Advective and topographic influences on eddy flux measurement

Using arguments are based on scale analyses, wind tunnel experiments, and large eddy simulations, several authors have argued that Bernoulli effects associated with flow over hills strongly affect subcanopy motions, possibly altering horizontal CO2 advection. We report on our efforts to detect these effects at a well-studied site with complicated topography.

Observations have been made over a period of three years aimed to describe how boundary layer flow over topography at Harvard Forest MA influences subcanopy flows. We have continued to operate the subcanopy array at the Little Prospect Hill (LPH) tract. In one season season, we installed two sodars, one upwind and one downwind of the prominent topography. We characterize above-canopy flows by direction and intensity, comparing the flow upwind and downwind of the major topographic features. Separately, we show a well-defined anabatic-katabatic subcanopy flow pattern. We examine the extent to which subcanopy flows may or may not be altered by the flow aloft, estimating a relevant Froude number to facilitate comparison with earlier work. During the growing seasons, we operated a subcanopy CO2 array to assess horizontal advection.The final step is to document any effects that may modify horizontal advection on the subcanopy CO2 budget.