Improved Wind Dynamics for Coastal Forecasting

Recent numerical and experimental studies, reviewed at the IUTAM symposium in 2017, have identified the connections between key processes of wind wave dynamics in order to model the transfer of wind stress to wave energy; for lower wave amplitudes, surface shear layers, interact with elevated critical layers within thin recirculating cats-eyes, while for steeper waves these recirculating zones first become fully separated turbulent layers. At even higher amplitude, these zones develop into entraining two phase flows.

The dynamics also depend critically on the waves’ time development, such as occurs with the formation of wave groups, whether driven by near shore shelving dynamics or the formation of wave groups driven by turbulent eddy structures, which differ between unidirectional shear flows and curving, recirculating flows near the eyewalls of near shore Tropical Cyclones, such as recent studies of Lixao et al. (2015) The surface roughness on the waves vary behind, within and ahead of the cyclones. Ahead of the cyclones, groups of waves become sharper and their slopes steepen. Within the eyewall region, waves are relatively calmer and thus not as sharp-crested. But just outside the eyewall the waves are sharper and steeper than within the eyewall. However, these waves are not as sharp as the waves ahead of the eyewall. We show that this anomaly is due to variations of surface roughness and drag forces acting on the waves in different areas regime within and around cyclones.

Here further research results on wind-wave dynamics are presented about how wind driven waves are formed into characteristic groups (with or without ``white caps") and what are their essential properties, depending on the local atmospheric and oceanic conditions. This requires in-depth study of wind-driven waves, especially through their effect on small-scale motions and turbulence near the surface. Nonlinear unsteady waves dynamics (with amplitudes varying with space and time) demonstrate how the waves become sharp-crested and form typical ocean wave groups The focus will be on unsteady waves travelling under extreme winds.

Previous theoretical research (Sajjadi et al. 2014) is applied for the development of analytical expressions for unsteady waves and their coding as numerical subroutines for parameterizations of momentum flux, and hence the energy-transfer rate, from wind to the sea as well as the sea surface roughness for unsteady waves. This leads to one-way and two-way coupling (of air-sea interaction) and how they have different effects on boundary layer physics differently. The latter case results in larger surface roughness and, consequently, to larger surface fluxes, with predictions for more intense hurricane waves.

This paper concludes with a proposal for developing numerical integration of the sum of total surface shear stresses (due to wind turbulence, wave-induced turbulence and molecular) thereby developing a subroutine which can be incorporated in WAVEWATCH. This would then act as an input to a ``Coupled Ocean-Atmospheric Mesoscale Prediction System''. Closely allied will be the goal of utilizing another subroutine for energy-transfer rate to be incorporated for unsteady waves in operational wave models.

References

Li, Ahsan Kareem, Yiqing Xiao^, Julian C. R. Hunt. 2015. Turbulence Spectra for Boundary-Layer Winds in Tropical Cyclones: A Conceptual Framework and Field Measurements at Coastlines. Boundary-Layer Meteorol., 154, 243–26.

S.G. Sajjadi, J.C.R.Hunt and F. Drullion. 2014. Asymptotic multi-layer analysis of wind over unsteady monochromatic surface waves. J.Eng.Math., 84, 73–85.