Stages in the energetics of baroclinic systems
Sheldon, John P.
- Publisher: Co-Action Publishing
(issn: 1600-0870, eissn: 0280-6495)
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 alternative to the more conventional descriptions of cyclone development. These additional insights are made possible largely because the local energetics approach permits one to define an energy flux vector which accurately describes the direction of energy dispersion and quantifies 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 downstream through the intervening ridge, generating a new energy center there. In Stage 2, this new energy center grows vigorously, at first due to the convergence of these fluxes, and later by baroclinic conversion as well. As the center matures, it begins to export energy via geopotential fluxes to the eastern side of the trough, initiating yet another energy center. In Stage 3, this new energy center continues to grow while that on the western side of the trough decays due to a dwinding supply of energy via fluxes from the older upstream system and also as a consequence of its own export of energy downstream. As the eastern energy center matures, it exports energy further downstream, and the sequence begins anew. The USA “Blizzard of'93” is used as a new case study to test the limits to which this conceptual sequence might apply, as well as to augment the current limited set of case studies. It is shown that, despite the extraordinary magnitude of the event, the evolution of the trough associated with the Blizzard fits the conceptual picture of downstream baroclinic evolution quite well, with geopotential fluxes playing a critical rôle 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 and modifying the jet, resulting in a large extension of the overall kinetic energy center well into Mexico. Second, energy fluxes from this extension of the northwesterly flow were strongly convergent east of the trough, producing explosive growth of kinetic energy over the northwestern Gulf of Mexico, with baroclinic conversion following shortly thereafter. Lastly, the kinetic energy generated by the vigorous baroclinic conversion in the cold advection on the west side of the trough was very effectively transferred to the energy center on the east side of the trough via geopotential fluxes.DOI: 10.1034/j.1600-0870.1995.00108.x