A new paradigm for intensity change of tropical cyclones in vertical wind shear

An important roadblock to improved intensity forecasts for tropical cyclones (TCs) is our incomplete understanding of the interaction of a TC with the environmental flow. The processes leading to intensity change of a TC in vertical wind shear in the classical idealized numerical experiment are revisited. The prevailing school of thought for this problem has been that interaction of environmental air with the storm core at mid- to upper levels leads to intensity modification. We propose a new paradigm and demonstrate a significant impact of vertical wind shear on the thermodynamic properties of the inflow layer. Persistent, vortex-scale downdrafts flush the inflow layer with cooler and dryer air quenching the energy supply of the storm. Evidence is presented that this process governs the intensity evolution in our experiment to a large extent.

The vortex-scale downdrafts are linked to a quasi-stationary convective asymmetry outside of the eyewall, formerly dubbed the ‘stationary band complex'. The formation of the convective asymmetry is, to zero order, driven by the balanced dynamical response of the TC vortex to the vertical shear forcing. Thus a close link is established between the thermodynamic impact in the near-core inflow layer and the asymmetric balanced dynamics governing the TC vortex evolution. We present some observational support that the processes identified in the idealized experiment apply to the real atmosphere.