Close Investigation on The Behavior of Second Moment Turbulent Closure Models

Abstract:

Turbulent mixing plays an important role in the transport of momentum, heat, and particles across the mixed layer, and is critical to the vertical distribution of heat and salinity in the water column. It can directly impact the dynamics and thermodynamics in the upper ocean and lower atmosphere including the vertical distributions of chemical, biological, optical, and acoustic properties.

The navy coast ocean model (NCOM) is one of the major ocean models that the Naval Oceanographic Office uses to provide detailed oceanic forecasts for naval operations. Several turbulent closure models are implemented in NCOM, including Mellor & Yamada 2.5, Kantha & Clayson (2004), and Harcourt (2015) scheme.

In this study, we address the coordinate and resolution dependence of these three second moment turbulent closure schemes based on evaluations of the performance of one dimensional NCOM simulations against observations and large eddy simulation (LES) results at Ocean Station Papa. Our results suggest that while finer vertical resolution can improve the accuracy of mixed layer simulations, their ability in deepening the mixed layer is still very limited when compared with LES simulations and observations. Although the Harcourt scheme can often produce eddy viscosity at similar magnitude as the LES model in the upper mixed layer, it diminishes quickly above the base of the mixed layer and thus has limited abilities to mix the cooler thermocline water into the mixed layer. We also found that for all three mixing schemes, when having limited grid levels, finer resolution around the base of the mixed layer is more efficient in mixed layer deepening versus finer resolution near surface.