The purpose of this study is to understand and model the diurnal cycle in the microphysics of boundary layer clouds that has been inferred from recent remote sensing studies. To assess the modeled diurnal variation in cloud drop size, a one-dimensional cloud microphysics model is initialized with dynamical conditions that are typical of the contrast between daytime and nighttime clouds. The model-generated microphysics show a shift in droplet sizes form day to night that is consistent with that observed in satellite retrievals. The results indicate that the diurnal cycle in droplet size results from the shift in dynamical structure associated with the cloud becoming decoupled from the subcloud boundary layer during the day, along with an associated decrease in cloud-top entrainment. The effect of decoupling on drop size is also enhanced by a decreased concentration of activated droplets at cloud base associated with the characteristically smaller vertical updraft velocities for the decoupled clouds.