Tropical cyclone spin‐up revisited

Abstract

Abstract We present numerical experiments to investigate axisymmetric interpretations of tropical cyclone spin‐up in a three‐dimensional model. Two mechanisms are identified for the spin‐up of the mean tangential circulation. The first involves the convergence of absolute angular momentum above the boundary layer and is a mechanism to spin up the outer circulation, i.e. to increase the vortex size. The second involves the convergence of absolute angular momentum within the boundary layer and is a mechanism to spin up the inner core. It is associated with the development of supergradient wind speeds in the boundary layer. The existence of these two mechanisms provides a plausible physical explanation for certain long‐standing observations of typhoons by Weatherford and Gray, which indicate that inner‐core changes in the azimuthal‐mean tangential wind speed often occur independently from those in the outer core. The unbalanced dynamics in the inner‐core region are important in determining the maximum radial and tangential flow speeds that can be attained, and therefore important in determining the azimuthal‐mean intensity of the vortex. We illustrate the importance of unbalanced flow in the boundary layer with a simple thought experiment. The analyses and interpretations presented are novel and support a recent hypothesis of the boundary layer in the inner‐core region. Copyright © 2009 Royal Meteorological Society