Two sets of three-dimensional simulations of a springtime Arctic boundary layer cloud observed during the FIRE/SHEBA 1998 spring IOP were conducted to study the influence of entrainment of cloud condensation nuclei (CCN) at cloud top on cloud microphysical and dynamical structure, radiative properties, and cloud evolution. The model is a large-eddy version of the Regional Atmospheric Modeling System (RAMS) with explicit representation of the CCN spectrum and cloud droplet spectrum. The initial CCN concentration is a constant value of 30 cm-3 in the control run, while it varies from 30 cm-3 below cloud base to a peak of 250 cm-3 at the inversion in the sensitivity run. Results from the sensitivity run show that droplet concentrations increase about twofold, effective radii decrease by 9--15% from cloud top to cloud base, liquid water content increases about 21%, and no drizzle reaches the ground in comparison with results from the control run. The dynamic response becomes significant by the end of the 5 hour simulation, as reflected in more vigorous eddies in the sensitivity run. The response of the cloud optical properties to entrainment occurs from the beginning of the simulations. Cloud albedo increases 12%, while cloud optical depth increases 33%. These results are consistent with both observations and modeling studies. It is stressed that knowledge of boundary layer deepening is critical to prediction of cloud optical properties, both from the thermodynamical perspective, because the properties of the entrained air affect bulk cloud features such as liquid water path, and from the microphysical perspective because aerosol gradients across the top of the boundary layer can alter microphysical properties and, in turn, cloud optical properties. ¿ 2001 American Geophysical Union