Coherent structures in turbulent atmosphere. Experiment and theory

Processes of the Bénard cell origination and disintegration in air are studied experimentally. Our data confirm the main scenarios of turbulence origination (Landau-Hopf, Ruelle-Takens, Feigenbaum and Pomeau-Menneville stochastic scenarios). It is found that the Bénard cell disintegrates according to the Feigenbaum scenario. In this case, the main vortex in the Bénard cell is decomposed into smaller ones as a result of ten period-doubling bifurcations. The resulting turbulence is shown to be coherent and determinate; the fractal character (local self-similarity) of its spectrum is found. These results allow us to enlarge the notion coherent structure by including small-scale turbulent components. In the paper, the coherent structure is defined as a compact formation containing a long-lived spatial hydrodynamic cell (originating from the long-term action of thermodynamic gradients) and products of its discrete coherent cascade disintegration. The coherent structure meets all the features of chaos occurrence (turbulence) in typical thermodynamic structures. The real atmospheric turbulence is shown to be a result of (incoherent) mixing of different coherent structures with incommensurable frequencies of the main vortexes that carry energy. Based on the results presented here, the coherent structure (in its enlarged definition) can be considered as a key turbulence element.