Langmuir Turbulence under Hurricane Gustav (2008)

Abstract

Extreme winds and complex wave fields drive upper-ocean turbulence in tropical cyclone conditions. Motivated by Lagrangian float observations of bulk vertical velocity variance (VVV) under Hurricane Gustav (2008), upper-ocean turbulence is investigated based on large-eddy simulation (LES) of the wave-averaged Navier-Stokes equations. To realistically capture wind-and wave-driven Langmuir turbulence (LT), the LES model imposes the Stokes drift vector from spectral wave simulations; both the LES and wave model are forced by the NOAA Hurricane Research Division (HRD) surface wind analysis product. Results strongly suggest that without LT effects simulated VVV underestimates the observed VVV. LT increases the VVV, indicating that it plays a significant role in upper-ocean turbulence dynamics. Consistent with observations, the LES predicts a suppression of VVV near the hurricane eye due to wind-wave misalignment. However, this decrease is weaker and of shorter duration than that observed, potentially due to large-scale horizontal advection not present in the LES. Both observations and simulations are consistent with a highly variable upper ocean turbulence field beneath tropical cyclone cores. Bulk VVV, a TKE budget analysis, and anisotropy coefficient (ratio of horizontal to vertical velocity variances) profiles all indicate that LT is suppressed to levels closer to that of shear turbulence (ST) due to misaligned wind and wave fields. VVV approximately scales with the directional surface layer Langmuir number. Such a scaling provides guidance for the development of an upper-ocean boundary layer parameterization that explicitly depends on sea state.

Surface Wave Effects on the Translation of Wind Stress across the Air–Sea Interface in a Fetch-Limited, Coastal Embayment

“…While numerous studies have shown that Stokes production is often a dominant term in the TKE budget during times when Langmuir turbulence is present (McWilliams et al 2012;Rabe et al 2015), an analysis of the TKE budget on this dataset indicates that the pressure work was the dominant transport term and balanced dissipation to first order in the surface layer of the estuary (Scully et al 2016). More detailed analysis of turbulent energetics observed during this experiment can be found in Scully et al (2016;A.…”

“…It has been shown that Langmuir turbulence cells align predominantly with vertical shear in the Lagrangian velocity, defined as the sum of the Eulerian velocity and the Stokes drift velocity associated with wave orbital velocities. This shear strains the vorticity field generated beneath breaking waves (Leibovich 1983;McWilliams et al 1997;Van Roekel et al 2012;Rabe et al 2015). Therefore, the relative roles of breaking impulses and mean total shear in determining the stress direction in the surface layer of the estuary can be qualitatively addressed using the observed directions of wave breaking, Lagrangian shear, and the Reynolds stress in the surface layer of the estuary.…”

Langmuir Turbulence Parameterization in Tropical Cyclone Conditions

“…where the model variables are SGS averaged, D/Dt 5 ›/›t 1 u Á ›/›x, x is the coordinate vector, u is the velocity vector, u S is the Stokes drift vector (the vertical component is set to 0), f is the Coriolis parameter, p is the generalized pressure [p/r 1 0.5(ju 1 u S j 2 2 juj 2 )], p is the pressure, and SGS are the subgrid-scale terms described in detail by Rabe et al (2015). Scalar (temperature) distribution is solved by the advectivediffusive equation:…”

“…The LES results suggest that such a regime may be due to large variability of the Langmuir turbulence, since the local wave field varies significantly (both in time and in space) under a tropical cyclone. Comparisons between the turbulent quantities from the LES and the observations are imperfect for various reasons, particularly because of uncertainty of the drag coefficient in tropical cyclone conditions (see Rabe et al 2015). Nevertheless, the results suggest that the Langmuir turbulence is an important, spatially variable source of mixing under a tropical cyclone and that the additional mixing due to the CL vortex force is necessary to best match the simulations and the field observations.…”

“…Recently, Rabe et al (2015) investigated Langmuir turbulence under Hurricane Gustav (2008) by combining in situ measurements of turbulence obtained by Lagrangian floats deployed ahead of the storm and LES hindcasts of the upper-ocean turbulence at the float locations. The observations show a regime behind the eye of a tropical cyclone where the turbulent vertical velocity variance w 02 is reduced compared to traditional wind-driven u * scaling.…”