Sunday, May 01, 2011 2:15:36 PM
Tornado Outbreak Raises Climate Change Questions
By Andrew Freedman
Published: April 29th, 2011, Last Updated: April 30th, 2011
With at least 340 dead from dozens of tornadoes [ http://news.yahoo.com/s/ap/us_severe_weather;_ylt=AuSrvk_IdkZe8YeNhOwD0a2s0NUE ], including at least 249 deaths in Alabama alone, the tornado outbreak that tore across the South on Wednesday was certainly one for the record-books. Storm surveys are still going on, but it appears likely that this outbreak spawned several violent tornadoes, of the EF-4 to EF-5 category on the Enhanced Fujita Scale [ http://www.spc.noaa.gov/faq/tornado/ef-scale.html ]. Already one tornado has been designated an EF-5 [ http://news.yahoo.com/s/ap/20110429/ap_on_re_us/us_severe_weather_tornado_rating ]. It destroyed much of the town of Smithville, Mississippi, killing at least 14. The tornado that decimated parts of Tuscaloosa and Birmingham, Ala. is likely to be rated at least an EF-4 [ http://forecast.weather.gov/product.php?site=BMX&product=PNS&issuedby=BMX ], with maximum winds of at least 165 mph.
Such tornadoes are capable of ripping a home off its foundation and leaving just a concrete slab behind. The 1974 "Super Outbreak", which was the most recent outbreak of a similar magnitude, killed 330 people and featured a whopping 24 F4 and F5 tornadoes, including the infamous Xenia, Ohio F5 tornado [ http://www.ohiohistory.org/etcetera/exhibits/swio/pages/albums/1974_tornado/1974_tornado_albumPage02.html ], which destroyed most of that Ohio town.
The images of the tornadoes and the destruction they have wrought have sparked plenty of questions: why did this outbreak occur, and why has April been such an active month for tornadoes? And, might global climate change be making tornadoes stronger or more frequent, or perhaps be shifting Tornado Alley out of the Great Plains and into more heavily populated areas of the South?
Those of us who write about climate change are often accused of attempting to link every unusual weather event to climate change, as if increasing air and ocean temperatures can explain everything from hurricanes to snowstorms. In this case, with the second-deadliest tornado outbreak in US history, and with the most tornadoes for any April since records began in the early 1950s, it's important to understand that the scientific evidence indicates that climate change probably played a very small role, if any, in stirring up this violent weather. This might disappoint some advocates [ http://www.huffingtonpost.com/peter-h-gleick/tornadoes-2011_b_855032.html ] who are already using this to highlight the risks of climate change-related extreme weather.
As I wrote at washingtonpost.com [ http://www.washingtonpost.com/blogs/capital-weather-gang/post/tornado-outbreak-for-the-record-books-how-did-deadly-destructive-event-happen-and-what-does-it-mean/2011/04/28/AFLQ942E_blog.html ], tornadoes form when a combination of ingredients are present in just the right amounts. These include: abundant moisture, winds that change speed and/or direction with height (also known as "wind shear"), and a trigger to force air to rise into towering thunderstorms — such as a cold front. All of these ingredients were present in abundance on Wednesday, and in fact they have come together again and again this month across the South, Southeast, and Carolinas. In fact, so much wind shear was present on Wednesday that nearly every thunderstorm that formed in Alabama and Mississippi showed signs of rotation, meaning they were capable of producing tornadoes, and aircraft approaching Atlanta's Hartsfield International Airport — the world's busiest — had to break off their landings [ http://www.washingtonpost.com/local/at-atlanta-airport-storms-create-harrowing-tests-for-pilots-and-controllers/2011/04/28/AFRNxX8E_story.html ] because of tailwinds that quickly turned into headwinds and back again, altering their airspeed and posing a serious risk of a serious accident.
A major factor in spawning all of the massive tornadoes was unusually powerful jet stream winds, which both helped trigger the storms and gave them a supercharged dose of wind shear to work with. The airflow in the upper atmosphere has steered numerous dips or "troughs" in the jet stream far to the south, helping to bring warm, moist air on a collision course with cold and dry air to the north.
