Print Email Facebook Twitter The minimum wind speed for sustainable turbulence in the nocturnal boundary layer Title The minimum wind speed for sustainable turbulence in the nocturnal boundary layer Author Van de Wiel, B.J.H. Moene, A.F. Jonker, H.J.J. Baas, P. Basu, S. Donda, J.M.M. Sun, J. Holtslag, A.A.M. Faculty Applied Sciences Department Multi-Scale Physics Date 2012-11-01 Abstract The collapse of turbulence in the nocturnal boundary layer is studied by means of a simple bulk model that describes the basic physical interactions in the surface energy balance. It is shown that for a given mechanical forcing, the amount of turbulent heat that can be transported downward is limited to a certain maximum. In the case of weak winds and clear skies, this maximum can be significantly smaller than the net radiative loss minus soil heat transport. In the case when the surface has low heat capacity, this imbalance generates rapid surface cooling that further suppresses the turbulent heat transport, so that eventually turbulence largely ceases (positive feedback mechanism). The model predicts the minimum wind speed for sustainable turbulence for the so-called crossing level. At this level, some decameters above the surface, the wind is relatively stationary compared to lower and higher levels. The critical speed is predicted in the range of about 5–7 m s21, depending on radiative forcing and surface properties, and is in agreement with observations at Cabauw. The critical value appears not very sensitive to model details or to the exact values of the input parameters. Finally, results are interpreted in terms of external forcings, such as geostrophic wind. As it is generally larger than the speed at crossing height, a 5 m s21 geostrophic wind may be considered as the typical limit below which sustainable, continuous turbulence under clear-sky conditions is unlikely to exist. Below this threshold emergence of the very stable nocturnal boundary layer is anticipated. Subject surface layerwindsurface observationsland surface modelnonlinear modelssingle column models To reference this document use: http://resolver.tudelft.nl/uuid:6bd4b059-b6fb-4b3c-8972-f39bfbedd791 DOI https://doi.org/10.1175/JAS-D-12-0107.1 Publisher American Meteorological Society Embargo date 2013-05-01 ISSN 1520-0469 Source http://journals.ametsoc.org/doi/abs/10.1175/JAS-D-12-0107.1 Source Journal of the Atmospheric Sciences, 69 (11), 2012 Part of collection Institutional Repository Document type journal article Rights (c) 2012 American Meteorological Society Files PDF jas-d-12-0107.1.pdf 842.12 KB Close viewer /islandora/object/uuid:6bd4b059-b6fb-4b3c-8972-f39bfbedd791/datastream/OBJ/view