Physical Model of Gusty Coherent Structure in Atmospheric Boundary Layer.

Coherent 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. Plain Language Summary: After the passage of a cold front in spring in northern China, there is a rather regular gusty wind superimposed on the basic strong wind, and its horizontal and vertical component are negatively correlated and make downward transport of momentum and matter more effective. It was later found that this gusty coherent structure is a phenomenon of mechanical turbulence. This paper points out the possible physical phenomenon underlying this negative coherence, gives a dynamic explanation of this phenomenon by the hydrodynamic stability method, and compares the solutions with the observation results. Key Points: The gusty coherent structure can be explained by quasi‐streamwise vortex pairs and high and low speed streaksThe physical model of the gusty coherent structure can be obtained by the hydrodynamic stability analysis methodThe position of maximum intensity of vortexes and streaks obtained from the solution is consistent with the observation [ABSTRACT FROM AUTHOR]