The Intersection

How Strong Was Cyclone George?

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Here we go again. This devastating storm, which rapidly intensified yesterday before striking Port Hedland in northwestern Australia, was estimated to have 110 knot sustained winds by the Joint Typhoon Warning Center. 115 knot winds are the cutoff for a Category 4 storm on the Saffir-Simpson Scale. By these lights, George was a strong Category 3 storm.

But at least according to the advisory preserved here, the Australian Bureau of Meteorology’s tropical cyclone warning center in Perth was saying at one point that the storm had a minimum central pressure of 910 hectopascals (or millibars). For comparison, that’s also the minimum central pressure given for Category 5 Hurricane Ivan in the Atlantic in 2004, when the storm had 145 knot winds. And at least according to Wikipedia, it would be the lowest pressure in any storm so far this year. Does something sound amiss here?

It certainly does to me. Using the so-called Atkinson/Holiday wind pressure relationship employed by the Joint Typhoon Warning Center–see e.g., here–a storm with 110 knot winds should have an estimated minimum pressure of 933 millibars/hectopascals. Every storm is different and the Atkinson/Holiday formula is just a rough guide. Nevertheless, if that 910 mb/hPa measurement reported by Perth is right, this storm might have been much stronger than a Category 3.

My guess is that with this hurricane, as with so many others, we’re going to wind up with a lot of uncertainty about how strong it actually was.

P.S.: I’m not sure what the basis for the 910 mb/hPa measurement was….

Comments

  1. #1 andy
    March 9, 2007

    I believe the pressure-windspeed relationships differ for the various basins and in general for the Pacidfic basin, given central pressures correspond to lower windspeeds there than for the Atlantic. I don’t know what the relationship is for the Indian Ocean basin though.

  2. #2 Chris Mooney
    March 9, 2007

    Hi Andy,
    I believe you’re right but the 933 mb/110 knot relationship I cited was an estimate for the South Pacific/South Indian; the storm was from 2004 and was named Monty. See here.

    I think the point general remains, though: If 910 mb is right then George was probably a stronger storm than 110 kt….

  3. #3 Alex
    March 9, 2007

    Looks like Jacob’s coming the same way, with plenty of space to intensify. The Aussie Bureau of Met bulletins reported gusts up to 275 km/h=148 kts near the centre of George.

  4. #4 llewelly
    March 9, 2007

    If you look at the ACTF track file for George , column 17 is the pressure of the outermost enclosing isobar, which is about 1000hpa . I am pretty sure that no Atlantic TC of 2004 or 2005, even with the unusually low pressures of those seasons, had an outermost enclosing isobar that low. I think Wilma’s bottomed out at 1002hpa or 1004hpa.

    Max wind speed is not a simple function of the minimum pressure – it’s the peak differential of pressure, so it’s affected by external conditions and the details of storm geometry. Recall that Wilma at the oct 20, 0000utc advisory had 892hpa MSLP and cat 4 winds.

  5. #5 Chris Mooney
    March 9, 2007

    Alex,
    According to Chris Landsea’s Hurricane FAQ, “Typically in a hurricane environment, the value of the maximum 3 second gust over a 1 minute period is on the order of 1.3 times (or 30% higher than) than the 1 min sustained wind.”
    http://www.aoml.noaa.gov/hrd/tcfaq/D4.html

    If that’s true, then simple math suggests that a storm with 148kt gusts had roughly 114 knot sustained winds….this seems inconsistent with the 910 mb pressure claimed.

    Perhaps we should be doubting the 910 mb?

  6. #6 Chris Mooney
    March 9, 2007

    Llewelly,
    I know the pressure/wind relationship is complicated by factors like storm size, etc, but don’t you still think the 910 mb/hpa sounds hard to reconcile with 110 knots?

  7. #7 Chris Mooney
    March 9, 2007

    Another point…if you trust Wikipedia, here’s what it says about 1995′s Hurricane Opal in the Atlantic: “It deepened to a Category 4 hurricane with sustained winds of 150 mph and a central pressure of 916 mbar (the lowest ever recorded in a hurricane that never reached Category 5 intensity), possibly due to crossing the Loop Current.”

  8. #8 llewelly
    March 9, 2007

    Most wikipedia TC people assume ‘hurricane’ to mean East Pacific and Atlantic only. Everywhere else is a typhoon or a tropical cyclone. I went through the old JTWC actrs, and I found a number of cat 4 southern hemisphere cyclones with 916 hpa or lower MSLPs – in 2005 you can see Meena (916 hpa / 125 kt), Nancy (916 hpa / 125 kt), Ingrid (904 hpa / 135 kt), Adeline-Juliet (910 hpa / 130 kt). In 2004 you can see Frank (916 hpa / 125 kt), in 2003 you can see Beni (916 hpa / 125 kt), Dovi (910 hpa / 130 kt), and Erica (910 hpa / 130 kt) .

