Impacts of Shoaling Ocean Surface Waves on Wind Stress and Storm Surge
Ocean surface waves have been known to affect the wind stress, or air-sea momentum flux, in the open ocean under tropical cyclones. As a tropical cyclone (TC) makes landfall, the surface waves shoal as they enter the shallower coastal waters (<30~50 m). The impacts of these shoaling surface waves on the wind stress have not yet been thoroughly investigated, but they have been postulated to be important for storm surge prediction. In this study, the WAVEWATCH III (WW3) model is used to simulate the depth modifications on the surface wave fields and the wind stress. Two wave-spectrum based wind stress estimation modules in the WW3 model are used to quantify the impact of wave shoaling on wind stress and drag coefficient under steady uniform wind and tropical cyclone conditions. The consequence of using the shoaling wave modified wind stress for modeling storm surge is investigated using the Advanced Circulation (ADCIRC) model. For the shoaling wave simulations, an accurate wave field generated by tropical cyclones in the open ocean needs to be established first. The sensitivity of tropical cyclone wave simulations in the open ocean to different spatial resolutions (1/3°, 1/6°, 1/12° and 1/24°) is evaluated using two wave models, WW3 and Simulating WAves Nearshore. Results from both models show that the coarsest resolution (1/3°) introduces significant errors in both the significant wave height (Hs) and the mean wavelength. Moreover, results reveal that sensitivity to spatial resolution strongly depends on storm characteristics. Waves simulated under the small and fast moving TC show the largest sensitivity to the coarse spatial resolutions. With the 1/3° resolution, maximum Hs can be underestimated by as much as 6% in WW3 and 16% in SWAN compared to those with the 1/24° resolution. The impacts of shoaling ocean surface waves on the wind stress and drag coefficient (Cd) in coastal waters are investigated with the WW3 under steady, uniform onshore wind and tropical cyclones. Our results show that under uniform onshore winds, as water depth decreases, the drag coefficient increases gradually to a peak value and then rapidly reduces compared to the deep-water value. The maximum Cd occurs roughly where depth-induced wave breaking starts. The magnitude of Cd enhancement is more significant on a steeper slope and can reach 40%, which is mainly due to the steepening of waves and reduction of the wave phase speed during the shoaling. Our results suggest significantly larger variability of Cd at a given wind speed in finite depth waters than in deep water, which is also found during idealized landfalling TCs. Specifically, Cd is enhanced in the right TC quadrants (due to shoaling fetch-dependent waves) and in the left TC quadrants (due to shoaling opposing-wind swells) compared to its deep-water value. However, Cd is reduced in the front/rear quadrants due to weaker wind seas. The misalignment between wind stress and wind speed directions is also enhanced in shallow water. In general, the shoaling wave effects on the wind stress and Cd are much stronger on steeper bottom slopes and in faster-moving storms. At last, the shoaling wave modified wind stress is applied to a two-dimensional steady-state model and the ADCIRC model to investigate its impacts on storm surge. In the steady onshore wind conditions, the shoaling wave modified (or sea-state dependent (SSD)) wind stress can increase the sea surface elevation by as much as 15% on a steep bottom slope compared to the result using the bulk wind stress as a function of wind speed. In TC conditions, the maximum impact of the SSD wind stress occurs to the left of the storm track near the radius of maximum wind around the time of TC landfall. The set-down of the sea level is significantly enhanced due to increased offshore wind stress. However, the SSD wind stress impact on the peak surge to the right of the storm track is negligible. Hence, our analysis suggests that the SSD wind stress has the biggest impact in uniform onshore wind conditions (such as under large, slow moving extratropical cyclones) than in TC conditions. Our results also suggest that the water level prediction at the shoreline is sensitive to the wind stress in the surf zone, which is not resolved explicitly in our model.
"Impacts of Shoaling Ocean Surface Waves on Wind Stress and Storm Surge"
Dissertations and Master's Theses (Campus Access).