From Savanna Links, Issue 34, 2007. Savanna Links is written and produced by the Tropical Savannas CRC.

Desert sands key to mystery landphoons

Radar imagery at Halls Creek
After an initial weakening of the system, Abigail deepened as it crossed more than 1000 km over land. Radar imagery at Halls Creek captured a system with a structure similar to that of a mature oceanic cyclone. Images: Bureau of Meteorology

In February 2001, Tropical Cyclone Abigail crossed the Australian coast in the Gulf of Carpentaria. However, instead of weakening, the cyclone deepened as it moved over more than 1000 km of land. It was a ‘landphoon’, or ‘agukabam’. These mysterious weather systems act like cyclones and scientists are only now getting a handle on how they are possible. Frances Bancroft reports.

Cyclones and hurricanes can only form over oceans yet some cyclones in northern Australia have re-intensified when over desert land. Nicknamed ‘landphoons’, these systems have puzzled scientists for years—they seem to have characteristics of ocean-based cyclones but are missing the inner wall and distinctive eye. However, a team from the Bureau of Meteorology and the US Massachusetts Institute of Technology believe they are on track to proving not only that the systems have the same structure as cyclones but why a rapid heat transfer from the desert soils allows the storms to re-intensify.

The team has also renamed landphoons ‘agukabams’ from the Cape York Ayapathu language: agu, meaning land and kabam, meaning storm. Agukabams may be unique to Australia, although there are reports of similar occurrences in the Arizona desert.

Their research is based on the assumption there is a rapid heat transfer from the desert soil that allows the systems to re-intensify. While desert sand is very hot, it is not normally associated with a rapid heat exchange. However, if the sand is sufficiently wetted by the first rains of the approaching system then this may be enough to generate the rapid heat exchange needed to feed a cyclone.  To test this theory the team adapted a tropical storm modelling system called CHIPS and combined it with a very simple soil model. They then compared predicted and observed results.

One such system was ex-Tropical Cyclone Abigail, which, in February 2001, deepened as it progressed westwards from the base of the Gulf of Carpentaria to the Great Sandy Desert in Western Australia. After travelling more than 1000 kilometres over land, radar imagery captured at Halls Creek showed the system possessed a structure similar to that of a mature oceanic tropical cyclone. By running a simulation of Abigail with the soil model, the researchers achieved strikingly similar results.

The desert soils of Australia are not all the same however, and there are few weather stations that measure soil temperature, meaning that more complex soil models are not feasible. Further tests with more complex modelling will be needed to verify these findings and to deepen our understanding of not only these systems, but also oceanic cyclones as well.

In Press: Development of Warm Core Cyclones over Land
Jeff Callaghan and Peter Otto, BOM, Kerry Emanuel, Massachusetts Institute of Technology

More information: Jeff Callaghan, Bureau of Meteorology


Microwave images of tropical cyclone were obtained courtesy of the US Navy Research Laboratory Monterey (California) Satellite Section:

Radiosonde data around the globe, both current and archived, were obtained from the University of Wyoming: