Annular hurricane
Encyclopedia
An annular hurricane, also known as a truck tire or doughnut hurricane, is a tropical cyclone
in the Atlantic
or Eastern Pacific Ocean
s that features a large, symmetric eye
surrounded by a thick ring of intense convection. This type of storm is not prone to the fluctuations in intensity associated with eyewall replacement cycles
, unlike typical intense tropical cyclone
s. Annular hurricanes also tend to persist, even when encountering environmental conditions which easily dissipate most other hurricanes.
along every radial axis, i.e. very circular in appearance. They lack the spiralform rainbands which are characteristic of typical tropical cyclones. After reaching peak intensity, they weaken much more slowly than non-annular storms of similar intensity. However, most annular hurricanes have annular characteristics for only a portion of their lifetimes.
While hurricanes retain annular characteristics, they also seem to be less affected by diurnal, or daily variations. Most annular hurricanes have peak intensities of greater than 85 knots (100 mph, 155 km/h) and more than 85% of their theoretical maximum potential intensity.
Annular hurricanes maintain their intensities longer than usual after their peaks. Statistics
show that forecasters significantly overestimate the lessening of wind velocities in annular hurricanes. In terms of the Dvorak technique
, annular hurricanes weaken very slowly after their peak (on average, less than 0.5 T after one day from their peak intensities).
Annular hurricanes are very rare. Few storms meet all of the criteria, although many strong storms resemble annular hurricanes in some criteria. Fewer than 1% of Atlantic tropical cyclones exhibit all of the environmental conditions associated with annular hurricanes. In the Eastern Pacific, such conditions are more common, but still very unusual — 3% of Pacific tropical cyclones exhibit them.
An algorithm
for objective identification, in real time, of annular hurricanes has been developed and shows skill, but it is not yet operational.
What meteorologists do know is that a normal hurricane, after undergoing an eyewall replacement cycle, fails to re-establish the standard hurricane appearance. The new eyewall thickens, and rainbands dissipate, and the hurricane takes on an annular structure. As compared to the formation of normal hurricanes, this happens under weaker wind shear
and, surprisingly, cooler sea surface temperature
s.
Some of the conditions associated with annular hurricanes are:
of the 1995 Atlantic hurricane season
, Hurricane Edouard
of 1996
and Hurricane Alberto
of 2000
may have been annular hurricanes. Hurricane Epsilon of the 2005 Atlantic hurricane season
had a similar structure to an annular hurricane, which partially explains the storm's longevity in the face of unfavorable conditions. Powerful Hurricane Isabel
from 2003
also generated an annular hurricane structure around the time of its peak intensity. Other storms with annular structures include Hurricane Erin (2001)
, Hurricane Kate (2003)
, Hurricane Frances
, and Hurricane Igor.
Hurricane Daniel (2006)
in the eastern Pacific (image to the right) exhibited annular hurricane features with a large, well defined eye and was described as such by the United States
' National Hurricane Center
. Hurricane Daniel was able to retain Category 4 intensity for a lengthy period of time, and in parts of the Eastern Pacific basin which at the time were unfavorable for such sustained intensity.
Other Pacific storms that showed annular features include 1998's Hurricane Darby, 1998's Hurricane Howard, ,1999's Hurricane Beatriz,1999's Hurricane Dora,2009's Hurricane Felicia
and three from the 2011 Pacific hurricane season
: Hurricane Adrian and Hurricane Dora and Hurricane Eugene.
Tropical cyclone
A tropical cyclone is a storm system characterized by a large low-pressure center and numerous thunderstorms that produce strong winds and heavy rain. Tropical cyclones strengthen when water evaporated from the ocean is released as the saturated air rises, resulting in condensation of water vapor...
in the Atlantic
Atlantic Ocean
The Atlantic Ocean is the second-largest of the world's oceanic divisions. With a total area of about , it covers approximately 20% of the Earth's surface and about 26% of its water surface area...
or Eastern Pacific Ocean
Pacific Ocean
The Pacific Ocean is the largest of the Earth's oceanic divisions. It extends from the Arctic in the north to the Southern Ocean in the south, bounded by Asia and Australia in the west, and the Americas in the east.At 165.2 million square kilometres in area, this largest division of the World...
s that features a large, symmetric eye
Eye (cyclone)
The eye is a region of mostly calm weather found at the center of strong tropical cyclones. The eye of a storm is a roughly circular area and typically 30–65 km in diameter. It is surrounded by the eyewall, a ring of towering thunderstorms where the second most severe weather of a cyclone...
surrounded by a thick ring of intense convection. This type of storm is not prone to the fluctuations in intensity associated with eyewall replacement cycles
Eyewall replacement cycle
Eyewall replacement cycles, also called concentric eyewall cycles, naturally occur in intense tropical cyclones, generally with winds greater than 185 km/h , or major hurricanes...
