On the temporal scale of the turbulent exchange of carbon dioxide and energy above a tropical rain forest in Amazonia

Abstract

The Large Scale Biosphere‐Atmosphere Experiment in Amazonia (LBA) project has been using the eddy covariance technique since 1998 to monitor energy, water, and carbon surface fluxes over Amazonia. The results obtained up‐to‐date indicate high level of uncertainties, especially regarding the role of the Amazonian ecosystem to the global carbon budget. Besides the problems related to the eddy covariance measuring system (systematic error and nighttime stable conditions), an extremely important factor is associated with the averaging time scale or “time window” used by the scientific community to determine the surface fluxes. This work presents initial efforts to determine the turbulence time scale for long‐term carbon and energy surface fluxes over the Amazon rain forest. A total of 198 nights and 218 days during 2006 were analyzed. The multiresolution decomposition technique was applied to project the signal into several time scales and determine when the spectral and cospectral gap occurred. This technique permitted evaluating and separating the real contribution from turbulent and mesoscale fluxes to the total surface fluxes at both diurnal and nocturnal periods. The average turbulence time scale was below 200 and 1200 s for all scalars at nighttime and daytime, respectively. In all cases, there is seasonal dependence. This result shows that the time scale commonly used to calculate nocturnal surface fluxes (30 min) includes a good portion of mesoscale flux in the estimates. The role of these mesoscale fluxes, in terms of seasonal dependence and the uncertainties they add to the estimates, is then analyzed.