Sea-State Dependent Drag Coefficient in Shallow Waters Under Tropical Cyclones

The air-sea momentum flux, or the drag coefficient, may be strongly modified by different sea states, particularly with a complex surface wave field under a tropical cyclone. As a storm approaches a coastline, the surface wave filed underneath is modified by the finite depth effects – mainly the depth induced breaking and the enhanced bottom dissipation. The wind forcing on waves, or the wave growth rate, also changes because the wave phase speed decreases in a shallow water. In this study, the sea-state dependent drag coefficient in a shallow water is investigated in high wind speeds, focusing on tropical cyclone conditions, by combining the wave model simulations and the two theoretical approaches to estimate the drag coefficient. Our results suggest the drag coefficient may be reduced in very shallow waters but be enhanced when opposing swell is present.