Annular tropical cyclone
An annular tropical cyclone, also known as a truck tire or doughnut hurricane, is a tropical cyclone 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 tropical cyclones also tend to persist, even when encountering environmental conditions which easily dissipate most other tropical cyclones.
Annular tropical cyclones 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 tropical cyclones have annular characteristics for only a portion of their lifetimes.
While tropical cyclones retain annular characteristics, they also seem to be less affected by diurnal, or daily variations. Most annular tropical cyclones have peak intensities of greater than 85 knots (100 mph, 155 km/h) and more than 85% of their theoretical maximum potential intensity. Rarely, tropical cyclones take on annular characteristics at lower intensities (see Hurricane Epsilon).
Annular tropical cyclones 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-number after one day from their peak intensities).
Annular tropical cyclones are very rare. Few storms meet all of the criteria, although many strong storms resemble annular tropical cyclones in some criteria. Fewer than 1% of Atlantic Hurricanes exhibit all of the environmental conditions associated with annular tropical cyclones. In the Eastern Pacific, such conditions are more common, but still very unusual — 3% of Pacific hurricanes exhibit them. Annular typhoons are also known to occur in the Western Pacific Basin, usually in the East China Sea, with 2% of all pacific typhoons are known to exhibit annular characteristics.
Research into the characteristics and formation of annular tropical cyclones 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 tropical cyclone, after undergoing an eyewall replacement cycle, fails to re-establish the standard hurricane appearance. The new eyewall thickens, and rainbands dissipate, and the tropical cyclone takes on an annular structure. As compared to the formation of normal tropical cyclones, this happens under weaker wind shear and, surprisingly, cooler sea surface temperatures.
Some of the conditions associated with annular tropical cyclones 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.
- Knaff, John A.; Kossin, James P.; DeMaria, Mark. "Annular Hurricanes" (PDF). Weather and Forecasting (American Meteorological Society) 18 (April 2003): 204–223. doi:10.1175/1520-0434(2003)018<0204:ah>2.0.co;2. ISSN 1520-0434. Retrieved 2012-02-28.
- Knaff, John A.; T.A. Cram, A.B. Schumacher, J.P. Kossin, M. DeMaria (February 2008). "Objective Identification of Annular Hurricanes" (abstract). Weather and Forecasting (American Meteorological Society) 23 (1): 17–28. Bibcode:2008WtFor..23...17K. doi:10.1175/2007WAF2007031.1.