Impact of Shoaling Ocean Surface Waves on Wind Stress and Drag Coefficient in Coastal Waters: 2. Tropical Cyclones. Issue 7 (24th July 2020)

• Record Type: Journal Article
• Authors: Chen, Xuanyu; Ginis, Isaac; Hara, Tetsu
• Is Part Of: Journal of geophysical research. Volume 125:Issue 7(2020)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2020-07-24
• Subjects: tropical cyclones -- shoaling ocean waves -- sea state‐dependent drag coefficient -- coastal water; Oceanography -- Periodicals; 551.4605
• DOI: 10.1029/2020JC016223
• Languages: English
• ISSNs: 2169-9275

Abstract

Abstract: This study investigates the impacts of shoaling waves on the wind stress and drag coefficient ( C d ) 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 C d ) is significantly increased in shallow water at a given wind speed. Compared to its deep‐water value, C d is enhanced in the right (due to shoaling fetch‐dependent waves) and in the left (due to shoaling opposing‐wind swells) TC quadrants. However, C d 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 C d are much stronger on steeper bottom slopes and in faster moving storms. Plain Language Summary: This study investigates the impact of shoaling ocean surface waves on the air‐sea momentum exchange, represented by wind stress and drag coefficient, under tropical cyclones during their landfall. Four idealized tropical cyclones with different characteristics (maximum wind speeds and translation speeds) are prescribed to make landfall normal to the coastline. The corresponding wavefields are simulated with the WAVEWATCH III model, from which the wind stress vector is calculated with two established methods. The results show that the variability of wind stress magnitude (or drag coefficient) at a given wind speed significantly increases as water depth decreases. Compared to its deep‐water value, wind stress magnitude is enhanced in shallow water in the right quadrant (due to shoaling fetch‐dependent waves) and in the left quadrant (due to shoaling opposing‐wind swells). However, wind stress magnitude can be reduced in the front/rear quadrants due to weaker wind seas. The wind stress direction is misaligned with the wind direction when the dominant waves are misaligned with wind and the misalignment angle is enhanced in shallow water. In general, the shoaling wave effects on the wind stress and drag coefficient are much stronger on a steeper bottom slope and are further enhanced with a faster‐moving storm. Key Points: During tropical cyclone landfall the wind stress and drag coefficient are enhanced in the storm right‐rear and left‐front quadrants The effects of shoaling on the wind stress and drag coefficient are stronger with steeper bottom slopes and faster moving tropical cyclones The variability of the drag coefficient at a given wind speed increases significantly in shallow waters