Date of Award
Doctor of Philosophy in Oceanography
There is a growing need to improve the prediction and parameterization of momentum, energy, heat, and gas exchanges between the ocean and atmosphere. It is well known that the coupling between turbulent wind and a spectrum of waves significantly modifies turbulent air-sea fluxes, thereby playing an important role in the development of wave, weather, and climate patterns. Due to the limitations of field measurements, especially in high-to-extreme wind conditions such as tropical cyclones, constraining air-sea fluxes in numerical wave, storm surge, weather, and climate forecast models continues to be a challenge due to our incomplete understanding of near-surface turbulence in the air and water that arises from complex surface wave dynamics in a range of environmental conditions.
In Chapter 1, we use LES of airflow over sinusoidal waves to investigate the causes and features of airflow separation over strongly forced (young) wind waves. Flow fields previously observed over laboratory waves are compared with those in LES under similar conditions (see Husain et al. 56). Many of the same flow features are observed, including strong cross-wave vorticity detached from the leeward crest followed by enhanced TKE aloft, and strong wind shear near the height of the wave crest, with weak velocity, recirculation and reduced TKE in the trough---features of airflow separation over moving waves. This study demonstrates fidelity of LES to represent turbulence in the airflow above strongly forced waves. It also highlights the important wave features that modulate airflow separation and its impact on turbulence in the wave boundary layer, including the alteration of the mean wind speed, drag coefficient, and wave growth rate.
Chapters 2 and 3 expand on the previous chapter by using the identical LES approach to investigate airflow turbulence over an extended range of sea states when waves are misaligned with the wind---a frequent occurrence in the open ocean, especially under transient high wind conditions coupled to complex seas. Despite the ubiquity of this scenario, our knowledge on the impacts of turbulent airflow in such conditions remain limited. In the following two chapters, we investigate misaligned wind and waves in the context of key air-sea momentum fluxes, similar to Chapter 1.
In Chapter 2, we employ LES for turbulent airflow over steep sinusoidal waves of a range of wave phase speeds (i.e., wave ages) both following and opposing the wind. We find that while the airflow dynamics and impacts rapidly evolve as the wave age increases for waves following wind, there is a rather smooth transition from the slowest waves following wind, to those opposing wind, to the fastest waves opposing wind. We see increasingly stronger flow perturbations as the opposing wave phase speed increases, suggesting that airflow is separating intermittently (or exhibiting separation-like patterns). This process alters the phase averaged wind and pressure fields, causing both the wave decay rate and effective roughness length (drag coefficient) to increase rapidly.
Finally, in Chapter 3, we employ LES for wind blowing over steep, strongly forced waves following and opposing oblique wind to elucidate the impacts on wind speed and direction, drag coefficient, and wave growth/decay. We find that oblique winds maintain a signature of airflow separation while introducing a cross-wave component that strongly modifies the overall flow fields. The direction of the mean wind speed and wind shear deviate from the direction of wind stress in the wave boundary layer, especially towards the surface. Wave growth and decay rates fall within the range of typical parameterizations. The most interesting finding of this study is the erasure of the effect of waves on the effective surface roughness (drag coefficient) in oblique wind.
This dissertation emphasizes the important role of surface waves in modifying airflow within the wave boundary layer and the evolution of the wave field in space and/or time. We demonstrate that LES can be used to investigate wave-driven turbulent processes in the air over complex seas, and discuss the implications of our findings for elucidating the impacts on sea-state dependent air-sea flux parameterizations and turbulence closure models for numerical forecasts.
Husain, Nyla Tauqir, "LARGE EDDY SIMULATION OF TURBULENT AIRFLOW OVER SURFACE WAVES AND AIR-SEA FLUXES" (2020). Open Access Dissertations. Paper 1221.