Natural integration of scalar fluxes from complex terrain

Large eddy simulations of turbulent flow and transport in the atmospheric b oundary layer were conducted over heterogeneous sources of heat and water v apor to identify the blending properties of the turbulent mixing in an unst ably stratified boundary layer. The numerical simulations show that the con cept of blending in the ABL is in fact a useful one, even under convective conditions, for a range of surface conditions. Since the transport eddies t hat are responsible for the blending have sizes that are constrained by the boundary layer depth, and since the vertical motion is so important under the unstable density stratification studied here, we see that a hen the len gth scales of the source variability on the land surface become significant ly greater than the ABL depth the blending is lost. In this case the source fields remain relatively uncoupled by the important eddy motion. However, for smaller surface length scales, the dynamic eddy motion couples the surf ace patches. Hence, there is good reason that the land surface exchange phe nomenon would not be scale invariant over the entire range of scales, Becau se of the active role of temperature the effects of inhomogeneous surface s ources of sensible heat persist higher into the ABL than do the effects of surface sources from more passive scalars, such as water vapor. Moreover, t he mean fields of potential temperature and specific humidity blend at much lower heights than do the vertical turbulent flux fields of these two scal ars. We propose a useful measure of blending efficiency for simulation stud ies and show how this bridges from the dynamics responsible for the blendin g to the horizontal homogeneity of scalar flux fields at measurement height s in the ABL. (C) 1999 Elsevier Science Ltd. All rights reserved.