PhD research examines the behaviour of cyclones

PhD research examines the behaviour of subtropical cyclones       

Subtropical cyclones (STCs) cause extensive damage in New Zealand on average three times a year, yet research into these weather phenomena has been largely absent. 

But Victoria University PhD graduate Susanne Schroeder’s research into the behaviour and characteristics of STCs show that they are influenced by seasonal changes in the atmospheric conditions in the subtropics, in particular changes in the upper-level, or higher altitude, wind flow of the region. 

“My research looked at the structure and development processes of STCs, including how the upper-level flow affects the frequency, track and intensity of these cyclones,” says Ms Schroeder. 

She analysed a cyclone track database covering the last 21 years, which provided an insight into the behaviour of STCs in this region, and with the help of the UK Meteorological Office’s global model data looked at the development process of STCs from 1999 to 2003 in more detail.     

“Distinct annual and seasonal patterns in frequency, track and intensity were revealed. Differences in the structure and development of STCs can be attributed to the seasonal variations in the atmospheric conditions in this region, in particular the upper-level flow, which takes on mid-latitudinal features in the subtropics during winter.”  

Ms Schroeder developed a system of categorisation, based on differences in the upper-level flow above the surface centres of developing STCs. 

“Certain types occur only in winter, while others are more frequent in the summer months.” 

The interaction with an upper-level trough is the mechanism that steers these cyclones into the vicinity of New Zealand, where the high rain that accompanies them, which can be attributed to their place of origin, represents a major threat in terms of flood related damage. 

It is hoped this research can be used to improve the forecast of these events, by providing a new way of looking at the systems.

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