Studies of ‘cva maxima’ south of Australia. Part 1: a long-lived enhanced cumulus cluster and induced cyclogenesis

A long-lived mesoscale area of enhanced cumulus clouds was observed on satellite imagery moving from far south of Australia to Victoria over a two-day period, where it appeared to interact with a frontal cloudband and induce surface cyclogenesis over the western Tasman Sea. Such features on satellite imagery have been known since the 1960s as ‘cva maxima’, as they have been shown to be frequently associated with mid to upper-tropospheric centres of cyclonic vorticity advection, and so to mark potential areas of upward vertical motion or height falls. This paper uses the objective analyses from an operational limited area data assimilation system, LAPS, to assess the ability of LAPS to resolve the structure of this subsynoptic-scale system over the Southern Ocean, and to compare this structure with previous studies of ‘cva maxima’. It is shown that the cloud feature was closely associated with a centre of height falls forced by cyclonic lsentropic Potential Vorticity (IPV) advection near the tropopause, and that this more dynamically based diagnosis shows a closer association with the enhanced cumulus cluster than does diagnosis based on cyclonic vorticity advection alone. Further, it is shown that satellite imagery-based 'instant occlusion’ and the NWP model-based ‘IPV thinking’ conceptual models of cyclogenesis complement each other in interpreting the NWP model forecast guidance.