involving the initial conditions (ICs), lateral boundary conditions (LBCs), and model (MO) perturbations have been conducted to evaluate the performance of different ensemble experiments and investigate the forecast uncertainties of the FR and WR, respectively. The results indicate that combined perturbation experiments produce greater precipitation dispersion than single perturbation experiments,and the introduction of MO perturbations can effectively modify the deviation of precipitation, especially in the WR scenario. The forecast uncertainties of FR primarily stem from the synoptic low-level jet (LLJ) and the convergence of cold and warm air masses from north and south. The three-dimensional structure of the LLJ such as intensity, location and height determines the position and strength of the FR. It is important to note that introducing MO perturbations enhances the ability of ensemble simulation to represent the uncertainty in forecasting the convergence locations of cold and warm air masses. In contrast, the forecast uncertainties of WR mainly due to boundary layer dynamics and local wind convergence near the leeward sides of mountains, and the model physics configurations is sensitive to the thermal and dynamic field of boundary layer. The appropriate MO perturbations can more effectively represent the forecast uncertainty associated with localized WR process, thereby improving the overall performance of ensemble forecasts.