Boundary-layer structure in flow over a heterogeneous surface

University dissertation from Uppsala : Acta Universitatis Upsaliensis

Abstract: Airborne measurements performed within the framework of NOPEX (Northern Hemisphere Land Surface Climate Processes Experiment) have been used to study processes related to fluxes of momentum, heat, and moisture in the boundary layer in the Swedish boreal region. The NOPEX area is characterized as heterogeneous with respect to vegetation. Here forests, farmlands, and lakes are interspersed over a large range of area scales. The atmospheric boundary layer constitutes a complex system where physical processes on different scales interact. These processes, which depend on conditions both at the surface and within the boundary layer, are important for the transport of energy and have previosly not been fully understood.The flight missions and the airborne measuring techniques in NOPEX are described. The calibrated data are shown to be accurate within acceptable limits. The footprint concept has been used to reconstruct the heat fluxes at aircraft level from surface observations. A comparison with aircraft measured fluxes shows that the sum of sensible and latent heat fluxes agree, while sensible and latent heat fluxes, respectively, disagree. The reason for the disagreement may be that tower measurements in one type of vegetation are not representative for all similar surfaces for the case studied.Atmospheric turbulence is not a chaotic process, but contains organized, or coherent, structures. It is shown that turbulence in the boundary layer is organized in structures whose geometry is a function of normalized height and stability. These structures are found to be responsible for 70-100% of the vertical energy transport, even though they are present only during two thirds of the time of observation.The mesoscale flow modification in the boundary layer due to the presence of a lake (34 km2) has been studied with numerical simulations and airborne measurements. In this case, it appears as differences in surface roughness are more important than differences in surface temperature for the flow modification.

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