Comparison of Various Levels of Closure Model on Evolution and Maintenance of the Upper Ocean Mixed Layer during TOGA-COARE IOP

Abstract

Sensitivity of parameterization schemes for upper ocean mixing in a large-scale ocean model has been examined by using observational data of the R/V Hakuho-maru during the TOGA-COARE TOP, which was conducted from November, 1992 until February, 1993 in the equatorial Western Pacific region. The turbulent energy dissipation rate (e) and mixed-layer structure have been intensively observed, associated with surface fluxes at (Equator, 156°E) from 12-26 November, 1992.Time evolution of the mixed layer has been simulated by using 1-dimensional level 2, 2.5, 3 and 4 of the Mellor-Yamada closure model (Mellor and Yamada, 1974; Yamada and Mellor, 1975), and has been compared to the observations conducted by the R/V Hakuho-maru. A special emphasis was placed on the comparison of the turbulent energy dissipation rate, because it is an observed quantity directly related to a turbulent process. In general, the results of the four models are similar and, if compared to the observation, it is noted that the mixing is weak. Especially, when the surface wind is very weak, the discrepancy becomes prominent. It is concluded that there are few differences between these schemes as long as a 1-dimensional model is applied.Sensitivity of the results to vertical resolution has been investigated. General features of the upper mixing layer (N100m) are not different between high vertical resolution (dz=1m) and ordinary resolution (Liz=20m), although the diurnal variation of SST (0.2C) cannot be simulated by dz=20m.Time evolution of the mixed layer was simulated by using the level-2 model with eddy diffusion coefficients derived by the observed e. This result explains approximately 70% of the increase of heat content during this period.