Subtropical cyclone

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Subtropical Storm Gustav in 2002

A subtropical cyclone is a weather system that has some characteristics of a tropical cyclone and some characteristics of an extratropical cyclone. They can form in a wide band of latitude, from the equator to 50°.

1 Types
2 Characteristics
3 Formation
4 Naming
5 See also
6 References
7 External links

[edit] Types

[edit] Upper-level low

The most common type of subtropical storm is an upper-level cold low with circulation extending to the surface layer and maximum sustained winds generally occurring at a radius of about 100 miles/160 kilometres or more from the centre. In comparison to tropical cyclones, such systems have a relatively broad zone of maximum winds that is located farther from the centre, and typically have a less symmetric wind field and distribution of convection.

[edit] Mesoscale low

A second type of subtropical cyclone is a mesoscale low originating in or near a frontolyzing zone of horizontal wind shear, also known as a dying frontal zone, with radius of maximum sustained winds generally less 30 miles/50 kilometres. The entire circulation may initially have a diameter of less than 100 miles/160 kilometres. These generally short-lived systems may be either cold core or warm core, and briefly in 1972 this type of subtropical cyclone was referred to a neutercane. As of 2006, the warm core variety were moved under the umbrella of the tropical cyclone definiton, and removed from the subtropical cyclone definition. ^[1]

[edit] Transition from extratropical

By gaining tropical characteristics, an extratropical low may transit into a subtropical depression/storm. A subtropical depression/storm may further gain tropical characteristics to become a pure tropical depression/storm, which may eventually develop into a hurricane, and there is at least one case of a tropical storm transforming into a subtropical storm. Generally, a tropical storm or tropical depression is not called subtropical while it is becoming extratropical, after hitting either land or colder waters. This transition normally requires significant instability through the atmosphere, with temperature differences between the underlying ocean and the mid-levels of the troposphere requiring over 40C of contrast in this roughly 20,000-foot/6000-metre layer of the lower atmosphere. ^[2]

[edit] Characteristics

These storms can have maximum winds extending further from the centre than in a purely tropical cyclone. The maximum recorded wind speed for a subtropical storm is 33 m/s (119 km/h, 65 knots, or 74 mph), also the minimum for a hurricane. In the Atlantic Basin, the United States NOAA classifies subtropical cyclones similar to their tropical cousins, based on maximum sustained surface winds. Those with winds below 18 m/s, 65 km/h, 35 kts, or 39 MPH are called subtropical depressions, while those at or above this velocity are referred to as subtropical storms.

Subtropical cyclones are also somewhat more likely than tropical cyclones to form outside of a region's designated hurricane season. Subtropical Storm Ana (which became Tropical Storm Ana) in mid-April of the 2003 hurricane season is such a case.

[edit] Formation

Most subtropical cyclones form when a deep cold-core extratropical cyclone drops down into the subtropics. The system becomes blocked by a high latitude ridge, and eventually sheds its frontal boundaries as its source of cool and dry air from the high latitudes diverts away from the system. Temperature differences between the 500 hPa pressure level, or 6000 meters/20000 ft above ground level, and the sea surface temperatures initially exceed the dry adiabatic lapse rate, which causes an initial round of thunderstorms at a distance east of the center. Due to the initial cold temperatures aloft, sea surface temperatures usually need to reach at least 20 degrees C for this initial round of thunderstorms. The initial thunderstorm activity moistens up the environment around the low, which destabilizes the atmosphere by reducing the lapse rate needed for convection. When the next shortwave or upper level jet streak moves nearby, convection reignites closer to the center and the system develops into a true subtropical cyclone. The average sea surface temperature that helps lead to subtropical cyclogenesis is 24 degrees C.^[3] If the thunderstorm activity gets deep and its initial low level warm core deepens, tropical cyclogenesis is possible.

[edit] Naming

Throughout the 1950's and 1960's, the term semi-tropical was used for what would become known as subtropical cyclones. In 1972, the National Hurricane Center (NHC) finally designated these storms as subtropical cyclones in real-time, and updated the hurricane database from 1968-1971 to include subtropical cyclones. Subtropical cyclones were initially named from the phoenic alphabet list in the early to mid 1970's. In the intervening years of 1975-2001, subtropical storms were either named from the traditional list and considered tropical or used a separate numbering system. Between 1992 and 2001, two different numbers were given to subtropical depressions or subtropical storms, one for public use, the other one for NRL and NHC reference. For example, Karen in 2001 was initially known as Subtropical Storm One as well as AL1301 (or 13L for short)[1]. In 2002, the NHC began giving numbers to subtropical depressions and names to subtropical storms from the same sequence as tropical ones. From 2002 onwards, Subtropical Depression 13L will be known as Subtropical Depression Thirteen instead. Subtropical Storm Nicole, from the 2004 Atlantic hurricane season was the first subtropical storm that did not become tropical since the policy change. A subtropical storm from the 2005 Atlantic hurricane season also did not become tropical, but was not named since it was not recognized until post-season analysis.