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ABSTRACT

The Shapiro (1982) conceptual model 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 studies presented herewithin. The early stages in the evolutions of these two cases are used to examine specific limitations of this conceptual model: (1) upper-level frontogenesis in southwesterly flow that evolves from a state of equivalent barotropy to a state of cold advection along the front, and (2) upper-level frontogenesis in northwesterly flow with along-front variation in the sign of the thermal advection, such that warm advection occurs upstream of cold advection in the thermal trough.

Vector-frontogenesis diagnostics for the Lagrangian rate of change of the magnitude and direction of the horizontal potential temperature gradient, including tilting due to vertical motion, are derived. These diagnostics are applied to the two cases to examine the maintenance of the potential temperature gradient and the development of cold advection along each upper-level front. The upper-level front in southwesterly (northwesterly) flow was maintained primarily by deformation (tilting) frontogenesis, in agreement with previous research. The increasing cold advection along the upper-level front in both cases was related to an upstream vorticity maximum. For the case in southwesterly flow, the preexisting vorticity maximum approached a downstream equivalent-barotropic upper-level front in a manner similar to an instant occlusion, resulting in cold advection along the length of the upper-level front. For the case in northwesterly flow, an intensifying vorticity maximum concentrated the cold advection in the base of the thermal trough, as warm advection developed upstream.

These two cases are compared to upper-level fronts in previous literature and 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 southwesterly and northwesterly flow. Therefore, a revised version of the Shapiro conceptual model is presented that more accurately represents the early evolutions exhibited in the present and previous studies.

Figure 1: Revised conceptual model: Idealized schematic depiction on an upper-tropospheric isobaric surface of the early evolution of an upper-level jet--front system through a midlatitude baroclinic wave over a 12-24-h period: (a) southwesterly flow case; (b) northwesterly flow case. Geopotential height contours (solid grey lines), isentropes (solid black lines), and relative vorticity (shaded).

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A follow-up comment on a hypothesis advocated by Rotunno et al. (1994) regarding how the along-front cold advection develops.

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