Theoretical models of radiative transfer are developed to simulate the 85 GHz brightness temperature (T85) observed by the Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI) radiometer as a function of rain rate. These simulations are performed separately over regions of the convective and stratiform rain. TRMM Precipitation Radar (PR) observations are utilized to construct vertical profiles of hydrometeors in the regions. For a given rain rate, the extinction in 85 GHz due to hydrometeors above the freezing level is found to be relatively weak in the convective regions compared to that in the stratiform. The hydrometeor profile above the freezing level responsible for the weak extinction in convective regions is inferred from theoretical considerations to contain two layers: 1) a mixed (or mixed-phase) layer of 2 km thickness with mixed-phase particles, liquid drops and graupel above the freezing level, and 2) a layer of graupel extending from the top of the mixed layer to the cloud top. Strong extinction in the stratiform regions is inferred to result from slowly-falling, low-density ice aggregates (snow) above the freezing level. These theoretical results are consistent with the T85 measured by TMI, and with the rain rate deduced from PR for the convective and stratiform rain regions. On the basis of this study, the accuracy of the rain rate sensed by TMI is inferred to depend critically on the specification of the convective or stratiform nature of the rain.