Tropical cyclones are intense, cyclonically-rotating, low-pressure weather systems that form over the tropical oceans. Cyclonic means counterclockwise in the northern hemisphere and clockwise southern hemisphere while intense means that sustained wind speeds exceed 17 m s-1 (60 km h-1, 32 kn) near the surface. The convention for the definition of a "sustained wind speed" is a 10 min average value, except in the United States, which adopts a 1 min average. Severe tropical cyclones have near surface sustained wind speeds equal to or exceeding 33 m s-1 (120 km h-1, 64 kn): these are called hurricanes over the Atlantic Ocean, the East Pacific Ocean and the Caribbean Sea, and Typhoons over the Western North Pacific Ocean. Typically the strongest winds occur in a ring some tens of kilometres from the centre and there is a calm region near the centre, the eye, where winds are light. For moving storms, the wind distribution is asymmetric with the maximum winds in the forward right quadrant in the northern hemisphere and in the forward left quadrant in the southern hemisphere. The eye obtains its name because, in a mature storm, it is normally free of deep clouds, but is surrounded by a ring of deep convective clouds that slope outwards with height. This ring is called the eyewall cloud or simply the eyewall. At larger radii from the centre, storms usually show spiral bands of convective clouds. Figure 1 shows a satellite view of the eye and eyewall of a mature typhoon, together with photographs looking out at the eyewall cloud from the eye during aircraft reconnaissance flights.
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The mature tropical cyclone consists of a horizontal quasi-symmetric circulation on which is superposed a vertical, or transverse circulation. These are sometimes referred to as the primary and secondary circulations, respectively. When combined, these two component circulations result in a spiralling motion with inflow at low and middle levels and outflow at upper levels. The secondary circulation is mostly thermally-direct, which means that warm air is rising, a process that releases potential energy. However subsidence occurs in the eye and the circulation there is thermally indirect, a process that requires energy to be supplied.
Figure 2 shows a schematic cross-section of prominent cloud features in a mature cyclone including the eyewall clouds that surround the largely cloud-free eye at the centre of the storm; the spiral bands of deep convective outside the eyewall; and the cirrus canopy in the upper troposphere. Other aspects of the storm structure are highlighted in Fig. 3. Air spirals into the storm at low levels, with much of the inflow confined to a shallow boundary layer, typically 500 m to 1 km deep, and it spirals out of the storm in the upper troposphere, where the circulation outside a radius of a few hundred kilometres is anticyclonic. The spiralling motions are often evident in cloud patterns seen in satellite imagery and in radar reflectivity displays.
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Latest version: Munich 12 December 2024