The study of Fengyun4A temperature profile data assimilation in a southwest vortex heavy rainfall case

A data assimilation and forecasting system was built to ensure operational safety along the Sichuan–Tibet Railway. Based on the system, the Fengyun4A satellite (FY-4A) temperature profile data quality control and assimilation were processed in a heavy rainfall event in Sichuan Province during June 25–27, 2020. The data quality of the FY-4A temperature profile in the clear-sky area was better than that in the cloudy area. The temperature profile assimilation after cloud detection reduced temperature biases in lower troposphere for the analysis and forecast field, further improving the 48 h precipitation forecast. Based on that, the corrected observation error of the FY-4A temperature profile in the clear-sky was obtained using sounding observations. After adopting the FY-4A temperature profile observation error to correct the original static temperature observation error, the temperature biases in the analysis and forecast field were further reduced, and the precipitation forecast was significantly improved. The southwest vortex (SWV) was the main weather system dominating the precipitation process. The analysis field after observation error correction had the adjustments of warming, humidifying and enhancing the southwesterly wind on the 700 hPa environmental field. After assimilation, the near-surface stratification tends to be unstable, and the positive vorticity at 700 hPa increases in the Sichuan Basin. With favorable dynamic and thermal conditions, the SWV intensity was enhanced and caused precipitation. In contrast, the original FY-4A temperature profile assimilation was drastic for temperature, humidity and wind field adjustments. The adjusted environmental field was not conducive to the development of the SWV and subsequent precipitation.