An Observational and Modeling Study of Tropical Cyclone Outer Rainbands

When a tropical cyclone is approaching, the general aviation community must typically respond before the arrival of the first outer rainbands if aircraft are to be safely evacuated. Therefore, there is not just one but two major forecast questions that must be answered. First, will your location be affected by the intense central portion of the circulation such that you should evacuate to avoid damaging winds? And second, if damaging winds are anticipated, when will the first outer rain bands arrive and begin making the safe the evacuation of general aviation aircraft problematic? Understanding the behavior and dynamics of the outer rainbands is crucial to answering this latter question.

Theoretical modeling studies have indicated the formation and evolution of outer rainbands may be understood in terms potential vorticity (PV) dynamics (e.g., Guinn and Schubert, 1993). For example, these idealized studies have demonstrated that the interaction of the high-PV inner circulation of the storm with pre-existing high-PV structures on the periphery of the circulation (i.e., vortex merger) leads to PV structures that strongly resemble outer rainbands.

In this study, specific cases of rainband formation and evolution are examined using satellite imagery and PV diagnostics calculated using high resolution model initialization grids. These results are compared with the evolution of similar but idealized PV structures using theoretical models to test hypotheses that attempt to explain observed rainband behavior (e.g., the observed rainbands result from the merger of the vortex with other regions of pre-existing PV).