In earlier work a Δ2-stream radiative transfer model was used in conjunction with ground-based measurements to retrieve the microphysical and shortwave radiative properties of boundary layer stratus. The effective radii output by the retrieval scheme for optically thick clouds have now been parameterized as a function of the cloud liquid water path, the transmission ratio, and the cosine of the solar zenith angle. The parameterization enables estimation of stratus cloud microphysical properties using ground-based measurements that are readily available at a number of locations. To validate the retrieved and parameterized cloud microphysics, ~5 hours of data were analyzed and compared to collocated in situ measurements made by a Forward Scattering Spectrometer Probe. On average, the retrieved values of the cloud droplet effective radius and the cloud droplet number concentration differed from the corresponding aircraft measurements by 7% and 27%, respectively, while the parameterized values differed from the aircraft measurements by 15% and 32%, respectively. Averaging all of the data to 30-min resolution significantly reduced the differences between the aircraft data and the retrieved and parameterized results, suggesting that both the aircraft and the ground-based data are capable of characterizing the cloud microphysics over this temporal scale. The parameterizations of the stratus shortwave radiative properties were generally within 5% of Slingo's four-band, model-derived parameterization when absorption above cloud top was incorporated into the Slingo parameterization. ¿ 1998 American Geophysical Union