Verification and bias analysis of multi-model forecast for the “7·22” extreme torrential rain in Gansu Province

The strongest regional rainstorm process since 1961 occurred in Hedong area of Gansu on July 22-24, 2024. The precipitation forecast performance of the global numerical model ECMWF, CMA-GFS, and regional numerical models CMA-SH9, CMA-MESO were evaluated by using the automatic weather station and ERA5 reanalysis datasets. And then deviation were analyzed from the perspective of rainstorm formation mechanism.The results are as follows. (1) The precipitation extremes of ECMWF, CMA-SH9, and CMA-MESO were better, while the magnitude and range of heavy precipitation in CMA-GFS were significantly smaller. The strong precipitation areas in each mode were biased towards the northeast compared to the actual situation. And the heavy rainfall forecast from the night of the 22nd to the early morning of the 23rd was significantly weaker, while it was significantly stronger during the day of the 23rd. (2) The heavy precipitation scores of CMA-SH9 and CMA-MESO were higher than ECMWF and CMA-GFS, especially the score of CMA-SH9 for heavy precipitation from 08:00 BT 22 to 08:00 BT 23 was significantly higher than other models. But the false alarm rate of both models were over 70%. (3) During the heavy rainfall period from the night of the 22nd to the early morning of the 23rd, CMA-SH9 model had the smallest shear position deviation at 500 hPa and formed a larger positive relative vorticity, and the ground wind field was also closest to the actual situation. Therefore, CMA-SH9 had the highest comprehensive score for heavy precipitation during this period. (4) In the downstream of the 500 hPa ridge region, there was a stronger development of a trough, accompanied by stronger cold advection and relative vorticity in each model. And there was a strengthening and stable maintenance of the 700 hPa vortexin and ground convergence in the northeast regions. Therefore the heavy rainfall during the day on the 23rd was significantly stronger and located towards northeast of the observation.