Large scale turbulence structures in the Ekman boundary layer

The Ekman boundary layer (EBL) is a non-stratifi ed turbulent layer of fl uid in a rotated frame of reference. The EBL comprises two sub-layers, namely, the surface sub-layer, where small scale well-developed turbulence dominates, and the core sub-layer, where large scale self-organized turbulence dominates. This study reports self-organization of large scale turbulence in the EBL as simulated with the large-eddy simulation (LES) model LESNIC. The simulations were conducted in a large domain (144 km in the cross-fl ow direction, which is an equivalent to about 50 EBL depths) to resolve statistically signifi cant number of the largest self-organized eddies. Analysis revealed that the latitude of the LES domain and, unexpectedly, the direction of the geostrophic wind forcing control the self-organization, turbulence scales, evolution and the quasi steady-state averaged vertical profi les in the EBL. The LES demonstrated destabilization of the EBL turbulence and its mean structure by the horizontal component of the Coriolis force. Visualisations of the EBL disclosed existence of quasi-regular large scale turbulent structures composed of counter-rotating vortices when the geostrophic fl ow was set from East to West. The corresponding structures are absent in the EBL when the geostrophic fl ow was set in the opposite (i.e. West to East) direction. These results fi nally resolve the long-standing controversy between the Leibovich-Lele and the Lilly-Brown instability mechanisms acting in the EBL. The LES demonstrated that the Lilly-Brown mechanism, which involves the vertical component of the Coriolis force, is working in the polar EBL where its impact is nevertheless rather small. The Leibovich-Lele mechanism, which involves the horizontal component of the Coriolis force, acts in low latitudes where it completely alters the turbulent structure of the EBL.