Some Estimates of Dissipation from Turbulent Velocity Component Gradients Over a Forest Canopy

Abstract

Measurements of vector air velocities at two levels above a conifer canopy for 2 separate days were used to estimate the dissipation rate for turbulent energy following an analysis relating the dissipation to the spatial structure function for turbulent scales small compared to the height above the canopy. The rates were averaged for half-hour intervals and compared with estimates of production and the local divergence of the advective and diffusive fluxes of energy for 2 days; one with a forest fetch and one with an upwind forest edge. The dissipation estimates for the forest fetch were similar in magnitude to published measurements over a comparable canopy and qualitatively consistent with estimates of the other terms in the turbulent energy balance. Estimated net transport divergence is small as is the production by the turbulent sensible heat flux. The results dcwnwind of the forest edge indicate larger dissipation values and a greater excess of dissipation over production apparently reflecting turbulence production at the edge.