Importance of Low-Frequency Contributions to Eddy Fluxes Observed over Rough Surfaces

Abstract

Eddy covariance flux observations at a deciduous temperate forest site (83 days) and at a boreal forest site (21 days) are analyzed for midday periods (1100–1400 LT). Approximate stationarity of the time series is demonstrated, and the ensemble-averaged roughness sublayer cospectra are presented. Spectral and cospectral forms in the roughness sublayer are more peaked than those found in an inertial sublayer. They exhibit similar forms dependent on (z − d)/(h − d), where d is the displacement height and h is the canopy height. The inertial-layer spectral forms are recovered when observations are made where this scaled height is approximately 4. For a sample summer at the midlatitude deciduous forest, large eddies with periods from 4 to 30 min contribute about 17% to surface eddy fluxes of heat, water vapor, and carbon dioxide (CO₂). Much larger contributions can occur in light-wind conditions. This effect, likely caused by the passage of convective boundary layer eddies, is not observed when using many currently popular averaging procedures. Several running-mean periods have been used to assess the effect of the mean removal procedure on flux estimates. Given the assumption that large eddies would have been sampled at the towers had an ensemble measurement been possible, a correction is proposed based primarily on the mean wind speed to adjust fluxes obtained using short averaging intervals. This correction is successful in achieving observational energy-balance closure at two dissimilar forested sites. Cospectral similarity is found for all scalars studied. Daytime fluxes of CO₂, for example, can be underestimated at standard flux towers by 10%–40%, depending on wind speed.