The spectral features of heat and momentum transfer within open canopies

The eddy covariance data have been analyzed to examine the spectral feature of canopy turbulence and the role of convection in heat and momentum transfer within open canopies in unstable condition. For this purpose, spectra, cospectra and multiresolution decomposition have been anlayzed. The results show that the normalized power spectra obey closely a-2/3 power law in the upper canopy but they deviate near the bottom. The ratio of transverse to longitudinal spectrum is less than 4/3 in the inertial subrange, indicating that turbulence is anisotropic within the canopy. Convection increases variance and flux over a wide range of frequency, which leads to reduced role of canopy scale eddy in heat and momentum transfer within open canopies in unstable condition compared to stable condition. Fourier analysis and multiresolution decomposition results give the following features of heat and momentum transfer in the lower canopy. Both sweep and boundary layer convection contribute to heat transfer with similar periodicity but with different duration. The momentum transfer is dominated by large-scale gust. And small-scale convection contributes to heat flux but not to significantly momentum flux in small averaging time less than 25 s.