极端天气预报指数和集合异常预报法在浙江台风和梅雨暴雨预报中的应用

利用2016—2021年ECMWF集合预报,评估了极端天气指数EFI(Extreme Forecast Index)、尾偏移指数SOT(“Shift of Tail”index)以及集合异常预报法在浙江台风和梅雨暴雨预报中的应用效果。通过研究极端天气预报指数对浙江台风和梅雨暴雨的最优预报阈值,发现梅雨暴雨阈值比台风暴雨明显偏小,且随预报时效增加减小速度偏慢。最优阈值预报相比确定性模式预报,在台风和梅雨暴雨预报检验中体现出更好的稳定性、提前性和准确性。进一步研究发现,通过区分天气类型确定的最优预报阈值,可作为台风和梅雨暴雨落区预报的参考依据。925 hPa水汽通量散度的集合异常度对浙江台风暴雨的落区和强度变化有较好的预报效果,850 hPa涡度和700 hPa垂直速度的集合异常度可以反映稳定性梅雨暴雨的过程演变。

Application of precipitation extreme forecast index and ensemble anomaly forecasting approach on typhoon and Meiyu rainstorm forecast in Zhejiang Province

The precipitation Extreme Forecast Index (EFI), "Shift of Tail" index (SOT) and ensemble anomaly forecasting approach were evaluated for typhoon and Meiyu rainstorms in Zhejiang Province during 2016-2021, based on European Center for Medium-range Weather Forecasting Ensemble Prediction System (ECMWF-EPS). The optimal thresholds of extreme forecast indexes for typhoon and Meiyu rainstorms were obtained. The thresholds for Meiyu rainstorm are significantly smaller and decrease more slowly than those for typhoon rainstorm. The optimal thresholds forecast show better stability, timeliness and accuracy, compared with a single model forecast. Further research found that the optimal forecast thresholds determined by different weather types can be used as good reference for typhoon and Meiyu rainstorms forecast. Ensemble anomaly forecasting approach was applied to diagnose the physical quantities for better understanding of the development of rainstorm. The standardized anomaly of water vapor flux convergence at 925 hPa indicates the variation of typhoon rainstorm, while the standardized anomalies of vorticity at 850 hPa and vertical velocity at 700 hPa reveal the development of stable Meiyu rainstorm.