Highly asymmetric structures in a landfalling hurricane can lead to the formation of heavy rains, wind gusts and tornadoes at preferred locations relative to the hurricane center. In this study, we examined the development of asymmetric structures in an explicitly simulated hurricane during landfall using potential vorticity (PV) dynamics. Previous studies have shown that the potential vorticity (PV) bands are well correlated with the inner spiral rainbands.

We found that the boundary layer friction produces positive PV ahead of the hurricane and negative PV behind. The positive PV generation tends to enhance the low-level convection and the associated diabatic heating, which further amplifies the low-level PV in the front quadrants. This asymmetric configuration breaks the low-level PV ring, allowing the low equivalent potential temperature air parcels entering the eye to stabilize the atmosphere in the core. Consequently, PV increases significantly in the core within and just above the boundary layer where weak stratification was present when the hurricane was offshore. Large PV bands shedding from this enhanced low-level PV core interact with the boundary layer to produce stronger vertical motions in the front quadrants, forming asymmetric rainbands over land. The dramatic changes in the boundary layer act to create a large vertical wind shear which is favorable for the generation of tornadoes especially in the right-front quadrant.

After the hurricane makes landfall, the reduced diabatic heating in the eyewall cannot generate enough PV to maintain the PV ring in the middle and upper troposphere. The PV ring evolves to a monopolar structure through the non-linear mixing process which is also responsible for the spin-down of the hurricane.