Heat and water vapour diffusivities near the base of a disturbed stable internal boundary layer

We present results from an experiment that was designed to investigate turb ulent transport relationships in a nearly homogeneous boundary layer distur bed by unsteady wind swings, as found at the base of an advective inversion with a convective boundary layer overhead. In such a situation we measured vertical gradients and eddy fluxes of temperature and humidity at two heig hts. From these, the turbulent diffusivities of heat and water vapour are o btained, and compared to the predictions of Monin-Obukhov similarity theory and those of a numerical second-order closure model. It is found that the measured diffusivities exceed both predictions. This is interpreted as a co nsequence of the unsteady conditions. It is also found that the diffusivity for heat is roughly 10% larger than that for water vapour. This is in agre ement with a theoretical treatment of the unsteadiness effects that we deve loped in an earlier publication. This result is not reproduced by the numer ical model because the model has no provision for unsteady conditions. Our result disagrees with that from an earlier, very similar, field experiment, which may be due to a systematic underestimation of sensible heat flux in the older experiment.