Intermittent Turbulence Associated with a Density Current Passage in the Stable Boundary Layer |
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Authors: | Jielun Sun Sean P. Burns Donald H. Lenschow Robert Banta Rob Newsom Richard Coulter Stephen Frasier Turker Ince Carmen Nappo Joan Cuxart William Blumen Xuhui Lee Xin-Zhang Hu |
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Affiliation: | (1) National Center for Atmospheric Research, USA;(2) NOAA Environmental Technology Laboratory, Boulder, Colorado, U.S.A;(3) Cooperative Institute for Research in the Atmosphere, Fort Collins, Colorado, U.S.A;(4) Argonne National Laboratory, Argonne, Illinois, U.S.A;(5) University of Massachusetts, Amherst, U.S.A;(6) Atmospheric Turbulence and Diffusion Division, Oak Ridge, Tennessee, U.S.A;(7) Instituto National Meteorologia, Barcelona, Spain;(8) Program in Atmospheric and Oceanic Sciences, University of Colorado, Boulder, U.S.A;(9) Yale University, New Haven, Connecticut, U.S.A |
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Abstract: | Using the unprecedented observational capabilities deployed duringthe Cooperative Atmosphere-Surface Exchange Study-99 (CASES-99),we found three distinct turbulence events on the night of 18October 1999, each of which was associated with differentphenomena: a density current, solitary waves, and downwardpropagating waves from a low-level jet. In this study, we focus onthe first event, the density current and its associatedintermittent turbulence. As the cold density current propagatedthrough the CASES-99 site, eddy motions in the upper part of thedensity current led to periodic overturning of the stratifiedflow, local thermal instability and a downward diffusion ofturbulent mixing. Propagation of the density current induced asecondary circulation. The descending motion following the head ofthe density current resulted in strong stratification, a sharpreduction in the turbulence, and a sudden increase in the windspeed. As the wind surge propagated toward the surface, shearinstability generated upward diffusion of turbulent mixing. Wedemonstrate in detail that the height and sequence of the localthermal and shear instabilities associated with the dynamics ofthe density current are responsible for the apparent intermittentturbulence. |
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Keywords: | Density currents Intermittent turbulence Nocturnal boundary layer Stable surface layer |
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