One climate cycle that is being eyed as a contributor to the deadly April weather is La Niña, which is a natural cycle of climate variability that is characterized by cooler than average waters in the Pacific Ocean. Here's how I described La Niña's influence on tornado season over at the Capital Weather Gang blog:
Like its sibling El Niño, La Niña can influence weather patterns far from the Pacific Ocean. A study that examined the relationship between sea surface temperatures in the Pacific and the number of tornadoes in the U.S. found a weak correlation between La Niña and a greater number of tornadoes. Another study [ http://pielkeclimatesci.wordpress.com/2011/04/24/la-nina-and-tornado-outbreaks-in-the-usa/ ] found tornadoes during La Niña years had longer than average track lengths, more violent tornadoes, and a good probability of having an outbreak of 40 or more tornadoes. Brooks points out that both the Palm Sunday tornado outbreak in 1965 [ http://www.crh.noaa.gov/dtx/palmsunday/ ] and the Super Outbreak in 1974 [ http://www.april31974.com/ ] occurred during La Niña years.
Another broader factor that may be aiding and abetting the destructive weather is a very warm Gulf of Mexico, where sea surface temperatures have been between 1 and 2.5°C above average for this time of year. This is important because it means there is more moisture flowing northward off the Gulf, and a humid environment is necessary for severe thunderstorms to form. As I wrote earlier this week:
“Tornado Alley” owes its existence in part to humid air transported northward from the Gulf of Mexico, since without this moisture, you would not have the combustible clash of warm, humid air and cool, dry air masses in the Plains. Farther east, into “Dixie Alley [ http://voices.washingtonpost.com/capitalweathergang/2011/03/re-defining_tornado_alley.html ]”, the Gulf also serves as the fuel for many tornadic storms.
How Might Climate Change Affect Tornadoes?
Climate change is already changing the environment in which severe thunderstorms and their associated tornadoes form, and it's bound to have some sort of influence on tornado frequency or strength. But as of now, no discernible trend has been detected in the observational data, and studies of how tornadoes will fare in a warmer world show somewhat conflicting results.
While overall tornado counts have been increasing in the U.S. in recent years, the number of strong tornadoes has actually been going down, leading researchers to conclude that they are catching more of the weaker tornadoes that are far more common than the mile-wide "wedge" tornadoes that decimated communities like Tuscaloosa, Ala. this week. The trends in tornado counts is likely a result of improvements in observational technology, such as Doppler radar networks and a national network of trained storm spotters. Also, historical tornado data is not considered very reliable, since it's thought that many tornadoes — particularly weaker ones — were missed in the early decades of record-keeping, and construction methods have also changed with time. This is important since the strength of a tornado is determined by a damage survey, rather than actual instrument measurements of tornadic winds (no wind-recording instrument has ever survived a direct hit by a tornado).
As I reported for the Capital Weather Gang:
As for global warming, this is an active area of scientific research, with some conflicting projections so far about whether a warming atmosphere will make it more or less likely that tornadoes will form. Since more moisture gets added to the atmosphere as the climate warms, additional water vapor may help severe thunderstorms and tornadoes to form. On the other hand, wind shear is expected to decline due to climate change, which would argue against an increase in tornado numbers.
According to some studies, though, by the end of the present century, the added water vapor will be enough to overcome the lower wind shear, and create more opportunities for severe thunderstorms to form.
Tornadoes are a bigger wild card for climate scientists than other types of extreme weather and climate events, such as heat waves and flooding. (Studies have consistently found that both of these hazards will occur more frequently and severely as the world warms.)
Online Resources:
Studies: A 2008 paper [ http://www.agu.org/pubs/crossref/2008/2008EO530001.shtml ] in EOS Transactions, a publication of the American Geophysical Union, discussed challenges involved in analyzing tornado trends and climate change.
This 2009 paper [ http://www.stanford.edu/~omramom/Trapp_GRL_09.pdf ] in Geophysical Research Letters found that climate change may make many parts of the U.S. more susceptible to severe thunderstorms, and possibly to tornadoes as well.
This 2007 study [ http://www.pnas.org/content/104/50/19719.short ] in Proceedings of the National Academy of Sciences found that severe thunderstorms may become more frequent as the climate warms, particularly in areas close to the Gulf of Mexico and along the Atlantic Seaboard.
Weather Underground chief meteorologist Jeff Masters detailed these studies and others in a 2008 post [ http://www.wunderground.com/blog/JeffMasters/comment.html?entrynum=910&tstamp=200802 ] on this topic.
New York Times blogger Andy Revkin has extended comments [ http://dotearth.blogs.nytimes.com/2011/04/29/killer-tornadoes-horrible-and-still-unknowable/ (below)] from some of the top researchers studying climate change and tornadoes.
