A comparison of three-dimensional versus axisymmetric tropical cyclones in the Cloud Model CM1

Axisymmetric fluid dynamics, either as a basis for theory, simulation or forecasting, has long served as a foundation for understanding hurricane intensification and mature intensity. Of course, there are certain limitations to this framework. For example, the intensity based on a local wind speed maximum is generally greater than that based on the axisymmetric mean. Nonetheless, fair metric comparisons between three-dimensional (3D) and axisymmetric (AX) flow can usually be found, such as comparing azimuthally-averaged tangential wind speed between both flow configurations. Deviations from axial symmetry can be viewed as perturbations on an axisymmetric mean state. Previous studies have suggested that flow asymmetries generally contribute adversely to the intensification rate and the mature hurricane intensity. One type of flow asymmetry results from the barotropic/baroclinic breakdown of an unstable ring vortex. The ensuing potential vorticity redistribution process has been shown to weaken the maximum tangential wind while simultaneously spinning up the flow within the eye towards solid body rotation. However, if the asymmetry results from a deep convective structure (perhaps initiated by a local buoyancy asymmetry), then we have considerably less theoretical guidance on the outcome of the 3D flow in comparison to AX. The most prominent theory for mature intensity is the axisymmetric theory of Emanuel, which essentially assumes a passive role for cumulus convection, slaved to the whims of the surface enthalpy fluxes.

In this study we conduct a suite of numerical simulations using the CM1 numerical model, which is ideal for 3D vs. AX comparisons. While our results support the foregoing picture of intensity reduction by barotropic/baroclinic instability processes, our results suggest also a fundamental difference between convective organization in 3D versus AX. On the one hand, cumulus convection in 3D is influenced by the differential rotation of the system-scale circulation in the radial and vertical directions. During the intensification process, the convection does not organize into convective rings and as a result the mean heating rate is considerably less than found in AX. However, convection in 3D can become more intense locally than found in AX. Even after an eyewall ring has formed in 3D, the eyewall convection still exhibits a tendency to concentrate into strong and localized convective updrafts. These localized updrafts have important effects on both the mean heating rate and the corresponding mean overturning circulation. These findings have important consequences for the formulation of a theory of tropical cyclone intensification and mature intensity.