• Stormtrack's forum runs on Xenforo forum software, which will be undergoing a major update the evening of Wednesday, Feb 28th. The site may be down for a period while that update takes place.

Hurricane Ivan: ground observations?

Brian Guppy

See the full post post by Bob Schafer at:
http://www.stormtrack.org/forum/viewtopic....p?p=39693#39693

Originally posted by Bob Schafer
Something is really, really fishy.

Seen the pics from Jamaica? Anything more than F1?
...
Someone show me some buoy archive data of wind speed greater than 90KTS. I bet you can't even find one of GUSTS.
...

We know that trying to get any kind of precise windspeed estimate based on damage isn't possible, so I'm not willing to make any inferences about hurricane intensity based on damage photos, especially since we have no way of knowing whether the photos captured areas that experienced the most intense winds. Regarding the (relatively) underwhelming observed windspeeds, it has been mentioned that the area of most intense winds is small and so it's at least plausible that the max wind speeds were significantly higher than anything officially observed.

Still, the lack of any ground-based observational data to confirm the hurricane's strength is troubling. Is the area of major hurricane-strength winds really so small that it would go completely unobserved by ground-based observation networks? Does anyone actually know how accurate a wind measurement taken from a buoy in 40+ foot seas is? How well "calibrated" are the Hurricane Hunter and satellite-based intensity estimates - ie, are ground observations typically collected that show the intensity estimates to be accurate? If so, then why didn't it happen this time (or did it)? If not, then how trustworthy are those estimates? Can anyone shed some light??

-Brian
 
They were talking about this on tv last night. The official maximum sustained winds are just that, the maximum. They are occuring in a very small area. They could easily slip by a ground based station. Especially in a place like Jamaica where the eye never made landfall. It seems like the past few years, the official max sustained is rarely achieved at landfall. Although, the people in Punta Gorda, FL might argue that. So would the people in Homestead, FL. It was only recently that Andrew was reclassified as a cat 5.

As for accuracy. I would think that most of the instrumentation the US goverment uses is very accurate. Not always within 1mph, but 5-10 mph off would be acceptable. On Mount Washington we take great pride in the accuracy of our equipment, the stuff we use is quie a bit better than a Davis Weather Station, and those are pretty good. Buoys could have errors, but I would guess they underestimate the wind speed. Satellite based intensity is really just an estimate, like radar rainfall totals.
 
I'm not on any mission to prove my belief that Ivan was never as intense as advertised (I couldn't possibly, anyway). This whole subject has me mystified, though.

Consider this evidence:

Ivan missed Grand Cayman(GC), but barely, and GC was in his NE quadrant as he tracked to the WNW. Rated cat4 at the time.......by 1mph.

AT 11 AM EDT (Sept 12)...1500Z...THE CENTER OF HURRICANE IVAN WAS LOCATED NEAR
LATITUDE 19.0 NORTH...LONGITUDE 81.5 WEST OR ABOUT 30 MILES...
45 KM...SOUTHWEST OF GRAND CAYMAN.....MAXIMUM SUSTAINED WINDS ARE NEAR 155 MPH

HURRICANE FORCE WINDS EXTEND OUTWARD UP TO 90 MILES...150 KM...
FROM THE CENTER...AND TROPICAL STORM FORCE WINDS EXTEND OUTWARD UP
TO 175 MILES...280 KM.

EYE DIAMETER 15 NM

Ivan's center to GC 30 miles (to closest point on GC?), using 7.5 miles radius from center to eyewall, and maximum distance from SW to NE on GC of ~20 miles, then no point on GC was essentially more than 42.5 miles from the eyewall, and "middle" of GC was about 30-35 miles from eyewall, i.e. 30-35 miles from 155mph sustained winds if they occurred in NE quad.

WINDS AND SEAS VARY GREATLY IN EACH QUADRANT.....duly noted.

HAM RADIO REPORTS INDICATE SUSTAINED WINDS
OF 50 TO 60 MPH...WITH GUSTS AS HIGH AS 110 MPH...HAVE BEEN
MEASURED ON GRAND CAYMAN DURING THE PAST COUPLE OF HOURS.

