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The simulation of horizontally homogeneous boundary layers that have characteristics of weakly and moderately stable atmospheric flow is investigated, where the well-established wind engineering practice of using ‘flow generators’ to provide a deep boundary layer is employed. Primary attention is given to the flow above the surface layer, in the absence of an overlying inversion, as assessed from first- and second-order moments of velocity and temperature. A uniform inlet temperature profile ahead of a deep layer, allowing initially neutral flow, results in the upper part of the boundary layer remaining neutral. A non-uniform inlet temperature profile is required but needs careful specification if odd characteristics are to be avoided, attributed to long-lasting effects inherent of stability, and to a reduced level of turbulent mixing. The first part of the wind-tunnel floor must not be cooled if turbulence quantities are to vary smoothly with height. Closely horizontally homogeneous flow is demonstrated, where profiles are comparable or closely comparable with atmospheric data in terms of local similarity and functions of normalized height. The ratio of boundary-layer height to surface Obukhov length, and the surface heat flux, are functions of the bulk Richardson number, independent of horizontal homogeneity. Surface heat flux rises to a maximum and then decreases. [ABSTRACT FROM AUTHOR]

In stably stratified flows vertical movement of eddies is limited by the fact that kinetic energy is converted into potential energy, leading to a buoyancy displacement scale z B . Our new mixing-length concept for turbulent transport in the stable boundary layer follows a rigid-wall analogy, in the sense that we assume that the buoyancy length scale is similar to neutral length scaling. This implies that the buoyancy length scale is: ℓ B = κ B z B , with κ B ≈ κ, the von Karman constant. With this concept it is shown that the physical relevance of the local scaling parameter z/Λ naturally appears, and that the α coefficient of the log-linear similarity functions $${\phi_{m,h}=1+\alpha z/\Lambda}$$ is equal to c/ κ 2, where c is a constant close to unity. The predicted value α ≈ 1/ κ 2 = 6.25 lies within the range found in observational studies. Finally, it is shown that the traditionally used inverse linear interpolation between the mixing length in the neutral and buoyancy limits is inconsistent with the classical log-linear stability functions. As an alternative, a log-linear consistent interpolation method is proposed. [ABSTRACT FROM AUTHOR]

Stable internal boundary layers form when warm air is advected over a cooler surface, a common occurrence in coastal areas. The internal boundary layer deepens with distance along-wind, eventually reaching equilibrium with the surface and becoming a fully developed marine boundary layer. We present observations of the late stages of internal boundary-layer evolution made by the U.K. Meteorological Office's C-130 Hercules research aircraft over the Persian Gulf in April 1996. Northwesterly winds brought warm dry air from the surrounding desert landmass across the cooler waters of the Gulf. Loss of heat to the surface resulted in the formation of a shallow, stable internal boundary layer downwind of the coast. The aircraft measurements were made several hundred kilometres downwind, by which point the original deep convective boundary layer had been eroded away and the internal boundary layer was well developed, effectively a new marine atmospheric boundary layer. Throughout most of its depth the boundary layer was statically stable and a downward heat flux of approximately 15 W m-2 was observed; however, an exceptionally strong latent heat flux, in excess of 250 W m-2 near the surface, was sufficient to overcome the downwards heat flux and maintain weak buoyant convection in the lower 30–50% of the boundary layer. Scaling of boundary-layer turbulence statistics using local similarity theory produces results in agreement with previous studies. Because of the strong humidity contribution to the buoyancy flux, however, care is required with the definition of the similarity scales. It is usual for either the sensible heat or buoyancy flux to be used in the definitions of both the temperature and length scales; the latter being used over water where humidity plays a significant role in determining stability. In the present case we find that while the buoyancy flux is appropriate in the definition of the length scale, the temperature scale must be defined in terms of the sensible heat flux. [ABSTRACT FROM AUTHOR]