Hurricane Isabel of 2003 showing annular hurricane structure. Notice the large eye (partially filled by eyewall mesovortices) and the relatively few spiral bands around the outside of the storm.

An annular hurricane 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. Forecasters have difficulty predicting the behavior of annular hurricanes; they are a recently recognized phenomenon, and as such, little is known about their tendencies. Because of this, they can be more dangerous than typical hurricanes.

Characteristics

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 encounter 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 encounter 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]

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 and, surprisingly, cooler sea surface temperatures.

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.^[1]

Notable annular hurricanes

Hurricane Daniel of the 2006 Pacific hurricane season.

Hurricane Luis of the 1995 Atlantic hurricane season, and Hurricane Edouard of 1996 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. While she retained outer rainbands, devestating 2005 Hurricane Katrina's inner core featured a very large eye and a symmetrical appearance at maximum Category 5 intensity.

2006's Hurricane Daniel in the eastern Pacific (image shown at the above at 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.

See also

• Tropical cyclone
• Eye (cyclone)

References

es:Huracán anular nl:Ringvormige orkaan pt:Ciclone tropical anular simple:Annular hurricane