, unlike typical intense tropical cyclone
Cyclone
In meteorology, a cyclone is an area of closed, circular fluid motion rotating in the same direction as the Earth. This is usually characterized by inward spiraling winds that rotate anticlockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere of the Earth. Most large-scale...
s. Annular hurricanes also tend to persist, even when encountering environmental conditions which easily dissipate most other hurricanes.
Characteristics
Annular hurricanes are axisymmetric — symmetricSymmetry
Symmetry generally conveys two primary meanings. The first is an imprecise sense of harmonious or aesthetically pleasing proportionality and balance; such that it reflects beauty or perfection...
along every radial axis, i.e. very circular in appearance. They lack the spiralform rainbands which are characteristic of typical tropical cyclones. After reaching peak intensity, they weaken much more slowly than non-annular storms of similar intensity. However, most annular hurricanes have annular characteristics for only a portion of their lifetimes.
While hurricanes retain annular characteristics, they also seem to be less affected by diurnal, or daily variations. Most annular hurricanes have peak intensities of greater than 85 knots (100 mph, 155 km/h) and more than 85% of their theoretical maximum potential intensity.
Annular hurricanes maintain their intensities longer than usual after their peaks. Statistics
Statistics
Statistics is the study of the collection, organization, analysis, and interpretation of data. It deals with all aspects of this, including the planning of data collection in terms of the design of surveys and experiments....
show that forecasters significantly overestimate the lessening of wind velocities in annular hurricanes. In terms of the Dvorak technique
Dvorak technique
The Dvorak technique is a widely used system to subjectively estimate tropical cyclone intensity based solely on visible and infrared satellite images. Several agencies issue Dvorak intensity numbers for cyclones of sufficient intensity...
, annular hurricanes weaken very slowly after their peak (on average, less than 0.5 T after one day from their peak intensities).
Annular hurricanes are very rare. Few storms meet all of the criteria, although many strong storms resemble annular hurricanes in some criteria. Fewer than 1% of Atlantic tropical cyclones exhibit all of the environmental conditions associated with annular hurricanes. In the Eastern Pacific, such conditions are more common, but still very unusual — 3% of Pacific tropical cyclones exhibit them.
An algorithm
Algorithm
In mathematics and computer science, an algorithm is an effective method expressed as a finite list of well-defined instructions for calculating a function. Algorithms are used for calculation, data processing, and automated reasoning...
for objective identification, in real time, of annular hurricanes has been developed and shows skill, but it is not yet operational.
Formation
Research into the characteristics and formation of annular hurricanes is still in its infancy. First classified and categorized in 2002, little is known about how they form, or why some are able to maintain their intensity in hostile conditions.What meteorologists do know is that a normal hurricane, after undergoing an eyewall replacement cycle, fails to re-establish the standard hurricane appearance. The new eyewall thickens, and rainbands dissipate, and the hurricane takes on an annular structure. As compared to the formation of normal hurricanes, this happens under weaker wind shear
Wind shear
Wind shear, sometimes referred to as windshear or wind gradient, is a difference in wind speed and direction over a relatively short distance in the atmosphere...
and, surprisingly, cooler sea surface temperature
Sea surface temperature
Sea surface temperature is the water temperature close to the oceans surface. The exact meaning of surface varies according to the measurement method used, but it is between and below the sea surface. Air masses in the Earth's atmosphere are highly modified by sea surface temperatures within a...
s.
Some of the conditions associated with annular hurricanes are:
- An intensity 85% or greater from their theoretical maximum potential intensity,
- Weak wind shear from the east or southeast,
- A cold east wind at a high altitude (the 200 mbar pressure level),
- Near-constant sea surface temperatures between 25.4 °C and 28.5 °C, and
- Lack of relative eddy flux convergence at the 200 mbar pressure level, relative to the storm.
Notable annular hurricanes
Hurricane LuisHurricane Luis
Hurricane Luis was one of the deadliest and most destructive hurricanes of the 1995 Atlantic hurricane season, with maximum sustained winds of 140 mph . The storm was the twelfth tropical storm, sixth hurricane, and second major hurricane of the season...
of the 1995 Atlantic hurricane season
1995 Atlantic hurricane season
The 1995 Atlantic hurricane season was the third most active Atlantic hurricane season on record. It officially began on June 1, 1995, and lasted until November 30, 1995. These dates conventionally delimit the period of each year when most tropical cyclones form in the north Atlantic ocean...
