Vertical Structure of the CO2 Exchange Between the Surface and the Atmosphere Within and Above An Araucaria Forest Canopy

Several studies have shown differences between the turbulent fluxes below and immediately above rough surfaces caused by a sub-layer that occurs at the transition from within to above the canopy. We used a cospectral analysis for estimating the turbulent carbon dioxide fluxes measured simultaneously at two heights, one inside and the other above an Araucaria forest canopy at South America in southern Brazil, during a two-month period (february and march, 2010). This site is part of Rede Sul-brasileira de Fluxos Superficiais e Mudança Climática (SULFLUX - http://www.ufsm.br/sulflux). The comparison of turbulent fluxes within and above forest canopy is important for parameterizing the surface layer exchange processes more realistically than merely assuming that the fluxes above canopy are representative of the processes over the whole vertical extension of the surface layer. Such misrepresentation implies an incorrect lower boundary condition for modelling efforts of the surface exchange of scalars, such as CO2. Firstly, the results were compared with those obtained from direct measurements through eddy covariance technique using a fixed 30 min averaging time. Subsequently, the fluxes were determined using the covariance taken over a more proper averaging time obtained, from the integral temporal scale, inferred from the cospectral shape analysis.