Sensitivity of tropical-cyclone models to the surface drag coefficient

摘要

Motivated by recent developments in tropical-cyclone dynamics, this paper reexamines a basic aspect of tropical-cyclone behaviour, namely, the sensitivity of tropical-cyclone models to the surface drag coefficient. Previous theoretical and numerical studies of the sensitivity in axisymmetric models have found that the intensity decreases markedly with increasing drag coefficient. Here we present a series of three-dimensional convection-permitting numerical experiments in which the intensification rate and intensity of the vortex increase with increasing surface drag coefficient until a certain threshold value is attained and then decrease. In particular, tropical depression-strength vortices intensify to major hurricane intensity for values of C_k/C_d as small as 0.1, significantly smaller than the critical threshold value of about 0.75 for major hurricane development predicted by Emanuel using an axisymmetric balance model. Moreover, when the drag coefficient is set to zero, no system-scale intensification occurs, despite persistent sea-to-air fluxes of moisture that maintain deep convective activity. This result is opposite to that found in a prior axisymmetric study by Craig and Gray. The findings are interpreted using recent insights obtainedon tropical-cyclone intensification, which highlight the intrinsically unbalanced dynamics of the tropical-cyclone boundary layer. The reasons for the differences from earlier axisymmetric studies and some potential ramifications of our findings are discussed. The relative insensitivity of the intensification rate and intensity found for drag coefficients typical of high wind speeds over the ocean calls into question the need for coupled ocean wave-atmospheric models to accurately forecast tropical-cyclone intensity.