Effect of Surface Waves on Air–Sea Momentum Exchange. Part II: Behavior of Drag Coefficient under Tropical Cyclones

Abstract

Present parameterizations of air-sea momentum flux at high wind speed, including hurricane wind forcing, are based on extrapolation from field measurements in much weaker wind regimes. They predict monotonic increase of drag coefficient (C d) with wind speed. Under hurricane wind forcing, the present numerical experiments using a coupled ocean wave and wave boundary layer model show that C d at extreme wind speeds strongly depends on the wave field. Higher, longer, and more developed waves in the right-front quadrant of the storm produce higher sea drag; lower, shorter, and younger waves in the rear-left quadrant produce lower sea drag. Hurricane intensity, translation speed, as well as the asymmetry of wind forcing are major factors that determine the spatial distribution of C d. At high winds above 30 m s 1 , the present model predicts a significant reduction of C d and an overall tendency to level off and even decrease with wind speed. This tendency is consistent with recent observational, experimental, and theoretical results at very high wind speeds.

Sensitivity of tropical‐cyclone intensification to perturbations in the surface drag coefficient

“…However, as noted in section 2, a moving vortex produces a response in the surface wave field that exhibits a prominent azimuthal wavenumber-one asymmetry around the storm, with a corresponding coherent structure component in the drag coefficient (e.g. Figures 9 and 12 of Moon et al, 2004).…”

“…The fields show a prominent pattern of enhanced drag coefficients in the front right quadrant. With our experiments in which the maximum drag is located to the right and front right of the vortex motion vector in two different experiments, we cover the situation shown by Moon et al (2004). The area of maximum drag coefficients is located between the right and front right of the motion vector in their experiments in which the surface drag coefficient was calculated from their wave model below a simulated tropical cyclone.…”

“…At this stage it is not possible to determine the relative contribution of these errors, in part because of the different methodologies used to estimate the surface drag coefficient. Extensive efforts have been devoted to including wind-wave coupling for the tropical-cyclone intensification problem (Moon et al, 2004(Moon et al, , 2007Chen et al, 2007;and references therein). However, the results of Montgomery et al (2010) and Smith et al (2012) provide a basis for questioning the need for such a complex approach and suggest that perhaps the results of numerical models will not be unduly sensitive to such spatial fluctuations in the drag coefficient.…”

“…corresponding distribution of surface stress (e.g. Moon et al, 2004). For this reason, we analyse also calculations of the drag coefficient in the idealized tropical-cyclone model with a uniform background flow described by Thomsen et al (2012).…”

Extreme Waves and Wave Run-up in Halifax Harbour under Climate Change Scenarios

“…The large waves simulated by Xu et al (2007) during, and after, the peak winds tended to be overestimated, particularly on the right side of the storm track. Thus, they used a modified drag coefficient, motivated by experimental and theortical results (Powell et al, 2003;Donelan et al, 2004;Moon et al, 2004), and winds were corrected by taking into account the swell orbital velocities and Stokes drift.…”