By Tom Frame
Tropical cyclones have featured heavily in the weather news over the past couple of weeks. Last week the first tropical storm of the Atlantic hurricane season (Arthur) struck the eastern seaboard of the USA. This week Typhoon Neoguri (Korean for Racoon Dog) hit southern Japan. Just in these first two sentences I have used many different terms, tropical cyclone, hurricane, tropical storm, typhoon, so I should take a minute to say what all these terms mean. The interesting thing here is that all these terms refer to the same thing: a tropical cyclone. Hurricane and typhoon are both terms for tropical cyclones with maximum sustained surface wind speeds exceeding 64 knots (33 metres per second, or Force 12 on the Beaufort scale), the only difference being that the term hurricane is used in the Atlantic and Eastern Pacific, whereas typhoon is used in the Western Pacific. A tropical storm (such as Arthur) is a tropical cyclone, occurring anywhere, with maximum sustained surface winds exceeding 34 knots (17 metres per second, or Force 8 on the Beaufort scale).
As a physical scientist my instinct is always to want to explain the causes of particular phenomena in as concise and general way as possible. Using multiple names for the same thing seems irrational and overly complicated. So why do meteorologists use different names for essentially the same object? The answer is twofold: these names are in part historical/cultural, but mostly the names are used to communicate the likely impact of the storm. For example in a tropical storm, ocean wave height is likely to reach 6-16 metres, whereas a hurricane or typhoon will produce waves exceeding 14 metres.
Figure 1. On July 7 at 2:41 a.m. EDT the TRMM satellite had a near perfect view as it passed above the center of Typhoon Neoguri. Heaviest rainfall was occurring at 106 mm per hour in feeder bands south-east of Neoguri’s eye. Image courtesy NASA/SSAI.
So tropical storms, hurricanes and typhoons are all tropical cyclones, but what are tropical cyclones and how do they form? Tropical cyclones are atmospheric vortices a few hundred to a few thousand kilometres in diameter. They are driven by the warm ocean surface which provides heat and moisture to the air causing it to rise rapidly leading to strong rainfall and cloud formation. Within a mature tropical cyclone the ascending air and associated clouds form into spiral bands rotating round a cloudless central region of descending air known as the “eye” (Figure 1). The supply of moisture and heat from the ocean surface provides the energy to maintain the cyclone. This is sometimes characterised in terms of a heat engine, with strong heating at the surface and cooling due to radiative processes at the top of the cyclone being converted in mechanical energy, creating strong winds. This means that the tropical cyclones always develop over the ocean and begin to weaken as they make landfall, essentially because their fuel is cut off.