NUMERICAL SIMULATIONS OF OBSERVED ARCTIC STRATUS CLOUDS USING A 2ND-ORDER TURBULENCE CLOSURE-MODEL

A high-resolution one-dimensional version of a second-order turbulence closure radiative-convective model, developed at Los Alamos National Laboratory, is used to simulate the interactions among turbulence, rad iation, and bulk cloud parameters in stratiform clouds observed during the Arctic Stratus Experiment conducted during June 1980 over the Bea ufort Sea. The fidelity of the model to the underlying physics is asse ssed by comparing the modeled evolution of the cloud-capped boundary l ayer against data reported for two particular days of observations. Ov er the period encompassed by these observations, the boundary layer ev olved from a well-mixed cloud-capped boundary layer overlying a stable cloudy surface layer to a shallower well-mixed boundary layer with a single upper cloud deck and a clear, diminished, stable surface layer. The model was able to reproduce the observed profiles of the liquid w ater content, cloud-base height, radiative heating rates, and the mean and turbulence variables over the period of observation fairly well. The formation and eventual dissipation of the surface cloud feature ov er the period of the simulation was found to be caused by the formatio n of a stable surface layer as the modeled air mass moved over the rel atively cold Beaufort Sea region. Condensation occurred as heat in the surface layer was transported downward toward the sea surface. Eventu al dissipation of the surface cloud layer resulted from the transport of moisture in the surface layer downward toward the sea surface. The results show that the subsidence was the major influence on the evolut ion of the cloud-top height but was not a major factor for dissipation of either cloud layer during the simulation.