Gravity Waves in a Forest: A Linear Analysis

Abstract

Wavelike oscillations are a common form of air motion in the forest canopy at night. This paper investigates the canopy wave phenomenon using a two-dimensional inviscid linear wave model taking into account the drag force exerted on the wave wind components by plant elements and the plant?air heat exchange induced by temperature wave oscillations. The model appears to have adequately reproduced the salient features of a wave event in a boreal aspen forest. The wave dynamics are investigated as functions of parameters of the background states expressed in analytical form. It is shown that canopy waves are generated by wind shear near the treetops and share features of a Kelvin?Helmholtz disturbance. Because it is located close to the inflection point of the mean wind, the ground exerts a strong stabilizing effect on the wave motions, particularly in a sparse forest. The main role of the canopy drag in the wave dynamics is the creation of the inflection point; its damping effect on wave oscillations themselves is limited to disturbances of wavelengths shorter than that of the fastest growing waves. Wavelength, phase speed, and period of the fastest growing waves, those that are most likely to dominate observations, appear insensitive to static stability.