A Fast Radiative Transfer Model and its Application in MM5 4Dvar

A fast radiative transfer model- RTTOV7 is used to calculate radiance from IRAS (InfraRed Atmospheric Sounder), MWTS (MicroWave Temperature Sounder) and MWHS (MicroWave Humidity Sounder) of FY3, a next generation of the polar orbit meteorological satellite of CMA. Fast coefficients for IRAS have been computed with a transmittances database from GENLN2. Correlative validation of 43 profiles and independent validation of 34 profiles show that the STD distribution of the brightness temperature between GENLN2 and RTTOV7 are less than 1K for all channels of IRAS which means that the coefficients can be used in RTTOV7. Coefficients for MWTS and MWHS have also been calculated with transmittances database in microwave from Liebe 1989/1993. Correlative and independent validations present that the brightness temperature's STDs for all channels of MWTS and MWHS are less than 0.5K. A new 4Dvar system to assimilate radiance of ATOVS instruments has been set up by introducing RTTOV7 into MM5 4Dvar. In RTTOV7 the initial profiles in this new data assimilation system are time-dependant while those fixed in 3Dvar for ATOVS' brightness temperature computing. In one case for a rainstorm, infrared radiance of ATOVS has been assimilated into MM5 4Dvar while the impact on the amount and location of precipitation is quite slight. In another case for a Typhoon, the impacts of microwave radiance assimilation show very obvious influence on the prediction of typhoon's track.