Turbulence mechanisms at an agricultural site

Abstract

An extensive set of turbulence data from the 3- and 12-m heights taken over an agricultural site (Marsta, Sweden) are analyzed and compared with data from ‘ideal’ sites. In unstable air, Monin-Obukhov similarity is found to be valid for the non-dimensional gradients of wind, φ m , temperature, φ h , and humidity, φ e , for φe (only a few data), for σ T /|T *|,/ E /|E *| and for the non-dimensionalized inertial subrange spectra of temperature and humidity. Where comparison is possible, the unstable data also agree with those found in the Kansas study, with one remarkable exception, the inertial subrange constant of the temperature spectrum, β1, being only 0.39, compared to the value 0.80 found at the Kansas site. On the stable side, most similarity predictions break down, with most of the data differing systematically from the corresponding Kansas results, the only exception being φe. The inertial subrange constants for temperature, β1, and for humidity, γ1 are found to have the same values, 0.39 and 0.30, respectively, as they do on the unstable side. Remarkable similarity is found for the shape of the stable u- and θ- and e-spectra. In addition, this shape is found to be identical with that found in Kansas. The peak wavelength of the stable u-, and θ-spectra is found to be about four times larger than it is for the corresponding Kansas spectra. This is interpreted to be a result of the increased macro-roughness at the Marsta site as compared with that at the Kansas site. A possible explanation for the low β1-value is discussed, suggesting that β1 is not a universal constant, but instead dependent on the turbulent structure.