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.
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Acknowledgments
We acknowledge Rolf Maser and Dieter Schell from the enviscope GmbH (Frankfurt/M, Germany) for technical support and the two pilots Alwin Vollmer and Oliver Schubert from the rotorflug GmbH (Friedrichsdorf, Germany) for great helicopter flights. RAS acknowledges support from the Alexander von Humboldt Society during the period in which this research was carried out, as well as support from NSF grant AGS-1026123. We thank Don Lenschow, Szymon Malinowski, Juan-Pedro Mellado, and Bjorn Stevens for enlightening discussions about stratocumulus clouds.
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Open Access This is an open access article distributed under the terms of the Creative Commons Attribution Noncommercial License (https://creativecommons.org/licenses/by-nc/2.0), which permits any noncommercial use, distribution, and reproduction in any medium, provided the original author(s) and source are credited.
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Katzwinkel, J., Siebert, H. & Shaw, R.A. Observation of a Self-Limiting, Shear-Induced Turbulent Inversion Layer Above Marine Stratocumulus. Boundary-Layer Meteorol 145, 131–143 (2012). https://doi.org/10.1007/s10546-011-9683-4
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DOI: https://doi.org/10.1007/s10546-011-9683-4