  Canopy flows in the atmospheric surface layer play important economic and
ecological roles, governing the dispersion of passive scalars in the
environment. The interaction of high-velocity fluid and large-scale
surface-mounted obstacles in canopy flows produces drag and causes intense,
inhomogeneous, and anisotropic turbulence. In this work, we focus on the
turbulent dispersion of passive scalars by studying the ``pair dispersion'' - a
statistical measure of relative motion between particles. We analyze the
results of a 3D-PTV experiment in a wind tunnel canopy flow, focusing on small
scales. We confirm the existence of local isotropy of pair dispersion at scales
smaller than a characteristic shear length scale
$L_\Gamma=(\epsilon/\Gamma^3)^{1/2}$, where $\epsilon$ and $\Gamma$ are the
mean dissipation rate and shear rate, respectively. Furthermore, we show that
pair dispersion in this locally isotropic regime is a scale-dependent
super-diffusive process, similar to what occurs in homogeneous isotropic
turbulent flows. In addition, we measure the pair relative velocity correlation
function, showing that its de-correlation occurs in the locally isotropic
regime, and discuss the implications of this observation for modeling pair
dispersion. Thus, our study extends the fundamental understanding of turbulent
pair dispersion to the anisotropic, inhomogeneous, turbulent canopy flow,
bringing valuable information for modeling scalar dispersion in the atmospheric
surface layer.
