Observations and Numerical Simulations of the Onset and Growth of Langmuir Circulations

We report novel observations of the onset and growth of Langmuir circulations (LCs) from simultaneous airborne and subsurface in situ measurements. Under weak, fetch-limited wind-wave forcing with stabilizing buoyancy forcing, the onset of LCs is observed for wind speeds greater than about 1 m s(-1). LCs appear nonuniformly in space, consistent with previous laboratory experiments and suggestive of coupled wave-turbulence interaction. Following an increase in wind speed from <1 m s(-1) to sustained 3 m s(-1) winds, a shallow (<0.7 m) diurnal warm layer is observed to deepen at 1 m h(-1), while the cross-cell scales of LCs grow at 2 m h(-1), as observed in sea surface temperature collected from a research aircraft. Subsurface temperature structures show temperature intrusions into the base of the diurnal warm layer of the same scale as bubble entrainment depth during the deepening period and are comparable to temperature structures observed during strong wind forcing with a deep mixed layer that is representative of previous LC studies. We show that an LES run with observed initial conditions and forcing is able to reproduce the onset and rate of boundary layer deepening. The surface temperature expression however is significantly different from observations, and the model exhibits large sensitivity to the numerical representation of surface radiative heating. These novel observations of Langmuir circulations offer a benchmark for further improvement of numerical models.