Conditions for the Formation of the Coherent Structure in Gusty Disturbances in the Atmospheric Boundary Layer

The discovery of coherent structures in wall turbulence is one of the significant progresses in modern research of turbulence. The coherent structure was found in the atmospheric boundary layer in the 1950s, which was called cloud street and has been studied extensively. In recent years, a coherent structure was also found in the surface layer. Based on analysis of strong wind and light wind data in the urban boundary layer collected at the 325 m meteorological tower in Beijing, it is revealed that gusty disturbances have a period equal to around 3-6 min and possess coherent structure. The coherent structure is also found in disturbances over different underlying surfaces and makes downward transport of momentum more effectively. Conditions for the formation of the coherent structure in gusty disturbances was investigated using the turbulent data from the ultrasonic anemometer-thermometers and the mean wind and temperature data collected at the monitoring tower in the Badan Jilin Desert in Minqin County of Gansu Province in April 2012. The time series of three dimensional atmospheric velocity were analyzed using conventional Fourier spectral analysis. The atmospheric velocity was decomposed into three components:basic mean flow (period>10 min.), gusty disturbance (1 min < period < 10 min.) and turbulence fluctuation (period<1 min.). The results show that the coherent structure occurs in neutral, unstable and even slightly stable boundary layer. In other words, the coherent structure occurs under the condition that the turbulence is mainly generated by mechanical action, and it can be suppressed or disturbed by buoyancy. Thus, the coherent structure in the gusty wind and that in wall turbulence occur in neutral boundary layer. It is generated by mechanical action and plays a major role in energy and momentum transport. The coherent structure in the gusty wind is not the same as cloud street, and it appears under different atmospheric stability conditions and has different scales.