Strongly sheared stratocumulus convection: an observationally based large-eddy simulation study

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.

Physics of Stratocumulus Top (POST): turbulence characteristics

“…They also proposed an empirically based division of the stratocumulus top region into sublayers based on the vertical profiles of wind shear, stability and the thermodynamic properties of the air. An analysis of the dynamic stability of the EIL using the gradient Richardson number Ri confirmed the hypothesis presented by Wang et al (2008Wang et al ( , 2012 and Katzwinkel et al (2011) that the thickness of the turbulent EIL changes based on meteorological conditions (temperature and wind variations between the cloud top and free troposphere (FT)) such that the Richardson number across the EIL and its sublayers is close to the critical value.…”

Physics of Stratocumulus Top (POST): turbulent mixing across capping inversion

“…Inversion capping a typical marine stratocumulus is usually too strong to allow buoyancydriven turbulence, produced elsewhere, to break the barrier of potential energy and mix across the stably stratified layer. This means that at least locally the gradient Richardson number across the inversion has to exceed the critical value, as shown, e.g., by Wang et al (2008Wang et al ( , 2012 and Kurowski et al (2009). Such a shear-generated turbulence in the EIL was observed in a great detail by Katzwinkel et al (2012) in the course of several penetrations of Sc top performed with an Airborne Cloud Turbulence Observation System ACTOS (Siebert, 2006).…”

UCLALES–SALSA v1.0: a large-eddy model with interactive sectional microphysics for aerosol, clouds and precipitation

“…It is typical for a stratocumulus topped boundary layer to shift towards a decoupled structure in the morning as shortwave heating by the rising sun begins to offset the long-wave cloud top radiative cooling and therefore reduces clouddriven mixing (Ghate et al, 2014). Although wind shear may also affect the entrainment and cloud top static stability (Wang et al, 2012), it is not surprising that the sharpest transition in LEV4 occurs around 5 h into the simulation, which is also close to sunrise at the assumed location. It is also noted that after the initial shift, the decoupled structure is subject to positive feedbacks as it reduces the supply of moisture from the surface to the cloud layer, which further reduces the cloud top radiative cooling and thus the clouddriven mixing.…”