Flux‐gradient relationships for momentum and heat over a rough natural surface

摘要

AbstractThe flux‐gradient relationships for momentum and heat have been investigated over extensive flat bushland (h= 2.3 m,zo= 0.4 m) with observations being made at several levels extending up to 10 times the canopy height. It has been found that: (1) Eddy diffusivities for momentum and heat,KHandKM, are enhanced by factors of 1.65 and 2.5 respectively under near‐neutral conditions and within the layer close to the canopy. (2) These enhancements distinctly depend on instability, and their variations with height are not simply monotonic, as expected in earlier investigations. (3) Even a counter‐gradient flux of heat could occur under some circumstances. (4) The level of enhancement remains substantially unchanged over different surfaces for heat transfer but decreases as the canopy density increases in the case of momentum transfer. Possible mechanisms are discussed. (5) The length scale of atmospheric turbulence should be taken into account in studies of turbulent transfer over very rough surfaces, where the large‐scale eddies may have a k