Forecast evaluation of a heavy precipitation by CMA-GFS and its diagnostic improvement
A large-scale heavy precipitation occurred in Jianghuai-South China from June 6 to 10,2022,causing severe floods and waterlogs.The precipitation mainly appeared near the shear line at 850 hPa.When the shear line moved to South China and coincided with the southwest warm and humid air,it resulted in a rainstorm in the region.In this paper,the performance of the China Meteorologi-cal Administration Global Forecast System(CMA-GFS)on this process is evaluated using ERA5 re-analysis and merging precipitation products.The diagnosis of forecast deviations and sensitivity experi-ments are also conducted.The results show that the movement of the South Asia high ridge,range of subtropical high,and the movement of the shear line at 850 hPa forecast by CMA-GFS that affect the location and shift of the rain belt are consistent with the results of ERA5 reanalysis.Therefore,the CMA-GFS performed well on the location and shift of this rain belt.However,the forecast has some deficiencies including that the precipitation along the shear line is weak,scattered heavy rainstorms in eastern Fujian,Northern Guangdong and central Guangxi are missed,the rain belt in northern Guangdong is located northward,and the magnitude of precipitation in warm sector is under estimated.The deviation diagnosis reflects that the wind speed forecast on the north side of the shear line is weaker by 3～8 m/s,and the water vapor convergence over the central and northern parts of Guangxi and Guangdong is weaker by 2×10-5～5×10-5 g/(m2·Pa·s),resulting in the weak precipitation on the shear line.The wind forecast on the south side is stronger by 3～5 m/s,resulting in the rain belt in northern Guangdong located northward by approximately 60 km.The jet over the South China coast at 850 hPa is weaker by 4～6 m/s,and the water vapor fluxes in the warm area are less by 4～10 g/(cm·hPa·s),resulting in the less rainstorms in the warm sector.Some numerical experiments are carried out to modify these deviations forecast by CMA-GFS.Using the analysis of the National Center for Environmental Predic-tion Global Forecast System to initialize the CMA-GFS significantly improves the forecast of rain belt location.Using the WSM6 microphysics scheme and tightening the trigger in the cumulus convec-tive parameterization scheme improve the forecast of centers and magnitude of precipitation on the shear line,and the magnitude of the precipitation in the warm sector increases from small and moderate to moderate and heavy rain.
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