USA Researcher Uncovers Second, Devastating Eyewall

[lurkey]

[url=http://www.southalabama.edu/publicrelations/pressreleases/2007pr/051707b.html]
USA Researcher Uncovers Second, Devastating Eyewall Inside Hurricane Katrina[/url]

Discovery Explains Horrific Damage Miles East of Hurricane’s Eye

Dr. Keith Blackwell

Mobile, Ala.--University of South Alabama hurricane expert Dr. Keith Blackwell used the latest in microwave satellite technology to look inside Hurricane Katrina’s storm clouds, leading to the discovery of a second, or outer, very potent eyewall, which extended severe hurricane winds far outward from the storm’s center.

His findings explain for the first time why the Category 3 hurricane delivered catastrophic damage over such a wide area of land, creating the biggest natural disaster in America’s history.

Conventional satellite imagery hid the dangerous outer eyewall that lurked beneath the dense clouds swirling counter-clockwise around the hurricane’s eye as it roared toward the Mississippi-Louisiana line in the early morning hours of Aug. 29, 2005.

According to Blackwell, discovery of the outer eyewall explains the extremely heavy damage to the coastline, extending unusually far to the east through Jackson County, Miss., miles eastward from Katrina’s eye. The outer eyewall came ashore on the Mississippi coast shortly after daylight while the hurricane’s eye was still well offshore. High winds pounded the coast for hours before the eye finally made landfall. Blackwell said instruments dropped into the outer eyewall from aircraft recorded winds between 140 to 145 miles per hour inside the eyewall at an altitude of 1,500 feet. He estimated sustained winds near the ground of approximately 105 miles per hour with much higher gusts in the torrential rain. This outer eyewall swept across the entire Mississippi coast as far east as Pascagoula. It was then followed by the more intense inner eyewall, accompanied by a record-breaking 28-foot storm surge, over the western Mississippi coast later that morning.

Blackwell, an associate professor in the USA department of earth sciences and a hurricane forecaster and tropical weather research center specialist with USA’s Coastal Weather Research Center, said hurricane researchers have known about outer eyewalls for a long time. Previous research has shown that a double eyewalls often form in intense hurricanes, but only persist for a day or two, thus many strong storms that previously exhibited two eyewalls over the open ocean may not retain both until landfall. However, the development of microwave satellite imagery is helping researchers to learn more about the evolving internal structure of hurricanes and their potential to grow quickly in size as the original eyewall becomes surrounded by a much more expansive outer eyewall.

“If residents of eastern Jackson County near Pascagoula had known about this outer eyewall, or even the possibility that such a thing could occur in the hours before Katrina’s eye made landfall, then their focus may have shifted from the New Orleans and western Mississippi coastline areas to their own safety. In addition, many people more directly in the path of the hurricane were suddenly surprised by the early arrival of strong damaging winds on the coast well in advance of Katrina’s center or extremely high tides,” said Blackwell.

Blackwell said as the second eyewall approached the Mississippi coastline, the strong easterly winds were actually pushing water to the west, parallel to the coastline, rather than to the north onto land as the hurricane would do later once the eye made landfall. “Initially, high winds in the outer eyewall struck the Mississippi coast up to three to four hours before the highest water arrived. The problem with water created by the storm’s devastating tidal surge arrived later,” explained Blackwell.

For thousands of Alabama and Mississippi residents still fighting insurance companies over whether wind or water damaged or destroyed their structures, Blackwell’s findings could mean a difference in how some cases are settled.

Other storms with double eyewalls have delivered devastating wallops to coastal areas, including the outer eyewall of Hurricane Ivan on Sept. 12, 2004, as the storm’s eye and inner eyewall passed to the south of Grand Cayman. The outer eyewall packed sustained winds of 150 miles per hour with gusts to 171 miles per hour, destroying 95 percent of the island’s buildings and leaving damages of $1.85 billion. The building codes on Grand Cayman at that time were similar to the strict codes enacted in south Florida following Hurricane Andrew.

Blackwell said developing data on second eyewalls can help public safety officials determine wider evacuation areas and give first responders life-saving information as they respond to storm emergencies. It can also help residents reach a better decision on evacuation plans.

“Traditionally, people have looked at hurricane warning areas and thought they were relatively safe if their residence was not in the direct path of the eye or not near the center of the warning zone. What we are learning about outer eyewalls can change how they perceive the threat a hurricane may pose to areas closer to the edge of that zone,” said Blackwell.

The USA Coastal Weather Research Center, a self-supporting operation of the department of earth sciences, began operation on Jan. 1, 1988. Located in the Mitchell Center on the main USA campus, the weather center consists of a meteorological laboratory, information center and archive.

[jrod]

This is not a new discorery. IIRC Gilbert in 1980 had a double eyewall observed and documented by the hurricane hunters. Many intense hurricanes have been observed with this phenomena. Besides the double eyewall, Katrina was a large storm and I am quite sure the NWS and NHC gave sufficient warning to the surrounding area. The mainsteam media were the ones focused on New Orleans.

