Natural integration of scalar fluxes from complex terrain

Abstract

Large eddy simulations of turbulent¯ow and transport in the atmospheric boundary layer were conducted over heterogeneous sources of heat and water vapor to identify the blending properties of the turbulent mixing in an unstably strati®ed boundary layer. The numerical simulations show that the concept 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 that 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 strati®cation studied here, we see that when the length scales of the source variability on the land surface become signi®cantly greater than the ABL depth the blending is lost. In this case the source ®elds remain relatively uncoupled by the important eddy motion. However, for smaller surface length scales, the dynamic eddy motion couples the surface patches. Hence, there is good reason that the land surface exchange phenomenon would not be scale invariant over the entire range of scales. Because of the active role of temperature the eects of inhomogeneous surface sources of sensible heat persist higher into the ABL than do the eects of surface sources from more passive scalars, such as water vapor. Moreover, the mean ®elds of potential temperature and speci®c humidity blend at much lower heights than do the vertical turbulent¯ux ®elds of these two scalars. We propose a useful measure of blending eciency for simulation studies and show how this bridges from the dynamics responsible for the blending to the horizontal homogeneity of scalar¯ux ®elds at measurement heights in the ABL. Ó

Heterogeneity-Induced Heat-Flux Patterns in the Convective Boundary Layer: Can they be Detected from Observations and is There a Blending Height?—A Large-Eddy Simulation Study for the LITFASS-2003 Experiment

“…For zero lag ψ s ,φ (0, 0, z) is equivalent to a one-point correlation. In contrast to the one-point correlation analysis used by Albertson and Parlange (1999), the spatially-lagged correlation analysis considers the spatial displacement by horizontal advection during vertical transport and is therefore better suited for conditions with a mean flow. …”

“…However, although a spectral analysis provides information regarding the spectral energy at a certain scale, it does not indicate the flow organization with respect to the underlying surface, which might be completely different above heterogeneous surfaces compared to homogeneous surfaces. Albertson and Parlange (1999), in their LES of idealized surface heterogeneities, showed that z b also exists under convective conditions, except for strong convective conditions and surface heterogeneities with a horizontal length scale significantly >z i . For such situations an influence of surface heterogeneity on the entrainment processes was also found.…”

Atmospheric stability effect on subgrid-scale physics for large-eddy simulation

“…Since it was ®rst introduced [11], LES has been used to study the impact of dierent surface hydrologic factors such as the spatial variability of surface heat, moisture and roughness, on the ABL transport of momentum, heat and water vapor [1,2,18,20,25,46,55]. In LES, the unsteady 3D equations governing turbulent transport are numerically solved for scales of motion larger than a given grid size D, usually on the order of meters in simulations of the ABL.…”

Evaluating Source Area Contributions from Aircraft Flux Measurements Over Heterogeneous Land Using Large-Eddy Simulation

“…Roth and Oke 1995;Sempreviva and Hojstrup 1998;Asanuma and Brutsaert 1999). Albertson and Parlange (1999a), using largeeddy simulation (LES) for an artificial patchwork of fields, showed that water vapour appeared to be carried more nearly horizontally over wet fields and ultimately carried upward in thermally driven eddies over downwind arid regions, while sensible heat was immediately convected upward in buoyant plumes above local sensible heat sources.…”