Topographic Effects on Flux Measurements at Harvard Forest

Forest-atmosphere exchange of energy and matter are affected by air movement inside the forest canopy. At flat sites, these movements are primarily determined by the vertical transfer of momentum from the flow above the canopy. However, at hilly sites, topographical effects such as flow around hills and nocturnal subcanopy drainage flows can limit the representativeness of observations.

Lee (1998) suggested that sustained drainage flows at the canopy floor lead to mean vertical motions, and that these motions are responsible for suspected underreporting of ecosystem respiration rates. The interpretation of flux measurements is also complicated when topographical features bias local wind directions, or result in mean vertical velocities.

Studies to date have hypothesized the existence of drainage flows under forests, but few observations of the phenomenon have been published. Drainage circulations are notoriously difficult to observe, and they rarely occur as sustained flows. Such flows at the forest floor are further complicated by the presence of tree trunks and subcanopy vegetation.

In this study, we deployed instruments at the Harvard Forest Environmental Measurement Site (EMS) for four months to observe subcanopy flow patterns in conjunction with detailed measurements of the flow field above the forest. Five sonic anemometers (four 2-D, one 3-D) were spatially distributed around the EMS tower at 1.5m above ground, and an acoustic sounder operating at canopy height from a nearby scaffolding tower reported wind speeds and turbulence intensity at 25m intervals up to 750m. Ongoing continuous measurements at the EMS tower include those from 3-D sonic anemometers at 30m, 18m and 11m, radiative budget components, temperature, CO2 and H2O profiles.

Results describing the effects of the local topography on the flow field above and below the canopy, and our evaluation of the implications on flux measurements at the EMS tower, will be presented.