Observation and simulation of flow in vegetation canopies

Abstract

Terrestrial vegetation canopies are obvious examples from the natural world of flow and transport in complex obstructed geometries. Observations of turbulence structure in vegetation canopies date back almost forty years (Wright and Lemon, [654]; McBean, [402]; Allen, [6]; Baines, [19]; Isobe, [307]; Shaw et al., [571]). One of the earliest observational studies of the budget of turbulent kinetic energy inside a forest is that of Lesnik [369]. This latter author appears to have been the first to illustrate the importance of the diffusion of turbulent kinetic energy between levels in the canopy, and to point to the significance of the large difference in scale of turbulence between that created by large-scale shear near the canopy top and that resulting from wakes behind individual canopy elements. It is now clearly apparent that turbulence over and within plant canopies differs significantly from that over smoother surfaces. In particular, velocity spectra are more sharply peaked, streamwise and vertical velocities are strongly (negatively) correlated and are each highly skewed, and transport is dominated by organized structures with ejections from the canopy preceding downward sweeps of high momentum fluid. Excellent reviews of flow and turbulence in plant canopies can be found in Raupach and Thom [522] and, more recently, in Finnigan [187].