Numerical Simulation And Analysis On Characteristic Of Boundary Layer Of A Torrential Rain During The Meiyu Period

Using the NCEP reanalysis data and China hourly merged precipitation analysis data combined with the WRF model, a Meiyu rainstorm occurred on the north of Jiangxi province is analyzed. The observational data analysis results show that the rainstorm occurred at a favorable weather condition and the processes happened in the boundary layer (especially the convergence in the boundary layer) is extremely important to this rainstorm. Based on the model data of high spatial and temporal resolution simulated from WRF model, we analyzed the characteristics of the boundary layer. The analysis results show that: PBL (planetary boundary layer)height has obvious diurnal variation characteristic during the rainstorm,but it's lower than that in areas without precipitation. Under general conditions, the turbulent motion occured in the boundary layer and the intensity is mainly related to the terrain,but significant turbulent motion can be found above the boundary layer in the developmental and mature stage of the storm. The turbulent mixing process plays an important role in momentum, heat and water vapor transport in the vertical direction.The vertical turbulent momentum flux is normally downward and upward transferring momentum flux appears in the process of vertical wind shift. The sensible heat flux is mainly downward transported. While the latent heat flux is basically upward transported, the latent heat release has an important role in the developmental stage of the storm. Reynolds averaged momentum equation is also employed to analyze. The result shows that the trend of the change of momentum gradient in the boundary layer and the evolution of the rainstorm are closely related.As an unique item in the boundary layer, the turbulent item is also very important in the torrential rainfall process. In order to analyze the influence of the turbulent mixing process on the rainstorm visually, the differences of low-level jet,convergence zone and water vapor condition between two different simulated results which are from models whether the turbulent exchange exists are discussed. The result shows that the turbulent exchange process can change the wind speed in the low layer, then causing the changes of the strength of low-level jet and the position of convergence zone and the water vapor condition, which affect the torrential rainfall process.