A Direct Assessment of Langmuir Turbulence Parameterizations in Idealized Coastal Merging Boundary Layers

Abstract Langmuir turbulence affects turbulent mixing in the ocean boundary layers and its effects require parameterizations in ocean circulation models. Most existing Langmuir turbulence parameterizations focus on the surface boundary layer in open oceans. In the shallow waters of coastal oceans, a surface boundary layer may interact and even merge with a bottom boundary layer. It is unclear how existing Langmuir turbulence parameterizations perform under such complex conditions. Here we assess the performance of two recent Langmuir turbulence parameterizations in an idealized case of merging boundary layers against turbulence‐resolving large‐eddy simulations (LES). In addition to assessing the solutions of free runs of single‐column model (SCM) simulations, in which errors in the mean fields and turbulent fluxes are entangled, we also compare the simulated turbulent fluxes in SCM simulations with their mean fields nudged to those of the LES. In doing so, we focus on the parameterized turbulent fluxes in different parameterizations given the perfect mean fields. Our comparison highlights the tendency of parameterizations to deviate from the LES at each time instance, and thereby reveals the deficiencies of parameterizations in an instantaneous sense. It is shown that both parameterizations overestimate the near‐bottom turbulent momentum flux when velocity shear is correct, resulting in too weak near‐bottom shear in a free run. Consistent with previous studies, a down‐Stokes drift shear momentum flux is necessary for capturing the momentum flux due to Langmuir turbulence but still misses the nonlocal momentum flux when coherent Langmuir supercells form.