Multi-scale decomposition of turbulent fluxes above a forest canopy

Multi-scale properties of upward and downward component contributions to the turbulent fluxes of momentum, sensible heat, water vapor, and CO2 over a forest canopy in different atmospheric stability conditions are examined. The technique uses an innovative wavelet cospectral decomposition of fluxes into their positive and negative components. Results show that both the frequency of occurrence and the intensity of upward and downward events in the wavelet cospectra, as well as the upward and downward global wavelet cospectra, are intimately tied to the scale of motion. The average frequency of occurrence of the events in both directions was close to 50% at small scales, with the main component dominating at larger scales. The averaged normalized global intensity of the main component of scalar wavelet cospectrum has a prominent peak at 30-70 s varying with the interested fluxes and the atmospheric stability, and decreases sharply for larger scales and more gradually for smaller scales. The intensity of the minor component is almost constant in the fine scales, and decreases to almost zero as the scale increases. Results from these analyses are indirectly supported by techniques such as the quadrant-hole analysis and the Fourier cospectrum. These properties suggest that the main component of wavelet cospectrum dominating at larger scales has a more important contribution to fluxes than the minor component. These results support the idea that the scalar exchange takes place mostly through the action of large-scale eddies.