Turbulent transport of carbon dioxide and water vapor within a vegetation canopy during unstable conditions: Identification of episodes using wavelet analysis

The net exchange of CO2 between the biosphere and atmosphere is realized as a difference between the fluxes associated with photosynthesis and respiration. This paper contrasts the turbulent transport mechanics of two dominant pathways affecting this exchange. Using high-frequency measurements from an experiment conducted at the Duke Forest in North Carolina, wavelet analysis is applied to time series of carbon dioxide and water vapor concentrations in order to (1) determine the dominant eddy sizes involved in the net exchange of these constituents, (2) resolve the eddy size and timescales involved in the intermittent release of CO2 from the forest floor to the atmosphere, and (3) relate the boundary layer turbulent characteristics to the transport of air enriched in CO2 from soil respiration. During the daytime hours, when photosynthesis and soil respiration are active in this pine forest and evapotranspiration is taking place, air enriched in both CO:! and water vapor is indicative of transport from the forest floor. Thus the coherent turbulent structures associated with these transport events are identified and conditionally analyzed from the time series by wavelet transforms, which retain information in the time domain as well as the frequency domain. The dominant flux-carrying eddies between the canopy and atmosphere were approximately 63 m in diameter, about four times the height of the canopy. Eddies that were most effective in transporting air enriched in CO2 from below the canopy to the atmosphere were found to be approximately 8 m in diameter, on the order of one half the canopy height. Conditional sampling shows that the prevalence of air enriched in both CO2 and water vapor is related to the rate of turbulent kinetic energy production measured from 24 approximately half-hour time series corresponding to unstable atmospheric conditions.