Georgetown, and the airport, are in SW GC. I don't know where on GC that ham was, but if near either of those places, then that puts the report from about 25 miles from the eyewall, and if those 155mph winds had a width of 5 miles, then 20 miles from them. Let's say I "struggle" with the notion that the winds may have been 155 in one place, and 50-60 (sustained) 20 miles away.....or 30, or 40.....in the NE quad.

Sure, there's all the arguments about "What do we know about that equipment, and how it was placed, and what time was that taken....?" and so forth.

I cyber-chased Ivan for about 10 hours a day for 11 days. I looked at soooooooo much data........

This sort of conundrum occurred over and over and over for a week.

http://www.miami.com/images/miami/miamiher...93281144914.jpg

That's a picture that the media is using to illustrate the damage on GC. ("Devastation beyond imagination", remember?)

The media will use the MOST dramatic pictures they can get their hands on.

That's it?

It has been well stated that GC is a wealthy place, and that construction standards are absolutely the most stringent and enforced there. Never mind the lack of structural damage. The palm trees still stand, with fronds still on them.

http://www.miami.com/images/miami/miamiher...93281173888.jpg

Structural damage. Okay. The cars haven't been moved or overturned from the spots where they were (semi)neatly parked.

The most stunning, dramatic pictures the media could get.

Submitted for your intellectual stimulation.

Bob
 
Originally posted by B Ozanne
It was only recently that Andrew was reclassified as a cat 5.

I don't want to go too far off-topic, but I have heard rumors that the Insurance Industry and Florida politicians looking to try and keep insurance rates in the state under control had a LOT to do with the posthumous upgrade.
 
I hear you. Just keep in mind that the hurricane hunters are on a mission to find the worst conditions in the hurricane. They use a myriad of sensors to find the most intense cells and to get data from them. Maybe it was just one small part of one cell that had the advertised winds...that's all they need.
 
Originally posted by Mike Hardiman+--><div class='quotetop'>QUOTE(Mike Hardiman)</div>
<!--QuoteBegin-B Ozanne
It was only recently that Andrew was reclassified as a cat 5.

I don't want to go too far off-topic, but I have heard rumors that the Insurance Industry and Florida politicians looking to try and keep insurance rates in the state under control had a LOT to do with the posthumous upgrade.[/b]

That may have been the case but ground ovservations supported the CAT 5 rating.
 
The area with the highest wind speeds not a particular large spatial region relative to the size of the entire hurricane. Also, there are a number of factors that affect the actual wind a "people" height - not the model used of 10 m winds (~30 ft). Let's look at Ivan winds near Grand Cayman:

ftp://ftp.aoml.noaa.gov/pub/hrd/hwind/200...30/col02deg.png

As you can see, only a small patch on the east side is showing winds of 100 knots, and the Georgetown area can expect winds (if it were actually open water instead of land) of 90-95 knots for a 1 minute average wind. So, high end CAT 2 winds is the most Grand Cayman was likely exposed to - so I don't see any scandalous implications there.


What about back at Jamaica? Maybe a scandal awaits there.

ftp://ftp.aoml.noaa.gov/pub/hrd/hwind/200...30/col02deg.png

Hmm..., seems Ivan was a much tighter circulation then - and more intense, with up to 135 knot winds in the NW quadrant - with 90 knot winds just clipping the extreme SW coast of Jamaica - so again, CAT 2 exposure.


Finally, let's see how Ivan appeared as it approached the Alabama coast early today:

ftp://ftp.aoml.noaa.gov/pub/hrd/hwind/200...30/col02deg.png

We can really see the effects of friction from land here - as highest winds are over the open water and bay regions, but still, 90 knot winds are what appear to be impinging on the coast, so CAT 2 winds at landfall. Escambia counties (one in each state) will bear the brunt based on this - and that appears to be the case from early reports. Stepping back just before landfall, winds were closer to 95 knots - maybe a bit higher.

ftp://ftp.aoml.noaa.gov/pub/hrd/hwind/200...30/col02deg.png

So, perhaps just into CAT 3 in the hours before landfall - and this is not unfair for NHC to go with CAT 3 expectations at landfall.

Glen
 
I don't want to push a conspiracy theory here or accuse anyone of overrating the hurricane. I'm not even saying that the observations and the intensity estimates/forecasts are at odds with each other. I'm just curious to know how inferences about hurricanes are made from the available observations and, if we don't exactly have a wealth of measurements that support or refute those inferences, how we know how reliable they are.

