Subcanopy fluxes are estimated from a minisonic in a mature aspen forest during BOREAS. Detailed vertical structure is estimated from thermocouple data. Since the aspen canopy is semi-open, a strong surface inversion forms in the lower part of the subcanopy layer with clear nocturnal conditions. A weaker secondary nocturnal inversion forms across and immediately above the canopy. The daytime subcanopy is weakly unstable.
The measurements indicate much more efficient transport of momentum compared to that for heat in the subcanopy, in contrast to previous models. The subcanopy drag coefficient and transfer coefficients reach a maximum at near-neutral conditions, where shear-generation is large, but generally decreases with increased thermodynamic instability of the subcanopy flow. Are previous theories wrong or are subcanopy flux measurements beyond the capability of present sonic anemometers? Both possibilities are examined in this study. Subcanopy fluxes are difficult to measure due to intermittent transport by weak eddies and sensitivity to uncertain tilt corrections. Toward the goal of physically interpreting the measured transport, the flux is partitioned into: a) larger scale coherent structures which mix with air above the canopy and b) small scale turbulence generated locally within the subcanopy.