Very large scale motions in the atmospheric surface layer: a field investigation

Abstract

A field observation array for the atmospheric surface layer (ASL) was built on a dry flat bed of Qingtu Lake in Minqin (China) as the Qingtu Lake Observation Array (QLOA) site, which is similar to the Surface Layer Turbulence and Environmental Science Test (SLTEST) site in the Utah (USA) Western desert. The present observation array can synchronously perform multi-point measurements of wind velocity and temperature at different vertical and streamwise positions. In other words, three-dimensional turbulent ASL flows can be measured at the QLOA station and Reynolds numbers as high as $Re_{\unicode[STIX]{x1D70F}}\sim O(10^{6})$ can be achieved with steady wind conditions. By careful selection and pretreatment for measured data of more than 1200 h, the QLOA data have been validated to be reliable for high Reynolds number turbulent boundary layer research. Results from correlation and spectral analysis confirm that very large scale motions (VLSMs) exist in the ASL at a Reynolds number up to $Re_{\unicode[STIX]{x1D70F}}\approx 4\times 10^{6}$. Through premultiplied spectral analysis, it is revealed that the spectral energy in the high-wavenumber region decreases with height, similar to turbulent boundary layers at low or moderate Reynolds numbers, while it increases with height in the low-wavenumber region resulting in a log–linear increase of VLSMs energy with height, which is different from turbulent boundary layers at low or moderate Reynolds numbers. The present analyses support the view that the evolution of the VLSMs cannot be fully attributed to a ‘bottom-up’ mechanism alone, and probably other mechanisms, including a ‘top-down’ mechanism, also play a role.

Statistics and spectra of turbulence under different roughness in the near‐neutral atmospheric surface layer

“…height at different roughness conditions was given in Figure 8. It should be noted that a distinct peak in the low-wavenumber region that corresponds to the VLSMs is observed in the spectra of all the heights (Wang and Zheng, 2016). Although another peak corresponding to the LSMs is also observed in other studies (Balakumar and Adrian, 2007;Hutchins and Marusic, 2007;Mathis et al, 2009), this peak is difficult to distinguish in the current ASL data.…”

“…The field observation of flat sand surface is carried out in QLOA (Qingtu Lake Observation Array, the detailed descriptions of this experimental site can be seen in Wang and Zheng (2016)). The field observation of flat sand surface is carried out in QLOA (Qingtu Lake Observation Array, the detailed descriptions of this experimental site can be seen in Wang and Zheng (2016)).…”

“…The data pretreatment method is same as Wang and Zheng (2016). That is, the stratification stability judgment (|z/L| < 0.1), wind direction adjustment (|α| < 30°) and nonstationary effects exclusion (IST less than 30%) were used to select the proper data to analyze the results in this paper.…”

“…The data obtained from the three kinds of roughness surface data are selected as 9 m/s at the heights of 5 m, and the data are shown in Table II. The smoothing operations for the raw power spectrum were performed, which can be found in Wang and Zheng (2016). The smoothing operations for the raw power spectrum were performed, which can be found in Wang and Zheng (2016).…”

“…In addition, the peak energy of premultiplied spectral energy and the wave number of the peak were extracted, and the variation of the average peak energy and wave number with The time "20130507_15-16 (as an example)" means pm15: 00-pm16: 00 on May 7, 2013; "U mean (5 m)" means the mean velocity at the heights of 5 m; "u * " means that we acquire the shear velocity by the MVP through the heights and the mean velocity (Clauser method); "u τ (5 m)" means the shear velocity is calculated by ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi -< u ' w ' > p at the height of 5 m. Other parameters (viscosity, z/L and IST) details can be seen Wang and Zheng (2016). height at different roughness conditions was given in Figure 8.…”