Effect of intense wind shear across the inversion on stratocumulus clouds

Abstract

[1] A large-eddy simulation model is used to examine the impact of the intense cross-inversion wind shear on the stratocumulus cloud structure. The wind shear enhanced entrainment mixing effectively reduces the cloud water and thickens the inversion layer. It leads to a reduction of the turbulence kinetic energy (TKE) production in the cloud layer due to the weakened cloud-top radiative cooling and the formation of a turbulent and cloud free sublayer within the inversion. The thickness of the sublayer increases with the enhanced wind shear intensity. Under the condition of a weaker inversion, the enhanced shear mixing within the inversion layer even lowers the cloud-top height and reduces the entrainment velocity. Finally, increasing wind shear or reducing inversion strength tends to create an inversion layer with a constant bulk Richardson number ($0.3), suggesting that an equilibrium value of the Richardson number is reached. Citation: Wang, S., J.-C. Golaz, and Q. Wang (2008), Effect of intense wind shear across the inversion on stratocumulus clouds, Geophys. Res. Lett., 35, L15814,

Observation of a Self-Limiting, Shear-Induced Turbulent Inversion Layer Above Marine Stratocumulus

“…Furthermore, these observations and the conceptual picture outlined in Fig. 5 are consistent with the LES studies of Brucker and Sarkar (2007) for a turbulent, stratified clear-air mixing layer and of Wang et al (2008) for a stratocumulus top with strong shear, who both showed monotonic layer growth until Ri ≈ 0.3. At least semi-quantitatively, the TIL evolution can be further analyzed by considering the terms of the balance equation for turbulent kinetic energy (e.g., Brucker and Sarkar 2007):…”

“…Interestingly, the ε τ averaged over the TIL does not show a clear trend with z sub . The estimated w e , however, hints at a decrease with z sub , which is consistent with the LES results of Wang et al (2008). The decrease can be understood by noting that the entrainment velocity scales as w e ∼ σ w (l e / z sub ), and that l e and σ w are observed to be rather constant for the measured profiles.…”

“…In six sampled profiles through the layer it is observed that the Richardson number increases with layer thickness to order unity, suggesting that the layer develops to the point where shear production of turbulence is sufficiently weak to be balanced by buoyancy suppression. This picture is consistent with the evolution of turbulence at localized stratified shear layers (Smyth and Moum 2000;Brucker and Sarkar 2007), and with the LES of Wang et al (2008). The large eddy scale is suppressed by buoyancy to be on the order of the Ozmidov scale, much less than the thickness of the TIL, such that direct mixing between the cloud top and the free troposphere is inhibited.…”

“…de Roode and Wang (2007) provide further observational evidence from FIRE I and refer to the process as detrainment, through which diluted, but still saturated air, forms a local environment at the cloud boundary. The dependence of the interface layer on shear was highlighted in the LES studies of Wang et al (2008), who observed that the layer thickness increases with shear intensity, and, consistent with the cloud-free work described in the previous paragraph, noted that the inversion layer tends to evolve to a constant bulk Richardson number of approximately 0.3.…”

“…The temperature inversion acts to suppress mixing but what little turbulence forms results in important coupling between the boundary layer and the troposphere, and the resulting entrainment is critical to cloud-layer evolution at the boundary-layer top (e.g., Stevens 2002). Generally, turbulence at the interface is thought to be generated through thermodynamic destabilization, for example due to radiative cooling and possible buoyancy reversal (e.g., Deardorff 1980;Mellado 2010), or through wind shear and the resulting Kelvin-Helmholtz instability (e.g., Wang et al 2008). Formation of turbulence and the evolution of turbulent mixing in a stably stratified environment are problems of broad geophysical interest, and have been widely explored.…”