Conceptual models of upper-level frontogenesis in south-westerly and north-westerly flow

The conceptual model proposed by Shapiro as it is applied to the evolution of an upper-level frontal zone within a baroclinic wave is reviewed and its limitations are investigated through previous literature and two case-stud ies presented in this paper. The early stages in the evolutions of these tw o cases are used to examine specific limitations of this conceptual model: (1) upper-level frontogenesis in south-westerly flow that evolves from a st ate of equivalent barotropy to a state of cold advection along the front, a nd (2) upper-level frontogenesis in northwesterly flow with along-front var iation in the sign of the thermal advection, such that warm advection occur s upstream of cold advection in the thermal trough. Vector-frontogenesis diagnostics for the Lagrangian rate of change of the m agnitude and direction of the horizontal potential-temperature gradient, in cluding tilting due to vertical motion, are derived. These diagnostics are applied to the two cases to examine the maintenance of the potential-temper ature gradient and the development of cold advection along each upper-level front. The upper-level front in south-westerly (north-westerly) flow was m aintained primarily by deformation (tilting) frontogenesis, in agreement wi th previous research. The increasing cold advection along the upper-level f ront in both cases was related to an upstream vorticity maximum. For the ca se in south-westerly flow, the pre-existing vorticity maximum approached a downstream equivalent-barotropic upper-level front in a manner similar to a n instant occlusion, resulting in cold advection along the length of the up per-level front. For the case in north-westerly flow, an intensifying vorti city maximum concentrated the cold advection in the base of the thermal tro ugh, as warm advection developed upstream. These two cases are compared to upper-level fronts in previous literature a nd a climatology of upper-level fronts associated with landfalling cyclones over the eastern North Pacific Ocean. The results indicate that these two cases are typical of early evolutions of upper-level fronts that can occur in south-westerly and north-westerly flow. Therefore, a revised version of the Shapiro conceptual model is presented that represents more accurately t he early evolutions exhibited in the present and previous studies.