Review of Global Tropical Cyclone Terminology
by Gary Padgett
Revised : August 2009 The following is a synopsis of the operational terminology used by the various TCWCs to describe the different stages of tropical cyclone development and intensification. This gives the formal terminology used to refer to a given cyclone in warnings, public advices, and discussion bulletins. The adjective "severe" is used in several regions with quite different meanings, and these are defined below if the term is formally applied to designate an intensity range. In the U. S., the term may be frequently used to describe the character of a hurricane, but is not formally used as the descriptor for a given cyclone intensity range. In the Atlantic and Northeast Pacific basins, the Saffir/Simpson Hurricane Wind Scale is utilized to further classify tropical cyclones with winds exceeding hurricane intensity. This is the same as the older Saffir/Simpson Scale but with the central pressure and storm surge descriptors removed. Information on the scale may be found at the following URL: http://www.nhc.noaa.gov/aboutsshs.shtml The TCWCs in Australia and Fiji also employ a five-category scale to further classify all tropical cyclones above gale force. The Australian Cyclone Severity Scale is based upon the peak gusts expected to occur within a cyclone. Information on this scale may be found at: http://www.bom.gov.au/weather/cyclone/about/about-tropical-cyclones.shtml#severity 1. ATLANTIC and NORTHEAST PACIFIC Basins - TPC/NHC (also CPHC) MSW Averaging Period: 1 minute Tropical Cyclone - generic term for systems of all intensities Tropical Disturbance - distinct area of disturbed weather but usually with no well-defined low-level circulation apparent/ regular advisories not issued/ usually corresponds to Dvorak ratings less than T2.0 Tropical Depression - fairly well-defined low-level circulation/ MSW less than 34 kts/ Dvorak rating usually T2.0 Tropical Storm - MSW in range of 34-63 kts/ Dvorak rating T2.5, T3.0, or T3.5 Hurricane - MSW exceeding 63 kts/ Dvorak rating T4.0 or higher The point at which regular advisories are initiated is a little subjective. A system with a Dvorak rating of T1.5 might be upgraded to a tropical depression if it were in a position to affect a populated area and/or if it seemed to be rapidly intensifying and its development potential was considered excellent. 2. JTWC for NORTHWEST PACIFIC Basin Only MSW Averaging Period: 1 minute Tropical Cyclone - generic term for systems of all intensities Tropical Disturbance - distinct area of disturbed weather but usually with no well-defined low-level circulation apparent/ regular advisories not issued/ usually corresponds to Dvorak ratings of T1.0 or less/ MSW generally less than 25 kts Tropical Depression - fairly well-defined low-level circulation/ MSW 25-34 kts/ Dvorak rating usually T1.5 - T2.0 Tropical Storm - MSW in range of 34-63 kts/ Dvorak rating T2.5, T3.0, or T3.5 Typhoon - MSW exceeding 63 kts/ Dvorak rating T4.0 or higher Super typhoon - MSW reaching or exceeding 130 kts 3. JTWC for NORTH INDIAN OCEAN and SOUTHERN HEMISPHERE MSW Averaging Period: 1 minute For the North Indian Ocean and all Southern Hemisphere regions, JTWC uses only the generic term "Tropical Cyclone" to refer to systems of all intensities in warning status. Warnings are usually initiated when the system is forecast to produce gale/tropical storm force winds within 48 hours. In many cases winds are already approaching this threshold when the first warning is issued and frequently the initial MSW is set at 35 kts. 4. NORTHWEST PACIFIC Basin - JMA (Japan) MSW Averaging Period: 10 minutes Tropical Cyclone - generic term for systems of all intensities Low-pressure Area - distinct area of disturbed weather but usually with no well-defined low-level circulation apparent/ regular warnings not issued Tropical Depression - fairly well-defined low-level circulation/ MSW less than 34 kts Tropical Storm - MSW in range of 34-47 kts/ Dvorak rating T2.5 or T3.0 Severe Tropical Storm - MSW in range of 48-63 kts/ Dvorak rating T3.5 or T4.0 Typhoon - MSW exceeding 63 kts/ Dvorak rating T4.5 or higher The Japanese Meteorological Agency (JMA) is the World Meteorological Organization's (WMO) official Regional Specialised Meteorological Centre (RSMC) for the Northwest Pacific Basin. While adhering to a 10-min averaging period for MSW, JMA normally equates 34 kts to a Dvorak rating of T2.5; thus, JMA and JTWC agree in principle on the threshold of tropical storm intensity. However, for very intense typhoons, JMA's MSW estimates are usually far below those assigned by JTWC due to a much lower 1-min to 10-min conversion factor for extreme intensities. The meteorological services of other Asian nations issue tropical cyclone warnings for portions of the Northwest Pacific region, including the Philippines, Hong Kong, China, Vietnam, Taiwan, and Korea. Warnings from these weather services are issued independently of JMA but utilize the same terminology and are usually reasonably close to JMA's positions and intensity estimates. 5. NORTH INDIAN OCEAN Basin - IMD (Indian Meteorological Department) MSW Averaging Period: 3 minutes Zone of disturbed weather - a zone in which the pressure is low relative to the surrounding region and there are convective cloud masses which are not organized Low-pressure Area - an area enclosed by a closed isobar with mean surface winds less than 17 kts Depression - well-defined low-level circulation but with MSW generally less than 28 kts/ Dvorak rating of T1.5 Deep Depression - depression with MSW in range of 28-33 kts/ Dvorak rating of T2.0 Cyclonic Storm - tropical cyclone with MSW in range of 34-47 kts/ Dvorak rating T2.5 - T3.0 Severe Cyclonic Storm - tropical cyclone with MSW in range of 48 - 63 kts/ Dvorak rating of T3.5 Very Severe Cyclonic Storm - tropical cyclone with MSW in the range of 63 kts - 119 kts/ Dvorak rating T4.0 or higher Super Cyclonic Storm - tropical cyclone with MSW 120 kts or higher The Indian Meteorological Department (IMD) is the WMO's RSMC for the North Indian Ocean (Bay of Bengal and Arabian Sea), although the meteorological services of other nations may also issue warnings for portions of the basin. The MSW values reported in warnings are considered to be over a 3-min averaging period; however, IMD does not modify the Dvorak scale. 6. SOUTHWEST INDIAN OCEAN Basin (West of 90E) MSW Averaging Period: 10 minutes Zone of Disturbed Weather - term used to describe weak, ill-defined systems with winds generally less than 25 kts and corresponding to Dvorak ratings of less than T2.0 Tropical Disturbance - MSW usually 25 kts near center/ Dvorak rating usually T2.0 (Beaufort Force 6) / such systems usually classified as tropical depressions by most TCWCs Tropical Depression - MSW in range of 28-33 kts (Beaufort Force 7 - Dvorak T2.5) Moderate Tropical Storm - MSW in range of 34-47 kts/ Dvorak rating of T3.0 - weak T3.5 Severe Tropical Storm - MSW in range of 48-63 kts/ Dvorak rating of strong T3.5 - T4.0 Tropical Cyclone - MSW in range of 64-89 kts/ Dvorak rating of T4.5 - T5.5 Intense Tropical Cyclone - MSW in range of 90-115 kts/ Dvorak rating T6.0 - T6.5 Very Intense Tropical Cyclone - MSW exceeding 115 kts/ Dvorak rating T7.0 - T8.0 The WMO's RSMC for the South Indian region is Meteo France on the French island of La Reunion; however, names are actually assigned by the Sub-regional Tropical Cyclone Advisory Centres on Mauritius (east of 55E) and Madagascar (west of 55E). The La Reunion TCWC employs a conversion factor of 0.88 to convert the 1-minute MSW Dvorak scale to an equivalent 10-minute average scale. (Prior to the 1999-2000 season, the conversion factor used was 0.80.) 7. AUSTRALIAN REGION (longitude 90E eastward to longitude 160E) MSW Averaging Period: 10 minutes Tropical LOW - term is used to describe disturbances ranging from diffuse, ill-defined low-pressure areas all the way to well- organized tropical depressions with MSW up to 33 kts Tropical Cyclone - MSW in range of 34-63 kts/ Dvorak rating ranging from a strong T2.5/weak T3.0 to T4.0 Severe Tropical Cyclone - MSW exceeding 63 kts/ Dvorak T4.5 or higher Warnings in the Australian Region are issued by three separate TCWCs at Brisbane (Queensland), Perth (Western Australia), and Darwin (Northern Territory) with Darwin being the RSMC for the region. In addition to these, a TCWC at Port Moresby, Papua New Guinea (formerly an Australian territory) issues warnings for a small portion of the region near and east of the island of New Guinea, and a TCWC at Jakarta, Indonesia, issues warnings for the region west of 125E and north of 10S. The Papua New Guinea and Indonesian regions have an extremely low incidence of tropical cyclone occurrences. The Australian centres avoid use of the term "tropical depression" in public advices primarily to reduce possible confusion with the use of the term "depression" in association with extratropical systems; and also possibly because until recently (early 1990's), in the Southwest Indian Ocean Basin, a "tropical depression" meant any system with winds up to 63 kts (hurricane force). The Australian TCWCs utilize a conversion factor of 0.88 or 0.90 to modify the 1-minute Dvorak scale to an equivalent 10-minute average scale. In the Australian Region to qualify as a tropical cyclone a tropical LOW must be accompanied by gales surrounding more than 50% of the center of circulation for a period of at least six hours. In order to insure that adequate warnings are provided, it is not at all unusual for a system to be named as a tropical cyclone, but later reduced to tropical LOW status after a careful post-storm analysis reveals that this spatial distribution of gales criterion was not met, e.g., Isobel and Odette, 2007, and Gabrielle, 2009. 8. SOUTH PACIFIC Basin (east of longitude 160E) MSW Averaging Period: 10 minutes Tropical Disturbance - distinct area of disturbed weather but usually with no well-defined low-level circulation apparent/ regular advisories not issued/ usually corresponds to Dvorak ratings less than T2.0 Tropical Depression - fairly well-defined low-level circulation/ MSW less than 34 kts/ Dvorak rating usually T2.0 or weak T2.5 Tropical Cyclone - MSW exceeding 33 kts/ Dvorak rating of strong T2.5/T3.0 to T4.0 Severe Tropical Cyclone - MSW exceeding 63 kts/ Dvorak T4.5 or higher The TCWC at Nadi, Fiji, is the RSMC for the South Pacific basin but the meteorological service of New Zealand (Wellington) issues warnings for systems that move (or very rarely form) south of latitude 25S. The Fiji TCWC employs a 1-minute to 10-minute MSW conversion factor of 0.90 when adjusting the Dvorak scale. A tropical system can have associated gales and still be classified as a tropical depression if the gales are well-removed from the center. In such cases the gales are usually found in only one or two quadrants. Prior to 2000, the WMO Region V definition of a tropical cyclone required that gales surround the center, but the definition was changed to allow classification as a tropical cyclone if gales were present near the center and likely to persist, even if in only one quadrant.
Updated: 9th August 2009
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