A LES study of wind-shear effect on turbulent structure of stratocumulus clouds

Turbulence dynamics of stratocumulus clouds has been investigated intensively for the past several decades. Traditionally, most of the studies have been focused on the physical processes driven by or closely coupled to the cloud-top longwave radiative cooling. Most of the dynamical theories and parameterization schemes have been based on the implicit assumption that the wind shear across the inversion only plays a negligible role. However, some observations have shown that this wind shear can be very significant. Particularly, in the west coast of central California, forced by the coast line and topography, the wind shear at the top of the stratocumulus-topped boundary layer could reach as high as 10 m/s across an inversion of only 50-100 m thickness. In this work, we used an LES model to simulate such an observed boundary layer and examined how turbulence and mean structure respond to the wind shear.

We performed sensitivity simulations with different wind shear across the inversion and analyzed the mean turbulence structures and instantaneous flow features. Our findings can be summarized as follows. A stronger wind shear produces a much thicker inversion with significantly more variability in the mixing interface due to the shear-generated distur-bances. Since the turbulence intensity within the inversion is significantly strengthened, the cloud-top entrainment is considerably enhanced, leading to a decrease of liquid water content and radiative cooling at the cloud top. Consequently, the cloud-top height is lower, which is in contrast to the conventional argument: stronger entrainment leading to a higher cloud top. It is suggested that much of the air undergoing mixing within the inversion re-mains there so that the mixing does not directly result in the entrainment of drier and warmer air into the cloudy and fully turbulent mixed layer. These results may have important implications on parameterizations of the cloud-top entrainment, since the cross-inversion wind shear is inevitable, given the presence of the turbulent and non-turbulent flows.