Langmuir Turbulence under Hurricane Gustav (2008)

Tyler J Rabe,Brandon G Reichl,Eric A D’Asaro,Tobias Kukulka,Tetsu Hara,Ramsey R Harcourt,Isaac Ginis,Peter P Sullivan

doi:10.1175/jpo-d-14-0030.1

Abstract

AbstractExtreme 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.

Highlights

Over the past few decades, forecasts for tropical cyclone track have improved significantly with the development of complex numerical models
A large eddy simulation was used to model the turbulence under Hurricane Gustav in an effort to better understand the importance of Langmuir turbulence (LT) during tropical cyclones
Comparison of large eddy simulation (LES) results with Lagrangian float field observations indicate that LT plays an important role in upper ocean mixing

Summary

Introduction

Over the past few decades, forecasts for tropical cyclone track have improved significantly with the development of complex numerical models. The ability to predict their strength has not progressed as quickly [Bender and Ginis, 2000, Emanuel et al, 2004] This is partially due to the incomplete understanding of turbulent upper ocean mixing. [Price et al, 1986, Large et al, 1994, Zedler et al, 2002]) have investigated the impact of high winds on upper ocean turbulence using one dimensional turbulence parametrizations. They focus on the importance of inertially rotating wind stress on mixing at the thermocline. The one dimensional parametrizations used in these models do not explicitly include the effects of surface gravity waves, which have been shown to play an important role in upper ocean mixing [McWilliams et al, 1997, Li et al, 2005]

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Results

Conclusion