AN IMPROVED MIXED-LAYER MODEL FOR GEOPHYSICAL APPLICATIONS

An improved mixed layer model, based on second-moment closure of turbu lence and suitable for application to oceanic and atmospheric mixed la yers, is described. The model is tested against observational data fro m different locations in the global oceans, including high latitudes a nd tropics. The model belongs to the Mellor-Yamada hierarchy but incor porates recent findings from research on large eddy simulations and se cond-moment closure. The modified expansion of Galperin, Kantha, Hassi d and Rosati (1988) that leads to a much simpler and more robust quasi -equilibrium turbulence model is employed instead of the original Mell or and Yamada (1974) model. Findings from ongoing research at the Nati onal Center for Atmospheric Research on large eddy simulations of the atmospheric boundary layer are utilized to improve parameterizations o f pressure covariance terms in the second-moment closure. Shortwave so lar radiation penetration is given careful treatment in the model so t hat the model is applicable to investigations of biological and photoc hemical processes in the upper ocean. But by far the major improvement is in the inclusion of the shear instability-induced mixing in the st rongly stratified region below the oceanic mixed layer that leads to a more realistic and reliable mixed layer model that is suitable for ap plication to a variety of geophysical mixed layers and circulation pro blems. The model appears to predict the mixing in the upper ocean well on a variety of time scales, from event scale storm-induced deepening and diurnal scale variability to seasonal time scales. With proper at tention to the heat and salt balances in the upper ocean, it should be possible to use it for simulations of interannual variability as well . While the model validation has been primarily against oceanic mixed layer data sets, it is believed that the improvements can be readily i ncorporated into a model of the atmospheric boundary layer as well.