-Brian
 
Originally posted by B Ozanne

That may have been the case but ground ovservations supported the CAT 5 rating.

Not from what I can tell (you may have a different source?)... did a little digging that may prove interesting to the true topic at hand as well.

From: http://www.nhc.noaa.gov/1992andrew.html
While a universally accepted value for Andrew's wind speed at landfall may prove elusive, there is considerable evidence supporting an estimate of about 125 kt for the maximum sustained wind speed, with gusts to at least 150 kt (Fig. 5).

The strongest reported sustained wind near the surface occurred at the Fowey Rocks weather station at 0800 UTC (Fig. 5). The station sits about 11 n mi east of the shoreline and, at that time, was within the northwest part of Andrew's eyewall. The 0800 UTC data included a two-minute wind of 123 kt with a gust to 147 kt at a platform height of about 130 ft.

**Note the platform height of the observing system**

Granted...
It is unlikely that this point observation was so fortuitously situated that it represents a sampling of the absolute strongest wind. The Fowey Rocks log (not shown) indicates that the wind speed increased through 0800 UTC. Unfortunately, Fowey Rocks then ceased transmitting data, presumably because even stronger winds disabled the instrumentation.

The article actually goes on to say that the instrument tower was bent 90-degrees...

Note the max winds measured by Recon:
http://www.nhc.noaa.gov/gifs/1992ed_andr2.gif

Now here's the kicker, this from:
http://www.disasterrelief.org/Disasters/020821andrew/
In 1992, the year of Hurricane Andrew, weather experts believed that hurricane winds were about 75 percent of the strength registered by the dropwindsondes. However, NOAA scientists have determined hurricane strength is actually 90 percent of what is registered, increasing Andrew's winds from 145 to 165 mph, and making it a Category 5.

Perhaps the results of this study are flawed?

I'm not 100% sure where I stand on this, but it does make for an interesting debate and dicussion with interesting points on both sides![/url]
 
One quick comment... The Saffir-Simpson scale is based on sustained winds. Now, a quick gust of of 140mph which can rip apart roofs, etc, DOES NOT make the storm an automatic Cat 4. Damage can look horrendous simply because they got 1 strong gust, which can be 20-40mph above sustained winds. So, even if an ASOS measured a max wind gust of 140mph does not justify a Cat 4 rating. Yes, the NHC did downgrade to Cat 3 (130mph sustained with gusts to 155mph I believe) right at landfall, I'd still be surprised to see if that verified. I certainly haven't seen gusts to 155mph... gusts to 130mph maybe, but again, gusts don't qualify the Saffir-Simpson scale.

I'm nearly certain that the I10 bridge feel apart because of the extreme waves and high surge. notice that the ONLY parts of the bridge that collapsed / are missing are the lowest parts of the bridge, indicating that it was largely waves and surge that caused that damage.
 
Originally posted by Bob Schafer
Something is really, really fishy.

Seen the pics from Jamaica? Anything more than F1?

Seen the pics from Grand Cayman? Hmmmmmmm?

Someone show me some buoy archive data of wind speed greater than 90KTS. I bet you can't even find one of GUSTS.




Remember that Ivan did not directly hit Jamaica or Grand Cayman. The center of the circulation (eye), where the hurricane hunters measured the MAX wind, was always at least 40 nautical miles from land until it came upon far western cuba where there are no observations.

Though I'm not sure how sound this logic is.....the maximum wind that is reported by the hurricane hunters is most often used to categorize the hurricane. So, because the area of intense (cat2 and higher) winds is confined to such a small area, you would literally have to have observing stations every square mile or more to even try and sample the strongest winds. Remember that the hurricane hunters can sample the hurricane for many hours to find the strongest winds.

Regardless of all this, flooding from heavy rains is far more deadly than direct impact from strong winds, and Ivan has the potential to dump and dump and dump huge amounts of rain as it stalls in the Appalachians. So, I don't think we are anywhere near done with Ivan yet.
 
No doubt that bridge failed from water/wave action.

On the main topic...I like the system, and so do the people who run it.

