An annular hurricane, also known as a truck tire or doughnut hurricane,[1] is a tropical cyclone in the Atlantic or Eastern Pacific Oceans 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 cyclones. Annular hurricanes also tend to persist, even when encountering environmental conditions which easily dissipate most other hurricanes.
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Annular hurricanes are axisymmetric — symmetric 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.[1]
An algorithm for objective identification, in real time, of annular hurricanes has been developed and shows skill, but it is not yet operational.[2]
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 and, surprisingly, cooler sea surface temperatures.
Some of the conditions associated with annular hurricanes are:
Hurricane Luis 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, [1],1999's Hurricane Beatriz,1999's Hurricane Dora,2009's Hurricane Felicia and three from the 2011 Pacific hurricane season: Hurricane Adrian[3] and Hurricane Dora and Hurricane Eugene.