Numerical nowcasting experiments for the simulation of a mesoscale convective system using a cloud model and radar data assimilation with 4DVar

The paper focuses on numerical nowcasting experiments for the simulation of a mesoscale convective system occurred in the Beijing-Tianjin-Hebei plain on 4 July 2013. A three-dimensional numerical cloud model is applied in this study. The rapid-refresh radar data of 6 CINRAD radar observations combined with regional automatic weather stations are assimilated with 4DVar to provide initial condition for model simulation. Results indicate that convective-scale numerical nowcasting has been improved greatly when radar observations are considered fully; however, several problems are also detected. It is found that: 1) the model is able to capture the characteristics of the organization and evolution of the moving storm system, and well forecast the moving direction and scale of the echo band, but cannot realistically simulate the strength and location of strong echoes; 2) the model can reproduce small-scale disturbances in the storm system, and provides reasonable forecast of changes in cold pool and outflow boundary (gust fronts); 3) the model has a great advantage of forecasting the heavy rain center and rain-belt location, and can well predict the distribution and total rainfall related to weak rain belts, but the area of heavy rain is overestimated; 4) the model forecast is more accurate for convective rainfall, and the forecast bias for areas of precipitation within 0.5-10 mm is reasonable , but areas of precipitation above 10 mm is overestimated. Besides, for a weak rain storm, the model performs better in weak echo forecast than in strong echo forecast; 5) large biases are found in the forecast of Piedmont wind, rainfall, and the development and evolution of the storm system due to the poor representation of the complex terrain in Beijing-Tianjin-Hebei region in the three-dimensional numerical cloud model.