Some observations of turbulence and turbulent transport within and above plant canopies

Abstract

Observations have been made of the structure of turbulence and turbulent exchange within plant canopy layers. A new three-dimensional anemometer was used to measure the eddy fluxes of heat and momentum, and the related cospectra, within and above a corn crop and above a red pine forest. Measured values of momentum and heat fluxes, at each height within the corn canopy, were relatively constant proportions of the flux above the canopy, for the period of a day's observation. Extensive regions obeying a -j power relation were found. Isotropy was found above the forest at high frequencies while above and within the corn crop, the ratios of the lateral and vertical spectral densities to the longitudinal component were less than the expected value in the -j region. In all situations, the vertical velocity spectra were more peaked than a 'universal' curve, particularly a vertical velocity spectrum from above the forest. It is suggested that the additional variance results from the mixing caused by the individual roughness elements. As expected, the spectra could not be normalized using the height above the soil surface to calculate a non-dimensional frequency, but scaling heights were estimated by matching the frequencies of the peak of each curve with that of the 'universal' curve. Cospectra of uw and wT within the corn canopy were of similar shape and frequency regime, and were basically similar in shape to cospectra above the crop. All of the cospectra were more sharply peaked than 'universal' cospectral curves.

Large-eddy simulation of plant canopy flows using plant-scale representation

“…The experimental data used here include our recent PIV measurements and previous experiments of Shaw et al (1974) and Wilson et al (1982). The normalized and curve-fitted data of Shaw et al (1974) and Wilson et al (1982) were extracted from the review paper of Finnigan (2000) and are quoted in this paper. Most figures presented here only show the vertical profiles below z/h = 2 because the available experimental data are for below this height.…”

“…To date, the complexity of turbulence structures and high turbulence intensities inside canopies have prevented the development of a general similarity scheme in canopy turbulence like, for example, the Monin-Obukhov theory in the atmospheric surface layer. Interests in quantifying turbulence statistics and exploring large-scale coherent structures in plant canopies have led to a number of experimental investigations for different types of canopies, such as forests (Shaw et al 1988;Gardiner 1994), cereal crops (Shaw et al 1974;Wilson et al 1982;van Hout et al 2007), and wind-tunnel canopy models (Seginer et al 1976;Brunet et al 1994;Zhu et al 2006), to name just a few. Some important turbulence structures have been observed in the experiments, e.g., coherent scalar ramp patterns (Gao et al 1989) and wavelike travelling disturbances in cereal canopies (Finnigan 1979).…”

“…Two sets of existing experimental data from corn canopies (Shaw et al 1974;Wilson et al 1982) are also used in the assessment of the present LES results. These data were collected at the Elora Research Station, Elora, Ontario, Canada, over different years.…”

Turbulence structure in a deciduous forest

“…The dissipation of turbulence, in higher-order closure models, is commonly parameterized with length scales, derived from surface boundary-layer arguments. Yet, some field data suggest that Boundary-Layer Meteorology 43 (1988) plant elements act to short-circuit the normal eddy cascade process, which may alter the dissipation rate of turbulence (Shaw and Seginer, 1985;Baldocchi and Hutchison, 1988) whereas other field data show that the eddy cascade proceeds according to Kolmogorov's scaling arguments (e.g., Wilson et al, 1982;Shaw et al, 1974b). Some Lagrangian models assume a homogeneous turbulence field inside the canopy (Raupach, 1987), yet field and wind tunnel measurements indicate that turbulence is actually inhomogeneous inside plant canopies (Wilson et al, 1982;Raupach et al, 1986).…”

Computational parameter estimation for a maize crop

“…In the calculations, the drag coefficient C, = 0.20 is the value given by Wilson and Shaw (1977) In our analysis, a value of the relative turbulent intensity of i,,, = 0.30 was used. As Shaw et al (1974b) state, the experiments were made in September when the structure of the crop was quite different from that of a healthy crop; hence the crop may not be representative of that during the major portion of its growth cycle. Consequently, uncertainty exists in applying the value i, = 0.30 in the model calculations.…”