Generation of Large-scale Semi-organized Structures in Turbulent Convection

In the last decades it has been recognized that the very high Rayleigh number convective boundary layer (CBL) has more complex nature than might be reckoned. Besides the fully organized component naturally considered as the mean flow and the chaotic small-scale turbulent fluctuations, one more type of motion has been discovered, namely, long-lived large-scale structures, which are neither turbulent nor deterministic. These semi-organized structures considerably enhance the vertical transports and render them essentially non-local in nature. In the atmospheric shear-free convection, the structures represent three-dimensional Benard-type cells composed of narrow uprising plumes and wide downdraughts. They embrace the entire convective boundary layer (∼ 2 km in height) and include pronounced large-scale (∼ 5 km in diameter) convergence flow patterns close to the surface (see, e.g., Atkinson and Wu Zhang, 1996; Etling and Brown, 1993, and references therein). In sheared convection, the structures represent CBL-scale rolls stretched along the mean wind. Life-times of the semi-organized structures are much larger than the turbulent time scales. Thus, these structures can be treated as comparatively stable, quasi-stationary motions, playing the same role with respect to small-scale turbulence as the mean flow. In a laboratory turbulent convection several organized features of motion, such as plumes, jets, and the large-scale circulation, are known to exist. The experimentally observed large-scale circulation in the closed box with a heated bottom wall (the Rayleigh-Benard apparatus) is often called the ”wind” (see, e.g., Niemela et al., 2001, and references therein). There are several unsolved theoretical questions concerning these flows, e.g., how do they arise, and what are their characteristics and dynamics.