Potential vorticity analysis of a multiple frontal cyclogenesis event during CASP II

A potential vorticity (PV) analysis is performed using a 60-h simulation of a family of mesoscale frontal cyclones which occurred over the Western Atlantic ocean from 0000 UTC 13 to 1200 UTC 15 March 1992. The frontal cyclogenesis events were characterized by an upper-level ring of PV anomalies of stratospheric origin, located on the cyclonic side of the jet stream. The shape of the "PV ring" resembled the tracks of the surface frontal cyclones, implying its role in determining the initiation and the tracks of the surface cyclones. Piecewise PV inversions indicate that low-level baroclinicity contributed most to the intensity of two of the frontal cyclones, followed by low-level latent heating, and upper-level PV anomalies (trough/ridge system). The upper-level PV anomalies contributed most to the depth of a third frontal cyclone. The major frontal cyclone (MFC), underwent explosive deepening (27 hPa in 24 h) and eventually absorbed the low-level circulation of the parent cyclone. The explosive deepening of the MFC can be attributed to the phase-locking between the upper-level and low-level PV anomalies.;A series of full physics sensitivity experiments is performed to test the impact of removing upper-level PV anomalies on the development of the MFC. It is found that the delay in the genesis of the MFC, as well as weakening in its final intensity, is directly related to the proximity of the removed upstream PV anomaly. In another series of "dry" experiment with the removal of upper-level positive PV anomalies, the model fails to generate the MFC when the upstream PV anomaly closest to the pre-MFC is removed. From the results of piecewise PV inversion and sensitivity experiments, low-level thermal advection induced by the flow of the upper-level PV ring appears to initiate and sustain the development of the MFC. (Abstract shortened by UMI.)...