Exploring Mechanisms Governing Dissipating Coastal Stratocumulus Using Large-Eddy Simulation

Large-eddy simulation (LES) is used to explore a case of dissipating stratocumulus sampled by the CIRPAS Twin Otter the afternoon of 31 Aug 2012 as part of the UPPEF2012 field campaign. Most previous studies of marine stratocumulus have revolved around exploring the behavior of quasi-steady-state cloud systems or the diurnal cycle, but the mechanisms governing details of the cloud dissipation process have not been extensively explored. We use LES to test, via a series of sensitivity experiments, the importance of mechanisms such as shear across the boundary-layer inversion, subsidence rate, surface fluxes, and large-scale temperature and moisture tendencies. The simulations attest to the difficulties of model spin-up for a transient (time-dependent) cloud system, in particular the tendency to dissipate the cloud too quickly, and we evaluate a number of different spin-up approaches to ameliorate these problems. A mixed-layer model budget is calculated from the LES simulation output to quantify the various cloud dissipation mechanisms.