Turbulent characteristics of a semiarid atmospheric surface layer from cup anemometers – effects of soil tillage treatment (Northern Spain)

Abstract

Abstract. This paper deals with the characteristics of turbulent flow over two agricultural plots with various tillage treatments in a fallow, semiarid area (Central Aragon, Spain). The main dynamic characteristics of the Atmospheric Surface Layer (ASL) measured over the experimental site (friction velocity, roughness length, etc.), and energy budget, have been presented previously (Frangi and Richard, 2000). The current study is based on experimental measurements performed with cup anemometers located in the vicinity of the ground at 5 different levels (from 0.25 to 4 m) and sampled at 1 Hz. It reveals that the horizontal wind variance, the Eulerian integral scales, the frequency range of turbulence and the turbulent kinetic energy dissipation rate are affected by the surface roughness. In the vicinity of the ground surface, the horizontal wind variance logarithmically increases with height, directly in relation to the friction velocity and the roughness length scale. It was found that the time integral scale (and subsequently the length integral scale) increased with the surface roughness and decreased with the anemometer height. These variations imply some shifts in the meteorological spectral gap and some variations of the spectral peak length scale. The turbulent energy dissipation rate, affected by the soil roughness, shows a z-less stratification behaviour under stable conditions. In addition to the characterization of the studied ASL, this paper intends to show which turbulence characteristics, and under what conditions, are accessible through the cup anemometer.

Wind fluctuations over the North Sea

“…Previous analysis of the spectral gap has addressed a variety of flow conditions and surface types. For example, Yahaya et al (2003) analysed wind speed time series for an inland site in North Spain, and found a very welldefined spectral gap with a period of around 12 min. Courtney and Troen (1990) calculated the wind speed spectrum in a flat, agricultural area of Denmark, and found a modest spectral gap, with only a factor of two separating the variance in the spectral gap from that in the turbulent peak.…”

“…Additionally, measurements of pressure and relative humidity were used to calculate the virtual potential temperature. Gaps in the measurements meant that Ri b could be calculated for most of 2001, and for parts of 2002and 2003. Following Grachev and Fairall (1997, the Obukhov length is related to Ri b according to…”

“…This part of the spectrum is often referred to as the 'spectral gap', since many authors have identified a local minimum in the wind speed spectrum at a period close to half an hour (for example, Van der Hoven, 1957;Petersen, 1975;Courtney and Troen, 1990;Yahaya et al, 2003). Based on the assumption of the spectral gap, we might expect there to be little variability in the wind speed on these time scales, but it has been shown that the spectral gap is not always well defined, and that in certain atmospheric conditions it may not exist at all (Gjerstad et al, 1995;Heggem et al, 1998).…”

Profile of the horizontal wind variance near the ground in near neutral flow – K-theory and the transport of the turbulent kinetic energy

“…), it was pointed out that the two tillage methods induced differences in both dynamic characteristics and energy budget partitioning between the two plots. In a second paper, Yahaya et al (2003) presented the influences of soil tillage treatment on some turbulent parameters, such as the Euler integral scales, the TKE dissipation rate, the spectral range of turbulence, the spectral energy containing-scale and the horizontal wind variance. Frangi and Richard (2000) and Gomes et al (2003).…”

“…Then, the question is, from which eddy scale can a measurement be considered as having integrated the substance of the wind velocity variance? In a previous study, relating to the same experiment, Yahaya et al (2003) showed that the length scales of the spectral peaks, range from 187 m (bottom level) to 522 m (upper level), on plot CT, and from 95 m to 450 m, on plot RT. So, in the light of these results, we can affirm that the sampling cut-off scales represent about a tenth of the energy containing ones.…”