Question about annular hurricanes

[John Peters]

I Understand that a hurricane with annular characteristics implies that there are very few feeder bands; essentially that the storm is just eyewall and CDO, sort of like a buzz saw. It's been mentioned that Isabel developed some annular qualities. The wiki page for annular hurricanes mentions that they do not go through eyewall replacement cycles, yet it seems that Isabel developed concentric eyewalls several times during its life (hence the back and forth between cat 4 and cat 5). Is there anyone here that knows a lot about annular hurricanes and would like to elaborate on their typical characteristics (and perhaps some examples)?

- John

 

[cdcollura]

Good day,

An annular hurricane is just as it's name implies, an annulus (Greek emptymology for large "circle").

Basically, a TRUE annular hurricane has only one major "ring" of convection, with little or no feeder bands outside of the hurricane core.

The main eyewall of an annular hurricane can still consist of concentric eyewalls, but the "space" between them (the moat) is often less discernable. Eyewall replacement will happen in such storms, but it is a slower and more subtle process.

In Isabel, in 2003, the storm was almost a perfect example of an annular hurricane as it was at category-5 intensity in the open Atlantic. There was only one main eyewall, and it was at one point a 70-mile wide eye.

Structurally, the storm looked more like a "truck tire" on radar (from the aircraft) with very little reflectivity outside the eyewall. These storms, because of slower (or lack of in some cases) eyewall replacement cycles, often do not fluctuate in intensity very much.

Annular hurricanes do, however, feel the effects of ocean temperature and wind shear, the latter taking a but longer to deteriment such a hurricane than one without annular characteristics.

Some hurricanes start out with a small eye, like Wilma first did in 2005. The outer eyewall can form, and the inner one weakens. Eventually, the larger eye clears out without shrinking very much and may form an annular type hurricane.

A strong hurricane that weakened over land, then re-strengthened over water again also can become annular as the wind core "loosened up" over land and became wider, only to re-form over water and concentrate convection in the now wider wind core.

Wilma off Florida in 2005 had some annular characteristics (although not entirely annular due to feeder bands north of it's core) ... But expressed a 60-Mile wide eye and only one main eyewall as it hit Florida.

Hurricanes Frances and Jeanne in 2004 also had some "annular" characteristics, both coming ashore in Florida with very large eyes (about 60-Miles wide). Both started out as small "more compact" storms with 20-Mile wide eyes, weakened, widened, then re-intensified.

There is also some evidence that annular hurricanes may form over warm ocean waters that are not "excessively" warm. At least 80 degrees (78.8 to be exact) is the criteria for hurricane formation. Often, temps near 90 F or higher cause intense hurricanes with (usually) small eyes. Annular hurricanes have been known to "favor" water temperatures in the 82 to 84 degree range, give or take.

One very important note is that in the average hurricane with a 20-Mile wide eye, about 85% of latent heat energy is released in the eyewall (or eyewalls if concentric), and about 15% from feeder-bands.

In an annular hurricane, nearly 100% of the energy transfer from sea surface to atmosphere occurs in the single ring around the eye (main eyewall).

Annular hurricanes contrast sharply to those with "pinhole" eyes, such as Charley in 2004 and Wilma (during its pre-annular, and far-more intense stages) in 2005 in the Caribbean. Both eyes of Charley and Wilma were no more than 2.5 miles wide and were strongly concentric with a 20-mile wide outer wall.

But sometimes, under the right conditions, small core hurricanes can "evolve" to larger, even annular ones. In the converse, annular storms can sometimes develop a smaller eyewall right in their centers and evolved to a smaller eye system with the larger outer wall shrinking and / or deteriorating. Water temperature, land interaction, and environment (shear, dry air, etc) all can play roles in this.

Examples of annular hurricanes (with pictures below)...

Above - Hurricane Epsilon in 2005 (formed over 80 F waters in central Atlantic).

Above - Isabel in 2003 (Open Atlantic, waters in 82-84 F range).

Above - Katrina in 2005 (Annular over Gulf at some point during cat-5 strength).

Above - Frances near Florida in 2004. The eye itself had annular characteristics but not an "annular hurricane".

Above - To the "opposite" type of hurricane - Charley in 2004. A concentric storm with feeder bands and "pinhole" inner eye!

 

[John Peters]

Good day,

An annular hurricane is just as it's name implies, an annulus (Greek emptymology for large "circle").

Basically, a TRUE annular hurricane has only one major "ring" of convection, with little or no feeder bands outside of the hurricane core.

