East coast lows and extratropical transition of tropical cyclones, structures producing severe events and their comparison with mature tropical cyclones

Abstract

Examination of events occurring over the last 53 years in the Australian Region have revealed in the minds of forecasters a common pattern in the development of severe extratropical cyclones which have affected the sub-tropical and temperate East Coast. To evaluate this theory 20 years of data were systematically examined and showed that this was true. To represent these many cases nine such events which delivered the largest impacts over the 53 years were chosen for study. These extratropical cyclones formed downstream of a tropopause undulation which can be easily identified as a warm region at the 200 hPa-level and the formation zone was in a region of heavy rain embedded in a region of warm air advection at 700 hPa. There were hardly any exceptions to this general rule, and one that occurred is presented and was also one of the most rapidly developing systems. This pattern is then evaluated against tropical cyclone events which move in the Australasian sub tropics and three different scenarios are described and compared with a mature severe tropical cyclone which intensified as it moved into the Australia sub tropics. Hurricane Sandy due to its devastating effect on the US sub-tropics in 2012 is examined as a benchmark case whose impact could affect the Australasian sub tropics in the future as sea levels rise with higher density populations.