J. Laubach et al., Heat and water vapour diffusivities near the base of a disturbed stable internal boundary layer, BOUND-LAY M, 94(1), 2000, pp. 23-63
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.