Turbulence statistics above and within two Amazon rain forest canopies

The turbulence structure in two Amazon rain forests was characterised for a range of above-canopy stability conditions, and the results compared with previous studies in other forest canopies and recent theory for the generat ion of turbulent eddies just above forest canopies. Three-dimensional wind speed and temperature fluctuation data were collected simultaneously at up to five levels inside and above two canopies of 30-40 m tall forests, durin g three separate periods. We analysed hourly statistics, joint probability distributions, length scales, spatial correlations and coherence, as well a s power spectra of vertical and horizontal wind speed. The daytime results show a sharp attenuation of turbulence in the top third of the canopies, resulting in very little movement, and almost Gaussian pr obability distributions of wind speeds, in the lower canopy. This contrasts with strongly skewed and kurtotic distributions in the upper canopy. At ni ght, attenuation was even stronger and skewness vanished even in the upper canopy. Power spectral peaks in the lower canopy are shifted to lower frequ encies relative to the upper canopy, and spatial correlations and coherence s were low throughout the canopy. Integral length scales of vertical wind s peed at the top of the canopy were small, about 0.15 h compared to a value of 0.28 h expected from the shear length scale at the canopy top, based on the hypothesis that the upper canopy air behaves as a plane mixing layer. A ll this suggests that, although exchange is not totally inhibited, tropical rain forest canopies differ from other forests in that rapid, coherent dow nward sweeps do not penetrate into the lower canopy, and that length scales are suppressed. This is associated with a persistent inversion of stabilit y in that region compared to above-canopy conditions. The inversion is like ly to be maintained by strong heat absorption in the leaves concentrated ne ar the canopy top, with the generally weak turbulence being unable to destr oy the temperature gradients over the large canopy depth.