Thom (1971) has described the importance of the canopy area density (CAD) to the mean and turbulent flow in the canopy. In this study, we test the hypothesis that turbulent moments exhibit an exponential decay similar to the Beer's law absorption for light. We use CAD profiles from several forest types and scale height using a vegetation-weighted thickness (zc). Special emphasis is placed on a case study of foliated vs. leafless conditions at a broadleaf forest site. We find that profiles of mean wind speed, variances, covariances, and skewness of wind components and selected scalars are generally similar when placed in this framework. However, the data separate naturally into two forms, according to vegetation type (e.g., coniferous vs. broadleaf forests). This indicates that other parameters, such as canopy top topography, are needed to parameterize the turbulent flow within the canopy.
The impact of the LAD on the roughness sublayer (RSL) is also studied. Most of tower-based measurements over forest are close to the canopy (z/h < 1.5, where z and h are the instrument and the canopy height respectively). Preliminary results indicate that the skewness of the vertical wind speed in the roughness sublayer (RSL) seems to follow a single curve, when an alternative displacement height is used. The displacement height was determined by using zc instead of z to calculate the mean momentum absorption level. Efforts now are concentrated to determine the characteristic RSL mixing length and explain differences from the observed profiles to the logarithm profiles observed in the inertial sublayer (ISL).
An extensive spectral analysis for a deciduous forest is also performed for the wind speed, temperature, humidity, and CO2. In the RSL the spectra and cospectral curves presented a more peaked shape than the one found in ISL. The subcanopy spectra and cospectra show that the upper canopy works as a low pass filter.
Thom, A.S., 1971. Momentum absorption by vegetation. Quarterly Journal of Royal Meteorological Society, 97, 414-428.
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