, Hurricane Edouard
Hurricane Edouard (1996)
Hurricane Edouard was the strongest hurricane in the 1996 Atlantic hurricane season, reaching winds of 145 mph on its path. Edouard remained a major hurricane for eight days, an unusually long amount of time. A Cape Verde-type hurricane, the storm formed near the coast of Africa in the middle...
of 1996
1996 Atlantic hurricane season
The 1996 Atlantic hurricane season was the first season since 1965 in which all tropical cyclones reached tropical storm status. It officially began on June 1, 1996, and lasted until November 30, 1996...
and Hurricane Alberto
Hurricane Alberto (2000)
Hurricane Alberto was an Atlantic Cape Verde-type hurricane that formed on August 3, 2000 and became extratropical on August 23. Alberto was the first named storm, first hurricane, and first major hurricane of the 2000 Atlantic hurricane season....
of 2000
2000 Atlantic hurricane season
The 2000 Atlantic hurricane season was the first season without a tropical cyclone in July since 1987. The season officially began on June 1, 2000, and lasted until November 30, 2000. The June through November dates conventionally delimit the period of each year when most tropical cyclones form in...
may have been annular hurricanes. Hurricane Epsilon of the 2005 Atlantic hurricane season
2005 Atlantic hurricane season
The 2005 Atlantic hurricane season was the most active Atlantic hurricane season in recorded history, repeatedly shattering numerous records. The impact of the season was widespread and ruinous with an estimated 3,913 deaths and record damage of about $159.2 billion...
had a similar structure to an annular hurricane, which partially explains the storm's longevity in the face of unfavorable conditions. Powerful Hurricane Isabel
Hurricane Isabel
Hurricane Isabel was the costliest and deadliest hurricane in the 2003 Atlantic hurricane season. The ninth named storm, fifth hurricane, and second major hurricane of the season, Isabel formed near the Cape Verde Islands from a tropical wave on September 6 in the tropical Atlantic Ocean...
from 2003
2003 Atlantic hurricane season
The 2003 Atlantic hurricane season was an active Atlantic hurricane season with tropical activity before and after the official bounds of the season – the first such occurrence in 50 years. The season produced 21 tropical cyclones, of which 16 developed into named storms; seven...
also generated an annular hurricane structure around the time of its peak intensity. Other storms with annular structures include Hurricane Erin (2001)
Hurricane Erin (2001)
Hurricane Erin was the longest-lived hurricane in the 2001 Atlantic hurricane season. The sixth tropical depression, fifth tropical storm, and first hurricane, Erin developed from a tropical wave on September 1. After strengthening to a 60 mph tropical storm as it moved to the west-northwest,...
, Hurricane Kate (2003)
Hurricane Kate (2003)
Hurricane Kate was the second-longest tropical cyclone in the 2003 Atlantic hurricane season. The eleventh tropical storm, fifth hurricane, and third major hurricane of the season, Kate developed from a tropical wave in the central tropical Atlantic on September 25. Its unusual track included four...
, Hurricane Frances
Hurricane Frances
Hurricane Frances was the sixth named storm, the fourth hurricane, and the third major hurricane of the 2004 Atlantic hurricane season. The system crossing the open Atlantic during mid to late August, moving to the north of the Lesser Antilles while strengthening. Its outer bands affected Puerto...
, and Hurricane Igor.
Hurricane Daniel (2006)
Hurricane Daniel (2006)
Hurricane Daniel was the second strongest hurricane of the 2006 Pacific hurricane season. The fourth named storm of the season, Daniel originated on July 16 from a tropical wave off the coast of Mexico. It tracked westward, intensifying steadily to reach peak winds of 150 mph on...
in the eastern Pacific (image to the right) exhibited annular hurricane features with a large, well defined eye and was described as such by the United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
' National Hurricane Center
National Hurricane Center
The National Hurricane Center , located at Florida International University in Miami, Florida, is the division of the National Weather Service responsible for tracking and predicting weather systems within the tropics between the Prime Meridian and the 140th meridian west poleward to the 30th...
. Hurricane Daniel was able to retain Category 4 intensity for a lengthy period of time, and in parts of the Eastern Pacific basin which at the time were unfavorable for such sustained intensity.
Other Pacific storms that showed annular features include 1998's Hurricane Darby, 1998's Hurricane Howard, ,1999's Hurricane Beatriz,1999's Hurricane Dora,2009's Hurricane Felicia
Hurricane Felicia (2009)
Hurricane Felicia was the third strongest tropical cyclone of the 2009 Pacific hurricane season, as well as the strongest storm to exist in the eastern Pacific at the time since Hurricane Daniel in 2006. Forming as a tropical depression on August 3, the storm supported strong thunderstorm...
and three from the 2011 Pacific hurricane season
2011 Pacific hurricane season
The 2011 Pacific hurricane season is an ongoing, annual cycle of tropical cyclone formation. The season officially started on May 15, 2011, for the eastern Pacific, and started on June 1, 2011, for the central Pacific, both of which ended on November 30, 2011. These dates conventionally...
: Hurricane Adrian and Hurricane Dora and Hurricane Eugene.