The sensitivity of numerical simulation to vertical mixing parameterization schemes: a case study for the Yellow Sea Cold Water Mass

The vertical mixing parameterization scheme,by providing the eff ects of some explicitly missed physical processes and more importantly closing the energy budgets,is a critical model component and therefore imposes signifi cant impacts on model performance.The Yellow Sea Cold Water Mass(YSCWM),as the most striking and unique phenomenon in the Yellow Sea during summer,is dramatically aff ected by vertical mixing process during its each stage and therefore seriously sensitive to the proper choice of parameterization scheme.In this paper,a hindcast of YSCWM in winter of 2006 was implemented by using the Regional Ocean Modeling System(ROMS).Three popular parameterization schemes,including the level 2.5 Mellor-Yamada closure(M-Y 2.5),Generic Length Scale closure(GLS)and K-Profi le Parameterization(KPP),were tested and compared with each other by conducting a series of sensitivity model experiments.The infl uence of diff erent parameterization schemes on modeling the YSCWM was then carefully examined and assessed based on these model experiments.Although reasonable thermal structure and its seasonal variation were well reproduced by all schemes,considerable diff erences could still be found among all experiments.A warmer and spatially smaller simulation of YSCWM,with very strong thermocline,appeared in M-Y 2.5 experiment,while a spatially larger YSCWM with shallow mixed layer was found in GLS and KPP schemes.Among all the experiments,the discrepancy,indicated by core temperature,appeared since spring,and grew gradually by the end of November.Additional experiments also confi rmed that the increase of background diff usivity could eff ectively weaken the YSCWM,in either strength or coverage.Surface wave,another contributor in upper layer,was found responsible for the shrinkage of YSCWM coverage.The treatment of wave eff ect as an additional turbulence production term in prognostic equation was shown to be more superior to the strategy of directly increasing diff usivity for a coastal region.

The role of surface waves in the ocean mixed layer

Previously, most ocean circulation models have overlooked the role of the surface waves. As a result, these models have produced insufficient vertical mixing, with an under - prediction of the ,nixing layer （ML） depth and an over - prediction of the sea surface temperature （SST）, particularly during the summer season. As the ocean surface layer determines the lower boundary conditions of the atmosphere, this deficiency has severely limited the performance of the coupled ocean - atmospheric models and hence the climate studies. To overcome this shortcoming, a new parameterization for the wave effects in the ML model that will correct this systematic error of insufficient mixing. The new scheme has enabled the mixing layer to deepen, the surface excessive heating to be corrected, and an excellent agreement with observed global climatologic data. The study indicates that the surface waves are essential for ML formation, and that they are the primer drivers of the upper ocean dynamics; therefore, they are critical for climate studies.

Effect of wind-current interaction on ocean response during Typhoon KAEMI(2006)

The Weather Research and Forecasting （WRF） model, the Princeton Ocean Model （POM）, and the wave model （WAVEWATCH III） are used to develop a coupled atmosphere-wave-ocean model, which involves different physical pro- cesses including air-forcing, ocean feedback, wave-induced mixing and wave-current interaction. In this paper, typhoon KAEMI （2006） has been examined to investigate the effect of wind-current interaction on ocean response based on the coupled atmosphere-ocean-wave model, i.e., considering the sea surface currents in the calculation of wind stress. The results show that the wind-current interaction has a noticeable impact on the simulation of 10 m-winds. The model involving the effect of the wind-current interaction can dramatically improve the typhoon prediction. The wind-current interaction prevents excessive momentum fluxes from being transferred into the upper ocean, which contributes to a much smaller turbulence kinetic energy （TKE）, vertical diffusivity, and horizontal advection and diffusion. The Sea Surface Temperature （SST） cooling induced by the wind-current interaction during the initial stage of typhoon development is so minor that the typhoon intensity is not very sen- sitive to it. When the typhoon reaches its peak, its winds can disturb thermocline, and the cold water under the thermocline is pumped up. However, this cooling process is weakened by the wind-current interaction, as ocean feedback delays the decay of the typhoon. Meanwhile, the temperature below the depth of 30 m shows an inertial oscillation with a period about 40 hours （-17°N） when sudden strong winds beat on the ocean. Due to faster currents, the significant wave height decreases as ignoring the wind-current interaction, while this process has a very small effect on the dominant wave length.