  We investigate the intermittent dynamics of momentum transport and its
underlying time scales in the near-wall region of the neutrally stratified
atmospheric boundary layer in the presence of a vegetation canopy. This is
achieved through an empirical analysis of the persistence time scales (periods
between successive zero-crossings) of momentum flux events, and their
connection to the ejection-sweep cycle. Using high-frequency measurements from
the GoAmazon campaign, spanning multiple heights within and above a dense
canopy, the analysis suggests that when the persistence time scales ($t_p$) of
momentum flux events from four different quadrants are separately normalized by
$\Gamma_{w}$ (integral time scale of the vertical velocity), their
distributions ($P(t_p/\Gamma_{w})$) remain height-invariant. This result points
to a persistent memory imposed by canopy-induced coherent structures, and to
their role as an efficient momentum transport mechanism between the canopy
airspace and the region immediately above. Moreover, $P(t_p/\Gamma_{w})$
exhibits a power-law scaling at times $t_{p}<\Gamma_{w}$ with an exponential
tail appearing for $t_{p} \geq \Gamma_{w}$. By separating the flux events based
on $t_p$, we discover that around 80\% of the momentum is transported through
the long-lived events ($t_{p} \geq \Gamma_{w}$) at heights immediately above
the canopy while the short-lived ones ($t_{p} < \Gamma_{w}$) only contribute
marginally ($\approx$ 20\%). To explain the role of instantaneous flux
amplitudes towards momentum transport, we compare the measurements with a
newly-developed surrogate data and establish that the range of time scales
involved with amplitude variations in the fluxes tend to increase as one
transitions from within to above the canopy.
