Preliminary study of the use of TRMM TMI data in the Japan Meteorological Agency numerical weather prediction model

We performed a preliminary experiment of assimilating TMI total column precipitable water (TCPW) into the global numerical weather prediction (NWP) system. TCPW was not produced as a TMI standard product. Hence, we retrieved it from the brightness temperature over the ocean using the Meteorological Satellite Center algorithm, which will be the standard TCPW retrieval algorithm for AMSR on board ADEOS- II. The analysis method of the NWP system used in the experiment was 3D optimum interpolation (3D-OI), and TCPW data couldn't be assimilated directly. We therefore introduced a method to modify the water vapor field of an analysis by 3D-OI according to TCPW analysis assimilated TMI TCPW data. Using the analysis system, we performed the TMI TCPW observation system experiment. The results revealed clear positive impacts on forecasted wind fields of 850 and 250 hPa height over the tropics and small positive impacts on same levels and region. Improvements of the one-day forecasted rainfall over the tropical region were also recognized. In addition, we found a strange seasonal variability in the TCPW field of the JMA operational global analysis in 1998. We also found 15-42-day oscillation cycles in the difference between TMI and SSM/I TCPW, which we assume originates from the bias of the TMI brightness temperature and the effect that TRMM flied on a non-sun- synchronous orbit.