Transition and turbulence in the stably stratified Ekman layer – TRANSTEK

It is difficult to attribute certain known characteristics of the stably stratified atmospheric boundary layer, such as the intermittency of turbulence, to purely dynamical effects in a stationary external flow, or to more complex interactions, with soil or with an evolving external flow. Our working hypothesis is that the secondary instability of stratified Ekman layer roll vortices can be a purely internal and dynamical source of intermittency, without invoking other causes that may be relevant depending on atmospheric conditions. Indeed if the time scale of secondary instability is dictated by local overturning, as suggested by stratified free shear layers, while the emergence of a roll vortex takes place within an advective time scale, the emergence/breakdown phases of the eddy life cycle become increasingly asymmetric as stratification increases, with shorter and shorter turbulent phases. Our main objectives are therefore to : 1.Establish a systematic bifurcation diagram of the transition to turbulence in the Ekman boundary layer with neutral to strongly stable stratification, together with the characteristic temporal and spatial scales and a qualitative understanding of the instability mechanisms. Indeed the transition to turbulence of such a fundamental flow as the Ekman layer has received a very incomplete description, especially in the presence of stratification. 2.Assess the ability of the instability mechanisms to explain the variability patterns and scales observed in laboratory experiments and ground-based atmospheric measurements performed at the SIRTA observatory and during the SHEBA campaign. 3.Complement the experimental and numerical data produced to meet the previous objectives in order to contribute a novel set of time-space resolved data suitable for the derivation of flux-gradient relationships and the assessment of SBL similarity hypotheses.