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Speakers: Part 1: Igor Rogachevskii, Nathan Kliorin. Part 2: Igor Ezau.
Date: February 19, 2014
Place: FL2, Room 1022
Titles: Part 1: New turbulence closure theory for shear-free and sheared convective boundary layers.
Part 2: Self-organized turbulence and its impact on the Ekman boundary layer characteristics.
We discuss an energy- and flux budget turbulence closure theory for shear-free and sheared convective boundary layers (CBL). This theory is based on the three-fold decomposition: mean flow + organised strictures + turbulence, in combination with analytical models of convective cells and convective rolls for the shear-free and sheared convective boundary layers, respectively. We consider CBL as consisted of the three basic parts: (i) surface layer strongly unstably stratified and dominated by small-scale turbulence of very complex nature including usual 3-D turbulence, generated by both mean-flow shears and structural shears, and unusual strongly anisotropic buoyancy-driven turbulence; (ii) CBL core dominated by the structural energy-, momentum- and mass-transport, with minor contribution from usual 3-D turbulence generated by local structural shears on the background of almost zero vertical gradient of potential temperature (or buoyancy); (iii) turbulent entrainment layer at the CBL upper boundary, characterised by essentially stable stratification with negative (downward) turbulent flux of potential temperature (or buoyancy). In this talk we focus on the first two parts: (i) and (ii). We discuss the self-organisation of convective turbulence, obtain analytical solution for typical structures, namely, cells - in shear-free CBLs and rolls - in sheared CBLs, formulate budget equations for turbulent energies and fluxes in the CBL core and in the surface layer, and outline a generalised turbulence closure method incorporating structural and turbulent transports.
The turbulent Ekman boundary layer (EBL) has been studied in a large number of theoretical, laboratory and modeling works since F. Nansen’s observations during the Norwegian Polar Expedition 1893–1896. Nevertheless, the proposed analytical models, analysis of the EBL instabilities, and turbulence-resolving numerical simulations are not fully consistent. In particular, the role of turbulence self-organization into longitudinal roll vortices in the EBL and its dependence on the meridional component of the Coriolis force remain unclear. A new set of large-eddy simulations (LES) are presented in this study. LES were performed for eight different latitudes (from 1 N to 90 N) in the domain spanning 144 km in the meridional direction. Geostrophic winds from the west and from the east were used to drive the development of EBL turbulence. The emergence and growth of longitudinal rolls in the EBL was simulated. The simulated rolls are in good agreement with EBL stability analysis given in Dubos et al. (2008). The destruction of rolls in the westerly flow at low latitude was observed in simulations, which agrees well with the action of secondary instability on the rolls in the EBL. This study quantifies the effect of the meridional component of the Coriolis force and the effect of rolls in the EBL on the internal EBL parameters such as friction velocity, cross-isobaric angle, parameters of the EBL depth and resistance laws. A large impact of the roll development or destruction is found. The depth of the EBL in the westerly flow is about five times less than it is in the easterly flow at low latitudes. The EBL parameters, which depend on the depth, also exhibit large difference in these two types of the EBL.