The distribution of mixed-layer heights, momentum and sensible heat fluxes are critical factors in the dilution and spread of air pollutants. In dispersion models, the individual horizontal grid cells often enclose regions of pronounced inhomogeneities. The estimation of the regional/aggregated mixed-layer height, momentum and heat fluxes is therefore a central issue for the parameterisation used in those models. A possible approach to evaluate regional fluxes is through a flux aggregation scheme. Point measurements are combined, in one way or another, to produce regional fluxes. Such methods has received some success for regional heat fluxes, likely because forest edge effects on the heat fluxes (temperature) are rather small as compared to fluxes over forest and grass land - at least for common patch-sizes of agricultural and forested areas typically for Europe. But for regional momentum fluxes it does not work, partly because the regional momentum flux of a patchy landscape of surfaces with different roughness length add up the local momentum fluxes in a highly non-linear way, and partly because the contribution to the regional momentum flux also is composed of effect by isolated trees, houses and sharp edges between forest and grass-land which are not necessarily small. Local momentum and heat fluxes traditionally are determined from mast measurements performed in locally homogeneous terrain, either directly by turbulence measurements, or by applying appropriate profile-flux relationships based on Monin-Obukhov scaling theory on measured profiles of temperature and wind. Such a network of flux measuring stations located over patches of typical and locally homogeneous terrain is inappropriate for the determination of regional surface momentum fluxes and large scale mixing-heights.
展开▼
