Impacts of radiance data assimilation on the Beijing 7.21 heavy rainfall

• AMSU (AMSU-A and AMSU-B) radiance data assimilation in the WRF model simulation improves the spatial distribution and intensity of precipitation in Beijing 7.21 heavy rainfall event of 2012.
• Data assimilation of AMSU-A only has similar impacts on simulated heavy rainfall pattern and intensity as the AMSU data assimilation run.
• Data assimilation of AMSU-B only improved the spatial distribution of heavy rainfall, but it had little impact on the intensity.
• Results from radiance data assimilations suggest that the initial conditions of temperature and humidity play a positive impact on heavy rainfall in 24 h simulation.

Precipitation simulation during the Beijing heavy rainfall event on 21 July 2012 has been investigated in this study. A control run of the Weather Research and Forecasting (WRF) model can roughly simulate the mesoscale features and spatial heterogeneity of precipitation over North China, but it cannot capture the locations, intensity and timing of maximum rainfall over the Beijing area compared with ground observations. To gain better initial conditions in the performance of the mesoscale model, Advanced Microwave Sounding Unit (AMSU) radiance data assimilation was carried out. The spatial distribution and intensity of precipitation have been significantly improved. In particular, data assimilation of Advanced Microwave Sounding Unit-A only has similar impacts on heavy rainfall simulation as the AMSU data assimilation run. Data assimilation of Advanced Microwave Sounding Unit-B only has improved the spatial distribution of heavy rainfall, but it has little impact on the intensity of rainfall. However, the timing of heavy rainfall in all simulations is about 5 to 6 h later than observations. Further analysis shows that the center and strength of the low-pressure system was not well simulated during the early development of heavy rainfall in the model. Results from radiance data assimilation suggest that improvement of the initial conditions plays a positive impact on heavy rainfall in 24 h simulation. Operational forecast system including satellite radiance data assimilation may improve heavy rainfall forecasting.