A Study on the Characteristics of the Coherent Structure of Turbulence in a Semi-Arid Area

The coherent structure (CS) of turbulence plays an important role in turbulent transfer. Combining the methods of Barthlott et al. (2007), Thomas and Foken (2005), and Zhang et al. (2011) to extract CS with the relevant study results, and considering the phenomenon of consecutive occurrence for CS, we repeated the experiments to obtain a specific method to extract CS using the wavelet transform technique. This method is effective and can achieve results consistent with existing research. Adopting this method, we applied the turbulence observation data of the Dingxi Arid Meteorology and Ecological Environment Experimental Station, Lanzhou Institute of Arid Meteorology, China Meteorological Administration, and performed quality-control and filtered the linear gravity wave to analyze the characteristics of CS in a semi-arid area. We found that (1) in the typical daytime CS pattern the air converges and rises, the temperature increases, and the water vapor density increases. The air then diverges and sinks, and the temperature and the water vapor density decreases. In the typical nighttime mode the air converges and rises, the temperature decreases, and the water vapor density increases. Next, the air diverges and sinks, the temperature rises, and the water vapor density declines. (2) The typical values for the number of CS per 30 min, duration, and interval were 15, 18.2 s and 18 s, respectively. The Taylor length scale was typically less than 15 m and the intermittency factor was 55% (stable) or 85% (unstable). Two consecutive CSs were most frequent. (3) The duration of the ejection and sweep processes exhibited a significant linear relationship with a slope of less than 1, and the CS was not symmetric. (4) The Taylor length scale and stability/frictional velocity also exhibited a significant linear relationship. The mechanical factors tended to generate large CSs, while the thermal factors generated more and smaller CSs. (5) There was also a significant correlation between the ramp intensity and stability/frictional velocity. When more stable or unstable, the scalar ramp intensity generally first decreased and then increased, and the opposite occurred with the vector. If the frictional velocity was higher, the scalar ramp intensity first increased and then decreased, and the vector ramp intensity exhibited a steady increasing trend. (6) The ratio of the inner and outer radii revealed that CS was more concentrated under the unstable condition than under the stable condition, with scalars relatively concentrated in the center of the CS and the vectors relatively scattered. (7) The average flux contribution of CS to momentum, heat, and water vapor density was 31%, 37%, and 32%, and the transfer efficiency was 65%, 79%, and 67%, respectively; i.e., the transfer of scalars by CSs was more effective than that by momentum.