Assimilating Himawari-8 All-sky Infrared Radiances to Improve Convective Predictability

The new generation geostationary satellite Himawari-8 has begun its full operations in July 2015. The Advanced Himawari Imager (AHI) onboard Himawari-8 provides visible and infrared observations with finer spatial, spectral, and temporal resolution compared with the previous generation geostationary satellites such as MTSAT-2/Imager. In this study, we examined the potential of assimilating Himawari-8 all-sky infrared radiances  to improve the analysis and prediction of the real-case sudden local severe rainfall events. In addition to the conventional observations, Himawari-8 infrared radiances were assimilated into the Japan Meteorological Agency Non-Hydrostatic atmospheric Model (JMA-NHM) at 10-km horizontal grid spacing using the Local Ensemble Transform Kalman Filter (LETKF) and the Radiative Transfer for TOVS (RTTOV). We performed a series of single observation experiments and investigated the sensitivities of the spectral and temporal resolution to the analyses of model state variables. In addition, we demonstrated the effect of inter-channel observation error correlations. We also performed an experiment in a real-world convective weather case. The results showed that assimilating Himawari-8 all-sky infrared radiances improved the representation of severe rainfall. By increasing the number of spectral bands sensitive to vapor from 1 to 3 and the temporal frequency from 30 to 10 minutes, we could improve the analysis although it is necessary to adequately consider the inter-channel observation error correlations. This improvements of the spectral and temporal resolution correspond to the upgrade of the JMA’s geostationary satellite from MTSAT2/Imager to Himawari-8/AHI.