In view of their large potential climate impacts, the role of cloud condensation nuclei (CCN) in determining the global distribution of clouds and their optical properties is of great current interest, but is not yet well understood. Insight into the nature of the steady states attainable in the marine boundary layer will be obtained by simulating the interactions between aerosols, clouds, and radiation using a simple model that describes the key components of the system in a parameterized way. The boundary- layer cloud model to be developed, implemented, and tested includes: (1) A physically based parameterization of CCN sources, sinks, and transports; (2) A parameterization of boundary-layer cloud formation and dissipation, including the effects of drizzle formation as regulated by the predicted CCN; and (3) A parameterization of radiative transfer in the cloudy boundary layer, including both solar and terrestrial radiation, and taking into account the cloud particle number density and size distribution and the cloud liquid water content. A case study from the 1992 Atlantic Stratocumulus Transition Experiment (ASTEX) will be selected and used to test the model. The proposed study will provide a foundation for future work to incorporate CCN physics into general circulation models.
