This study assesses the impact of introducing passive microwave data into traditional retrievals of droplet effective radius and optical depth from visible and near-IR reflectances. An optimal estimation retrieval approach is used to help quantify this impact. The retrievals are restricted to marine water clouds and involve the simultaneous estimation of droplet effective radius, visible cloud optical depth, cloud top temperature, total precipitable water, liquid water path, and near-surface wind speed (or surface emissivity). The retrieval method is being developed for the Global Imager (GLI) and Advanced Microwave Scanning Radiometer (AMSR) soon to be launched on the Advanced Earth Observing Satellite (ADEOS)-II. The method uses the GLI’s channel 13 (0.68 microns), channel 30 (3.72 microns), and channel 35 (10.8 microns) and the AMSR 10.65 GHz, 23.8 GHz, and 89 GHz channels. As an early test of the retrievals, we use coincident MODIS and TRMM Microwave Imager data at similar wavelengths as replacements for the GLI and AMSR. A suitable case of extensive stratocumulus was found off the west coast of South Africa on March 22, 2000 at 1016 UTC. Results will be presented at the conference.