Currently both RTTOV and CRTM have been used in the WRF data assimilation (DA) system for radiance assimilation under all-sky weathers. To well know the influence of both fast radiative transfer models on radiance, GPM/GMI is assimilated in WRFDA system for one storm case occurred on July 19-21, 2016, in Beijing, China，then compare the simulated brightness temperatures to their observed equivalents in this work. The comparisons show that at GMI low frequencies(10-23 GHz) the simulated Tbs from RTTOV seems better than those from CRTM due to using emissivity atlas in RTTOV since the low frequency channels are more sensitive to the land surface. For rainy pixels over land, both simulations for GMI mid-high frequency channels are significantly affected by the cloud and precipitation. For GMI 5-9 channel, the simulated Tbs from RTTOV is more consistent with the observations than those from CRTM which is dramatically lower than both the observations and the simulated Tb from RTTOV when observed Tb is lower than 260 K. For GMI channel 10-13, both simulations are quite close but far away from the observations. In a word, for GMI 1-7 channels, simulated Tbs from RTTOV are much close to the observations, while for GMI high frequency, such as higher than 89GHz, both RTTOV and CRTM still need more improved in rainy condition.
A study on sensitivity of TMI brightness temperatures (Tbs) to the different vertical profiles of hydrometeor in precipitating cloud has been made by using a microwave radiative transfer model VDISORT. The different hydrometeor profiles are obtained from the simulations of precipitating convective cloud by means of a sophisticated meso-scale meteorological model MM5. The results show that at high frequency scattering effect of ice crystal and graupel are stronger than that of snow among the ice phase hydrometeors. When changing the ice phase water content the corresponding scattering coefficient has significant change however the extinction coefficient with obvious change is found only for changing ice crystal content. The analyses show that the Tbs at TMI high frequency calculated by the VDISORT model are very sensitive to the ice crystal concentration and much less sensitive to the snow and graupel concentrations. In addition comparison of measured and calculated TMI Tbs indicates that the MM5 might produce too much snow and less ice crystal content in the convective cloud for medium or heavy rainfall.