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This study utilizes a large-eddy simulation (LES) approach to systematically assess the directional variability of wave-driven Langmuir turbulence (LT) in the ocean surface boundary layer (OSBL) under tropical cyclones (TCs). The Stokes drift vector, which drives LT through the Craik–Leibovich vortex force, is obtained through spectral wave simulations. LT’s direction is identified by horizontally elongated turbulent structures and objectively determined from horizontal autocorrelations of vertical velocities. In spite of a TC’s complex forcing with great wind and wave misalignments, this study finds that LT is approximately aligned with the wind. This is because the Reynolds stress and the depth-averaged Lagrangian shear (Eulerian plus Stokes drift shear) that are key in determining the LT intensity (determined by normalized depth-averaged vertical velocity variances) and direction are also approximately aligned with the wind relatively close to the surface. A scaling analysis of the momentum budget suggests that the Reynolds stress is approximately constant over a near-surface layer with predominant production of turbulent kinetic energy by Stokes drift shear, which is confirmed from the LES results. In this layer, Stokes drift shear, which dominates the Lagrangian shear, is aligned with the wind because of relatively short, wind-driven waves. On the contrary, Stokes drift exhibits considerable amount of misalignments with the wind. This wind–wave misalignment reduces LT intensity, consistent with a simple turbulent kinetic energy model. Our analysis shows that both the Reynolds stress and LT are aligned with the wind for different reasons: the former is dictated by the momentum budget, while the latter is controlled by wind-forced waves.
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On the shear-current effect: toward understanding why theories and simulations have mutually and separately conflicted
ABSTRACT The shear-current effect (SCE) of mean-field dynamo theory refers to the combination of a shear flow and a turbulent coefficient β21 with a favourable negative sign for exponential mean-field growth, rather than positive for diffusion. There have been long-standing disagreements among theoretical calculations and comparisons of theory with numerical experiments as to the sign of kinetic ($$\beta ^u_{21}$$) and magnetic ($$\beta ^b_{21}$$) contributions. To resolve these discrepancies, we combine an analytical approach with simulations, and show that unlike $$\beta ^b_{21}$$, the kinetic SCE $$\beta ^u_{21}$$ has a strong dependence on the kinetic energy spectral index and can transit from positive to negative values at $$\mathcal {O}(10)$$ Reynolds numbers if the spectrum is not too steep. Conversely, $$\beta ^b_{21}$$ is always negative regardless of the spectral index and Reynolds numbers. For very steep energy spectra, the positive $$\beta ^u_{21}$$ can dominate even at energy equipartition urms ≃ brms, resulting in a positive total β21 even though $$\beta ^b_{21}\lt 0$$. Our findings bridge the gap between the seemingly contradictory results from the second-order-correlation approximation versus the spectral-τ closure, for which opposite signs for $$\beta ^u_{21}$$ have been reported, with the same sign for $$\beta ^b_{21}\lt 0$$. The results also offer an explanation for the simulations that find $$\beta ^u_{21}\gt 0$$ and an inconclusive overall sign of β21 for $$\mathcal {O}(10)$$ Reynolds numbers. The transient behaviour of $$\beta ^u_{21}$$ is demonstrated using the kinematic test-field method. We compute dynamo growth rates for cases with or without rotation, and discuss opportunities for further work.
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- PAR ID:
- 10332340
- Date Published:
- Journal Name:
- Monthly Notices of the Royal Astronomical Society
- Volume:
- 507
- Issue:
- 4
- ISSN:
- 0035-8711
- Page Range / eLocation ID:
- 5732 to 5746
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1093/mnras/stab2469
