Adiabatic Rearrangement of Hollow PV Towers

Diabatic heating fromdeepmoist convection in the hurricane eyewall produces a towering annular structure of elevated potential vorticity (PV). is structure has been referred to as a hollow PV tower. e sign reversal of the radial gradient of PV satises the Charney-Stern necessary condition for combined barotropic-baroclinic instability. For thin enough annular structures, small perturbations grow exponentially, extract energy from the mean žow, and lead to hollow tower breakdown, with signicant vortex structural and intensity change. e three-dimensional adiabatic rearrangements of two prototypical hurricane-like hollow PV towers (one thick and one thin) are examined in an idealized framework. For both hollow towers, dynamic instability causes air parcels with high PV to be mixed into the eye preferentially at lower levels, where unstable PV wave growth rates are the largest. Little or no mixing is found to occur at upper levels. e mixing at lower and middle levels is most rapid for the breakdown of the thin hollow tower, consistent with previous barotropic results. For both hollow towers, this advective rearrangement of PV ašects the tropical cyclone structure and intensity in a number of ways. First, the minimum central pressure and maximum azimuthal mean velocity simultaneously decrease, consistent with previous barotropic results. Secondly, isosurfaces of absolute angular momentum preferentially shiŸ inward at low levels, implying an adiabaticmechanism bywhich hurricane eyewall tilt can form. irdly, a PV bridge, similar to that previously found in full-physics hurricane simulations, develops as a result of mixing at the isentropic levels where unstable PV waves grow most rapidly. Finally, the balanced mass eld resulting from the PV rearrangement is warmer in the eye between 900 and 700 hPa. e location of this warming is consistent with observed warm anomalies in the eye, indicating that in certain instances the hurricane eye inversion is enhanced through PVmixing on isentropic surfaces and is thus partially dynamically controlled.