Boundary-Layer Meteorol (2006) DOI 10.1007/s10546-006-9064-6 ORIGINAL ARTICLE The relative importance of ejections and sweeps to momentum transfer in the atmospheric boundary layer Gabriel Katul · Davide Poggi · Daniela Cava · John Finnigan Received: 9 June 2005 / Accepted: 8 December 2005 © Springer Science+Business Media B.V. 2006 Abstract Using an incomplete third-order cumulant expansion method (ICEM) and stan- dard second-order closure principles, we show that the imbalance in the stress contribution of sweeps and ejections to momentum transfer (S o ) can be predicted from measured pro- files of the Reynolds stress and the longitudinal velocity standard deviation for different boundary-layer regions. The ICEM approximation is independently verified using flume data, atmospheric surface layer measurements above grass and ice-sheet surfaces, and within the canopy sublayer of maturing Loblolly pine and alpine hardwood forests. The model skill for discriminating whether sweeps or ejections dominate momentum transfer (e.g. the sign of S o ) agrees well with wind-tunnel measurements in the outer and surface layers, and flume measurements within the canopy sublayer for both sparse and dense vegetation. The broader impact of this work is that the “genesis” of the imbalance in S o is primarily governed by how boundary conditions impact first and second moments. G. Katul (B ) · D. Poggi Nicholas School of the Environment and Earth Sciences, Duke University Box 90328, Durham, NC, USA e-mail: gaby@duke.edu G. Katul Department of Civil and Environmental Engineering, Duke University, Durham, NC, USA D. Poggi Dipartimento di Idraulica Trasporti e Infrastrutture Civili, Politecnico di Torino, Corso Duca degli Abruzzi, 24 10129, Torino, Italy D. Cava CNR - Institute of Atmospheric Sciences and Climate, Section of Lecce, Polo Scientifico dell’Università Strada Prov. Lecce-Monteroni km 1,200 73100 Lecce, Italy J. Finnigan CSIRO Marine and Atmospheric Research, FC Pye Laboratory, Black Mountain, Canberra, ACT 2601, Australia