11th Conference on Atmospheric Radiation and the 11th Conference on Cloud Physics

Thursday, 6 June 2002: 9:15 AM
A Turbulence Closure Model With Explicit Cloud Microphysics for Stratocumulus-topped Boundary Layers
Qing Wang, NPS, Monterey, CA; and S. Wang, H. Zuo, and D. L. Westphal
Marine stratocumulus-topped boundary layers are characteristic of strong interactions between cloud microphysics, radiation, and boundary layer turbulence. Recent modeling studies on the evolution of marine stratocumulus have incorporated size-resolved cloud microphysics in the boundary layer model in both one-dimensional and three-dimensional (large-eddy-simulation) framework. However, results from the 1-D simulations indicated spurious predictions of supersaturation that lead to unrealistic growth of the cloud droplets. This is considered a result of neglecting the interactions between cloud microphysics and boundary layer turbulence, which has been thought to be important, but has not been studied in detail.

A one-dimensional third-order turbulence closure model has been developed that includes predictions of size-resolved number concentrations of cloud droplets. The third-order turbulence closure model makes it possible to resolve the interaction between supersaturation (activation and growth) and boundary layer turbulence, a process that was neglected in all previous 1-D models. Our results indicated that the new terms included in the current model are fairly significant in modifying liquid water flux and droplet size distribution in different regions of the cloud, such as cloud base and cloud top. Including the turbulence-microphysics interaction in the model yields more realistic results than those from previous models. The limitation of the approach will also be discussed.

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