The main eyewall of an annular hurricane can still consist of concentric eyewalls, but the "space" between them (the moat) is often less discernable. Eyewall replacement will happen in such storms, but it is a slower and more subtle process.

In Isabel, in 2003, the storm was almost a perfect example of an annular hurricane as it was at category-5 intensity in the open Atlantic. There was only one main eyewall, and it was at one point a 70-mile wide eye.

Structurally, the storm looked more like a "truck tire" on radar (from the aircraft) with very little reflectivity outside the eyewall. These storms, because of slower (or lack of in some cases) eyewall replacement cycles, often do not fluctuate in intensity very much.

Annular hurricanes do, however, feel the effects of ocean temperature and wind shear, the latter taking a but longer to deteriment such a hurricane than one without annular characteristics.

Some hurricanes start out with a small eye, like Wilma first did in 2005. The outer eyewall can form, and the inner one weakens. Eventually, the larger eye clears out without shrinking very much and may form an annular type hurricane.

A strong hurricane that weakened over land, then re-strengthened over water again also can become annular as the wind core "loosened up" over land and became wider, only to re-form over water and concentrate convection in the now wider wind core.

Wilma off Florida in 2005 had some annular characteristics (although not entirely annular due to feeder bands north of it's core) ... But expressed a 60-Mile wide eye and only one main eyewall as it hit Florida.

Hurricanes Frances and Jeanne in 2004 also had some "annular" characteristics, both coming ashore in Florida with very large eyes (about 60-Miles wide). Both started out as small "more compact" storms with 20-Mile wide eyes, weakened, widened, then re-intensified.

There is also some evidence that annular hurricanes may form over warm ocean waters that are not "excessively" warm. At least 80 degrees (78.8 to be exact) is the criteria for hurricane formation. Often, temps near 90 F or higher cause intense hurricanes with (usually) small eyes. Annular hurricanes have been known to "favor" water temperatures in the 82 to 84 degree range, give or take.

One very important note is that in the average hurricane with a 20-Mile wide eye, about 85% of latent heat energy is released in the eyewall (or eyewalls if concentric), and about 15% from feeder-bands.

In an annular hurricane, nearly 100% of the energy transfer from sea surface to atmosphere occurs in the single ring around the eye (main eyewall).

Annular hurricanes contrast sharply to those with "pinhole" eyes, such as Charley in 2004 and Wilma (during its pre-annular, and far-more intense stages) in 2005 in the Caribbean. Both eyes of Charley and Wilma were no more than 2.5 miles wide and were strongly concentric with a 20-mile wide outer wall.

But sometimes, under the right conditions, small core hurricanes can "evolve" to larger, even annular ones. In the converse, annular storms can sometimes develop a smaller eyewall right in their centers and evolved to a smaller eye system with the larger outer wall shrinking and / or deteriorating. Water temperature, land interaction, and environment (shear, dry air, etc) all can play roles in this.

Wow, thank you - Very informative. Is the formation of annular characteristics related to the conclusion of the ERC and the widening of the storm's Core? I guess what' i'm asking is, are there any specific atmospheric conditions that would promote development of annular characteristics, or are they caused mainly by restructuring of the hurricane's core?

This image of Katrina comes to mind as an example of somewhat annular characteristics:

Edit: Didn't realize you had already posted an image of Katrina

 

[cdcollura]

Good afternoon,

Wow, thank you - Very informative. Is the formation of annular characteristics related to the conclusion of the ERC and the widening of the storm's Core? I guess what' i'm asking is, are there any specific atmospheric conditions that would promote development of annular characteristics, or are they caused mainly by restructuring of the hurricane's core?

Good questions ... The answer to that question is basically BOTH (or either).

The eyewall is an EXTREMELY complex phenomenon of fluid dynamics. Little is understood about ERC and "internal" changes in a hurricane / typhoon's core. In fact, we know more about tornadoes / supercells than eyewall dynamics!

With that said, the atmosphere can dictate annular hurricane formation conditions, as it did with Epsilon in 2005. Epsilon formed over water temperatures BARELY capeable of hurricane formation. In fact, after Epsilon formed, it encountered cool water temperatures (in the 70's) but still retained hurricane status - In late December! How'd that happen?

