Physical Model of Gusty Coherent Structure in Atmospheric Boundary Layer

Abstract

AbstractCoherent structure is an important phenomenon in the fluid field, and also exists in atmospheric boundary layer. For example, after the passage of a cold front in spring in northern China, there is a rather regular gusty wind with a period of approximately 3 min superimposed on the basic strong wind, and the gusty wind possesses a coherent structure: the vertical velocity is upward when horizontal velocity is in the valley phase, but downward when horizontal velocity is in the peak phase. The coherence makes transport of momentum and matter more effective. It was later found that this gusty coherent structure is a phenomenon of mechanical turbulence, which occurs in weakly stable, neutral and unstable stratification. However, research on the physical mechanism of this coherent structure is still lacking. This paper shows that the gusty coherent structure can be explained by quasi‐streamwise vortex pairs and high and low speed streaks induced by quasi‐streamwise vortexes. The coherent structure can be obtained from the three‐dimensional Navier‒Stokes (N‒S) equations. The maximum intensities of vortices and streaks are located at 20.7% and 12.1% of the boundary layer heights respectively, which are consistent with the observed results at 200 and 120 m.