The Ejection-sweep Character of Scalar Fluxes in the Unstable Surface Layer

In the atmospheric surface layer, it is widely accepted that ejection and sweep eddy motions, typically associated with coherent structures, are responsible for much of the land-surface evaporation, sensible heat, and momentum fluxes. The present study analyzes the ejection-sweep properties using velocity and scalar fluctuation measurements over tall natural grass and bare soil surfaces. It is shown that momentum ejections and sweeps occur at equal frequencies (Deject Dsweep 0:29) irrespective of surface roughness length or atmospheric stability conditions. Also, their magnitudes are comparable to values reported from open channel velocity measurements (Dsweep 0:33;Deject 0:30). The scalarDeject is constant and similar in magnitude to the momentumDeject( 0:29) over both surfaces and for a wide range of atmospheric stability conditions, in contrast to the scalarDsweep. The scalar sweep frequency is shown to depend on the scalar skewness for the dynamic convective and free convective sublayers, but is identical toDeject for the dynamic sublayer. The threshold scalar skewness at which theDsweep dependence occurs is 0.25, in agreement with the accepted temperature skewness value at near-neutral conditions. In contrast to a previous surface-layer experiment, this investigation demonstrates that the third-order cumulant expansion method (CEM) reproduces the measured relative flux contribution of ejections and sweeps ( S0) for momentum and scalars at both sites. Furthermore, a linkage between S0 and the scalar variance budget is derived via the third-order CEM in analogy to momentum. It is shown that S0 can be related to the flux divergence term and that such a relationship can be estimated from surface-layer similarity theory, and the three sublayer model of Kader and Yaglom and proposed similarity functions.