An embedded bottom boundary layer formulation for z-coordinate ocean models

An embedded bottom boundary layer (EBBL) scheme is developed to improve the bottom topographic representation in z-coordinate ocean general circulatio n models. The EBBL scheme is based on the combined techniques of an embedde d topography-following slab, an explicit turbulent bottom boundary layer (B BL), and a generalized pressure gradient formulation. The coupling between the interior z-level model and the EBBL model is achieved by exchanging ent rainment/detrainment and pressure gradients at the bottom layer surface, wh ich allows temporal and spatial variations. The EBBL is implemented into one of the most widely used z-coordinate model s, the Modular Ocean Model (MOM). A test problem with a source of dense wat er on a slope is used. The new EBBL products significantly more realistic p lume spreading than the existing BBL scheme of Killworth and Edwards and is comparable to the results from a topography-following coordinate model (SC RUM). which is believed to be more suitable for such a problem. Calculation of the momentum budget demonstrates that the improved representation of th e downslope pressure gradient formulation plays an important role in the si mulations of dense slope flows. Sensitivity experiments with different grid sizes, model parameters, and de nsity contrast between the cold source water and the warm interior water ar e carried out to lest the robustness of the EBBL scheme. In contrast to the BBL model of Killworth and Edwards, which tends to diffuse too much dense water along isobaths. the EBBL model allows dense water to sink across isob aths through a very thin bottom layer into the deep ocean. Even in the coar ser-resolution case (1/4 degrees and 15 levels) the EBBL produces more real istic deep water than the existing BBL with higher resolution (1/8 degrees and 30 levels), and at only one-eighth the computational cost. It is theref ore concluded that the EBBL scheme presented here is cost effective and rob ust to model resolution and mixing parameters, and should be easily impleme nted in any nontopography-following coordinate ocean model.