Towards understanding the tropical cyclone life cycle

Conceptual frameworks are discussed for understanding the physics of the tropical cyclone life cycle in an idealized, three-dimensional, numerical simulation in a quiescent environment. Both axisymmetric and three-dimensional frameworks are discussed. A central feature of one of the two axisymmetric frameworks is the assumption that absolute angular momentum is materially conserved above the frictional boundary layer, at least in the classical Eliassen balance formulation. Such conservation implies that vortex spin up requires radial inflow above the friction layer, while radial outflow there leads to spin down. Many of the ideas are illustrated by two simple laboratory experiments.

In the other axisymmetric framework, the so-called WISHE framework the material conservation of absolute angular momentum is dispensed with in favour of assuming that the saturated moist equivalent potential vorticity is everywhere zero. This assumption limits the applicability of the WISHE framework at best to a small portion of the storm’s life cycle, even if one were able to justify the implicit angular momentum source thereby introduced. Analysis of a recent three-dimensional numerical simulation of the tropical cyclone life cycle unveils a causality problem with the assumptions underlying these models.

In a three-dimensional framework, the rotating-convection paradigm highlights the importance for vortex spin up of the deep, convectively-induced overturning circulation being strong enough to generate inflow above the frictional boundary layer in the presence of the ubiquitous tendency of the boundary layer to generate outflow there. When deep convection is too weak to ventilate all the mass that is converging in the boundary layer to the upper troposphere, there is net outflow above the boundary layer and the vortex weakens. This behaviour appears to be ruled out in the WISHE models by their assumption of global moist neutrality, but is a feature of the classical Eliassen model.