Structuring of turbulence and its impact on basic features of Ekman boundary layers

Abstract. 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. Thus, this study supports the need to include the horizontal component of the Coriolis force into theoretical constructions and parameterizations of the boundary layer in models.