Impact of Shoaling Ocean Surface Waves on Wind Stress and Drag Coefficient in Coastal Waters: 2. Tropical Cyclones

Abstract

This study investigates the impacts of shoaling waves on the wind stress and drag coefficient (Cd) in coastal waters during tropical cyclone (TC) landfall. Numerical experiments are conducted using idealized TCs with two intensities, Category 1 and 5, and two translation speeds, 5 and 10 m/s, propagating toward and normal to the shoreline over two bottom slopes, 1:200 and 1:2000. The wave spectra are simulated using the WAVEWATCH III wave model. The unresolved high‐frequency spectral tail is parameterized as a function of wind speed, and the full wave spectrum is used to calculate the wind stress and drag coefficient. Our results show that the sea state dependence of wind stress magnitude (or Cd) is significantly increased in shallow water at a given wind speed. Compared to its deep‐water value, Cd is enhanced in the right (due to shoaling fetch‐dependent waves) and in the left (due to shoaling opposing‐wind swells) TC quadrants. However, Cd is reduced in the front/rear quadrants due to weaker wind seas. The misalignment between the wind stress and wind speed directions is 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.

Observed Drag Coefficient Asymmetry in a Tropical Cyclone

“…, field observations (French et al, 2007;Hsu et al, 2017;Jarosz et al, 2007;Potter et al, 2015;Zhao et al, 2015), and numerical simulations (Chen, Ginis, & Hara, 2020;Moon et al, 2004;Reichl et al, 2014) confirmed the saturation and the consequent attenuation of 𝐴𝐴 𝐴𝐴D , although the critical wind speeds 𝐴𝐴 𝐴𝐴10𝑐𝑐 for the saturation were quite different. Holthuijsen et al (2012) found that the 𝐴𝐴 𝐴𝐴10𝑐𝑐 was ∼40 m/s based on the wind profiles measured by GPS drop-sondes.…”

“…The long fetch allowed for the development of wind‐wave, which in turn enhanced $${C}_{\mathrm{D}}$$. Chen, Ginis, and Hara (2020) attributed this enhancement to the shoaling wave effect. Furthermore, the wave age in the right front quadrant was larger than that in the right rear quadrant, which was another factor that affects the wind stress.…”

“…In the right front quadrant of TC, swells generated by the TC cross the local wind-wave. Under this situation, the energy of wind-wave is probably reduced by swells (Chen, Ginis, & Hara, 2020;García-Nava et al, 2012). Since 2016) also showed that the attenuation of the wind-wave spectrum can lead to the saturation of 𝐴𝐴 𝐴𝐴D .…”

“…(2003) found that the $${C}_{\mathrm{D}}$$ over the ocean was saturated at $${U}_{\mathit{10}}\approx $$33 m/s, and then the $${C}_{\mathrm{D}}$$ attenuated with the wind speed. Since then, several studies based on laboratory experiments (Donelan et al., 2004; Takagaki et al., 2016), field observations (French et al., 2007; Hsu et al., 2017; Jarosz et al., 2007; Potter et al., 2015; Zhao et al., 2015), and numerical simulations (Chen, Ginis, & Hara, 2020; Moon et al., 2004; Reichl et al., 2014) confirmed the saturation and the consequent attenuation of $${C}_{\mathrm{D}}$$, although the critical wind speeds $${U}_{\mathit{10}c}$$ for the saturation were quite different. Holthuijsen et al.…”

“…However, their $${C}_{\mathrm{D}}$$ was estimated using the profile method which had large uncertainty. Chen, Ginis, and Hara (2020) also simulated the asymmetric $${C}_{\mathrm{D}}$$ distribution, however, with no observational support.…”