Abstract
Abstract. 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 forcing tends to reduce the dynamic stability of the inversion, characterized by the bulk (or gradient) Richardson number. This decrease enhances the entrainment mixing, leading to reduced 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 Richardson number. The weakened inversion tends to enhance the turbulence buoyant consumption and simultaneously lead to a reduced buoyant production in the cloud layer due to less radiative cooling. These effects may result in a decoupling process that creates the different heating/moistening rates between the cloud and subcloud layer, leading to a two-layered structure in the strongly sheared stratocumulus-topped MBL.
Highlights
VOCALS-REx (Variability of the American Monsoons Ocean-Cloud-Atmosphere-Land Study-Regional Experiment) took place off the west coast of Chile/Peru in the Southeast Pacific during October and November 2008
It occurs because the shear of WS is concentrated within a considerably smaller zi under the weak shear forcing such that the mean thermal stability dominates the shear term in Richardson number (Rib), whose value is larger for WS than that for strong shear forcing (SS)
Wind shear exists across the inversion most of the time in the stratocumulus-topped marine boundary layer (MBL), either due to surface friction or baroclinicity associated with the sloping MBL
Summary
VOCALS-REx (Variability of the American Monsoons Ocean-Cloud-Atmosphere-Land Study-Regional Experiment) took place off the west coast of Chile/Peru in the Southeast Pacific during October and November 2008. Unlike many other boundary layers where the shear is mainly a result of surface friction, the shear documented during VOCALS-REx is primarily caused by baroclinicity within the inversion, which is linked to the large-scale horizontal gradient of the inversion height (Bretherton et al, 2010; Wang et al, 2011) Perhaps for this reason, the wind shear tends to be unusually intense and persistent. Wind shear across the inversion is well known 1f4or its significant role in generating turbulent mixing and enhancing the entrainment rate in cloud-free convective boundary layers. One specific issue that has drawn much attention is about decoupling in the cloudtopped MBL, a process that tends to create different heating/moistening rates between the cloud and subcloud layer, leading to a two-layered structure (e.g., Nicholls et al, 1984) In these studies, roles of the shear have been rarely examined.
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