Date of Award

2007

Degree Type

Dissertation

Degree Name

Doctor of Philosophy in Oceanography

Department

Oceanography

First Advisor

Isaac Ginis

Abstract

In existing ocean models, momentum and turbulent kinetic energy (TKE) fluxes through the air-sea interface are parameterized by the bulk formulae, neglecting their dependence on different sea states. However, when the ocean surface wave field is young and complex, the fluxes may significantly differ from the bulk estimates. Furthermore, the fluxes from wind and the fluxes into subsurface currents may be different when the wave field varies in space and in time, since waves take or give up momentum and energy. Numerical experiments are performed to investigate the energy and momentum flux budget across the air-sea interface when wave fields develop in time (duration limited) or in space (fetch limited) under steady homogenous wind forcing. Similar numerical experiments are performed under idealized stationary and moving hurricanes. The wave fields are simulated using the WAVEWATCH III model for all experiments. The air-sea momentum and TKE fluxes from the wind are estimated using a coupled wave-wind model and the fluxes into waves and currents are estimated using an air-sea momentum and energy budget model. The results show that the normalized significant wave height increases with a unique power law dependence on the wave age, being consistent with existing empirical formulae. For the uniform wind experiment, the difference between the momentum flux from wind and the flux into currents is negligible under duration limited cases but it is up to 5% when the wave field is fetch limited. The difference between the TKE flux from wind and the flux into currents can be as large as 20% under growing was (both in pace and in time). The re ult under idealized hurricanes suggest that the spatial variation of the hurricane-induced surface waves plays an important role in reducing TKE and momentum fluxes into subsurface currents in the rear-right quadrant of the hurricane. For a hurricane with a maximum wind speed (MWS) of 45 ms-1, the reduction can be as much as 10% in the vicinity of the radius of maximum wind (RMW). The percentage of the reduction is insensitive to the changes of the RMW, hurricane translation speed and the asymmetry in the wind field, but varies with the change of MWS. Our results also suggest that the TKE flux varies with wave age and friction velocity, and it is roughly proportional to the 3.5th power of the friction velocity rather than the 3rd power found in other studies. However, the wave age dependence become more complex under hurricanes suggesting it is necessary to explicitly resolve the wave field and calculate the wave spectrum in accurately predicting the momentum and TKE fluxes into currents.

The wind/wave/current coupling mechanisms and their effect on the ocean and surface wave field response is also investigated through a set of numerical experiments. The Princeton Ocean Model is used to simulate the ocean response. The results show that both variation in the surface wave field and wave-current interaction significantly reduce the momentum flux into the currents in the rear-right quadrant of the hurricane and consequently weaken the ocean response. During the wave-current interaction, the momentum flux is mainly affected by reducing the wind speed relative to currents while the wave field is mostly affected by refraction due to the current shear. Convergence in the current field increases wave height and decreases wave length while divergence in the current field decreases both the wave height and wave length. Both effects shift the wave energy to higher frequency and widen the angular distribution of the wave energy.

The performance of the wave model WA VEW ATCH III under a very strong tropical cyclone wind forcing is investigated with different drag coefficient parameterizations and ocean current inputs. The model results are compared with field observations of the surface wave spectra from a scanning radar altimeter in Hurricane Ivan (2004). Comparison of the directional wave spectra between model results and NASA Scanning Radar Altimeter (SRA) measurements in Hurricane Ivan (2004) suggests that an improved drag parameterization and the wave-current interaction yields improved wave forecast of significant wave height and wave spectral energy but tends to underestimate dominant wave length.

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