You are also forgetting there is a pressure component to the hurricane. The public only really sees the requirements for a CAT 5 in terms of wind speed, but a lot of forecasters look at the central pressure. With a known pressure of x they can argue that sustained winds of y are physically possible. Whether or not they happen is another note, but if conditions were perfect they could be reached.
 
It was only recently that Andrew was reclassified as a cat 5.

I don't want to go too far off-topic, but I have heard rumors that the Insurance Industry and Florida politicians looking to try and keep insurance rates in the state under control had a LOT to do with the posthumous upgrade.

As a member of the best track supervisory committee that had to approve the upgrade, I can say that no politicians or insurance industry people pressured me (or anyone else that I know of) to upgrade Andrew.

The intensity of Andrew has been a hot topic ever since it made landfall. There have been people who have suspected it was a Category 5 ever since we were able to pin down the landfall central pressure at 922 mb. However, it took the advent of the GPS sondes in the eywall to let us go back and re-evaluate the available aircraft data to determine it was likely a Category 5.

That's wasn't all we looked at, though. We also looked at radar cell tracking in the eywall, and looked in great and gory detail at the observations from Fowey Rocks. My own studies of the Fowey Rocks obs compared to the radar position of the eyewall suggest that the last observation was taken somewhere outside the radius of maximum winds, and that it was anywhere from 5 to 30 minutes before the maximum winds reached the station. Since the last ob showed the winds had increased almost 20 kt in the 10 minutes prior to the failure, you can see that the peak winds could have been significantly higher that what was measured. Unfortunately, we'll never know for sure.

Even now, the debate continues as to Andrew's landfall intensity, as a scientist at the Hurricane Research Division is arguing based on boundary layer premises that it was a Category 4. Hopefully we'll eventually get enough data about what happens to a hurricane's surface windfield at landfall to find out whether he's right or not.

More information on the Andrew upgrade is available at http://www.aoml.noaa.gov/hrd/hurdat/andrew.html

To bring this back to Ivan, I should point out that the three closest stations east of where the center came ashore are the Pensacola Naval Air Station, the Pensacola Airport, and the Pensacola National Ocean Service station. All of them stopped reporting before the worst of the conditions arrived. One of the DOW's was at the Gulf Shores airport and has reported gusts of 105-110 mph. However, that airport is several miles inland and it uncertain whether the strongest winds Ivan had to offer went over it or somewhere further east. At the very least, the wind gusts at the open coast would be higher that that.

Several portable mesonet stations and chasers were in the landfall area, so when their data comes in we should get a better picture of the landfall winds.

As for buoy 42040, I saw something similar in Hurricane Floyd back in 1999. Floyd ran right over a buoy east of Cape Canaveral, which reported a 938 mb pressure, 70 kt sustained winds and 54 ft seas. Although no one at the National Data Buoy Center can tell me for sure, I suspect that the smaller buoys (the 3 and 5 meter types) measure too low of winds in such seas.

There's a lot of work to be done before we make a final determination of Ivan's landfall intensity. Stay tuned, folks!

Jack Beven
Tropical Prediction Center
 
If insurance companies were part of the big conspiracy to reclassify Andrew a 5, I'm not sure of their reasoning. It seems like it would be in their best inerest to keep Andrew a CAT 4. If the event was proven to more common then they can easily raise rates.
 
As for buoy 42040, I saw something similar in Hurricane Floyd back in 1999. Floyd ran right over a buoy east of Cape Canaveral, which reported a 938 mb pressure, 70 kt sustained winds and 54 ft seas. Although no one at the National Data Buoy Center can tell me for sure, I suspect that the smaller buoys (the 3 and 5 meter types) measure too low of winds in such seas.

As an addendum to my previous post, buoy 42040 broke loose from its mooring during Ivan and is now adrift. It's anyone's guess what effect that had on the wind measurements.

Jack Beven
Tropical Prediction Center
 
Well, just getting caught up on stuff. We had a great chase. I would have posted sooner but after Hurricane Frances, was not sure which topic this hurricane stuff goes into anymore... So this time I blew off posting anything to stormtrack til now. Anyways, we chased Ivan, it was fun in Mobile and sure glad we stayed there with this night time storm and wicked surge. Heard a few chasers got trapped out on the island and may still be out there. Got to know when to hold your ground or run like hell from these storms. This time we took option B.

The video and images are up on http://www.lightningboy.com
 
Back
Top