4.4      Improving forecasts of the record-breaking Guangzhou “57” rainstorm by assimilating every 10-min AHI radiances with WRF 4DVAR.

Wu, Yali, Zhiquan Liu, National Center for Atmospheric Research, and Deqin Li, Institute of Atmospheric Environment, Shenyang, China

On 6 May 2017, a record-breaking warm-sector torrential rainfall (WSTR) event initiated over Guangzhou city around 1600 UTC and lasted for 20 hours. To replicate this rainfall event, both three-dimensional and four-dimensional variational (3DVAR/4DVAR) data assimilation strategies were used to initialize the convection-allowing Weather Research and Forecasting (WRF) model in an attempt to evaluate precipitation forecasts with or without the assimilation of Advanced Himawari Imager (AHI) radiances from three water vapor (WV) channels. Case study yielded subjective impressions that the 4DVAR experiments prominently improved convection initiation (CI) forecasts, while extra every 10-min AHI radiance data assimilation (DA) greatly improved the second-stage convection evolution and precipitation forecasts. Both 4DVAR experiments with and without the assimilation of AHI radiances replicated the observations accurately in predicting domain precipitation totals, but changes in precipitation location were better reflected by the former one. Objective verification showed that 4DVAR technique combined with AHI radiances improved the fractional skill scores (FSS) of 20-h precipitation forecasts, which were 2%–4.5%, 1%–3%, 6%–20%, and 8%–24% higher than those only assimilated conventional observations for 5 mm, 20 mm, 50 mm, and 80 mm thresholds, respectively. Reasons for these improvements were attributed to better temperature, moisture and wind analyses and forecasts. This study may have some significance in improving forecasts of typical WSTR events in the coastal areas of South China.