The benefit of hindsight: re-examining the maximum winds during tropical cyclone Tracy

Abstract

Tropical cyclone Tracy (Tracy) in December 1974 is arguably the most significant tropical cyclone in Australia’s history accounting for 71 lives and the destruction of most of Darwin. While several other cyclone events have caused more deaths including the 1899 Bathurst Bay (Qld) event, Tracy’s impact on Darwin profoundly affected Australian perspectives to the tropical cyclone threat leading to new building codes and enhancement of tropical cyclone warning services. Although Tracy passed directly over Darwin’s observation site, the anemometer was damaged after registering a peak gust of 217 kmh–1. The comprehensive 1977 report of Tracy used techniques available at the time to conclude that ‘peak gusts associated with Tracy were most likely in the range of 217 to 240 kmh–1, corresponding to maximum mean winds (10- minute average) over Darwin of 140 to 150 kmh–1.’ This study examines the techniques used in the 1977 report and re-evaluates the maximum winds using current tropical cyclone science. In particular the following aspects are investigated: • Satellite interpretation using current understanding of the Dvorak technique. • Wind-pressure relationships, given that the central pressure of 950 hPa was accurately measured. • The impact of tropical cyclone size on cyclone intensity estimates. Damage and storm surge assessments are not incorporated into this reanalysis although it is acknowledged that this information should be used with the results of this study to revise Tracy’s maximum intensity at landfall in the Australian tropical cyclone database. Indeed many of the issues raised in this study should be used to re-evaluate the intensity of other historical tropical cyclones in the Australian region and should be understood by planning and building designers. The consequences of this re-examination demonstrate the value of re-evaluating historical cyclones even if they were highly scrutinised following the event.