EDIT:
whoops, i didnt read the entire piece, and assumed it was saying a double eyewall was something not documented before.

[MWatkins]

This might explain how we were able to get a 135mph wind reading from the tahoe up near I-10 in Gulfport.

Given the wind conditions we experienced we were a bit surprised to find out later on that the eye was further west than we thought.

One thing is almost assured...the weakening allowed Katrina to spread out in terms of wind and surge. Had that dry air not intruded...things would have been even worse for extreeme western MS and eastern LA...but Mobile and eastern MS would have fared better.

MW

[wxmann_91]

This is not a new discorery. IIRC Gilbert in 1980 had a double eyewall observed and documented by the hurricane hunters. Many intense hurricanes have been observed with this phenomena. Besides the double eyewall, Katrina was a large storm and I am quite sure the NWS and NHC gave sufficient warning to the surrounding area. The mainsteam media were the ones focused on New Orleans.

EDIT:
whoops, i didnt read the entire piece, and assumed it was saying a double eyewall was something not documented before.

Outer eyewalls occur all the time with mature, strong tropical cyclones. Not a new phenomenon.

[MGC]

This is typical when a hurricane has an eyewall replacement cycle. A new outer eyewall forms. Willima did the same thing and had a huge eye when she hit Florida. 135mph winds 5 miles inland, Pearl River County EOC had the strongest wind gust well inland. I just can't imagine the winds right on the beach before the effects of land..........MGC

[JonathanBelles]

I hadnt thought about this possibility before. Could that 2nd eyewall have been a category higher? That was an interesting article.

[Derek Ortt]

There was no secondary eyewall in Katrina

If that feature was a secondary eyewall, why then did the aircraft never report it as such as why was its radius EXPANDING at landfall (expanded from about 85km to 100km... a 200km wide eye?)

I contend this was a wind streak, very similar to what occurred with Hurricane Fran. In both cases, strong shear hit the storm in the hours before landfall and the eyewall collapsed on its southern side. The maximum winds in Fran were not even in the eyewall, but in a streak about 100km from the center (see the Fran NHC report)

[curtadams]

IIRC, the last good recon pass through Katrina showed *3* wind velocity peaks and we were having brisk discussions about an ERC as she went in. So this doesn't surprise me at all.

[Derek Ortt]

There was also land and shear affecting the storm then

The recon and radar showed that the southern eyewall totally collapsed at landfall (very similar to Fran), but that there was not a true secondary eyewall (and the NHC report also states as such, it never could fully form.

This is something interesting that probably should be investigated further in future studies, how does the shear cause wind streaks 100km away from the eye with winds close to or at those in the eyewall

[otowntiger]

Interesting, but from all that I've seen, heard and read, the vast majority of the damage from Katrina was indeed from the storm surge. This is the first time I've seen any reports of winds that high anywhere on land.

[MGC]

The alleged second eyewall was only located about 10-15 miles away from the original eyewall. The original eyewall passed over western Harrison County while the alleged second eyewall passed near Biloxi. There was a wind maxima reported by recon as Katrina was making landfall along the LA/MS border. The aircraft was flying west over the Mississippi Sound when it observed the wind maxima. Personally, I doubt much of this alleged second eyewall's winds mixed down to the surface. The worst observed wind damage was in western Harrison County and eastern Hancock County. The wind damage in Biloxi was no where near as bad as the wind damage in the Bay of Saint Louis area. But, these are just my personal observations of wind damage along the coast here. Also, I've viewed the radar loop of Katrina coming ashore about a million times and still contend that convection in the RFQ eyewakk was becoming quite intense as Katrina came ashore. I'm certain that winds in this area were mixing down to the surface. It is the only explanation I can find that can explain the wind damage in this area. For example, at the intersection of 1-10 and MS 603 in western Hancock County, all the tall light poles were blown down by strong ESE winds, yet in Gulfport at the I-10 US 49 intersection the identical light poles remaind standing. I also didn't observe any well constructed homes futher east than Long Beach with complete roof failure. So, without question the strongest winds reaching the surface were in conjuction with the intense convection associated with Katrina's eyewall.....MGC

[Derek Ortt]

Here are a couple of flight level plots I made

http://orca.rsmas.miami.edu/~derek/rain ... 3space.jpg

Outer wind maximum about 85-90km from center about about 60 from eyewall


http://orca.rsmas.miami.edu/~derek/rain ... 8space.jpg

Outer wind maximum expands to 100km and is 70km from the eyewall (NE on left)

Not a true secondary eyewall as this was not contracting, but expanding. Seems more like a Fran wind streak

[wobblehead]