    However that is all JTWC data and we are talking about an Australia BOM MSLP estimate. The JTWC estimate for George is 933 hpa / 110 kts ( here ) – not at all unusual.

    I agree that the BOM numbers – 105 10-min wind max with 910 hpa MSLP are unusual (maybe a record, I don’t have enough data, maybe a mistake, again I don’t know enough) , but I don’t think it should be naively compared to Opal’s numbers, which probably occurred among much higher surrounding pressures. (Note that for Australian TCs, JTWC uses the West Pacific Dvorak / Wind / pressure relationship – see here for table that has both relationships.)

    Unfortunately I have no idea where the BOM got their 910 hpa MSLP estimate. (105 kt 10-min wind, equivalent to 115 kt 1-min wind, or SS cat 4, came from Bedout Island obs ) . I assume the low environmental pressures (1000 hpa outermost isobar is low for that area – normal is more like 1004 hpa) play a role, but I don’t know.

  9. #9 Dark Tent
    March 9, 2007

    I’m certainly no storm expert, but Wikipedia indicates that in many cases, the stated central TC pressure is actually estimated from wind speed using the Atkinson/Holiday wind pressure relationship:

    “It should be noted that as is the case in many tropical cyclones, direct measurements from storm’s centre are not available and this number is an estimate from estimated maximum surface winds using Atkinson/Holiday wind pressure relationship”

    The pressure from wind-speed estimation is made, even though it is known that there is no precise direct correspondence between max wind speed and central pressure with tropical cyclones. The relationship between pressure and max wind speed actually depends on latitude, storm size and local environmental (pressure) conditions:

    “Larger and higher latitude storms produce lower MSLP for the same maximum wind speed. Environmental pressure is additive, or in other words storms occurring in a higher pressure environment have higher MSLP.”

    http://rammb.cira.colostate.edu/quarterly_reports/3qtr06/TCRProductDevelopment.htm

    I’d say that the upshot is that the stated central pressure of a given storm has to be taken with a grain of salt — or perhaps a “grain of probability”, if the above reference is any indication — unless the pressure is known to have been measured directly, that is.

    To most accurately compare one storm to another based on pressure alone, one really has to be sure that the central pressures were all measured directly.

  10. #10 llewelly
    March 9, 2007

    Dark Tent’s words are especially important with respect ot the Southern Hemisphere, where direct MSLP measurements are available for very few TCs (e.g., I can’t find one for George – that’s why I keep referring to the 910 hpa figure as an ‘estimate’), versus Atlantic storms like Opal and Wilma, which were both measured directly.

  11. #11 Chris Mooney
    March 10, 2007

    Yeah, I’m not clear on where the 910 comes from…but Wikipedia is now using it even as it also classifies George as a Category 3 storm…weird.

  12. #12 Blair Trewin
    March 11, 2007

    Western Australia use the standard Western Pacific Dvorak-wind-pressure relationship. In the Australian region (as mentioned above) the intensity estimates are almost always based on the Dvorak number only – it’s rare to have any sort of ground-truthing data.

    The 275 km/h gusts/910 hPa noted in the warnings prior to landfall corresponds to the upper end of Dvorak 6.0 (which, in the Western Australian tables, corresponds to 10-min mean wind 195 km/h, maximum gust 275, and delta-P – difference from environmental pressure – 89 hPa). Again all this is consistent with the standard western Pacific tables.

    I’m assuming JTWC’s 110 knots was a 1-minute mean – if so they’ve rated it as a 5.5. (This level of scatter in Dvorak estimates is perfectly normal – in Monica JTWC had it 0.5 points higher than the Bureau). Western Australia also calculate pressures as a difference from environmental pressure, whereas the table which Dark Tent posted a link to assumes an environmental pressure of 1010 hPa. In brief, the difference between the Bureau and JTWC pressures looks like it can be reconciled, partly by the low environmental pressure around George, and partly by a 0.5 point difference in Dvorak number.

    I suspect (although I don’t know for sure) that, unusually for an Australian TC, the surface data from Bedout Island played at least some role in the intensity assessment (normally the intensity estimates are purely satellite-based). Unfortunately Bedout Island’s automatic station only reports 10-minute mean winds once an hour and doesn’t record a log of higher-resolution data – this means that the 195 km/h (105 kt) mean winds reported at 1000 UTC are a lower bound on the maximum mean wind but don’t preclude the possibility that something stronger might have occurred between 1000 and 1100 UTC.