Copyright © 2011 Climate Central
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Killer Tornadoes, Horrible and Still Unknowable
By ANDREW C. REVKIN
April 29, 2011, 10:27 am
The awesome power of great tempests is bad enough. But the terrifying destructiveness of the hundreds of roaring funnels that have ripped apart dozens of communities [ http://www.nytimes.com/interactive/2011/04/28/us/map-of-the-tornadoes-across-the-south.html ] this year in the worst tornado season in decades is unimaginable except for those who’ve experienced it. *
My heart goes out, particularly, to people who, out of poverty or lack of awareness, live without access to a basement or shelter — particularly those who live in trailer parks. As Walker Ashley, a meteorologist at Northern Illinois University, found in 2008, 44 percent of deaths in tornadoes [ http://www.niu.edu/PubAffairs/RELEASES/2008/feb/tornado.shtml ] occur in mobile homes [1:27 p.m. | Updated He just told me it's up to about 50 percent now.]
And, as Kirk Johnson discusses in an excellent overview of the mix of hazard and vulnerability in Tornado Alley, the fast growth in many parts of the South has generated precisely the wrong kind of housing in the wrong kinds of places [ http://www.nytimes.com/2011/04/29/us/29tornadoes.html ].
Johnson’s piece, sifting for a cause for this terrible storm season, explores the climate-change question and finds no easy answer. (The story I reported in 2002 from the wreckage left by an F4* [ http://www.nytimes.com/2002/05/07/science/finding-the-what-when-where-and-why-of-the-supertwister.html ] tornado in La Plata, Md., has more of the basic science.) While there’s evidence that increasing greenhouse heating of the planet is exacerbating hot spells and extreme downpours, and may be related to hurricane intensity (but not frequency), a combination of imprecise records and deep complexity in the mix of forces that generate killer tornadoes has clouded any link to global warming. Anyone implying such a connection is in the spin zone [ http://humanitariannews.org/20110428/peter-h-gleick-cost-denying-climate-change-accelerating-climate-disruptions-death-destructi ].
There’s certainly no reliable trend in the categories of tornadoes that matter — those designed 2 to 5 on the Fujita scale of twister ferocity. The graph below, of data from the National Severe Storms Laboratory in Norman, Okla., shows trends over the last 60 years. The big increases in weak tornadoes and slight dip in the F2 class are thought to be mainly the result of changes in reporting methods and also a far greater reporting rate as the South has grown more populous and radar has helped identify tornadic storms.
Trends in reported tornadoes by intensity on the Fujita scale (F0 weakest; F5 worst).
NOAA
Last January, when there was a rare winter tornado outbreak, and some talk of human-driven global warming playing a role, I consulted a batch of meteorologists and climate scientists who have studied trends in the categories of tornadoes that kill people, which are those designated F2 through F5 on the five-step Fujita scale of intensity (gauged by the amount and type of damage that is wrought).
I’ve strung their thoughts at the end of this post for those wishing to dig deeper.
Despite a lot of new technology and intensive research, including flying sturdy aircraft in and around potent thunderstorms (I got to go on one such flight long ago [ http://lh5.googleusercontent.com/_gmR8fkmAnjw/Tbq1GbLhhqI/AAAAAAAAD4M/7ni8_Us3Wzw/RevkinStormFlight.jpg ]), the mix of conditions that produce these monstrous funnels remain beyond human capacity to understand.
The bottom line is very similar to that in earthquake country. There are regions of clear risk, and practices — like building trailer parks or shoddy houses in known danger zones — that are guaranteed to produce high losses. But knowing when a particular quake or killer funnel strikes remains impossible.
[3:32 p.m. | Updated Roger Pielke, Jr. [ http://rogerpielkejr.blogspot.com/2011/04/bill-hooke-on-tornadoes.html ], drew my attention to an important recent post by Bill Hooke [ http://www.livingontherealworld.org/?p=257 ], the director of the American Meteorological Society's policy program, on the steady rise in vulnerability in America's tornado hot zone. The takeaway line?
Tornadoes hitting downtown areas in the past? Rare – almost unheard of. But tornadoes hitting downtown areas in the future? Increasingly common.]
The biggest risk, for the moment, still lies in mobile home parks. That means that the imperative in tornado country is to get far more serious about toughening standards for shelters and, particularly, trailer parks. I’m trying to find out whether “tie downs” for mobile homes — straps that secure them to the ground — have mad much of a difference in survivability.
Please post any info on research along these lines. For the moment, trailer park residents seem quite concerned [ http://www.topix.com/forum/city/greenville-ky/TP6M48NVJSSVPIUUR ] about trailers lacking this added protection.
