  The transport of energy, mass, and momentum in the atmospheric boundary layer
(ABL) is regulated by coherent structures. Although past studies have primarily
focused on stationary ABL flows, the majority of real-world ABL flows are
non-stationary, and a thorough examination of coherent structures under such
conditions is lacking. To fill this gap, this study examines the topological
changes in ABL turbulence induced by non-stationarity and their effects on
momentum transport. Results from a large-eddy simulation of pulsatile open
channel flow over an array of surface-mounted cuboids are examined with a focus
on the inertial sublayer, and contrasted to those from a corresponding constant
pressure gradient case. The analysis reveals that flow pulsation primarily
affects the ejection-sweep pattern. Inspection of the instantaneous turbulence
structures, two-point autocorrelations, and conditionally-averaged flow fields
shows that such a pattern is primarily influenced by the phase-dependent shear
rate. From a turbulence structure perspective, this influence is attributed to
the changes in the geometry of hairpin vortices. An increase (decrease) in the
shear rate intensifies (relaxes) the hairpin vortices, leading to an increase
(decrease) in the frequency of ejections and an amplification (reduction) of
their percentage contribution to the total momentum flux. Moreover, the size of
the hairpin vortex packets changes according to the hairpin vortices comprising
them, while the packet inclination remains unaltered during the pulsatile
cycle. Findings underscore the important impact of non-stationarity on the
structure of ABL turbulence and associated mechanisms supporting momentum
transport.
