NUMERICAL SIMULATIONS OF THE MARINE STRATOCUMULUS-CAPPED BOUNDARY-LAYER AND ITS DIURNAL-VARIATION

A high-resolution one-dimensional version of a second-order turbulence radiative-convective model, developed at Los Alamos National Laborato ry, is used to simulate the diurnal cycle of the marine stratocumulus cloud-capped boundary layer. The fidelity of the model to the underlyi ng physics is assessed by comparing the model simulation to data taken at San Nicolas Island during the intensive field observation (IFO) of the First International Satellite Cloud Climatology Project (ISCCP) R egional Experiment (FIRE), conducted during June and July 1987. The mo del is able to reproduce the observed diurnal cycle of the liquid wate r content, cloud-base height, radiative heating or cooling rates, and the mean and turbulence variables fairly well. The mechanisms that cau se the diurnal variation and the decoupling of the boundary layer are examined. The possible role of an imposed diurnal cycle for the subsid ence in inducing the cloud-top diurnal cycle observed during the FIRE IFO is also addressed. Three regimes of subsidence influence are ident ified for the stratocumulus-capped boundary layer. Regimes I and III a re characterized by vertical propagation of the inversion height and e rratic fluctuation of turbulence in the region of the inversion. Regim e II is characterized by a continuum of quasi-equilibrium states that can exist for a range of subsidence values. In this regime, the bounda ry layer height is fairly insensitive to changes in the subsidence. Th e boundary layer behavior implied for these regimes is used to explore the effect of a diurnally Varying subsidence rate on the diurnal cycl e for the cloud-top height.