The ratio of eddy diffusivities for heat and water vapour under conditions of local advection

Abstract

Both experimental (La Crau experiment) and modelling results (second-order closure) are used here to study the eddy diffusivities for sensible heat and water vapour under conditions of local advection. The main finding is that the eddy diffusivity for sensible heat is smaller than that of water vapour in the stable Internal Boundary Layer and larger in the unstable IBL. Since flux divergence cannot be neglected for this case, down to height-to-fetch ratios of at least 0.005, it is essential that the gradients and the fluxes are measured at the same height. These results also explain the apparently contrasting findings in earlier experiments concerning the behaviour of the eddy diffusivities. The overestimate due to flux divergence in the IBL of the surface Bowen ratio when estimated from vertical gradients measured just above the surface is partly compensated for by the variation of the ratio of eddy diffusivities with height for the case studied here.