Performance of the two-equation turbulence models in the Red Sea

Mehmet Ilıcak(1), Tamay M. Özgökmen(1), Hartmut Peters(1), Helmut Z. Baumert(2), Mohamed Iskandarani(1).

1)RSMAS, University of Miami, Miami, USA.
2)Freie Universität, Dept. Mathematik and Computer Science, Berlin, Germany.

Mixing between overflows and ambient water masses is a critical problem of deep-water mass formation in the downwelling branch of the meridional overturning circulation of the ocean. Modeling approaches that have been tested so far rely either on algebraic parameterizations in hydrostatic ocean circulation models, or on large eddy simulations that resolve most of the mixing using nonhydrostatic models.

In this study, we examine the performance of a set of turbulence closures in Regional Ocean Modeling System (ROMS), that have not been tested in comparison to observational data for overflows before. We employ k-ε, k-ω, KPP, and Mellor-Yamada turbulence closures with different stability functions. We also use a new k-ε turbulence model without any complex stability functions but only Richardson number dependent turbulent Prandtl number (Peters and Baumert 2007). To our knowledge, this is the first time that this new model adopted for a 3D ocean model.

The performance of different closure models are evaluated by conducting numerical simulations of the Red Sea overflow and comparing them to observations from the Red Sea Outflow Experiment (REDSOX). It is found that, most of the two-equation turbulence models capture the basic structure of the overflow, consisting of a well-mixed bottom layer (BL) and entraining interfacial layer (IL). KPP model leads to less mixing and thin IL, and on the contrary, Mellor Yamada leads to high mixing and BL signal becomes weak. The other models including the new k-ε give reasonable result in error analysis with respect to the REDSOX observations.