Structure and evolution of an isolated semi‐geostrophic cyclone

The evolution and structure of an indealized low-pressure system is studied within the framework of the semi-geostrophic dynamics and in the limit of uniform potential vorticiy. An isolated cyclone is grown from suitably chosen initial conditions, rather than studying the evolution of a longitudinally periodic train of baroclinic systems. It is shown that the resulting development is able to produce a range of flow features that compare favourably with observationally based conceptual models of cyclogenesis. These features include in particular: (i) the simultaneous occurrence of both a cold and a warm front whose alignment shows some of the characteristics of occluded frontal systems and is skin to the notion of a frontal fracture, (ii) a dry descending air-stream to the rear of the cyclone, (iii) a narrow region of maximum ascent within the warm front an its bent-back portion, (iv) a poleward traveling air-stream ahead of the cold front, and (v) a γ-shaped pattern of vertical lifting comparable with the cloud patterns as commonly observed in satellite pictures. The evolving cyclone is analysed both form Lagrangian and Eulerian viewpoints. It is demonstrated that Lagrangian criteria exist that allow for the objective definition of air-streams and flow patterns within developing cyclones. The structures of cold and warm fronts at low levels are significantly affected by the different nature of the Lagrangian trajectories within each of these regions. In particular, the air parcels in the warm-frontal region are transported rapidly towards the centre of the low, resulting in a low-level warm front with an intrinsically three-dimensional structure and an associated vorticity gradient in the along-front direction.

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