The role of entrainment in axisymmetric tropical cyclones

The intensification of tropical cyclones (TCs) results from the transport of conserved angular momentum, at least in an axisymmetric context. While there is general agreement on the role of moist cloud convection in driving the system, its precise contribution to intensification remains unclear. Additionally, the mechanisms by which convection facilitates angular momentum transport are still not well understood.

Two prominent but seemingly contradictory explanations for this phenomenon exist in the literature: the Conditional Instability of the Second Kind (CISK) and Wind-Induced Surface Heat Exchange (WISHE). Although these models offer different perspectives, we propose that they represent limiting, asymptotic scaling regimes of the same underlying physical process.

To reconcile these differing views, we use matched asymptotics to combine the three distinct regimes suggested by CISK and WISHE, thus providing a unified framework. Our analysis shows that the transport of angular momentum plays a crucial role in ensuring consistency with the asymptotic matching principle.

Interestingly, this work uncovers a new, previously undocumented pathway for angular momentum transport that may serve as a plausible mechanism for TC intensification. A key element of this process is the special role of the top-of-boundary-layer (BL) inflow, which is closely linked to the entrainment of convective cloud towers.

Through this combined approach, we offer a fresh perspective on TC intensification dynamics, confirming the validity of CISK and WISHE within their respective scopes and reconciling them into a more general theory.