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

Abstract

High-resolution measurements of thermodynamic, microphysical, and turbulence properties inside a turbulent inversion layer above a marine stratocumulus cloud layer are presented. The measurements are performed with the helicopter-towed measurement payload Airborne Cloud Turbulence Observation System (ACTOS), which allows for sampling with low true air speeds and steep profiles through cloud top. Vertical profiles show that the turbulent inversion layer consists of clear air above the cloud top, with nearly linear profiles of potential temperature, horizontal wind speed, absolute humidity, and concentration of interstitial aerosol. The layer is turbulent, with an energy dissipation rate nearly the same as that in the lower cloud, suggesting that the two are actively coupled, but with significant anisotropic turbulence at the large scales within the turbulent inversion layer. The turbulent inversion layer is traversed six times and the layer thickness is observed to vary between 37 and 85 m, whereas the potential temperature and horizontal wind speed differences at the top and bottom of the layer remain essentially constant. The Richardson number therefore increases with increasing layer thickness, from approximately 0.2 to 0.7, 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 prior numerical simulations of the evolution of turbulence in localized stratified shear layers. It is observed that the large eddy scale is suppressed by buoyancy and is on the order of the Ozmidov scale, much less than the thickness of the turbulent inversion layer, such that direct mixing between the cloud top and the free troposphere is inhibited, and the entrainment velocity tends to decrease with increasing turbulent inversion-layer thickness. Qualitatively, the turbulent inversion layer likely grows through nibbling rather than engulfment.

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

“…This is also true for the whole EIL. Similar experimental values of Ri at Sc top are reported by Lenschow et al (2000) and Katzwinkel et al (2012). Histograms in Fig.…”

“…Observed Ri ≈ Ri C across the EIL (TISL) suggests a dynamical adaptation of z to forcings. This suggestion, formulated in a stratocumulus context by Katzwinkel et al (2012) (see Fig. 5 therein and accompanying discussion), is well documented in oceanic statically stable shear layers (see Smyth and Moum, 2000, cf.…”

“…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).…”

“…Data from in situ measurements (e.g., Caughey et al, 1982;Nicholls, 1989;Lenschow et al, 2000;De Roode and Wang, 2007) and the results of numerical simulations (e.g., Moeng et al, 2005;Yamaguchi and Randall, 2008) clearly indicate that the top of Sc is located below the capping inversion and does not directly interact with the FT above. The thickness of the EIL, related to the distance between the cloud top and inversion, varies from a few meters to a few tens of meters (e.g., Haman et al, 2007;Kurowski et al, 2009;Katzwinkel et al, 2012), and differs for different Sc cases (Carman et al, 2012).…”

Wind speed response of marine non-precipitating stratocumulus clouds over a diurnal cycle in cloud-system resolving simulations

“…A number of studies (e.g., Wang et al, 2008Wang et al, , 2012Katzwinkel et al, 2012;Mellado et al, 2014) have investigated the effect of strong wind shear at the inversion on stratocumulus clouds. Such shear is often caused by a jump in large-scale wind speed and direction across the inversion, and differs qualitatively and quantitatively from shear that arises from the interaction of a constant large-scale wind speed with the surface and with the potential temperature gradient at the inversion.…”