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A better understanding of forest subcanopy flows is needed to evaluate their role in the horizontal movement of scalars, particularly in complex terrain. This paper describes detailed measurements of the canopy structure and its variability in both the horizontal and vertical directions at a deciduous forest in complex terrain (the Harvard Forest, Petersham, Massachusetts). The effects of the trunks and subcanopy shrubs on the flow field at each of six subcanopy array locations are quantified. The dynamics of the subcanopy flow are examined with pragmatic methods that can be implemented on a small scale with limited resources to estimate the stress divergence, buoyancy, and pressure gradient forces that drive the flow. The subcanopy flow at the Harvard Forest was driven by mechanisms other than vertical stress divergence 75% of the time. Nocturnal flows were driven predominantly by the negative buoyancy of a relatively cool layer near the forest floor. The direction of the resulting drainage flows followed the azimuth of the longest forest-floor slope. Similar results were found at a much flatter site at Borden, Ontario, Canada. There was no clear evidence of flow reversals in the subcanopy in the the of ridges or hills at the Harvard Forest even in high wind conditions, contrary to some model predictions.

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.

A study of the canopy influence on rainfall fields' microscale structure conducted in the tropical forests of French Guyana using a network of rain gauges reveals that microscale rainfall is continuous over the open deforested meadow region, while the rainfall field under the canopy of the forested area is unstructured. The random rainfall distribution in the forested area is due to the vegetation there, but if the canopy retention capacity is saturated, the throughfall field will become continuous.

Pressure fluctuations generated by coherent motions in a forest canopy at Camp Borden, Canada, are composed of mean shear wind components and velocity-phase relations inside the coherent structure. Pressure gradient and pressure transport interaction terms in the momentum flux and turbulent kinetic energy systems are involved in the transportation of the flux and kinetic energy in the forest canopy.

Utilizing the concepts of localized near-field diffusion, a modeling system was developed to estimate isoprene emissions from foliage of a mixed deciduous forest. The model determined radiation disposition and foliage temperature inside the canopy using as input forcing variables measured above the forest. Such model outputs were then combined with an isoprene emission algorithm to estimate emissions from the forest ecosystem and thus assess uncertainties in inventory emissions. Under conditions of atmospheric stability, modeled temperature in the bottom half of the forest stand was overestimated by as much as 4 [degrees] C. For unstable conditions modeled and measured temperatures agreed more closely, the model overestimating temperature by less than 2 [degrees] C. based on an uncertainty analysis, parameters exerting the greatest influence on calculated leaf temperature and light levels inside the forest included radiation extinction coefficients and the partitioning between sunlit and shaded leaves. The uncertainties associated with the estimation of isoprene forcing variables gave overestimated isoprene emissions of 30% compared to measurements during the middle of the growing season. To estimate seasonal isoprene emissions, two emission rates were considered. Using a constant emission rate representing middle of the growing season conditions, modeled isoprene flux densities were greater by a factor of 2 compared to gradient-diffusion-derived fluxes during the early part of the growing season. Once applied to the entire growing season, the estimated isoprene emissions were within 20% of measured quantities when seasonally adjusted emission rates were included in the flux calculations.