Simulations of marine stratocumulus using a new microphysical parameterization scheme

A new microphysical scheme that uses lognormal basis functions to represent cloud and drizzle drop spectra is presented. The scheme is incorporated in a two-dimensional (2-D) eddy-resolving model and applied to the simulation of a stratocumulus cloud deck based on a sounding from the Atlantic Stratocumulus Transition Experiment. Firstly, the philosophy behind the design of the scheme is discussed with emphasis on special problems that arise when simulating stratocumulus clouds. These include (but are not limited to) simulation of collection at low liquid water contents (a few tenths of g m−3), and the importance of correctly simulating drop sedimentation in weakly convective clouds. The scheme is then applied to two cases; one exhibiting weak precipitation (<1 mm d−1 at the surface) and the other, a more strongly precipitating case (1–3 mm d−1 at the surface). Results show that the scheme captures the main features of a precipitating stratocumulus system when compared with the same model using a detailed bin microphysical scheme. The new scheme uses less than a fourth of the time required by the bin microphysical model and thus enables one to perform 2-D and 3-D simulations of the cloudy boundary layer over much larger domains, or much longer timescales than was previously possible.