Study On ETKF Initial Condition Perturbation In Storm-allowing Ensemble Forecast

In order to improve the low-spread of Ensemble Transform Kalman Filter(ETKF) in initial perturbation, we consider the introduction of physical uncertainty to analyze the difference of the effect of two initial perturbation methods and test the causes of the difference, with a view to better understanding of the improvement mechanism. The sensitivity difference of the storm-scale in rainstorm to initial perturbation errors was studied. The relative degree of the two ETKF initial perturbation schemes was tested. The prediction ability of the different schemes to the spatial and temporal uncertainties and the contributions of the different scale perturbations to the prediction errors were analyzed.The WRF model was used to construct an updated forecast system. One extreme heavy precipitation which happened in May, 2014 was simulated and it was made ensemble forecasting. Two initial perturbation schemes were designed by using ETKF method. One was a single physical scheme called mono; the other was on the basis of scheme one called multi, adding the multi-physics process in the analyzing perturbation, and the same setting is adopted after the beginning of the forecasting process. The conclusions are written as follows,(1) Compared with mono, the spread of multi-scheme is obviously larger. It is obvious that the experiments with multi-physical perturbation scheme can improve the results, and provide more forecast information including forecast uncertainty to user decision. In the analysis of spread, the improvement effect of multi-scheme in strong convection area is better than that in whole area, and the ratio of spread to root-mean-square error of each variable is equal, and the rationality of ensemble forecasting system is explained.(2) The results show that the multi-physics program shows better performance ability in precipitation forecast score.(3) The multi-scheme with multi-physical perturbation in the analysis cycle is much better than mono-ETKF scheme in all aspects.Up till now the optimal design of storm-allowing ensemble forecast systems remains widely unknown in the world, this study has important scientific significance not only for ETKF initial perturbation schemes in the storm-allowing ensemble forecast systems,but also providing the basis for operational application.