Surface heterogeneities and the structure of the turbulent flow: a puzzle of timescales

Flow over surface heterogeneities has been mostly investigated through the lenses of internal boundary layers, to understand how much, or over how long, the characteristics of the flow over a certain surface patch transport over to the neighboring patches. Interestingly, there remains a limited understanding on whether the resulting flow presents a roll-type structure due to persistent shear with the surface, or instead is characterized by a cell-type structure resulting from the strong land-atmosphere coupling. Furthermore, little differentiation has been made on whether the land-atmosphere convective forcing leads to stationary, or transient convective cells. Here, we investigate the flow developed in different land-atmosphere configurations, as well as the controlling mechanisms that enable the transition from one type of flow to the other. We leverage the use of a quadruple flow decomposition in parallel with the definition of four different time scales that characterize each of the processes described by such decomposition. We then hypothesize that a combination of these timescales in a set of non-dimensional Pi-groups allow to determine, a-priori, under what land-atmosphere configurations the different terms of the quadruple decomposition become relevant, and what are the competing flow mechanisms to mixing. For this work, we exploit several existing Large-Eddy Simulation datasets of ABL flows over different types of surface heterogeneity.