A Case Study for the Downstream Development of Baroclinic Waves in the Midlatitude Jet Induced by Extratropical Transition

This study uses eddy kinetic energy analysis and targeting method to investigate how an extratropical transition event (ET) induced downstream cyclogenesis in the midlatitude jet environment. The downstream development showed clear characteristics of coupling development and boundary effect. Eddies first developed at upper levels, and then transported vertical ageostropic geopotential fluxes downward to trigger lower-level eddy development. At the early stage, these eddies all developed near the boundaries (i.e., the tropopause or surface), and their amplitudes increased to reach peaks at the boundaries. Thereafter, an upper-level (or lower-level) eddy would caught up with a lower-level (or upper-level) one ahead of it, and then they coupled with each other strongly to develop to a strong cyclone that extended to the whole troposphere. Based on the analyses, a new conceptual model for the downstream cyclogenesis is proposed. No matter in the coupling development or in boundary effect the vertical ageostrophic fluxes may be a crucial dynamical process.

Furthermore, the downstream development did not follow classic DBD theory. Ageostropic geopotential fluxes that were transported from Hurricane Fabian to the midlatitudes by the outflow below the tropopause led to the downstream ridge development in the upper-level jet. Barotropic conversion also became the primary energy source after the ridge growth was well under way. It is difficult for barolinic energy to convert to eddy kinetic energy directly. Baroclinic energy first converts to mean kinetic energy of the jet, and then mean kinetic energy converts to eddy kinetic energy through barotropic conversion. The strong downstream advection of eddy kinetic energy in the exit region of the jet streak triggered the downstream trough development. The well known ridge–trough couplet thus formed in the downstream jet.