Outside the radius of maximum winds tropical cyclones are generally characterised by weak cyclonic relative vorticity in the mid-low levels, despite the large-scale circulation. This is due to the opposing contributions from the shear and curvature terms. Examination of the absolute vorticity fields simulated with a triply-nested movable mesh high resolution tropical cyclone model show a region of anticyclonic absolute vorticity occurring in the rainbands at the 600-700 hPa level. Thus regions of air below the freezing level are inertially unstable. Low inertial instability is common in the upper levels of tropical cyclone rainbands where anticyclonic flow dominates and there are observations of low values on the outer side of rainbands, however the lower troposphere is generally viewed as inertially stable.

The regions of inertial instability are a consistent feature with the model simulated rainband, both in location and time. To investigate what mechanisms maintain the anticyclonic motion against the strong inertially stable flow of the vortex core the vorticity budget has been calculated. Initially the tilting of horizontal vorticity into the vertical generates most of the anticyclonic vorticity. The region coincides with maximum values of both the horizontal and vertical shearing of the tangential winds and is where a large pulse of radial inflow meets the outflowing air. Once the anticyclonic region is developed it weakens as it is advected along the band at the rate of the background flow. Over time the tilting spins down the anticyclonic vorticity, while the divergence term works to maintain the anomaly through convergence of the already anticyclonic flow.

A well-defined jet is apparent in the rainband and its location is between the inner cyclonic region and the outer anticyclonic air. Thus this feature in the band may be a mechanism by which an acceleration of the tangential winds and the development of a mid-level jet could feed potential vorticity generated in the rainband into the storm core, intensifying convection in the eyewall and the inner core winds. Discussion on how the anticyclonic anomaly may excite inward propagating vortex Rossby waves will also be made.