Marine boundary layer clouds and their interaction with the large-scale atmospheric circulation: an idealized-GCM study

Cloud feedback is one of the central uncertainties in climate modeling, and the short-wave radiative feedback of marine boundary layer (MBL) clouds is the most significant contributor to these uncertainties. We have incorporated a physically motivated eddy diffusivity/mass flux (EDMF) closure for convective turbulence coupled to a probabilistic cloud scheme in an idealized aquaplanet GCM to develop improved turbulence and cloud parameterizations and to study and constrain the physical mechanisms giving rise to cloud feedbacks.

Subtropical MBL clouds are observed in simulations with the idealized GCM. We study their response to changes in the longwave optical depth and in the ocean heat flux to determine the sign and magnitude of MBL cloud feedback over a wide range of climates. We compare our results with previous modeling and observational studies, such as the reported dependence of the MBL cloud fractions on the lower troposphere stability. We discuss the mechanisms underlying the MBL cloud feedback and the degree to which they can be expected to be robust, as well as the interaction of MBL processes with the large-scale atmospheric circulation.