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G. R. Foley and B. N. Hanstrum

Abstract

A climatology of all tropical cyclones that have affected the subtropical west coast of Australia during the 82-year period 1909–1990 is presented. A total of 39 events were recorded, an average of 0.5 per year, although a more realistic estimate of the frequency of occurrence, based on a dataset that included satellite surveillance (1964–1990), suggested a value of 0.7 cyclones per year. Northern sections of the west coast experienced nearly twice the number of cyclones as central and southern sections. Cyclone occurrence was confined to the later part of the tropical cyclone season (nominally November to April) and was most common in March.

An examination of mean sea level pressure (MSLP) analyses associated with tropical cyclones in the period 1964–1990 revealed two distinct synoptic patterns. One group of cyclones was characterized by a steady southward movement into a region of environmental easterly flow, and the other by rapid acceleration due to the approach of a frontal system from the central Indian Ocean.

The MSLP patterns for the latter group were composited nearest to the time of acceleration. The key feature of the composite analysis was a highly meridional front/ridge pair over the eastern Indian Ocean to the southwest of the cyclone. The acceleration toward the southeast began, on average, when the surface cold front was located approximately 1700 km to the southwest. On the basis of the composite analysis, interpretation of satellite imagery and a case study of a recent tropical cyclone event, a conceptual model for acceleration and transformation of tropical cyclones into the midlatitudes is proposed. Weather changes accompanying the transformation process are described.

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B. N. Hanstrum, K. J. Wilson, and S. L. Barrell

Abstract

A climatology of the prefrontal westerly trough of southern Australia is presented, based on data for the 10-yr period 1976–85. Trough formation was confined to the longitudes of the Australian continent in the warmer months of the year. An average of approximately 15 troughs/yr were observed, evenly distributed each month from September to April, with appreciable interannual variability in incidence. Troughs formed in advance of a major Southern Ocean cold front in a region of differential thermal advection. Genesis occurred primarily over the west of the continent in spring and autumn, but over southeastern Australia in summer.

Variability in the location of formation at different times of year was related to the seasonal shift of the subtropical ridge over the continent, the shape of the southern continental coastline, and periods of low zonal-index flow. A class of troughs prone to rapid intensification after formation was also identified.

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B. N. Hanstrum, K. J. Wilson, and S. L. Barrell

Abstract

A case study of frontogenesis within a surface prefrontal trough over southern Australia is presented. The trough developed ahead of a surface cold front and, over a period of approximately 24 h, intensified into a mature summertime frontal system while the original front underwent total frontolysis. Two-hourly rawinsonde ascents at three locations were used to examine the structure of the trough both before and after frontogenesis. Diagnosis of the frontogenetic tendencies showed that horizontal deformation of the potential temperature field was the main forcing mechanism.

The process studied occurs frequently in the warmer months of the year, and a conceptual model is proposed for the process of frontogenesis within the trough and frontolysis of the preexisting midlatitude front. Important components include differential thermal advection, the action of Coriolis turning in association with ageostrophic vertical circulations, and horizontal deformation of the thermal field.

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