A numerical modeling study of the effects of variations in aerosol concentrations on stratiform clouds in the marine boundary layer

Marine stratiform clouds play an important role in the global radiative heat budget of the Earth because they overlie about a third of the oceans and they reflect much more sunlight than the ocean surface. The reflectivity of these clouds depends in part on the concentrations of cloud condensation nuclei (CCN) on which cloud droplets form. Here a numerical model is used to investigate interactions between aerosol and cloud microphysics, radiative transfer, and turbulent mixing in the stratiform cloud-topped marine boundary layer.;Results from model simulations are found to be in general agreement with airborne measurements of marine stratocumulus clouds. However, the model underpredicts the concentrations of small cloud droplets in the lower region of the cloud layer. This is consistent with the lack of a peak supersaturation near cloud base in the model results, which is attributable to horizontal averaging in the model.;The model simulations indicate that equilibrium CCN concentrations are sensitive to their formation rate. The times required to reach equilibrium were found to increase with increasing CCN concentration, suggesting that cloud layers can maintain high CCN concentrations long after the supply of CCN is reduced.;The results of the model show cloud albedo to be more sensitive to cloud droplet concentrations than under the assumptions that cloud water is fixed and unactivated haze particles are ignored. Increased droplet concentrations generally (but not always) produce increased cloud water due to reduced drizzle. The number of haze particles increases with droplet concentrations due to decreased peak supersaturations in the cloud.;The model simulations show that when droplet collisions reduce droplet concentrations to extremely low values, a cloud layer can become so optically thin that cloud-top radiative cooling is unable to drive vertical mixing. The stratocumulus-topped marine boundary layer can then collapse to a shallow fog layer over the course of a day or more.;The model was also used to investigate long-lived, linear regions of enhanced cloud reflectivity that appear in satellite imagery downwind of ships. We have found that injections of CCN, which are present in ship exhaust, can account for many of the observed properties of ship tracks. Ship tracks are classified as Type 1, which are observed in visible satellite imagery, and Type 2, which are more common and are observed in near-infrared imagery. The distinction between the two types is attributed to differences in ambient concentrations of CCN that cause variations in turbulent mixing in the marine boundary layer, through the effect of cloud droplet concentrations on cloud-top longwave radiative cooling.