A diagnostic case study of self-development as an antecedent conditioning process in explosive cyclogenesis

A diagnostic study of two, successive operational model forecasts of a case of explosive cyclogenesis is presented, with the goal of understanding the rather substantial differences in the simulations. The rapid cyclogenesis, which occurred to varying degree in both forecasts, can be explained as a moist baroclinic response to a strong 500-mb trough embedded within the polar airstream. The variability in the forecasts in related to differential growth of low-level cyclonic vorticity in association with amplification of the 500-mb vorticity gradient between the upstream trough and a locally forced downstream short-wave ridge, prior to the period of most rapid deepening. This antecedent vorticity growth was initiated by advection offshore of the east coast of North America of a tongue of stratospheric potential vorticity, identifiable in the conventional constant-pressure analysis as a weak short-wave trough at 500 mb. Once initiated, low-level development continued as a result of a self-development process involving an interaction between quasigeostrophic forcing of ascent and latent heat release: upon the arrival of the polar trough, rapid surface deepening ensured. The self-development process during the antecedent stage effectively lengthened the time scale of intensification, leading to greater increases in surface relative vorticity through vortex stretching. In addition, the upstream 500-mb trough was amplified during this period. The weaker development in the less successful simulation of this case occurred as a result of damped self-development and consequently reduced low-level vorticity and weaker cyclonic vorticity advection during the rapid deepening stage. The impact on predictability of such nonlinear interactions between processes during cyclogenesis is discussed, with reference to short- and medium-range forecasts.