An ensemble-average boundary-layer model coupled with explicit aerosol and cloud microphysics is presented. When deriving the model equations, two correlation terms arise from unresolved scale perturbations in supersaturation and droplet nu mber concentration. These two terms, for the condensational growth and collision /coalescence processes, have previously been neglected in ensemble-average models. An earlier investigation, using LES results, found the collision/coalescence correlation to be negligible.

The model that is presented is based on an extensively tested and evaluated mesoscale three-dimensional model. The dynamical part of the model, including radiation and surface flux schemes, is augmented with explicit calculations of CCN and cloud droplets/drops. These calculations include condensational growth, collisi on/coaleence, and gravitational settling. The supersaturation-number concentration correlation term is parameterized using a traditional boundary-layer flux-gradient relationship. The simulations presented in this paper clearly show the impact of this previously neglected term.

The simulated supersaturation exhibits a maximum at cloud base, decreasing to zero at the cloud top, as is found when conditionally averaging over the up-drafts in a LES. Simulated cloud droplet spectra are realistic in the entire cloud; the cloud droplet number concentration is constant with height in the cloud, and the radius of the mean droplet volume increases with height, as is found in observations of marine stratocumulus.

The temperature, turbulent kinetic energy, water vapor, and cloud water mixing ratio are also well simulated by the model. The simulated maximum supersaturation is higher than in an identical simulation when the correlation term is neglected. This means that fewer CCN are activated when the term is neglected. Conclusions drawn from previous studies with ensemble-average models neglecting this term are likely to be influenced by unphysical behavior of the model.