The sensitivity of microwave brightness temperatures at the frequencies between 89 and 190 GHz to surface emissivities and hydrometeors in a tropical deep convective cloud system is investigated by simulations, using the Goddard Cumulus Ensemble model data of a simulated oceanic tropical squall line as input for a radiative transfer model. It is found that only the window channel at 89 GHz has an apparent dependence on the surface emissivity. The three water vapor channels around 183 GHz (i.e., 183.3 ¿ 1, ¿3, and ¿7 GHz) are completely independent, and the window channel at 150 GHz is nearly independent of the surface emissivity because of the atmospheric opacity at these frequencies. All channels are apparently influenced by deep convective clouds and their outflowing thick cirrus clouds. The channels at 89, 150, and 183.3 ¿ 7 GHz are strongly sensitive to variations in the liquid water content at levels above 5 km. The sensitivity of the channel at 150 GHz to liquid water is about twice that at 183.3 ¿ 7 GHz. All channels are generally sensitive to variations in the frozen hydrometeor content at levels above 7 km. The 183.3 ¿ 1 GHz channel has virtually no influence from the frozen hydrometeors at levels below 7 km. The sensitivity suggests that it should be possible to estimate the frozen hydrometeor properties in levels above 7 km in tropical deep convective clouds using the water vapor channels around 183 GHz.