By Robert Hallberg and Brandon Reichl
The ocean’s surface boundary layer is critical for modeling the climate system, and can profoundly influence the overall stability and reliability of coupled climate models. Often the parameterizations that are used to describe the physical principles and processes that govern the dynamics of the planetary boundary layer can make it challenging to understand what is happening. This talk presents a description of the dominant physical processes governing the planetary boundary layer drawing directly upon fundamental physical principles of conservation of energy and geometrically or dynamically derived mixing length-scales.
This talk shows how these fundamental physical ideas can be combined with careful numerical implementations to derive a new parameterization of ocean boundary layer mixing that is appropriate for use in global-scale ocean climate or near-term forecast models. This new parameterization, the energetic Planetary Boundary Layer (ePBL), combines a vertically integrated energy budget (similar to a traditional bulk mixed layer) with finite diffusivities in the boundary layer (similar to more elaborate two-equation closures or the widely used K-profile parameterization), in a computationally efficient form appropriate for use in climate models. The nondimensional scaling factors of ePBL are calibrated to reproduce both the results of a more elaborate two-equation turbulence closure and from highly resolved “Large Eddy Simulations” of small-scale turbulent motions across a wide range of parameter space. ePBL is demonstrated to work well in NOAA/GFDL’s new CM4 coupled climate model by various metrics. Finally, these same ideas are shown to have promise for a unified, flexible, and energetically consistent treatment of mixing throughout the ocean, which can harmoniously blend energetically constrained mixing from many diverse sources.
This talk is intended to be accessible to an generally scientifically literate audience without previous exposure to theories or terminology of ocean turbulence.