The evaluation of flux aggregation methods using aircraft measurements in the surface layer

Abstract

The use of the bulk aerodynamic relationship to predict the turbulent flux of momentum, heat or moisture at the surface over heterogeneous terrain may lead to a dependence of the exchange coefficient on averaging scale, i.e. on model grid size. This study investigates the scale dependence of exchange coefficients for momentum and heat transfer in a forest dominated heterogeneous boreal landscape by comparing the results of four different flux aggregation methods with aircraft data in the atmospheric surface layer. We have used a total of 41 aircraft runs (with a length of #approx#60 km each) from 11 flights performed during NOPEX (a NOrthern hemisphere land-surface climate Processes Experiment). We find a different behavior for momentum and scalar transfer. While momentum flux is governed by the roughest elements (in our case forest) in any single grid box (or aircraft run segment), the sensible heat flux is determined by the dominant land cover type. The effective exchange coefficients for momentum transfer appear to be independent of the averaging scale. This implies that an effective roughness length can be used for momentum flux parameterization purposes and standard similarity theory can be used. The effective roughness length for momentum transfer is on the order of 1.5 m. In contrast, the effective exchange coefficient for sensible heat transfer does depend on averaging scale. Therefore, the subgrid scale heterogeneity of the landsurface and the overlying boundary layer must be resolved explicitly in order to predict the correct grid averaged surface heat flux. Sensitivity analysis and comparison with data taken over a warmer and drier land-surface show, that this scale dependence seems to be confined to land surfaces with a surface temperature close to the temperature of the overlying air. This is particularly the case when sub-grid water surfaces are present. (orig.)Available from TIB Hannover: RR 6341(99) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman