# Annular tropical cyclone > Mediated Wiki article. Canonical URL: https://mediated.wiki/source/Annular_tropical_cyclone > Markdown URL: https://mediated.wiki/source/Annular_tropical_cyclone.md > Source: https://en.wikipedia.org/wiki/Annular_tropical_cyclone > Source revision: 1184030071 > License: Creative Commons Attribution-ShareAlike 4.0 International (https://creativecommons.org/licenses/by-sa/4.0/) An **annular tropical cyclone** is a [tropical cyclone](/source/Tropical_cyclone) that features a normal to large, symmetric [eye](/source/Eye_(cyclone)) surrounded by a thick and uniform ring of intense [convection](/source/Convection_(meteorology)), often having a relative lack of discrete [rainbands](/source/Rainband), and bearing a [symmetric](/source/Symmetry) appearance in general. As a result, the appearance of an annular tropical cyclone can be referred to as akin to a [tire](/source/Tire) or [doughnut](/source/Doughnut).[1] Annular characteristics can be attained as tropical cyclones intensify; however, outside the processes that drive the transition from asymmetric systems to annular systems and the abnormal resistance to negative environmental factors found in storms with annular features, annular tropical cyclones behave similarly to asymmetric storms. Most research related to annular tropical cyclones is limited to [satellite imagery](/source/Satellite_imagery) and [aircraft reconnaissance](/source/Aircraft_reconnaissance) as the conditions thought to give rise to annular characteristics normally occur over open water, well removed from [landmasses](/source/Landmass) where surface observations are possible. ## Characteristics and identification The annular hurricane was first defined as a subset of [tropical cyclones](/source/Tropical_cyclone) by John Knaff of [Colorado State University](/source/Colorado_State_University) and James Kossin of the [University of Wisconsin–Madison](/source/University_of_Wisconsin%E2%80%93Madison) in 2002 by use of [infrared](/source/Infrared) satellite imagery, which serves as the visual means of ascertaining annular characteristics within tropical cyclone. Knaff and Kossin defined an annular tropical cyclone as a tropical cyclone that maintains either an average or larger-than-average [eye](/source/Eye_(cyclone)) surrounded by deep [convection](/source/Convection_(meteorology)) containing the storm's inner core and a lack of convection occurring outside the [central dense overcast](/source/Central_dense_overcast) for at least three hours. As a result, annular storms lack the [rainbands](/source/Rainband) characteristic of typical tropical cyclone. These features lend the storm an [axisymmetric](/source/Axisymmetric) appearance common to annular tropical cyclones. However, this definition is only applicable while a storm maintains these characteristics—when and while a storm does not feature annular characteristics, the tropical cyclone is considered *asymmetric*. In addition to the primary defining characteristics, the [diurnal](/source/Diurnal_cycle) pulsation of the [cirrus cloud](/source/Cirrus_cloud) canopy associated with [outflow](/source/Outflow_(meteorology)) is subdued once storms become annular.[1] Some annular tropical cyclones may also display a "pinwheel eye", a feature in which conditions in the storm causes its [eye](/source/Eye_(cyclone)) to take the appearance of a [spoked wheel](/source/Spoke).[2] An algorithm for identification of annular tropical cyclones in real-time by objective criteria—the Annular Hurricane Index—has been developed, and shows some power, but was not yet operational, as of 2008.[3] However, the [National Hurricane Center](/source/National_Hurricane_Center) (NHC) later started used the Annular Hurricane Index for the purpose of assessing annular tropical cyclones. The index takes into account the cloud height of the storm, the radii of the eye, and other similar factors, using satellite imagery, and then produces a number, with the maximum value on the scale being 100. Any storm with a value of zero or lower are not annular, while storms with positive indices are considered to be annular. Tropical cyclones with higher indices have more annular characteristics than those with lower indices.[3][4][5] Although tropical cyclones can achieve annular characteristics across a wide spectrum of intensities, annular storms are typically strong tropical cyclones, with average [maximum sustained windspeeds](/source/Maximum_sustained_wind) of 108 kn (124 mph; 200 km/h). In addition, storms attaining annular characteristics are less prone to weakening as a result of [negative environmental factors](/source/Tropical_cyclone#Dissipation). Annular cyclones can maintain their respective peak intensities for extended periods of time unlike their asymmetric counterparts. Following peak intensity, such systems will tend to gradually taper off. This unusual intensity persistence makes their future intensities difficult to [forecast](/source/Tropical_cyclone_forecasting) and often results in large forecast errors. In an analysis of hurricanes in the [East Pacific](/source/Pacific_hurricane) and [North Atlantic](/source/Atlantic_hurricane) between 1995 and 1999, Knaff and Kossin observed that the [National Hurricane Center](/source/National_Hurricane_Center) underestimated the intensity of annular hurricanes 72 hours out by 18.9 kn (35.0 km/h, 21.7 mph).