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The development of tropical cyclones in Northwest Australia

Foster, Ian J. (1986) The development of tropical cyclones in Northwest Australia. PhD thesis, Murdoch University.

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Case studies of developing and nondeveloping tropical depressions in the northwest Australian region during the 1979/80 and 1980/81 seasons have been constructed using conventional and satellite observations. Overland and near coastal depressions were selected because of their forecasting importance. The daily evolution of synoptic situations associated with the two types of depression were described without the loss of individuality inherent in the compositing approach. The correlation of cyclogenesis with surges in the low level westerly and easterly wind regimes was investigated via longitude-time sections along 10°S and 25°S.

A major finding is that enhanced low level winds can occur during the lifetime of nondeveloping disturbances, they are not exclusively associated with cyclogenesis. Rather, the emphasis is on the three dimensional structure of the atmosphere around a cloud cluster and how that structure was maintained or altered through time. The requirement for flow out to 10 degrees radius and below 500 mb is for a sustained cyclonic organization of the winds rather than their magnitudes to exceed a critical value. In contrast, winds above 250 mb within 5 degrees radius must remain at less than 15 m/s. There was less dependence on the horizontal organization of the large scale upper flow. Intensification could occur under a light easterly flow as well as under a vigorous upper anticyclone. The most common cause for the failure of the nondevelopers was the strong upper easterlies and attendant large vertical wind shear.

The importance of both upper and lower level regimes of the Southern Hemisphere subtropical ridge also became apparent. This ranged from the effect of dry air advection on near coastal convection to the inhibitory effect of strong upper easterlies. The ridge could also affect cyclogenesis through its influence on the on or offshore movement of tropical depressions. Upper troughs played an indirect role in cyclogenesis through their interaction with the upper ridge rather than by providing outflow channels to the westerlies.

This study suggests that it is the large scale flows. primarily in the Southern Hemisphere, that ultimately control tropical cyclogenesis. Accurate forecasting of genesis cannot be achieved without greater understanding of the influence of large scale mid-latitude events on the environment of tropical depressions.

Item Type: Thesis (PhD)
Murdoch Affiliation(s): School of Environmental and Life Sciences
Notes: Note to the author: If you would like to make your thesis openly available on Murdoch University Library's Research Repository, please contact: Thank you.
Supervisor(s): Lyons, Tom
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