The effects of cloud horizontal inhomogeneity on solar radiative transfer have been studied extensively but almost all studies have only considered variability of cloud liquid water content (LWC) in conjunction with constant cloud droplet size distribution or effective radius (r_e). Aircraft observations indicate, however, that r_e varies much in space and is often positively correlated with LWC. The present work addresses the relative roles of variations in LWC and r_e for stratiform liquid phase clouds. Simulations are based on idealized conditions and data from in-situ observations collected in the Radiation, Aerosol, and Cloud Experiment (RACE) that took place over the Bay of Fundy during August-September 1995. A broadband Monte Carlo solar radiative transfer scheme is used that accounts for atmospheric attenuation by cloud droplets, aerosols, gases, and Rayleigh scattering.

Results indicate that the dominant impact of horizontal variability comes generally from variations in cloud water content (or path), which reduces cloud albedo and absorptance relative to a corresponding horizontally homogeneous cloud. However, when r_e and LWC are positively correlated, variations in r_e counteract these effects mainly because variations in extinction are less than when r_e is assumed to be constant. For example, for a 80 km aircraft transect in marine stratus observed during RACE on 16 Aug 1995, variations in r_e reduce the effect of cloud horizontal variability by about 40%. For a solar zenith angle of 45 degrees, this corresponds to about an 8 Wm^-2 difference in top-of-atmosphere and surface cloud radiative forcing. It is concluded that the radiative effects of horizontal variability in cloud microphysical parameters such as r_e deserve to be considered seriously.