[1] A survey of [Pacific typhoons](/source/Pacific_typhoon) between 1990 and 2009 found only 12 with annular characteristics, representing an occurrence rate of 4 percent.[6] ## Transition from asymmetric cyclones and necessary conditions Tropical cyclones can become annular as a result of [eyewall mesovortices](/source/Mesovortices#Eyewall_mesovortices) [mixing](/source/Entrainment_(meteorology)) the strong winds found in the eyewalls of storms with the weak winds of the [eye](/source/Eye_(cyclone)), which helps to expand the eye. In addition, this process helps to make the [equivalent potential temperature](/source/Equivalent_potential_temperature) (often referred to as theta-e or \theta_e) within the eye relatively uniform. This transition takes roughly 24 hours to complete and can be considered a type of [eyewall replacement cycle](/source/Eyewall_replacement_cycle). Winds have also been found to decrease in a stairstep like fashion within the [radius of maximum wind](/source/Radius_of_maximum_wind), which may indicate that more wind is mixed between the eye and eyewall as cyclones strengthen, which helps to explain why annular characteristics are generally exclusive to storms of higher intensities.[1] The intensity of annular systems is typically greater than 83.5% of the [maximum potential intensity](/source/Maximum_potential_intensity), suggesting that the conditions in which storms gain annular characteristics are generally conducive for tropical cyclone persistence and intensification. Annular tropical cyclones also require low [wind shear](/source/Wind_shear), and of the storms in the East Pacific and North Atlantic studied by Knaff and Kossin, all exhibited easterly winds and cold air in the upper [troposphere](/source/Troposphere). In addition to strong [outflow](/source/Outflow_(meteorology)), suggesting that the conditions that give rise to annular tropical cyclones are most optimal towards the equatorward side of a [subtropical ridge](/source/Subtropical_ridge) and within the [tropics](/source/Tropics). However, warmer [sea surface temperatures](/source/Sea_surface_temperature) (SSTs) are not required for annular tropical cyclones, with annular characteristics developing only within a narrow range of modest SSTs, ranging from 25.4–28.5 °C (77.7–83.3 °F).[1] Conditions favorable for annular typhoon development in the Western North Pacific are localized within two areas within a zonal belt between 20°N−30°N; one of these areas lies over the central part of the basin, while the other is located east of [Taiwan](/source/Taiwan).[6] Within the Eastern North Pacific, such conditions were present only 3 percent of the time between 1998 and 1999. In the same timeframe, the North Atlantic basin only exhibited conducive conditions for annular development 0.8 percent of the time.[1] ## See also - [Tropical cyclogenesis](/source/Tropical_cyclogenesis) - [Tropical cyclone forecasting](/source/Tropical_cyclone_forecasting) - [Central dense overcast](/source/Central_dense_overcast) - [Coriolis force](/source/Coriolis_force) ## References 1. Knaff, John A. & Kossin, James P. (April 2003). "Annular Hurricanes". *Weather and Forecasting*. **18** (2): 204–223. [Bibcode:2003WtFor..18..204K](https://ui.adsabs.harvard.edu/abs/2003WtFor..18..204K). [doi:10.1175/1520-0434(2003)018<0204:AH>2.0.CO;2](https://doi.org/10.1175/1520-0434(2003)018%3C0204:AH%3E2.0.CO;2) 1. Montgomery, Michael T. (2014). [*Advances in Tropical Cyclone Research: Chapter 21: Introduction to Hurricane Dynamics: Tropical Cyclone Intensification*](https://met.nps.edu/~mtmontgo/papers/Chapter_21.pdf). [Naval Postgraduate School](/source/Naval_Postgraduate_School). Retrieved 18 May 2019. 1. Knaff, John A.; Cram, T.A.; Schumacher, A.B.; Kossin, J.P.; DeMaria, M. (February 2008). ["Objective Identification of Annular Hurricanes"](https://journals.ametsoc.org/view/journals/wefo/23/1/2007waf2007031_1.xml). *[Weather and Forecasting](/source/Weather_and_Forecasting)*. **23** (1): 17–28. [American Meteorological Society](/source/American_Meteorological_Society). [Bibcode:2008WtFor..23...17K](https://ui.adsabs.harvard.edu/abs/2008WtFor..23...17K). [CiteSeerX 10.1.1.533.5293](https://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.533.5293). [doi:10.1175/2007WAF2007031.1](https://doi.org/10.1175/2007WAF2007031.1) 1. Louis Quibb (15 April 2015). ["Annular tropical cyclones"](http://quibb.blogspot.com/2015/04/annular-tropical-cyclones.html#:~:text=Using%20satellite%20data%20to%20measure,than%20zero%2C%20it%20is%20annular.). Blogspot. Retrieved 4 March 2021. 1. Lixion A. Avila (7 September 2018). ["Hurricane Olivia Discussion Number 26"](https://www.nhc.noaa.gov/archive/2018/ep17/ep172018.discus.026.shtml). *www.nhc.noaa.gov*. Miami, Florida: National Hurricane Center. Retrieved 4 March 2021. 1. Chu, Kekuan & Tan, Zhe-Min (April 2014). "Annular Typhoons in the Western North Pacific". *Weather and Forecasting*. **29** (2): 241–251. Boston, Massachusetts: American Meteorological Society. [Bibcode:2014WtFor..29..241C](https://ui.adsabs.harvard.edu/abs/2014WtFor..29..241C). [doi:10.1175/WAF-D-13-00060.1](https://doi.org/10.1175/WAF-D-13-00060.1) --- Adapted from the Wikipedia article [Annular tropical cyclone](https://en.wikipedia.org/wiki/Annular_tropical_cyclone) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Annular_tropical_cyclone?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.