Ever wonder why people who have a cat2 or 3 hurricane pass over them swear it was a cat4 or 5? What they experienced may indeed have been stronger than the official classification. Take hurricane Ivan as an example. Today a local hurricane researcher showed me a dopplar radar image of Ivan at landfall. A cross sectional image looking east from Mobile. The coast on the left and GOM on the right. Upper right corner displayed a moisture mass in the upper levels. The next image displayed this same moisture mass down at the surface. As explained to me this was a huge mass of moisture that becomes so heavy it plummets downward hitting the strongest wind levels below it and pushes these winds down to the surface creating a huge and powerfull wind blast. For some reason this event is more common with exposed eyewall systems.
Also I learned that Katrina hit Ms with 2 eyewalls as proven in microwave imaging of the storm shortly before landfall. As the new eyewall forms outside the original eyewall the energy in the space now taken by the new eyewall must go somewhere so naturally it displaces outward (can't go back to lp center) thus increasing the overall size of the system ie Katrina.
You all may find this boring but I found it to be fascinating so I share it.
I posted this topic back in May after a meeting with Dr. Williams and Dr. Blackwell. Not much response to it so I thought I'd bring it up again. Derek if you ever venture up to Mobile I'm sure you would enjoy speaking with Dr. Williams and Dr. Blackwell. The research they are doing is fascinating.

[Derek Ortt]

A secondary eyewall does not expand, it contracts. I have flight level data of Rita, along with model cross sections that show this very clearly (as do many other studies, especially Willoughby et al. 1982 with great examples from David and Allen)

As for the microwave, two points. I have presented on a couple of occasions that Katrina's "double microwave ring" did not meet a secondary eyewall distinction (at either 2130 UTC, Aug 28 or at 0230 on Aug 29). It may have been made up of multiple bands and there was not the well defined moat region that is present in a true secondary eye scenario juding by the flight level data

Here is a radar of Fran at landfall, demonstrating that Katrina was not unique

http://weather.unisys.com/hurricane/atl ... 10906r.gif

Not a secondary eyewall, but a partial convective band where the RMW was located (NHC operational definition is 50 percent coverage to qualify as an eyewall, recon never found that in Katrina or Fran)

I'll likely have more on this topic in my M.S. defense (hoping that is coming soon), as well as some modeling results showing that you CANNOT look at a single microwave image and conclude secondary eyewall, you can be fooled by a transient feature

[WxProf]

A much more complete description of the outer eyewall in Katrina (than what is in the USA press release for public consumption) is provided in the paper I presented to the 61st Interdepartmental Hurricane Conference (IHC) in New Orleans in early March of this year. My paper is the first one in Session 3.

http://www.ofcm.gov/ihc07/linking_file_ihc07.htm

There is overwhelming evidence of an outer eyewall in Katrina from Dropsondes, SFMR, Microwave MIMIC loop (from CIMSS, made up of multiple microwave satellite images), and on-board radar profiles from the NOAA P-3's flying around in the storm the morning of landfall. I show examples from these data sources in the IHC presentation. Both the microwave satellite imagery and onboard P-3 Doppler radar show a completely closed outer eyewall when the storm was near (or just south of) the mouth of the Mississippi River. Also, the aircraft crew mission summary from from the P-3 aircraft that morning (29 August) makes mention of both an inner and outer eyewall. By the time the storm made landfall on the Mississippi Coast, the southwestern portions of both the inner and outer eyewalls had "opened up", probably due to a combination of wind shear and dry air entrainment. But, the fact that they were "open eyewalls" (i.e., not completely closed circles), does not mean they were not eyewalls. And, up to at least 12Z, 29 August 2005, Microwave MIMIC imagery shows that there was still more than 50% of the outer eyewall remaining around the storm in the northern and eastern semicircles... by that time, parts of the Mississippi coast were already feeling some impacts. Eyewall wind profiles were being found in dropsonde data in the outer eyewall after 12Z as the outer eyewall was impacting the Mississippi Coast. I think the facts as presented in my presentation at the IHC speak for themselves.

[Derek Ortt]

I'll have a look at the full presentation tomorrow morning

[docjoe]

Speaking as a rank amateur I think winds away from the center often get underestimated. Where I live in Santa Rosa County is about 30-35 miles from the path of the eye of Ivan...as the crow flies. Yet the tree and roof damage here looked every bit as intense as places 10-15 miles closer to the eye. My personal uneducated opinion is that although a locale may not have sustained cat 2 or cat 3 winds (as in Ivan)but having gusts in this range that last 10 seconds or more and occuring multiple times can cause damage similar to sustained winds in that range.

docjoe

[CrazyC83]

Didn't one storm (Edouard in 1996) have a triple-eyewall?

[Derek Ortt]

don't know about Edouard, but Juliet in the EPAC in 2001 had 3 eyewalls as reported by aircraft. Iris that year also had 3 reported (though I have doubts on whether the third band was an eyewall)

[Derek Ortt]

I have had a chance to look at the presentations and have a couple of questions regarding it, though I'd perfer to ask (since they are very technical and research orientated) not in this particular forum