A study of the subcanopy flow kinematics in two forests

A better understanding of the kinematics and dynamics of subcanopy flows is desperately needed to evaluate their role in the transport of scalars, particularly in complex terrain. Both the canopy structure and the local topography are important influences that can produce significant forcing terms in the momentum equation, through drag, buoyancy and pressure effects.

Most previous studies have focussed on the downward mixing of momentum into the canopy. In this study, we consider the more complex realities where this feature is joined by other momentum sources and sinks. Measurements were performed at the Harvard Forest (Massachusetts) and Borden (Ontario) using networks of sub- and above-canopy sonic anemometers as well as sodar and wind profiler instrumentation. The data sets, spanning 3 years and consisting of over 6000 hours of data, showed that the subcanopy flows were directionally decoupled from flows aloft 75% of the time. Nocturnal flows were driven predominantly by negative buoyancy (drainage flows), even at the relatively flat Borden site. The slope that determines the drainage flow direction at these two sites was shown to be the longest rather than the steepest slope. Flow reversals, expected in the subcanopy in the lee of ridges and hills, were not observed.

The kinematics in the subcanopy were examined in light of detailed horizontal and vertical canopy structure measurements conducted in both foliated and leafless seasons. The concept of the relative magnitude of the buoyant forcing term in the momentum equation was shown to be useful as a predictor of conditions when significant horizontal transport of scalars, such as carbon dioxide, can occur.