Flux aggregation at large scales: on the limits of validity of the concept of blending height

The concept of blending height is known to be applicable to estimate areally-averaged surface heat and momentum fluxes over heterogeneous terrain with horizontal scales of surface variations much smaller than 10 km. Here, the performance of this concept is explored beyond this limit of validity. This is accomplished by analysing a three-dimensional meso-scale simulation of land-sea breeze systems which originate at large-scale temperature differences between water and land surfaces and by employing a one-dimensional version of the meso-scale model as a vertical column of a hypothesized macro-scale model. It appears that areally-averaged surface fluxes can be reproduced reasonably well. This is valid for area-averaged fluxes obtained by the average of surface fluxes on each land type, i.e. for so-called flux aggregation, and for a combination of flux aggregation and so-called parameter aggregation where similar land types are combined into an aggregated land surface. The weak dependence of averaged surface fluxes on secondary, meso-scale motions agrees with earlier theoretical considerations. (orig.)