A quantitative analysis of baroclinic instability in extratropical cyclogenesis

Abstract

Piecewise tendency diagnosis (PTD) is employed to quantify the contribution made by the baroclinic instability (BI) mechanism, as described by Hoskins et al. (1985), to extratropical cyclogenesis occuring near the United States' eastern coast. The PTD developed by Nielsen-Gammon and Lefevre (1996) is extended to enable diagnosis of low-level cyclone development. We introduce a method for partitioning the cyclone growth into parts attributable to vertical superposition and amplification of the participating potential vorticity (PV) anomalies, thereby allowing a complete quantitative estimation of the contribution made by cyclogenetic mechanisms such as Bi to cyclone growth. In all three episodes examined, it is found that baroclinic amplification of the low-level cyclone-scale thermal wave by winds associated with the upper perturbation PV field is the process most contributory to cyclogenesis. Upper-level wave evolution is dominated by processes involving interactions between the basic-state flow and perturbation PV field, rather than by baroclinic amplification. We infer that processes associated with non-conservation of PV modulate the adiabatic forcing for cyclogenesis, particularly during the latter stages of cyclone growth, resulting in observed growth rates which differ significantly from those which are diagnosed. Downstream translation of the upper wave is found to contribute significantly to low-level height falls but it is argued that this process is not one which contributes directly to cyclone growth.