Wind Shear, Convective Circulation, and Stratocumulus Organization

It is well known that the stratocumulus convection in marine boundary layers is primarily driven by the radiative cooling at the cloud top, which strongly controls overall turbulence intensity as well as the entrainment process across the inversion. Observations from recent field campaigns, such as VOCALS in southeast Pacific and UPPEF near the California coast, revealed that the wind shear across the inversion can be very significant because of strong baroclinicity within the inversion. Furthermore, satellite observations have shown various stratocumulus morphologies including quasi-linear cloud streets and open/closed cellular patterns. It is likely that the wind shear both near the surface and across the inversion play important roles not only affecting the turbulence dynamics such as convective circulation and entrainment but also regulating how the clouds are organized and how they appear. In this work, we use large-eddy simulation (LES) approach combined with observations to study effects of the wind shear on the convective circu-lation and the cloud organization. A LES model is configured with the horizontal grid resolution of 10m and the vertical of 5m to simulate the case of RF02 from the field program UPPEF (Unified Physical Parameterization for Seasonal Forecast). The simulation results are compared with observations in terms of mean and turbulence statistics profiles and PDF of turbulent heat and moisture fluxes. Various sensitivity simulations have been performed to analyze the effect of an individual process, such as the sur-face wind shear, inversion wind shear, or no wind shear, on the cloud circulation as well as cloud morphology. Preliminary results demonstrate that the surface wind shear enhances the turbulence intensity, produces the smaller turbulence scale ( 200m) near surface as opposed to the larger convective circulation (2km), and generate a quasi-linear cloud street pattern. The inversion wind shear weakens the convective circulation and slightly increases its scale. More detailed results will be presented in the conference.