Strongly Sheared Stratocumulus Convection: A LES Case Study

Unusually large wind shears across the inversion in the stratocumulus-topped marine boundary layer (MBL) were frequently observed during VOCALS-REx. To investigate the impact of wind shear on the MBL turbulence structure, a large-eddy simulation (LES) model is used to simulate the strongly sheared MBL observed from Twin-Otter RF 18 on 13 November 2008. The LES simulated turbulence statistics agree in general with those derived from the measurements, with the MBL exhibiting a decoupled structure characterized by an enhanced entrainment and a turbulence intensity minimum just below the clouds. Sensitivity simulations show that the shear tends to reduce the dynamic stability of the inversion, enhance the entrainment mixing, and decrease the cloud water. Consequently, the turbulence intensity in the MBL is significantly weakened by the intense wind shear. The inversion thickens considerably and the MBL top separates from the cloud top, creating a finite cloud-free sublayer of 10-50 m thickness within the inversion, depending on the shear intensity. The wind shear enhances the turbulence buoyant consumption within the inversion, and simultaneously weakens the buoyant production in the cloud layer. These effects may result in different heating rates between the cloud and subcloud layer, leading to a process that tends to decouple the cloud from the subcloud layer. The decoupling process occurs even without solar radiation in the case of an intense wind shear similar to the observations.