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°.
Types
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 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.
Mesoscale low
A second type of subtropical cyclone is a mesoscale low originating in or near a frontolyzing zone of horizontal wind shear, with radius of maximum sustained winds generally less 30 miles. The entire circulation may initially have a diameter of less than 100 miles. These generally short-lived systems may be either cold core or warm core.
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 however, a tropical storm or tropical depression is not called subtropical while it is becoming extratropical, after hitting either land or colder waters.)
Characteristics
These storms can have maximum winds extending further from the center 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 just as with tropical ones, based on maximum sustained surface winds: below 18m/s, 65km/h, 35 knots, or 39 MPH is a tropical or subtropical depression, and at or above this velocity is a tropical or subtropical storm.
Subtropical cyclones are also somewhat more likely than tropical cyclones to form outside of a region's designated hurricane season, as was the case with Subtropical Storm Ana (which became Tropical Storm Ana) in mid-April of the 2003 hurricane season.
Naming
In 1972, the National Hurricane Center (NHC) designated these storms as neutercanes, but this term was quickly discontinued the following year in 1973. In 2002, the NHC began giving numbers to subtropical depressions and names to subtropical storms from the same sequence as tropical ones. 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]. 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 discovered until post-season analysis.
See also
External links