Numerical Study of Wind Stress in Coastal Water Under a Tropical Cyclone

Understanding of wind stress in finite depth during the passage of a tropical cyclone (TC) is important for improving tropical cyclone models as well as storm surge models. This study conducts idealized numerical experiments to investigate impacts of shoaling TC waves on wind stress. Shoaling tropical cyclone wave simulations are conducted on a sloped bottom (slope varying from 1:200 to 1:2000) using the WAVEWATCH III model with shallow water physics. Idealized TCs with maximum wind speeds of 35m/s and 65m/s and translation speeds of 5m/s and 10m/s (normal to the coastline) are used as wind forcing. Resolved part of the shoaling wave spectrum is combined with an empirical spectral tail to compute the sea-state dependent Cd using two existing methods. Our results show that the drag coefficient typically is increased in the right-rear and the left-front quadrants and reduced in other quadrants compared to those in deep water. The misalignment between wind and wind stress is also increased compared to that in deep water. These modifications are caused by reduced wave phase speeds, swells misaligned with wind, as well as reduction of offshore-propagating wind waves. Overall, our numerical results show increased spatial variability of the magnitude and direction of wind stress at finite water depths compared to that in deep water. This depth-enhanced variability is more significant on a steeper bottom slope.