Anomalies in Flux-Gradient Relationships Over Forest

Abstract

Simultaneous profile and eddy correlation flux data gathered over Thetford Forest, U.K., have been analysed to find values of the vertical turbulent diffusivitiesKM,KH andKE (for momentum, heat and water vapour transfer, respectively) at a reference heightzR, nine roughness lengths above the zero-plane displacementd. The results show: (i), that values ofKM over the forest are not significantly different from these predicted by semiempirical diabatic influence functions appropriate to much smoother surfaces such as short grass; and (ii), thatKH andKE exceed their values predicted from the semiempirical functions by an average factor of 2 or more in unstable, near neutral and slightly stable conditions. These conclusions are strongly dependent on the assumed behaviour ofd, here taken as 0.76 tree heights, independent of both property and stability. Consideration is given to an alternative analysis procedure, in which values of the zero-plane displacementsdH anddE for heat and water vapour respectively, are obtained from the data by assumingKH andKE to be given by semiempirical diabatic influence functions; this procedure is shown to be unacceptable on both practical and physical grounds. To account for the anomalies inKH andKE, a mechanism is proposed in which the horizontally inhomogeneous temperature structure of the canopy causes free convection to be maintained by discrete; localized heat sources and/or sinks, effectively enhancing turbulent transport processes even in nearneutral conditions.