Potential vorticity diagnostics of cyclogenesis

We develop a diagnostic system based on the relative smallness of the irrotational part of the horizontal wind, which allows us to calculate the balanced flow given the three-dimensional distribution of Ertel's PV. The close agreement between the observed and balanced flows, even for intense cyclones, illustrates the practical utility of the PV approach. Furthermore, we present a technique for determining the flow associated with individual perturbations of PV. Insight gained from these diagnostics is demonstrated by examining a particular case of cyclogenesis. -from Authors