STAGES IN THE ENERGETICS OF BAROCLINIC SYSTEMS

The results from several idealized and case studies are drawn together to form a comprehensive picture of ''downstream baroclinic evolution' ' using local energetics. This new viewpoint offers a complementary al ternative to the more conventional descriptions of cyclone development . These additional insights are made possible largely because the loca l energetics approach permits one to define an energy flux vector whic h accurately describes the direction of energy dispersion and quantiti es the role of neighboring systems in local development. In this view, the development of a system's energetics is divided into three stages . In Stage 1, a pre-existing disturbance well upstream of an incipient trough loses energy via ageostrophic geopotential fluxes directed dow nstream through the intervening ridge, generating a new energy center there. In Stage 2, this new energy center grows vigorously, at first d ue to the convergence of these fluxes, and later by baroclinic convers ion as well. As the center matures, it begins to export energy via geo potential fluxes to the eastern side of the trough, initiating yet ano ther energy center. In Stage 3, this new energy center continues to gr ow while that on the western side of the trough decays due to a dwindi ng supply of energy via fluxes from the older upstream system and also as a consequence of its own export of energy downstream. As the easte rn energy center matures, it exports energy further downstream, and th e sequence begins anew. The USA ''Blizzard of '93'' is used as a new c ase study to test the limits to which this conceptual sequence might a pply, as well as to augment the current limited set of case studies. I t is shown that, despite the extraordinary magnitude of the event, the evolution of the trough associated with the Blizzard fits the concept ual picture of downstream baroclinic evolution quite well, with geopot ential fluxes playing a critical role in three respects. First, fluxes from an old, decaying system in the Pacific were convergent over the west coast of North America, creating a kinetic energy center there an d modifying the jet, resulting in a large extension of the overall kin etic energy center well into Mexico. Second, energy fluxes from this e xtension of the northwesterly flow were strongly convergent east of th e trough, producing explosive growth of kinetic energy over the northw estern Gulf of Mexico, with baroclinic conversion following shortly th ereafter. Lastly, the kinetic energy generated by the vigorous barocli nic conversion in the cold advection on the west side of the trough wa s very effectively transferred to the energy center on the east side o f the trough via geopotential fluxes.