Shear-forced vertical circulations in tropical cyclones

Abstract

The forced secondary circulation (FSC) by the vertical shear of horizontal winds is isolated from the latent heating and friction FSCs associated with a model-simulated hurricane vortex. This is achieved by use of a newly developed potential vorticity inversion and quasi-balanced vertical motion equations system. Results show that latent heating forces intense updrafts in the eyewall and slow subsidence in the eye, whereas the friction-FSC is similar to that of the Ekman pumping, with the peak ascent occurring near the top of the boundary layer in the eye. In contrast, when an environmental westerly shear is superposed with an axisymmetric balanced vortex, an anticlockwise FSC appears across the inner-core region with the rising motion downshear and easterly sheared horizontal flows in the vertical. The resulting horizontal flows act to reduce the influence of the vertical shear inside the storm by as much as 30-40%, thus opposing the destructive roles of the vertical shear. Copyright 2005 by the American Geophysical Union.