The relative importance of ejections and sweeps to momentum transfer in the atmospheric boundary layer

Using an incomplete third order cumulant expansion method (ICEM) and standard second-order closure principles, we showed that the imbalance in the stress contribution of sweeps and ejections to momentum transfer (DS0) can be predicted from measured profiles of the Reynolds stress and the longitudinal velocity standard deviation for different boundary layer regions. The ICEM approximation was independently verified using flume data, surface layer measurements above a grass and ice sheet surfaces, and within the canopy sublayer of a maturing Loblolly pine and an alpine hardwood forests. The model skill for discriminating whether sweeps or ejections dominate momentum transfer (e.g. sign of DS0) agreed well with wind tunnel measurements in the outer- and surface- layers, and flume measurements within the canopy sub-layer for both sparse and dense vegetation. The broader impact of this work is that the "genesis" of the imbalance in DS0 is primarily governed by how boundary conditions impact first and second moments.