Grapes Mesoscale Model System Rainstorm Simulation Error Diagnosis

In this paper, the numerical simulation errors of the non-hydrostatic version GRAPES-Meso (Mesoscaleof the Global and Regional Assimilation and Prediction System) at the resolution of 0.18o to the torrentialrain case, which happened in May 31st to June 1st of 2005 over Hunan province, are diagnosed andinvestigated by using the radiosondes, intensive surface observation and the operational global analysis data,and the sensitivity experimental results as well.It is shown that the GRAPES-Meso could reproduce quite well the main features of the large-scalecirculations, especially the 500hpa circulation patterns and their evolutions, while the distribution of theaccumulated 24h precipitation and the key locations of the torrential rainfall are captured reasonably well bythe model. Seeing from the point of the view of synoptic scale, the model could provide valuable numericalguidance for short-range weather forecasting.However, errors exist in the simulation on the mesoscale features of the torrential rain and details of therelevant systems, for example, the inveracious rainfall in the earlier model integration stage and theremarkable underprediction of the peak value of rainfall rates over the heaviest rainfall region, the weaknessof the upper jet simulation and the overpreprediction of the south-west wind in the lower troposphere etc.The investigation reveals that the sources of the simulation errors are different. The inveracious modelrainfall in the earlier integration stage over the heaviest rainfall region is induced by the model initialcondition errors of the wind field at about 925hPa over the torrential rainfall region, where the errors growrapidly and spread upward to about 600hPa level within the few hours into the integration and result in theabnormal convergence of the wind and moisture, then the irreal rainfall over that region. Whereas the largebias on the simulated rainfall intensity over the heaviest rainfall region could be imputed to the combinedfactors, (1) the simulation errors on the strength and detailed structures of the upper-level jet core whichbring on the significant underpredictions of the dynamic conditions (upper-level divergence and the upwardmotion as well) for heavy rainfall due to the unfavorable mesoscale vertical coupling between the strongupper-level divergence and lower-level convergence;and (2) the inefficient coupling of the cumulousparameterization scheme and the explicit moisture in the integration, which causes the failure of the explicitmoisture scheme on generating grid-scale rainfall in a certain extent through the inadequate convectiveadjustment of the CPS to the grid-scale. In addition, the interaction of the combined two factors could form anegative feedback to the rainfall intensity simulation, and eventually lead to the obvious underprediction ofthe rainfall rate.