Balanced thermal structure of an intensifying tropical cyclone

This study tests the hypothesis that the formation of a virtual potential temperature dipole in a developing tropical cyclone is a balanced response to the growth of an associated mid-level vortex. The dipole is collocated with the vortex and consists of a warm anomaly in the upper troposphere and a cool anomaly in the lower troposphere. An axisymmetric approximation to the observed potential vorticity distribution is inverted subject to non-linear balance for two successive days during the formation of typhoon Nuri in 2008. Good agreement is found between the area-averaged actual and balanced virtual temperature dipoles in these two cases. Furthermore, a strong correlation exists between the degree of bottom-heaviness of convective mass flux profiles and the strength of the balanced virtual potential temperature dipole. Since the dipole is balanced, it cannot be an immediate artefact of the existing convection, but rather is an inherent feature of the developing cyclone. Cloud resolving numerical modelling suggests that the dipole temperature anomaly actually promotes more bottom-heavy convective mass flux profiles, as observed. Such profiles are associated with low-level mass and vorticity convergence via mass continuity and the circulation theorem, resulting in low-level spin-up. The present work thus supports the hypothesis that the low-level spin-up associated with tropical cyclogenesis is made possible by the thermodynamic environment created by a strong mid-level vortex.