Langmuir turbulence and its parameterization in tropical cyclone conditions

The upper-ocean turbulence is significantly modified by the Stokes drift of the surface waves because of the Craik-Leibovich vortex force (Langmuir turbulence). Under tropical cyclones the contribution of the surface waves varies significantly depending on complex wind and wave conditions. Therefore, turbulence closure models used in ocean models need to explicitly include the sea-state dependent impacts of the Langmuir turbulence. In this study the K Profile Parameterization (KPP) 1st order turbulence closure model is modified to include the Langmuir turbulence effect, and its performance is tested against concurrent Large Eddy Simulation (LES) experiments under tropical cyclone conditions. First, the KPP model is retuned to reproduce LES results without Langmuir turbulence to eliminate any implicit Langmuir turbulence effects that is included in the standard (default) KPP model. Next, the Eulerian currents are replaced by the Lagrangian currents in the KPP equations for calculating the bulk Richardson number and the vertical turbulent momentum flux. Finally, an enhancement to the turbulent mixing is introduced as a function of the non-dimensional turbulent Langmuir number corrected for misaligned wind and waves. The retuned KPP, with the Lagrangian currents replacing the Eulerian currents and the turbulent mixing enhanced, significantly improves the prediction of the upper-ocean temperature and currents compared to the default (unmodified) KPP model under tropical cyclones. This modified KPP model also shows improvements over the default KPP at moderate winds.