Wave-modified turbulence fluxes above and below a forest canopy in the stable boundary layer

This presentation presents the results of a study of wave-modified fluxes of heat, momentum, water vapour, and carbon dioxide in the stable boundary layer above and below a forest canopy at a site near the Savannah River Laboratory near Aiken, SC. It is shown that failing to filter wave signals from the data results in over estimations of fluxes. This is because vertical fluxes due to waves are quite small when compared with turbulence fluxes; however, in Reynolds averaging these waves will be considered turbulence fluctuations, a process we term ‘turbulence inflation'.

Our analysis consists of first identifying periods of wave activity using wavelet analyses of surface pressures which reveal the times, durations, and frequency ranges of wave-like activity. The data are then band-passed filtered to estimate the amplitudes of wave-like perturbations of vertical velocity, temperature, water vapour, and carbon dioxide. These wave perturbations are then removed from the original time series, and the true turbulence fluxes are calculated. Our results will evaluate the potential errors in calculating fluxes with inflated turbulence.

Surface pressures were measured with an array of seven microbarographs. Turbulence data were obtained from sonic anemometers located at 30, 60, and 300 m above ground level on a tower in a forest clearing. At these same heights, water vapour and carbon dioxide concentrations were measured with Li-Cor 7500 instruments. The mean height of the forest was 10 m.

Analysis periods were May and December 2009. Wave-like motions were observed on almost all nights. Disturbances with periods from 2 to 4 minutes occurred almost continually on all nights. Disturbances with periods ranging from 10 to 25 and persisting for an hour or so were seen on many nights, but not every night. Our analysis focuses on these longer period disturbances since the time scales for these are close to the time scale used for flux calculations.