Well, Epsilon formed over extremely interesting atmospheric conditions. High up, at about 200 MB (39,000 feet or so), the temperatures were very cold due to an upper level system / low. However, the upper winds right above Epsilon were light and slighly difluent with the flow around the upper low. More importantly, the upper low remained in a position so that wind shear and subsidence would not destroy the hurricane. In such an environment, little feeder bands would form away from the main "forcing" associated with the eyewall, due to the low heat content of the ocean there. The convection would ONLY be the eyewall, due to the extra lift due to the upper system and central core convection of the hurricane. Ofcourse, this is describing an annular storm (no feeder bands, just an eyewall) as with Epsilon ;-)

Sometimes, in other cases where eyewall replacement occurs (as with Wilma / Katrina) ... If the hurricane is in a similar atmospheric environment, avoids land, and is over a sea temperature that does not change much, the eyewall replacement cycles stop (kind of like a wobbling top that reaches equillibrium and the "wobble" slows or even stops).

Once the last ERC is completed, as in Wilma, the inner wall deteriorates and the outer wall becomes the main eyewall, but does not contract. Eventually, the storm "stabilizes" and is in equilibrium with the atmosphere and sea temperatures it is within, and the storm has a virtually "stable" and large eye. If this happens, the outflow of the storm can be so strong aloft that subsidence develops around the storm and feeder bands may be supressed.

Another VERY important note ... You cannot have fronts, boundaries, other systems nearby the hurricane as that will cause convection around the storm and defeat the puspose of it becoming annular. A nearly constant high pressure all around the storm is best, as was the case with Isabel in 2003.

 

[Gregg Gallina]

Classic annulars typically develop between Taiwan and Japan and move due west usually after crossing Okinawa... example Typhoon Jelawat 2000 (there are other much more recent, but I can't recall off the top of my head). Usually with HUGE eyes (sometimes wider than the width of the surrounding ring of convection!)

 

[Glenn Rivers]

Landfall Hazards

I get the impression that hurricanes with large eye diameters are more likely to loose at least 1 or 2 categories of intensity before making landfall.

In retrospect, it seems that all category 5 landfalls in recorded history in the Americas have been from very compact storms that have a narrow radius of hurricane and gale force winds (and probably small eye diameters too)

-The great labour day hurricane (1935, Florida Keys)
-Camile (1969, Billoxi MS)
-Andrew (1992 Homestead FL)
-Dean (2007, lower Yucatan penn. Mexico)

Besides the size of these storms, all of them seem to have had a rather fast speed of translation i.e. 20 mph.

I wonder how difficult it would be for a large diameter hurricane to make landfall as a category 5. I am not aware of any historic precedent in the Americas. Any thoughts about this?

 

[John Peters]

I get the impression that hurricanes with large eye diameters are more likely to loose at least 1 or 2 categories of intensity before making landfall.

In retrospect, it seems that all category 5 landfalls in recorded history in the Americas have been from very compact storms that have a narrow radius of hurricane and gale force winds (and probably small eye diameters too)

-The great labour day hurricane (1935, Florida Keys)
-Camile (1969, Billoxi MS)
-Andrew (1992 Homestead FL)
-Dean (2007, lower Yucatan penn. Mexico)

Besides the size of these storms, all of them seem to have had a rather fast speed of translation i.e. 20 mph.

I wonder how difficult it would be for a large diameter hurricane to make landfall as a category 5. I am not aware of any historic precedent in the Americas. Any thoughts about this?

Dean had a slightly wider eye/radius of max winds/circulation when it made landfall than the others you listed i believe.

Gilbert hit the yucatan with a central press of 900 mb, winds of 140 kts, and well defined concentric eyewalls. I'm assuming that the radius of maximum winds would have been pretty wide with a double maxima. Had the replacement cycle occurred earlier in gilbert's track, the outer eyewall may have had a chance to contract and gilbert could have come ashore with a very large eye.

Ivan in 2004 cam pretty close to landfalling on cuba with a very large eye, 140 kt winds, and a central press of 912 mb.

so yes, i think it is possible.

 

[Glenn Rivers]

Cat 5 landfalls

Thank you for your reply John. My comment was meant to apply to Hurricanes making landfall on large land masses, but Cuba certainly counts, and I guess I failed to appreciate how large Dean was at landfall. I could easily be missing other examples on my list. Also, the Florida Keys would certainly have less effect on a hurricane than Cuba

 

[Gregg Gallina]

Glenn,

Large diameter eyes (which include but are not limited to annular cyclones) typically have a large broad area of lower pressures...so the pressure gradient force is much less and so the max wind (which is the determining factor for category...not pressure) tends to be much less...