Turbulent Fluxes and their Relation to Surface Heterogeneity and Transient Mesoscale Motions

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9310576 Mahrt The objective of this work is to analyze several new and existing data sets toward the goal of improving our understanding of the response of boundary-layer flow to heterogeneous surfaces ranging from multiple, well defined, discontinuities to more complex surface heterogeneity. The new data include aircraft, tower and limited remote sensing observations and allow examination of classes of heterogeneity in detail not previously considered. One of the goals is to examine the main influences on the area averaged surface fluxes. These influences include: 1) enhancement of the momentum flux associated with pressure drag induced by scatter trees, forest edges and other obstacles, 2) non-equilibrium effects in transition zones downstream from changes of surface conditions, 3) extra Reynolds terms associated with spatial variability and the nonlinear relationship between fluxes and gradients and 4) direct transport by mesoscale motions. To approach this problem, the flow observed by repeated aircraft runs over the same track is formally decomposed into turbulence, transient mesoscale motions and stationary mesoscale motions (primarily associated with surface heterogeneity). The modulation of the turbulence flux by the two mesoscale components will be considered in detail. From a more practical point of view, the above influences on the area averaged flux will be implicitly incorporated into calculation of effective roughness lengths and exchange coefficients corresponding to the area averaged fluxes. These calculations will be preceded by an analysis of flux sampling errors which will include some research on the techniques for estimating flux errors. Proposed theoretical work will be limited to interpretation of the mesoscale circulations over heterogeneous terrain in terms of several existing linear and nonlinear predictions. ***