Characteristics of Rapid Intensification of Tropical Cyclones in the Australian Region

As in other basins around the world, rapid intensification (RI) of tropical cyclones continues to be a forecasting challenge in the Australian region. This process causes particular difficulty for cases ahead of landfall when communities and industry require adequate lead time for preparations.

This investigation describes the TC RI characteristics within the Australian region, highlighting distinctive aspects within the global context. This includes:

• location and frequency;
• the distribution of RI events by intensity;
• influence of TC size on RI;
• characteristics of the most extreme RI events;
• key synoptic influences; and
• forecasting challenges including the identification of upper bound (95%) intensity forecasts.

Key outcomes include:

• Most Australian TC events reaching 65kn or more undergo RI at some point in their lifetime.
• The onset of RI commences at lower intensities than in most other basins.
• Extreme RI events are associated with smaller sized storms.
• RI typically occurs in low wind shear environments and strong upper-level divergent flow.
• The most difficult forecasting challenges relate to RI in the following scenarios: onset in moderate wind shear, during the early stages of development, when circulations develop after moving off the Australian mainland, and timing of RI for systems near the coast.

The demands for forecasts with longer lead times means that forecasters are often attempting to predict RI in the early stages of development with little to no RI guidance available to them. Furthermore, standard NWP guidance typically varies greatly ahead of formation, leading to lower confidence in forecasts. This can also contribute to higher track uncertainty, and hence lower confidence in wind, storm surge and rainfall predictions.

Case studies will be used to describe different synoptic influences on RI in the region and to demonstrate forecast challenges.