Potential vorticity mixing and tropical cyclone motion

The concentration of convective heating in the eyewall of a hurricane tends to generate high values of potential vorticity (PV) there, which in turn can lead to a reversal in the radial gradient of PV and thus the possibility of barotropic instability. Such instabilities may ultimately be removed by complex nonlinear redistribution of PV. Recent studies have shown that the PV mixing process may play a role in hurricane spiral bands, polygonal eyewalls, asymmetric eye contraction, intensity change, and the development of mesoscale vortices. However, both numerical and theoretical studies of this process have focused on stationary vortices on an f-plane, and thus have not addressed questions of vortex motion.

To what extent might chaotic PV mixing within a hurricane vortex affect its motion? To what extent must the details of the nonlinear flow evolution be resolved in order to adequately prediction the motion? To address these questions, we use an adaptive multigrid barotropic model (MUDBAR) to study the motion and evolution of a barotropically unstable vortex embedded in a large-scale zonal flow. The model uses a sequence of nested overlapping grids with mesh spacings from 0.5 km to 32 km, allowing it to represent both the details of the PV mixing and the environmental flow accurately and efficiently. A series of numerical experiments is used to quantify the sensitivity of the track. In particular, experiments with similar unstable and stable vortices help address the question of whether the ability to resolve the flow inside the vortex is important for tropical cyclone track prediction.