The dynamical lifecycle of tropical cyclones from the perspective of potential vorticity structure

Abstract. Although intense tropical cyclones (TCs) are widely associated with a potential vorticity (PV) tower, the relative contributions of diabatic and frictional processes to their evolving PV structure remain poorly quantified. Conceptualising TCs as meso-scale PV anomalies, we use online model diagnostics to decompose the PV budget into diabatic and frictional components, revealing how these processes generate the evolving PV structure that controls tropical cyclone intensification, maintenance and decay. TCs Katrina (2005) and Usagi (2013) serve as exemplary cases to identify common dynamical mechanisms governing PV evolution across TC life cycles. During TC intensification, convection acts as the main source of PV production, thereby driving TC development and intensification with the characteristic cloudless eye of TCs gradually becoming a spatially distinct substructure. A previously unknown dynamical balance emerges in the lower eye between negative PV generated by longwave radiative processes at the ocean surface, which is advected into the eye, and positive PV generated by the interaction of frictional processes with the strengthening eyewall baroclinic zone. The two contrasting contributions are large in absolute PV value, with the positive PV generated by frictional forces outweighing the negative PV contribution from longwave radiation. PV inversion diagnostics performed using a novel machine learning approach suggest a relatively weak net contribution of these contrasting processes to the cyclone's maximum intensity of ~10–15 %. The PV within the upper part of the TC eye is dominated by positive contributions to the PV budget from diabatic processes and frictional forces, with the latter also contributing an additional 10–15 % to the maximum intensity of TCs. Within the eyewall, latent heat release retains its positive feedback on TC intensification throughout the TC life cycle, with surface sensible and turbulent heat fluxes either counterbalancing or enhancing this positive feedback depending on altitude. Finally, we show that the spatial structure of PV plays a paramount role in the intensification and maintenance of a TC. TC decay commences when this structure can no longer be sustained owing to the accumulation of negative PV within the cyclone eye due to longwave radiation, highlighting ocean surface processes as critical to the cyclone's life cycle.