Analysis of the deviation of precipitation forecast of ECMWF model over the Yangtze-Huaihe River Valley in summer 2020

Based on CMPAS precipitation products and ERA5 reanalysis datasets, the performance of the ECMWF global high-resolution products (ECMWF) have been analyzed during the typical Meiyu (from 10th June to 20th July) period in 2020. Using the method of object-based diagnostic evaluation (MODE), the performance of the ECMWF in simulating the spatial characteristics of heavy precipitiation, such as the centroid latitude, the centroid longitude, the rainfall area, the major axis length, width and axis angle of precipitation, have been comprehensively revealed. The results show that the ECMWF tends to have a northward and westward bias in reproducing the Meiyu rain belt, in terms of the daily precipitation. Regarding the spatial morphology characteristics of the rain belt, the ECMWF has a larger rainfall area and has a steeper slope than that in the observation. The diurnal variation of precipitation within the rain belt peaks around early-morning in the observation, and the ECMWF can well forecast the diurnal variation of precipitation. In addition, the results also show that the frequency of the northward bias in forecasting the rain belt peaks in the evening to midnight and afternoon, while the bias of the southward mainly occurs in the morning. The low-level jet exhibits similar diurnal variations but with a 3 hours earlier peak than the observed precipitation. The peak time of low-level jet in the ECMWF forecast is 3 hours earlier than that in observed. Further analysis reveals that the zonal location of the rain belt is highly correlated with the intensity of low-level jet. The rain belt moves further northward when the low-level jet is strengthened, and visa-versa. Finally, two heavy rainfall events were chosen to represent the typical northward bias and relatively accurate forecast of the zonal position of the rain belt. The results show that the northward bias of the rain belt results from the stronger low-level jet 3 hours ahead in the ECMWF forecast.