An approach to detect turbulence coherent structures based on quadrant analysis

Abstract

<p>Turbulence coherent structures play an important role in the transport of momentum, sensible heat, water vapour and carbon dioxide fluxes over atmospheric surface layer (ASL) . Using eddy covariance system measurements on a 50m tower in Zengcheng, Guangdong province, we develop a novel method based on quadrant analysis to detect turbulent coherent structures. We presume that turbulent flux events’ durations smaller than threshold t are isotropic turbulence. Therefore, the durations of small-time-scale (duration< t) turbulent flux events of each quadrant are expect to be equal, which can be regarded as the criterion of threshold . A deviation of the similarity between four quadrant small-time-scale turbulent flux events’ durations is set to determine the  value. Contour map of momentum flux joint probability density function on quadrant domain proves our hypothesis. Coherent structures can be identified from large-time-scale (duration>t) turbulent flux events.</p><p>We apply this method to the momentum, sensible heat, water vapour and carbon dioxide fluxes and obtain individual turbulent coherent structures time-series of different fluxes. It is found that numbers and durations of turbulent coherent structures are similar. Secondly, threshold  is not sensitive to the change of ASL satiability. Compared with k method (NARASIMHA 2007), our  method stands for more physical background as it can be seen as the time-scale of isotropic turbulence, which makes our detecting method more efficient.</p>