A scale-wise analysis of intermittent momentum transport in dense canopy flows

Abstract

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.