Laboratory Simulation of Field Measurements of the Bottom Boundary Layer

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The turbulent region near the ocean floor is rich in dynamical processes. This region can be conveniently thought of as consisting of overlapping turbulent bottom boundary layers (TBBL) and turbulent bottom mixed layers (TBML). Because of the ubiquitous nature of flows along isobaths and their relative simplicity, much of the recent research on turbulent regions along boundaries has been performed under these conditions; e.g., flows along coastlines and ridges. Field experiments, coupled with theoretical and numerical modeling efforts during the past several decades have sharpened our understanding of this boundary region. There are gaps in our knowledge, however. There is a need, for example, to (i) have an improved understanding of the relation between the TBBL and the TBML, (ii) to have more information on Ekman layer arrest for turbulent flows because it is observed in certain field programs and not in others, (ii) there is controversy on the occurrence of double log boundary layers and (iv) it is desirable tobetter organize and consolidate the knowledge gained from various field measurements on oceanic turbulent boundary layers into some sort of unified whole. The intellectual merit of the project lies in the possibility of demonstrating clearly that laboratory simulations employing turbulence can or cannot quantitatively simulate oceanic boundary layers. The physical system considered is the along isobath flow of a stratified fluid in the presence of background rotation. The first objective of the proposed research is to show that the ocean's TBBL can be simulated in the laboratory using a sufficiently large rotating platform. To accomplish this, the laboratory simulations are to be compared directly with existing field data from studies of the deep water flow of the North Atlantic Deep Western Boundary Current along the Blake Outer Ridge and the winter and summer-time along isobath currents along the Northern California Shelf. These direct comparisons with ocean data will also shed light on the relation between the TBBL and the TBML. The second objective is to generalize data on the bottom shear stress, boundary layer depth and angle between the geostrophic flow and the bottom shear stress in terms of the external parameters of the system; these include the slope Burger number, the normalized time from the beginning of the experiment and parameters defining the surface roughness. Similarity will require the proof that, as in many engineering flows that as the Reynolds number becomes large, the flow characteristics become independent of the Reynolds number, a condition termed Reynolds number similarity.Broader impacts: This project will have broad impact on the range of oceanographic sub-disciplines if it can be successful in defining gross features of the bottom boundary layer in terms of a clear set of parameters, many of which can be measured remotely. Additionally the project will support international science and provide US and French students opportunities to interact in an international atmosphere.