On Kinematic Isolation in Stable Stratification: The CASES-99 Tower Observations

Tower observations made during the Cooperative Atmosphere–Surface Exchange Study 1999 investigation of the nocturnal planetary boundary layer support the finding reported elsewhere of kinematic isolation of the conventional surface boundary layer wind field from the surface itself in conditions of strong stability. The measurements indicate that surface flow is strongly influenced by local topography and that the larger-scale velocity field is displaced by the locally-imposed katabatic velocity regime. The CASES-99 tower data reveal that the larger scale wind profile implies a skimming flow that retains much of its upwind characteristics above a height in the range 10–25 m, depending on circumstances. The night-time wind profiles so affected are additionally influenced by intermittency most likely arising from the interaction between separate night-time katabatic flows rather than from any cause associated with a single one of them. It is concluded that kinematic isolation in strong stability negates the applicability of Monin–Okukhov scaling, with the remaining possibility that such scaling might apply if the relevant zero plane is allowed to respond to the elevation of the oncoming wind field by local flows and/or obstructions.