The Impact of GRAPES-MESO Model different Spatial Grid Resolution on China Summer Rainfall Forecasts in July 2012

Global/Regional Assimilation Prediction System (GRAPES-MESO) V4.0 with new spatial grid resolution (SGR) has been put into formal operation use in Numerical Weather Prediction Centre (NWPC) of China Meteorologcal Adiministration (CMA). Then it became nessary to appropriately address and determine forecast uncertainty with increasing spatial grid resolution(SGR), in particular, when convective scale motions start to be resolved. Because blunt increases in the model resolution will quickly become unaffordable and may not lead to improved NWP forecasts. This paper examined the computational accurcy of vertical grid resolution (VGR) and horizontal grid resolution (HGR), as consistency requirement exists between VGR and HGR. And the results showed the refined SGR is more accurate and consistent. Then sensitivity simulations were performed to determine the impact of different SGR on the forecast skill of China summer rainfall in July 2012, when there were 8 extreme heavy rainfall cases. The results indicated that a refined SGR, while adopting the widely used NCEP Final Operational Model Global Tropospheric Analyses on 1x1 degree grids for initial and lateral boundary conditions, does not necessarily result in a consistent improvement in quantitative precipitation forecasts. Equitable threat score (ETS) and bias values actually worsened with a greater overpredicted rainfall for half of the rainfall thresholds when the SGR was refined. Averaged over extreme heavy rainfall cases, ETS values worsened for all rainfall thresholds while biases mostly increased, indicating a further overprediction of rainfall when the VGR was increased, especially when the resolution in the surface layer was increased, which could attributed to better-resolved thermal fluxes indicated by increasing anomaly correlation coefficient (ACC) of temperature and decreasing root mean square error (RMSE) of temperature in lower levels. And in other rainfall cases, the refined SGR resulted in improvements, which maybe governed mostly by thermodynamic forcing and are sensitive to vertical profiles of temperature.