Influence of SST, surface stability and radiation on coastal stratus

The response of a stratocumulus-topped marine boundary layer (SCT MBL) to spatial and temporal forcing is investigated using idealized simulations that progressively become more complex. Irregular periods of cloud layer coupling and decoupling are found to occur even within horizontally homogeneous, one-dimensional SCT MBL simulations undergoing repetitive diurnal forcing. Prior to conducting a variety of extended simulations and sensitivity studies, results from an SCT MBL inter-comparison case involving numerous large eddy simulation models are used as a benchmark. A series of two-week long simulations are then conducted, differing only by their fixed sea surface temperature (SST) values. Cases with fixed SST reveal the sensitivity of SCT MBL decoupling to surface layer stability and the importance of longwave cloud base warming. Subsequent simulations introduce realistic temporal SST variations based upon buoy observations; a ≈3 K SST ramp-up midway through the two-week simulation is shown to be capable of dramatically altering the cloud layer dynamics. In a simulation with upward surface fluxes, the SST increase results in the re-couple of subcloud to cloud layer, while in another case initialized with weak downward surface fluxes, the increase results in a sharp reduction in integrated cloud liquid water. Finally, three-dimensional idealized simulations are performed exploring the impact of coastal orographic influences upon the cloudy MBL.