Typhoon's long-period swells affect operations far away

The Pacific Ocean to the east of the Philippines is the most prolific breeding ground in the world for tropical revolving storms.

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The Pacific Ocean to the east of the Philippines is the most prolific breeding ground in the world for tropical revolving storms. These features are conceived by thermodynamic processes over the warm tropical ocean, where the circulation may be very weak and insignificant at first.

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The illustration shows an image of Typhoon Dan, the lastest typhoon to affect Eastern Asia. The typhoon developed from tropical depression status on October 2, increased to a tropical storm on October 3, and became a fully-fledged typhoon on October 4.
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The embryo cyclonic circulation, however, begins to gather the ample quantities of energy supplied by the high sea temperatures of the Pacific and its central vortex begins to intensify. This then progresses rapidly through the developing stages of tropical depression and tropical storm, nurtured by indrawn water vapor, and is often accompanied by violent thunderstorms and torrential showers. The strengthening vortex reaches the classification of severe tropical storm then - finally - typhoon.

Mature typhoon

The mature typhoon is among the most destructive of all natural phenomena, capable of annihilating coastal towns and causing catastrophic disasters offshore, the effects of which are well known locally within the offshore industry.

It is paramount importance that a developing circulation can be identified as a potential typhoon threat as early as possible. The main problem in the past was the lack of observations of wind speed and direction in areas away from the main shipping lanes, combined with the fact that - in its early development stages - a weak cyclonic circulation is difficult to classify just from satellite imagery.

Satellite technology, however, is not limited only to visual images. The latest polar-orbiting satellites are capable of scanning and isolating small movements of the atmosphere beneath their orbits. This facility has resulted in the ability to identify remote cyclonic circulations in their very early stages - in areas where there are no ship observations. The detection rate of the position and growth rate of these features has shown a massive improvement.

Recently, Typhoon Dan developed east of the Philippines and moved across the north end of Luzon Island towards Hong Kong. The storm then turned northeastward and crossed the coast of China, south of Xiamen. At its mature stage, maximum sustained winds near the center reached over 100 kts - which is by no means the strongest typhoon experienced in this region - but still caused substantial disruption. At the time of this writing, Dan has moved inland.

Long-period swells

Apart from the effects of the wind and seas in the proximity of typhoons such as Dan, the offshore industry also has to contend with swell trains generated by the winds near the vortex. These swells contain huge amounts of energy and can affect offshore operations hundreds of miles away in the southern part of the South China Sea.

The swells are of a long period (15-17 seconds is not unknown) and, although they are of relatively low amplitude (less than a meter), they travel very rapidly (45-50 kts) and reports of substantial vessel motion have already been received from operations off the coast of peninsular Malaysia, as a result of Dan. The problem is caused by an effect similar to resonance, where a vessel will respond much more vigorously to a particular swell period - according to its construction characteristics.

Swell train studies

These swell trains and other storm effects are studied by Fugro GEOS and the UK Met Office, which provide detailed weather forecasting services for SE Asia, the Indian Ocean, Australasia, and Pacific Ocean as Marine Weather Services (MWS) Pte Ltd. MWS also contributes the operational planning for typhoon evacuation procedures, assessing the safe distance for evacuation away from the passage of a tropical storm as well as the intensity and duration of storm conditions.

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