    The technical summary posted around the time of George’s landfall rated George at T6.5 and commented that on reanalysis George might turn out to be a low category 5 (Australian scale – still a Saffir-Simpson 4). If this does happen it would correspond to a central pressure of 905 hPa.

    As far as I know there is virtually no ground-truth data supporting (or otherwise) the use of the Western Pacific Atkinson-Holliday relationship in Western Australia – rather it’s the best information available and there is no evidence (yet) to suggest it is inappropriate. Had Bedout Island maintained continuous pressure observations throughout those would have been priceless, but it didn’t.

  13. #13 Chris Mooney
    March 12, 2007

    Thanks Blair, Llewelly, and everyone else who has contributed. From all this I infer that using Saffir-Simpson categories, George may well have intensified all the way into the Category 4 range before landfall, but we don’t really know. I guess it’s a borderline storm.

    Those studying the global number of Category 4 and 5 storms annually take note: It’s going to be hard to figure out how to classify George. This is very similar to the problem I encountered with Larry last year. I left a mental note about this, of sorts, here.

    Blair, when do you think reanalysis of George will happen?

  14. #14 Blair Trewin
    March 15, 2007

    Normally it doesn’t get done until the end of the season (sometime from May onwards).

  15. #15 Richard
    March 17, 2007

    I manage a tourist facility in South Hedland, which bore the brunt of the storm, and there are major inconsistencies in all the reports I have seen so far.
    Having grown up in the Pilbara, namely Exmouth, I have weathered a few cyclones, but I have never witnessed anything like TC George.
    I am not ashamed to admit that I spent a good portion of the early hours of Friday morning huddled under a mattress in the central hallway of my house, waiting , dreading the moment the roof succumbed to the ferocity of the wind.
    The most disturbing thing was the time it took to pass over the town, slowing considerably as it made landfall, George battered the town for over 10 hrs. The first severe winds started around 1900 hrs, and steadily increased until a little after midnight when the eye reached us, and continued to rage until first light at 0600hrs.
    Our facility is located roughly 3km north of South Hedland, here we caught the edge of the eye. a scant few minutes of complete calm, and a dawn-like lightening of the night, eerily warning of things to come. What followed was indescribable.
    The ceilings literally rose and fell inches as the wind whipping over the building sucked at the roof. Walls and windows flexed visibly, as debris, like bullets, peppered the house, tearing long gashes in the colourbond steel cladding. and shattering double glazed windows behind security grills designed to stop impacts.
    Anyone contemplating how much fun a cyclone would be, my advice is to find and ride out a category 2 then decide wether you would like a cat4 or 5. I know a few people here that have forever changed their opinions about storms being fun.
    I am still working my way through the mountain of work created by the storm. 97 trees down on the property, dozens of smashed windows, steel doors torn free of their welded steel hinges, colourbond sheets ripped off roofs and vanished or speared into walls, airconditioners wrenched from walls and hurled across roads, a fridge door, brick, chair and tree debris in the pool, bitumen roads stripped back to soil, with knee deep potholes carved out by the 200 mm of rain that fell in 7 hrs.
    More than anything, cyclones are just a pain in the butt. In the 9 days since George, I have had 2 full nights without sleep, the night of George And the night Jacob approached us, and have done 140 hrs of work, extra work, more to the point, as I still have my regular duties to perform in there somewhere. It is nearly 0100hrs now, after a 15 hr day swinging a chainsaw and knocking in starpickets, winching trees upright in an attempt to save as many as possible, and dealing with the public, I am only now finishing my paperwork, and prising my eyes open to finish my rant here.
    I cannot help with your discussion on millibars and central pressure, though I can say that unofficially, the monitoring station near the airport recorded winds to 285km/hr (177mph) or I believe around 156 knots for a considerably sustained period after 0030 hrs when the eye past.
    So far I am yet to read two news reports that say the same thing, and none of them have actually got it right. Anyone who has been through a storm like that will know what I mean when I say there is no way to describe it. It is a miracle that the town is still functioning at all.. I know… I rode out Vance in ’99, the destruction of Exmouth was almost complete, and being a similar sized storm, if not bigger according to BOM, Hedland got off lightly.
    I think I have used up my “get out of cyclone relatively free” cards, and am not looking forward to going through anymore Cat 4 or 5 storms.
    My deepest sympathies to those that lost loved ones in George, and any storm, wherever it may be.

    Rich.

    http://profile.imageshack.us/user/zakalwe/images/detail/#110/1001294pc6.jpg

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