Weather by Malcolm RileyYasi image from the US Geostationary Satellite.In the wake of Tropical Cyclone Yasi



A man in Atlantic City, USA received his new barometer in the mail. He opened the package and the needle was stuck as low as it could go. With much anger he wrote a terse note to the manufacturer and went to the post office to return the faulty barometer.
When he got home from the post office his house near the beach was gone. The barometer was correct and he was a victim of the 1938 hurricane that without warning devastated much of the New England coastal population centres killing 600.
This is a good yarn and I have seen many references to this story that are all slightly different, which probably puts it into urban myth category. I mention it to highlight the advances in meteorological forecasting since1938, evidenced by the recent example of tropical cyclone Yasi.
The image above is from the Bureau of Meteorology website on Monday January 31 forecasting for 10pm on Wednesday night 2nd February.Before I go any further hurricanes, tropical cyclones and typhoons are different names for the same phenomenon; tropical revolving storms.
With the advances in science of the detection and forecasting of cyclones, the changes to the building code and the procedures for emergency services to manage the population, the human impact of Yasi was a lot less than Category 5 storms that have made landfall either previously in Australia or in more recently in other locations worldwide.
Computer model data were predicting the area of the storm several days in advance.
Tropical cyclones are low pressure systems that have at least gale force winds (sustained winds of 63 km/h or greater and gusts in excess of 89 km/h) near the centre. Cyclones are initially a cluster of thunderstorms that form between 5 and 10 degrees south latitude, usually in an area of lower pressure.
As this cluster of storms move further south the earth’s rotation starts the cyclonic rotation.
Schematic of the a tropical cyclone (courtesy of the ABC).The air rises through the system and adjacent air rushes in towards the centre of the storms giving strong surface winds. If the conditions are favourable the surface air pressure of the systems starts to fall dramatically and the eye, an area of clear skies forms at the centre of the system.
Winds rush in towards the centre of the system in a clockwise direction. In and around the storm bands of heavy rain and thunderstorms form.
Mariners that I talk to seem to be fascinated by the eye of the cyclone. The border of the eye is called the ‘eye wall’ and is made up of a ‘wall’ of large thunderstorms and generally has the strongest winds.
The strong winds do not transfer into the eye due to Coriolis force (see Afloat Sept/Oct 2008), as the winds get faster the leftwards moving force of the wind is increased. This means the wind flows around the eye rather than into it. Outside of the eye there are vast amounts of rising (hot) air.
Carnage at Port Hinchinbrook.A lot of the moisture has been removed by the time it gets to the top of the system; so it is relatively dry. Most of the air that has risen moves away from the cyclone and sinks. Some, however, moves into the middle of the cyclone and sinks, into the eye. This dry sinking air brings cloud free conditions.
One question often asked is: “as the winds in the eye are relatively calm can I sail in the eye?”
In the eye the wind are light, however, large waves caused by the strong winds outside the eye wall move into the eye from many directions. The sea conditions may be worse in the eye due to the erratic direction of the waves.
The erratic direction of large waves was one of the features of the loss of vessels in the 1979 Fastnet race. The conditions in this race were caused by several small lows pressure systems in close proximity to each other rather than a tropical revolving storm.
An interesting tale to emerge from TC Justin (March 1997) that made landfall near Cairns concerned Eastern Curlews which migrate from Queensland to China each year.
The Queensland Wader Study Group fitted satellite transmitters to 12 of the birds. The birds left Moreton Bay as cyclone Justin was brewing in the Coral Sea. Some of the birds turned up exhausted in New Guinea a couple of days later.
One bird appeared to be trapped in the eye of the cyclone and flew around in the eye for five days before the transmissions ceased. It is believed the bird perished. From the total 12 birds only two made it to China. The others turned back.
It is believed that the weather conditions from Justin weakened the birds and the birds realised that surviving a flight to China in their weakened state was unlikely, returning to Moreton Bay.
A really good animation of TC Yasi and TC Anthony can be found at: http://www.theweatherchaser.com/videos/201101-tropical-cyclone-yasi

*Malcolm Riley is the Public and Marine Officer for the Bureau of Meteorology in Hobart. He has worked in all States with the exception of Qld and is a Master V. He gives education courses on Marine Meteorology.