THE VALIDITY OF SIMILARITY THEORY IN THE ROUGHNESS SUBLAYER ABOVE FORESTS

Flux-gradient relationships based upon similarity theory have been rep orted to severely underestimate scalar fluxes in the roughness sublaye r above forests, as compared to independent flux estimates (for exampl e, eddy covariance or energy balance measurements). This paper present s the results of a unique three-month investigation into the validity of similarity theory in the roughness sublayer above forests. Eddy cov ariance and flux-gradient measurements of carbon dioxide (CO2) exchang e were compared above a mixed deciduous forest at Camp Borden, Ontario , both before and after leaf senescence. The eddy covariance measureme nts used a Li-Cor infrared gas analyzer, and the flux-gradient (simila rity theory) measurements featured a tunable diode laser Trace Gas Ana lysis System (TGAS). The TGAS resolved the CO2 concentration differenc e to 300 parts per trillion by volume (ppt) based upon a half-hour sam pling period. The measured enhancement factor gamma (the ratio of inde pendent flux estimates, in this case eddy covariance, to similarity th eory fluxes) was smaller and occurred closer to the canopy than in mos t previous investigations of similarity theory. Very good agreement be tween the eddy covariance and similarity theory fluxes was found betwe en 1.9 and 2.2 canopy heights (h(c)), and the mean enhancement factors measured before and after leaf senescence were 1.10 +/- 0.06 and 1.24 +/- 0.07, respectively. Larger discrepancies were measured closer to the canopy (1.2 to 1.4 h(c)), and mean enhancement factors of 1.60 +/- 0.10 and 1.82 +/- 0.11 were measured before and after leaf senescence , respectively. Overall, the Borden results suggest that similarity th eory can be used within the roughness sublayer with a greater confiden ce than previously has been believed.