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Tethered Balloon Observations Of The Nocturnal Stable Boundary Layer In A Valley
Authors:J J Holden  S H Derbyshire  S E Belcher
Institution:(1) Department of Meteorology, University of Reading, U.K.;(2) Department of Meteorology, University of Reading, U.K.
Abstract:Detailed measurements of profiles of mean and turbulent variablesthrough the nocturnal stable boundary layer over a valley arepresented. Two nights of data are analysed in detail, one with only aweakly stable boundary layer and one with a strongly stable boundarylayer. The weakly stratified night shows high levels of turbulence inwhich the flow remains attached to the valley and the boundary layeracts as a single coherent layer. On the strongly stratified night, twoflow regimes are identified: attached flow, as on the weaklystratified night, and decoupled flow in which the air in the valleybecomes so dynamically stable that there is no turbulent interactionwith the mean flow aloft. Because the valley is sloping, the decoupledlower stagnant air then forms a drainage current. We find that theFroude number evaluated at the hill height, FH = U(H)/N(H) H,diagnoses the flow regime: when FH = 2, the flow remainsattached and when FH lap 2 the flow in the valley becomesdecoupled from the flow aloft. The dynamics of the flow regimes areshown to be understandable in terms of the gradient Richardson number,which indicates the turbulent mixing. We show that the gradientRichardson number is a key parameter in diagnosing each flow regime.
Keywords:Boundary layer  Decoupling  Flow over hills  Stable stratification  Valley flow
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