Eye of the Storm

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Sunday, August 28, 2005

Is Katrina an annular hurricane?

One thing frequently mentioned in discussion from the NHC about intense hurricanes is that they are prone to fluctuations in intensity such that category five strength is not held for a long period of time. The exception to this is a special classification called an anular hurricane. Such hurricanes do not fluctuate as rapidly. The most recent case was Ivan, which retained category five status for 30 consecutive hours.

The tell-tale sign of an annular hurricane is that the convection is uniform, making a perfect circle, i.e. there are no spiraling bands, just a donut. That is almost exactly the case with Katrina, as one can see from infrared satellite. Another feature is a larger than average eye (average being 14 miles). The message from the recon plane shown in my last full update shows that Katrina's eye is 25 nautical miles in diameter, so it is well above average. As far as conditions associated with annular hurricanes go, some of the conditions are most certainly there. Average Sea Surface Temperatures associated with them are 26.9° C. Katrina is certainly on the more favorable side of that (ref). Weak vertical wind shear is another key factor. That is certainly the case in the immdiate vicinity of Katrina(ref)

For a technical paper on annular hurricanes, see the paper 'Annular Hurricanes' (PDF) by John Knaff, James Kossin and Mark DeMaria (Demaria, by the way developed the Statistical Hurricane Intensity Prediction Scheme (SHIPS) that has been mentioned so much with regards to the potential intensity of storms.

It will be over the head of most people, but it is worth checking out to compare the satellite imagery of known annular hurricanes with that of Katrina.


So enough of the theory, what does this mean practically?

It means that the situation has gotten even worse. If Katrina is indeed annular, then the chances of her retreating down to say category three status are nil. If we make the somewhat unlikely presumption that she has reached her maximum strength (and again if she is annular), then the averages of such hurricanes suggest that she would only weaken to 145 mph at landfall. That would be a storm with Charley-like intensity, but on a much larger scale as Charley was puny compared to the present size of Katrina.

I do hate repeating myself when it comes to evacuating, but the only way I've been thinking of not repeating myself involves a very salty string of compound-complex expletives that would not be appropriate for this family publication.

If you are on the coast, or in a low-lying area, or otherwise in a structure of dubious integrity, and are in the cone of uncertainty, you need to leave now. Otherwise, you will become a statistic.

One last thing to share: In my high school earth science class, my teacher stated 'There is only one tool needed for disaster recovery from a category five hurricane: A bulldozer".


At 10:49 PM, Anonymous Anonymous said...

Hi. Just crusing Google a bit tonight looking for info on annular hurricanes, since I'm tracking Rita (along with x-million other people, right?). I first heard of annular hurricanes during Katrina (another common story, I guess).

It's interesting, the comment you made about Katrina hitting with 145mph winds, based on averages. IIRC wasn't that just what happened?

I didn't experience Charley first-hand, but did Ivan. Very grateful that storm had dropped down somewhat in its vast, long trek. The power it still possessed was unbelievable. Dennis passed right over my head this year but Ivan was a much worse experience. I can't imagine going through a Katrina.

At 9:03 PM, Anonymous Anonymous said...

This is a bit late, but Katrina had enormous rainbands - it critically failed one of the most important criterion for annular hurricanes. Unfortunately, looking back, that didn't help much.


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