Impacts of Virtual Train-based Atmospheric Vertical Profile Data Assimilation on the Forecast of the “21.7” Zhengzhou Rainstorm

A record-breaking prolonged and extreme rainstorm occurred in Henan province, China on 18–23 July 2021. Almost all global and regional numerical weather prediction (NWP) models underpredicted the 24-h accumulated rainfall amount and the 1-h extreme precipitation in Zhengzhou city. To investigate the potential impacts of data assimilation (DA) of atmospheric vertical profiles based on the train-based mobile observation (MO) platform on precipitation forecast, we assimilated the virtual train-based air temperature (Ta), relative humidity (RH), U and V components of wind profile data based on the ERA5 reanalysis datasets into the Weather Research and Forecasting (WRF) model using the three-dimensional variational (3DVar) method. The results show that Ta, RH and wind speed (WS) profiles from ERA5 reanalysis datasets are reliable. RH of the analysis field is improved with the assimilation of virtual train-based moisture profiles. Additionally, the coverage and intensity of the 6-hour and 24-hour accumulated precipitation forecasts, as well as the predicted areas with maximum rainfall durations exceeding 20 hours, were closer to the measurements. In addition, TS and bias of 6-h, 12-h and 24-h accumulated precipitation forecast for heavy rain to torrential rain in Henan province improved significantly, especially for the Central and Northern regions (hereinafter referred to as region CNH). TS of 24-h accumulated precipitation forecast for 50 and 100 mm rainfall levels in Henan province increased by 0.17 and 0.18, and those in region CNH raised by 0.13 and 0.18. Water vapor increased significantly during the rainstorm period and more moisture were transported from south of Henan province to region CNH under the influence of the southwesterly wind, and the updraft in region CNH was obviously enhanced. Improvements of water vapor and upward motion ultimately enhanced the forecasts of this extreme rainstorm event.