#291 Postby AussieMark » Sun Mar 26, 2006 6:19 am
Cyclone Larry was one of the most intense – and least expected – weather events in Australian history. At first it was simply a low pressure system, but conditions coalesced and it intensified almost overnight into a category five cyclone.
“There was a favourable environment with a ridge to the south that helped keep it on a westerly trajectory,” says Ben Matson, meteorologist and founder of independent weather company SwellNet. “The surface temperature in the Coral Sea was also good and there was nothing to slow it down. It was a very special weather event.”
Although this is peak cyclone season, often a pressure ridge along the Queensland coast creates windshear in the upper atmosphere. This acts to slice the low pressure system in half and the embryonic cyclone is dead before it gets underway.
With Larry, the ridge wasn’t there, and there was nothing to slow it down. The pressure dropped to 915 hecto-pascals and peak wind gusts hit 215kmh. In layman’s terms, it was a very low-pressure system with very high winds.
But it wasn’t just the local conditions. The broader atmospheric signs were also favourable for Larry’s formation, and point to more heavy-duty cyclones before the season closes at the end of April. Larry was the offspring of a phenomenon called Madden-Julian Oscillation, a global, eastward-moving pattern of upper and lower air circulation that operates over a 30- to 60-day cycle.
“It enhances convection in the region and helps spawn tropical lows,” says Matson. The oscillation was in the ideal part of its cycle to help kickstart the convection needed to sustain a cyclone.
Cyclones need a sea surface temperature of at least 27.5°C to get going. Hot air rises, taking moisture with it. A low-pressure system and relatively light winds aloft help its formation, and the Earth’s rotation adds the spin. As long as the energy keeps pouring in, the cyclone keeps going. Fortunately, as soon as it crosses land – as Larry did on Monday – the heat source dries up and the cyclone withers into a tropical depression.
We’re in the middle of the season, the Madden-Julian Oscillation is favourable and the ocean is warm, so more cyclones are inevitable, but another of Larry’s intensity is hard to forecast. The perfect storm requires perfect conditions.
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