Small scale Langmuir circulations

Langmuir instability in small scales (mm to cm) is examined using the surface wave data obtained during the Coastal Ocean Processes (CoOP I, 1995) experiment. Both wind field and surface wave field are assumed to be stationary. To include contributions of multiple surface wave harmonics, Stokes drift profiles are calculated by adding contributions from a wavenumber range in the vicinity of the scale of resulting Langmuir cells. Two subsurface current profiles - linear and logarithmic - are examined. In both cases the dissipation of the Langmuir cells is assumed to be due to the kinematic viscosity only, that is, it is assumed to be unaffected by the shear turbulence. Results obtained from numerical simulations indicate that small scale Langmuir circulations are likely to develop regardless of the subsurface current profiles, as long as the shear turbulence affects only the profile of the subsurface current and does not contribute to the dissipation of the cells. The predicted lateral spacing of the fastest growing Langmuir cells is about 1-2 cm, which is somewhat smaller than 5-6 cm observed during the CoOP I experiment. A possible explanation of this discrepancy is that the numerical model examines the initial growth stage of Langmuir instability, whereas observations are made at the finite-amplitude stage of the instability. The growth rates of Langmuir cells may become negative if dissipation due to the eddy viscosity is included.