Sensitivity of tropical-cyclone models to the surface drag coefficient in different boundary-later schemes

Abstract

The recent study of the sensitivity of tropical-cyclone intensification to the surface drag coefficient in a three-dimensional model by Montgomery et al. is extended to include a wind-speed-dependent drag coefficient and one of four boundary-layer parametrization schemes: the bulk, Blackadar, MRF and Gayno–Seaman schemes. The schemes are slightly modified to have the same drag coefficient formulation and the same constant exchange coefficients for sensible heat and moisture. Interest is focused on the change in intensity of the azimuthally-averaged tangential wind speed and change in the low-level vortex structure when the standard value of the drag coefficient is halved or doubled. Changing the drag coefficient provides insight into unbalanced effects in the boundary layer and their impact on the vortex evolution and structure. The changes in vortex behaviour with changing drag coefficient are qualitatively similar for all schemes, the maximum intensification occurring for a value somewhere near the standard value of the drag coefficient. The interpretation given to explain this behaviour underlines the intrinsically unbalanced nature of the boundary-layer dynamics, although, for reasons discussed, a complete theory for the behaviour does not exist. The behaviour found is at odds with the predictions of Emanuel’s (balance) theory for the maximum intensity of a tropical-cyclone, which predicts a monotonic decrease in intensity with the drag coefficient if the enthalpy exchange coefficient is held fixed. It is at odds also with a recent numerical study of the maximum intensity by Bryan and Rotunno. The study underscores the importance of boundary-layer dynamics in models for forecasting tropical-cyclone intensity and the need for care in choosing a boundary layer scheme. However, it is not yet known which boundary-layer formulation is the most appropriate for this purpose, highlighting the need for a concerted research effort in this direction.