Comparison of an axisymmetric hurricane model with the corresponding slab‐symmetric ITCZ model

We examine the difference between the evolution of a hurricane‐like vortex in an axisymmetric model and that of an intertropical convergence‐zone‐ like disturbance in a slab‐symmetric model, starting from an initial disturbance with the same lateral structure. The main calculations are carried out using a hurricane model similar to that formulated by DeMaria and Pickle (1988), supplemented by a few calculations using a recently developed model by Nguyen et al. (2002). We show that, although the two flow configurations have many similarities, the slab‐symmetric model does not provide a dynamical surrogate for the hurricane. The main difference can be attributed to a geometrical factor in the formula for the conservation of absolute angular momentum in the axisymmetric model, which for an inward‐moving air parcel permits much larger tangential wind speeds to be attained than in the slab‐symmetric model. As a result, the sea‐surface latent‐heat flux, which is wind‐speed dependent, is much larger in the axisymmetric model, providing a larger energy supply to the growing disturbance per unit area than in the slab‐symmetric case. A further geometrical effect is that for the same inflow velocity profile in the boundary layer, there is larger convergence in the axisymmetric model. Because this convergence determines the deep cumulus mass flux in the DeMaria and Pickle model, the cumulus heating is larger in the axisymmetric configuration. The non‐development of the slab‐symmetric perturbation is a feature also of calculations using the Nguyen et al. (2002) model, even though two of the closures on the deep‐cumulus mass flux used do not depend on the mass convergence in the boundary layer. A few numerical details of the main model are described, including the method of solution, which differs from that used by DeMaria and Pickle (1988). Copyright © 2002 Royal Meteorological Society