Shear and curvature vorticity and potential-vorticity interchanges: interpretation and application to a cutoff cyclone event

Abstract

Major midtropospheric cyclogenesis begins as a jet embedded in northwesterly flow, identified as a maximum of cyclonic shear potential vorticity, propagates toward the base of a diffluent trough, identified as a maximum of cyclonic curvature potential vorticity. The interchange terms contribute both to the amplification of the trough and to the formation of a maximum of cyclonic shear potential vorticity on the downstream side of the trough. The potential-vorticity interchange process is shown to play a key role in transforming the asymmetric configuration of shear and curvature potential vorticity characteristic of the diffluent trough stage, where the cyclonic shear maximum lags the cyclonic curvature maximum, to the relatively symmetric configuration characteristic of the cutoff stage. A second important structural change occurring during midtropospheric cyclogenesis is the transformation of the potential-vorticity anomaly corresponding to the cutoff cyclone into a circularly symmetric configuration, which is accomplished by the contraction of the northwestern extension of the potential-vorticity anomaly toward the cyclone center. This contraction process results in the detachment of the potential-vorticity anomaly from the "stratospheric reservoir' of potential vorticity located north of the cyclone. -from Authors