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Typhoon Kirogi displays a rare concentric eye-wall Kirogi: July 1-9, 2000 Not since Typhoon Damrey in early May has the South China Sea been subject to such a powerful storm. Typhoon Kirogi and its sparring partner Kai-tak, brought soaking rains to the northern Philippines killing 42 and forcing 800,000 to evacuate. Saturated soil later caused a garbage heap to tumble, killing and trapping people living on its massive slopes near Manila. Kirogi then moved away from Kai-tak, travelling along the north-eastern flank of the sub-tropical Pacific High to hit southern Japan with heavy rains, killing four people, knocking out power and flooding streets. In the Port of Mikayu in Northern Honshu, the remains of Kirogi dumped 312mm of rain in 24 hours, more than double the July average of 127mm. What made meteorologists stand up and really take notice of Kirogi as it approached the south coast of Japan was the presentation of an unusual phenomena in the very heart of this typhoon - a double band of cloud around the eye. The double eye-wall, or concentric eye-wall, is seen in very intense cyclones and causes a chain reaction within the storm. Power hungry According to severe storm forecaster Jeff Callaghan, when the outer eye develops, it steals moist air as it is sucked radially into the storm, leaving the inner eye to shrink and collapse. Callaghan says pressure in the very centre of the cyclone can rise 30 hPa in as little as six hours when the concentric eye-wall develops, because convection is no longer happening there. It is taking place within the outer eye-wall. The inner eye literally dries up, as moist air is channeled around it. When the inner eye fades, the outer eye becomes the main centre of the storm. At this point the cyclone's eye is very large and the whole storm slows down - much like a ballet dancer as she spreads her arms outward. The eye usually shrinks again and the storm speeds up, just like when the dancer pulls her arms inward. "The angle of momentum is conserved if it is in a smaller area," notes Calllaghan. "It is the ballet dancer effect." In the case of Kirogi, the storm did appear to intensify after the concentric eye-wall merged into one eye, but mountains on the south coast of Japan and cooler northern water also affected its progress. Soon after making landfall it collapsed and degenerated into a tropical storm. Why, why, why? The dynamics of the concentric eye-wall are poorly understood, says Callaghan. In fact, debate still rages over the construction of the eye-wall itself. "Cyclones, typhoons or hurricanes are very hard to reproduce in the laboratory," explains Callaghan. Anxious in Hong Kong Kai-tak, which also showed a concentric eye-wall on July 7, later confounded forecasters with its route, stalling north of Luzon in the Philippines. Anxious typhoon fans in Hong Kong hoped and prayed that it would intensify and move over the city (Kai-tak is a name associated with Hong Kong. The old airport at Kowloon was also called Kai-tak). Instead, Taiwan took the brunt of Kai-tak's rain, as the typhoon faded into a tropical storm. The town of Heng-Chun on the southern tip of the island saw 282mm of rain fall in four days - almost three-quarters of the average July rainfall of 381mm. Previously, in the Philippines, the town of Laoag in the far north was hit by torrential rain from slow-moving Kai-Tak, depositing 444mm of rain in 60 hours ending midday GMT on July 8. That figure is close to the average for the whole of July (461mm). Other Western Pacific and Eastern Indian Ocean cyclones with Susan, Drena, Thelma, and even Vance at one stage before it hit the Western Australia coast.
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Cyclone Kirogi with a clear double eye-wall as it approaches southern Japan. (070703gmt) Cyclone Winnie in 1997 also showed double-eye characteristics. Here the cyclone's eye is north-east of Taiwan. Pictures courtesy of the
Kai-tak, to the left, and Kirogi team up to dump rain over the northern Philippines on July 6. Picture
courtesy of Yahoo
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