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Buoyant Turbulent Kinetic Energy Production in Steep-Slope Katabatic Flow

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Abstract

We develop several critical concepts that should be considered when interpreting, modelling and designing future experiments for flows over sloping terrain. Vertical buoyancy fluxes in katabatic flows can be positive and a source of turbulent kinetic energy (TKE) despite the statically stable, thermal stratification that drives these flows. This phenomenon occurs when the ratio of along-slope to slope-normal kinematic heat fluxes is greater than the cotangent of the slope angle, suggesting a critical value of slope-angle steepness found in earlier studies. We provide field-data-based evidence that the along-slope heat flux may dominate the variables in this inequality, and therefore in generating buoyant TKE production or suppression over a steep slope. These data show the along-slope heat flux can be more variable and significantly larger in magnitude than the slope-normal component. The gradient Richardson number does not include the effects of the along-slope buoyancy; furthermore, none of the canonical stability parameters can properly reflect the TKE redistribution from turbulent transport divergence and the sink of TKE in cases of counter-gradient momentum fluxes, which we frequently observe near the peak of the katabatic jet. In such cases, canonical stability parameters inadequately represent the physical mechanisms associated with stability. These results have broad implications related to accurately modelling turbulence and surface exchanges over sloping terrain and illustrate the need to more thoroughly investigate the along-slope heat flux and its drivers, the meaning and definitions of stability, and the effects of non-local turbulent transport.

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Acknowledgments

This research was primarily funded by the Swiss National Science Foundation (SNSF-200021-134892) and the Competence Center for Environmental Sustainability (CCES-SwissEx) of the ETH domain. This work was partially supported by the United States Office of Naval Research Award #N00014–11–1–0709, the National Science Foundation Division of Environmental Biology Grant # DEB-1440409 and the NSERC Discovery Grant. The authors wish to thank Dr. Andrey A. Grachev, Dr. Daniel F. Nadeau and Dr. Larry Mahrt for helpful discussions. Thanks go also to our colleagues from the EFLUM laboratory for outstanding assistance in the Val Ferret field campaigns, and special thanks to the Commune d’Orsières for general logistical support for field campaigns in Val Ferret. We are grateful to the anonymous reviewers, whose comments have helped to improve our manuscript.

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Authors and Affiliations

  1. School of Architecture, Civil and Environmental Engineering, École Polytechnique Fédérale de Lausanne (EPFL), Lausanne, Switzerland

    Holly J. Oldroyd

  2. Department of Civil and Environmental Engineering, University of California, Davis, CA, USA

    Holly J. Oldroyd

  3. Department of Mechanical Engineering, University of Utah, Salt Lake City, UT, USA

    Eric R. Pardyjak

  4. Department of Biological and Ecological Engineering, Oregon State University, Corvallis, OR, USA

    Chad W. Higgins

  5. Department of Civil Engineering, University of British Columbia, Vancouver, BC, Canada

    Marc B. Parlange

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  1. Holly J. Oldroyd
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  2. Eric R. Pardyjak
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  3. Chad W. Higgins
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  4. Marc B. Parlange
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Correspondence to Holly J. Oldroyd.

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Oldroyd, H.J., Pardyjak, E.R., Higgins, C.W. et al. Buoyant Turbulent Kinetic Energy Production in Steep-Slope Katabatic Flow. Boundary-Layer Meteorol 161, 405–416 (2016). https://doi.org/10.1007/s10546-016-0184-3

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