ANOMALIES IN FLUX-GRADIENT RELATIONSHIPS OVER FOREST

Simultaneous profile and eddy correlation flux data gathered over Thetford Forest, U.K., have been analysed to find values of the vertical turbulent diffusivities KM, KH and KE (for momentum, heat and water vapour transfer, respectively) at a reference height zR, nine roughness lengths above the zero-plane displacement d. The results show: (i), that values of KM 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), that KH and KE 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 of d, 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 displacements dH and dE for heat and water vapour respectively, are obtained from the data by assuming KH and KE 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 in KH and KE, 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. © 1979 D. Reidel Publishing Co.