Simulations of temperature and turbulence structure of the oceanic boundary layer with the improved near‐surface process

Abstract

Abstract. An improved model for the oceanic boundary layer is presented in view of the recent observation of the microstructure of the upper ocean including the high dissipation rate near the sea surface. In the new model the surface boundary conditions for both the turbulent kinetic energy flux and the roughness length scale are modified. The parameterization of stratification effects on turbulence is improved, and the convective process is reformulated on the basis of the observation of uniform temperature and velocity profiles within the convective mixed layer. Evolutions of the profiles of both the dissipation rate and temperature of the observation data Patches Experiment as well as the time series of the sea surface temperature over the observation days, are successfully simulated during a diurnal cycle for the first time. It is also shown that the model reproduces various important features of the oceanic boundary layer, for example, the formation of a diurnal thermocline, the profiles of buoyancy flux, and the magnitudes of the buoyancy gradients both within the mixed layer and at the diurnal thermocline. Performance of the model is compared with that of the widely used Mellor-Yamada model. A high level of TKE near the sea surface also makes the downward flux of TKE important in the energy budget of the oceanic boundary layer, rendering the contribution from shear production relatively unimportant. Hence, in the upper part of the oceanic boundary layer the balance is made between TKE flux and dissipation, and it leads to the relation for the variation of e with depth z as e -z -n with n -3.0-4.6. This is contrary to the case of the wall boundary layer in which the The significant difference between the oceanic and atmospheric boundary layer is also evident when the surface heat flux is stabilizing. In the atmospheric boundary layer a strong but continuous temperature gradient appears near the surface during the nighttime. On the other hand, in the oceanic boundary layer a diurnal thermocline is formed at a certain depth during the daytime, and the well-mixed surface layer is usually maintained above the diurnal thermocline, unless the wind is very weak. It was demonstrated by Noh [1996] that this significant discrepancy is again attributed to whether the dominant source of turbulence is TKE flux or shear production and that the TKE flux from the sea surface plays a critical role for the formation of a diurnal thermocline. Meanwhile, the large eddy simulation of the oceanic boundary layer by Skyllingstad and Denbo [1995] also showed that extremely underestimated TKE appears near the surface during the stabilizing heat flux as long as the wall boundary layer is applied, and they introduced the wave current interaction in order to resolve the problem. IntroductionAll these evidences from the near-surface process in the oceanic boundary layer, both from observations and numerical simulations, strongly suggest that the model and boundary conditions for the oceanic boundary layer must be fundamen-15,621

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