The Intersection

Strong Hurricanes, Weak Categories

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As Cyclone Favio makes landfall in an already flooded Mozambique–striking the provinces of Inhambane and Sofala as a Category 3–I am prompted to reflect a bit on what the South Indian cyclone season of 2006-2007 has shown us so far.

There have now been three storms that we can classify as Category 4s according to traditional Saffir-Simpson categorization: Bondo, Dora, and Favio. Being a Category 4 in this particular basin means that at some point, based upon estimates from satellite images, these storms were determined to have maximum sustained winds of 115 knots or higher for at least one advisory. (The forecasting center that officially does the advisories for this region of the world, or at any rate the one I’ve been listening to, is the Joint Typhoon Warning Center.)

A glance back over records that I’ve been keeping, however, shows that Dora and Favio have this in common: Each storm was only a Category 4 for one official advisory. Dora briefly reached 115 knots on February 3; Favio jumped up to 125 knots on February 20 but then dropped down again to 100 knots (or Category 3) the next day.

By contrast, Bondo of last year apparently reached Category 4 strength on three separate occasions. Other Category 4 and of course Category 5 storms have lasted at these intensities considerably longer than any of these three 2006-2007 storms, the record-setter being last year’s Hurricane and Supertyphoon Ioke, which was a Category 4 or higher for 198 consecutive hours.

The question then becomes: Even though we call Bondo, Dora, and Favio Category 4s as if they’re interchangeable, aren’t we missing something important by doing so? I would argue that we probably are; more importantly, so would MIT’s Kerry Emanuel, who has been a strong critic of classifying hurricanes according to traditional attributes such as intensity and frequency.

In terms of the amount of power dissipated, a storm that reaches weak Category 4 status for one advisory doesn’t really compare with a long-lived Category 4 or 5 storm. Metrics like Accumulated Cyclone Energy (ACE) or Emanuel’s Power Dissipation Index (PDI), which try harder to take into account how long storms are intense rather than merely sampling the peak intensity that they achieve, are much better at capturing that. And get this: In terms of its ACE rating, Ioke was apparently more powerful than all 2006 storms in the Atlantic combined.

So in short, while catchphrases like “Category 3” and “Category 5” may capture our attention and are easily understood by the public, they’re really fairly limited in terms of what they tell us about storms. Not only do these classifications ignore how long a storm lasts at a strong intensity; they also don’t say anything about the incredibly important factor of storm size.

Speaking of which, the next South Indian storm to worry about after Favio is Gamede, which looks considerably larger (as you can see from the image below), and which may be on a track to intensify and hit Madagascar….

UPDATE 4 pm ET: Gamede has already intensified faster than the Joint Typhoon Warning Center expected, and is now a strong Category 1 storm and expected to grow considerably stronger.

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Comments

  1. #1 Ahcuah
    February 22, 2007

    I wonder if a better measure of hurricane oomph (to use the techical term 🙂 ) might be to integrate its wind speed over its lifetime?

  2. #2 Chris Mooney
    February 22, 2007

    I guess I should have said…that’s precisely what Emanuel’s Power Dissipation Index does. I should have provided a link. See here:
    ftp://texmex.mit.edu/pub/emanuel/PAPERS/NATURE03906.pdf

  3. #3 Ahcuah
    February 22, 2007

    OK, so what the issue really is is that the media advertises things like Category, when what they should be advertising is something like the Power Dissipation Index.

    (Side note: how does Katrina compare on that measurement?)

  4. #4 llewelly
    February 22, 2007

    (The forecasting center that officially does the advisories for this region of the world, or at any rate the one I’ve been listening to, is the Joint Typhoon Warning Center.)

    I thought the WMO-designated official forecaster for that area was Meteo France’s La Reunion office. See here , for example.

    As Harold Brooks (thanks, Harold) pointed out to me, there is at least one paper showing different trends in different forecasting agency’s databases.

    JTWC is nice because it covers all basins, and uses more or less the same methods (but for Atlantic and Epac it copies the NHC forecasts).

    A good justification for using JTWC would be that you are interested in global trends. Justifying on the basis of the JTWC being official appears to be wrong (except for the US military).

  5. #5 Jon
    February 22, 2007

    You have hit upon an important point in discussing hurricane classification, that we lose a lot in trying to lump different hurricanes into various discrete categories. Another example is earthquakes, where you also have peak intensity versus total energy dissipated.

    The ultimate issue is that highly complex phenomena are not easily summarized with a single number or category. This is a problem faced by lots of fields, well beyond science. It’s especially an issue any time you have specialists trying to talk to lay persons. One can always provide more information, giving a more complete and clearer description. At some point, however, anyone who doesn’t also specialize in a field loses either interest or an ability to follow the discussion.

    How much information is needed to strike the right balance between too much and too little is an important question, but that doesn’t mean there’s an easy answer — or even a general purpose answer at all.

  6. #6 Jonathan Vos Post
    February 23, 2007

    I hope that “Storm World” at least mentions John Barnes’ “Mother of Storms” — science fiction, but impressive speculations!

  7. #7 Chris Mooney
    February 23, 2007

    Llewelly,
    Thanks, you can tell from my sentence that I was hesitant about calling JTWC “official” but I did use the word and shouldn’t have.

    The main reason I use JTWC for this part of the world is that I am not good with the La Reunion website and prefer English to French….

    Jonathan….no, I’m afraid Storm World doesn’t mention it….sorry.

    Ahcuah–I don’t know how much power Katrina dissipated but it was assuredly much more than storms like Dora and Favio. I’m pretty positive the Atlantic storm that would have dissipated the most power would be Hurricane Ivan of 2004 as it was very intense for a very long time, though Katrina was somewhat more intense at its peak….

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