Multiple potential-vorticity inversions in two FASTEX cyclones

This paper investigates pre-existing synoptic-scale disturbances responsible for cyclogenesis, using the manipulation of initial conditions through quasi-geostrophic potential vorticity (QGPV) and its inversion. The use of a QGPV inversion together with a T63 model allows us to study the sensitivity of some FASTEX events to several QGPV patterns present within the analysis. It has been found that Ertel potential-vorticity modifications of the analysis are preserved by the dynamical initialization process. The QGPV inversion method is applied to cyclones occurring in Intensive Observation Periods (IOPs) 11 and 12 of FASTEX (Fronts and Atlantic Storm-Track EXperiment) to demonstrate which potential-vorticity anomalies, present within the atmosphere prior to the development, are involved in the cyclogenesis. The choice of the boundary condition when an upper-level precursor is removed from the initial conditions has little impact on the cyclone development. The linear interactions (advection/deformation of the anomaly by its environment and downstream development) between these upper-level precursors and the surrounding flow dominate the dynamics of IOP11 cyclone development, whereas nonlinear processes are particularly strong in the IOP12 case. The cyclone development is only weakly sensitive to low-level initial structures unless they are shaped like adjoint model sensitivities; collocation with another shape has little influence on the development.