Three-dimensional variational assimilation of Lidar extinction profiles: Application to PM_2.5 prediction in north China

The capability of assimilating aerosol optical properties (AOP) using the three-dimensional variational (3DVAR) data assimilation (DA) approach is developed for the Weather Research and Forecasting (WRF) model's data assimilation (WRFDA) system by introducing WRF-Chem-based AOP observation operator and its tangent linear and adjoint. The new AOP DA capability of WRFDA- Chem is firstly applied to assimilate Lidar extinction coefficient profiles in Beijing City from 26 February to March 5, 2019. Cycling strategy of Lidar DA is important to allow a substantial positive impact on surface PM_2.5 forecast over the whole evaluation domain (D02, 100 times area of Beijing city covered by the eight Lidar sites) for the entire 24-h forecast range. Compared to the control run without aerosol DA, the mean RMSE reduction and correlation coefficient increase of surface PM_2.5 forecast can be up to 15% for the one-day partial cycling strategy. Lidar DA exhibits a clear benefit in improving the vertical distribution of aerosols over Beijing when comparing to surface PM_2.5 DA. With the very limited coverage of Lidar data, however, adding Lidar data above surface PM_2.5 data (covering the whole D02) does not show the further improvement of surface PM_2.5 forecast over D02.