The Intersection

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This year, after being increasingly frustrated with poor or inaccessible records of past hurricanes (in basins other than the Atlantic and East Pacific, anyway), I started keeping my own records. And so I have noted every Joint Typhoon Warning Center advisory this year (er, almost). And in the process, I’ve noticed something interesting that I’d like to comment further upon, in the hope that some experts may drop in and share their thoughts.

With both Cyclones Indlala and Kara of this month, the same thing happened. In between official advisories, the storms substantially intensified and then weakened again. Thus, with Indlala, the JTWC reported on March 14 that the storm was at 115 knots in advisory 6, and then that it was at 105 knots in advisory 7. But note that in between, the storm went up to 125 knots before going back down again.

Something similar happened with Cyclone Kara (pictured above). In between JTWC advisories 4 and 5 on March 26, the storm apparently revved all the way up to 105 knots, or Category 3 strength, before declining back down to 75 knots by the time of the official advisory. Or as JTWC put it at the bottom of the advisory (but not in the section where the storm’s official intensity is given):

PEAK INTENSITY OF 105 KNOTS OCCURRED NEAR 26/06Z WHEN SYSTEM HAD A PINPOINT EYE AND HAS DECREASED SINCE THAT TIME.

So my question: Is this kind of between-advisory behavior included in the official “best track” records? And if not, could that lead to a bias in studies of hurricane intensity?

The problem seems to have infected at least one archive. Unisys Weather, which I’ve often found to provide a pretty good record, only lists Kara as a Category 1 storm….

Comments

  1. #1 andy
    March 28, 2007

    I don’t know about the other basins, but the NHC has always been known to upgrade storms as required by their post-storm analysis. TS get upgraded to H, categories get increased and “new” storms (usually but not always subtropical) get added at the end of the year. The NHC official site does reflect this, but as you say, the Unisys site appears to just automatically collect the advisories and the site is updated by software.

  2. #2 llewelly
    March 28, 2007

    For pre-satellite coverage storms, measurements from aircraft, ships, bouys, etc, was often not available as frequently as every 6 hours, let alone at the higher frequencies necessary for between advisory fluctuations.

    So I suspect including data on inter-advisory behavior results in more error (with respect to long-term trends), because it was not consistently available.

  3. #3 Chris Mooney
    March 28, 2007

    Hi Andy,
    The folks at the NHC are the super duper ultra pros at this. They’re the best in the world, and I have utter confidence in their post-season analyses. Plus, they actually keep handy archives of everything they do. So I have no doubt that for the Atlantic and East Pacific all this is taken into account.

    But for the rest of the world, where the Joint Typhoon Warning Center is the main source (not the only one, but the most consistent one), I am dubious that between-advisory storm behavior gets adequately recorded.

    Llewelly….your comment makes sense, but I’ll just add, for some basins, JTWC only releases advisories every *12* hours, which seems to make this an even bigger issue.

  4. #4 llewelly
    March 29, 2007

    Sorry, I had forgot JTWC only issues advisories every 12 hours for some basins. But at least for Kara and Indlala , their track files contain entries every 6 hours, and Kara’s 105 kt peak and Indlala’s 125 kt peak are both in th e linked ACTF tracks – see the 2007 03 26 06 and 2007 03 14 18 entries respectively.
    I don’t recall noticing a southern hemi storm or a west pacific storm in the last few years that didn’t have track entries every 6 hours.

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