Downwind evolution of surface fluxes over a vegetated surface during local advection of heat and saturation deficit

The horizontal advection of warm, dry air to a vegetated surface and the resulting effects on evaporation were examined. Contrary to the decreasing trend of evaporation with downwind distance predicted by traditional models, the eddy covariance measurements made in this study indicated that evaporation remained nearly constant with downwind distance over the few hundred meters considered. Under stronger advection there was even some evidence of increases in evaporation with distance. Canopy conductance values calculated from the measured values of sensible and latent heat fluxes, wind speed and canopy temperature, suggested that canopy conductance increased with downwind distance, likely in response to the reduction in saturation deficit. The stomatal response to increases in humidity may have compensated for the effects of the humidity on transpiration, allowing the evaporation flux to continue at a high rate. These results support similar assertions made by recent studies. The mechanisms of heat and water vapor transfer between the surface and the atmosphere, downwind of the transition were analyzed using spectral analyses. Power spectra of temperature and humidity scalars showed changes with downwind distance that appear to be governed by both the upwind conditions and the growing local boundary layer. Cross-spectra of the vertical fluxes suggested that at lower frequencies the vertical motions preceded changes in temperature and humidity, whereas at higher frequencies the changes in the scalars led the changes in vertical motions. This suggests that large eddies generated over the upwind surface were essentially independent of the stability of the local boundary layer growing over the new surface. These findings suggest possible mechanisms for the recent observations that local similarity is not valid under these conditions. Observations of ramps of humidity and temperature at all downwind locations suggest that coherent structures were common. Intermittency of fluxes was estimated with a simple procedure, and suggested that a substantial portion of the fluxes occurred during intermittent events. More analyses of coherent structures and intermittency are needed to better understand the mechanisms of transport under advection conditions. (C) 1997 Elsevier Science B.V.