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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
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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
Email: J.Callaghan@bom.gov.au
nomad3.ncep.noaa.gov/ncep_data/index.html
Links
Microwave images of tropical cyclone were obtained courtesy of
the US Navy Research Laboratory Monterey (California) Satellite
Section:
www.nrlmry.navy.mil/tc_pages/tc_home.html
Radiosonde data around the globe, both current and archived,
were obtained from the University of Wyoming:
weather.uwyo.edu/upperair/sounding.html
