Mesoscale cyclogenesis in reversed shear - The 1998 Sydney to Hobart yachtrace storm

The 1998 Sydney-Hobart yacht race was affected by storm-force winds on the afternoon of 27 December 1998. Of the 115 yachts that contested the race, 7 1 retired, six were abandoned, five sank, 55 crew were rescued, and six liv es were lost. This band of storm-force winds was associated with a secondar y cyclone that developed on the western side of an occluding deep-troposphe ric low-pressure system. A high-resolution (approximately 5 km) horizontal resolution mesoscale numerical weather prediction model is used to model th is storm, and is shown to reproduce many of its observed features. These mo del fields are then used to both diagnose the processes that led to the dev elopment of the low and to describe its mesoscale structure and evolution. The synoptic environment prior to the development of the storm shows an amp lifying mid-latitude trough that negatively tilted and cut off as an equato rward-directed jet stream on its western flank moved to the apex of the tro ugh. An unusual feature of this case was the interaction of this developing cut-off low with the circulation of a lower-latitude extratropical cyclone in the northern Tasman Sea. This interaction led to the development of a s trong thermal gradient through Bass Strait prior to the development of the secondary low, with the direction of the gradient directed opposite to the direction of the pressure gradient - a reversed shear structure. Warm, mois t air moving over Tasmania from the northeast also provided a deep layer of weak static stability in the lower troposphere over Tasmania and southern Bass Strait. As the upper-tropospheric low cut off, a filament of cyclonic Isentropic Potential Vorticity (IPV) air was advected to the rear poleward side of this low. The development of the secondary low is hypothesised to b e associated with the vertical circulation induced by the advection of this IPV filament over the weak static stability air-mass over Bass Strait. The evolution of this IPV filament in the forecast model is supported by the w ater vapour imagery from the Japanese Geostationary Meteorological Satellit e (GMS-5), which shows dark (dry) features in close agreement with the IPV patterns. The secondary low had a warm-core structure in its lowest levels, but this was overlain by a cold-core in the middle troposphere. The revers ed shear environment on the northern flank of the low led to an extreme low -level jet.