For weather and climate geeks, here’s that roundup of meteorologists on tornadoes and climate change (jargon alert):
Howard Bluestein [ http://som.ou.edu/peopleDetails.php?facID=25 ], a meteorologist at the University of Oklahoma and author of “Tornado Alley [ http://books.google.com/books?id=jajQ6YsesYoC&pg=PR6&lpg=PR6&dq=bluestein+tornadoes+fax&source=bl&ots=ZOq8YSNAxS&sig=G6xct5foQ2LtjK73f9kic3xnSWE&hl=en&ei=OLq6TZzyIJS4tweDzoHDBQ&sa=X&oi=book_result&ct=result&resnum=2&ved=0CCAQ6AEwAQ#v=onepage&q&f=false ]”:
We simply do not understand the relationship between tornado intensity and the environment. We do know that supercells, which spawn the most intense tornadoes, need strong vertical wind shear and adequate buoyant energy. I think that Jeff Trapp (at Purdue) has done some studies showing how vertical shear and buoyant energy (CAPE) may vary in the future based on climate models, but even if the climate models were correct, we simply don’t know how the change in the environment would affect tornado intensity. I suggested doing such a study a number of years ago, but my colleague Kevin Trenberth questioned the ability of climate models to deal well enough with moisture, so that the estimates of buoyant energy would be seriously questioned.
I asked Harold Brooks [ http://www.nssl.noaa.gov/users/brooks/public_html/ ] of the National Severe Storms Laboratory (who provided the data on tornado trends) to explain the changes in tornado reporting over the decades that produce the distortions in the graph above:
The primary changes appear to occur ~1975, most likely as a result of the retrospective rating process that assigned ratings to tornadoes prior to the near-real-time ratings that began when the [National Weather Service] adopted the F-scale operationally in the mid-1970s, and ~2000, for reasons that aren’t completely clear, but are likely due to an increased emphasis on examining construction details and policies that changed the nature in how the ratings are created for the strongest tornadoes. Both have lead to a decrease in probability of a tornado being very strong, given that it’s strong. It’s possible that there’s a meteorological component, but the reporting practice changes are large enough that I don’t think we can pull a physical signal out, even if it’s fairly large.
Here’s Brooks on climate change:
As far as climate change and tornadoes go, there’s no clear expectation. The two most important large-scale variables severe thunderstorms are convective available potential energy (CAPE-a thermodynamic measure) and the vertical wind shear (magnitude of the difference between the horizontal winds near the surface and aloft, say, near 6 km above the ground.) A zeroth order expectation for the changes in the mean behavior of those two is that the favorable thermodynamics will increase as surface temperature increases and the favorable wind shear will decrease, as the equator-to-pole temperature gradient decreases. The models indicate that this will occur over the US, although there are sub-continental scale regional variations, and the signal in the thermodynamics is weak in many other regions of the globe. The models support the notion that the increase in thermodynamics will be greater than the decrease in shear and that the frequency of favorable conditions for severe thunderstorms will increase over the US, east of the Rockies.
There are important caveats, however. First, it is possible that initiation of thunderstorms won’t follow that trend (we can’t model it well at this point) and that the atmosphere won’t ‘take advantage’ of the favorable conditions at the same rate as it does now. For tornadoes, a second important issue is that tornadoes are much more dependent on wind shear that non-tornadic severe storms. As a result, the increased thermo/decreased shear implies that the fraction of storms that are tornadic could decrease. How that compares to a increase in the base rate of severe storms is unknown, but does imply tornado frequency is less likely to increase than non-severe storms.
As far as intensity goes, there’s only been a little work done and even less published. In the US, the vertical wind shear is a strong factor in estimating intensity, with stronger tornadoes associated with higher wind shear. It would appear, then, that the distribution of tornadoes by intensity could shift slightly to less intensity at the high end. It’s not clear that the differences would be large enough to detect without very large datasets and a stable reporting system.
There’s been some work done on hail in other countries (China, Italy, France) that suggests that hail occurrence has decreased in China because of the raising of the level in the atmosphere at which freezing temperatures are reached and that the size distribution of hail has shifted slightly to larger hail over the last 25-40 years (Italy, France.) A possible consistent explanation is that the CAPE has increased, allowing larger hail to form, but the raising of the freezing level allows for longer melting, with the smallest stones, that fall more slowly, preferentially melting the most.
Kevin Trenberth [ http://www.cgd.ucar.edu/cas/trenbert.html ], National Center for Atmospheric Research:
In Congressional testimony is 2009 I wrote:
"The record breaking numbers of tornadoes and deaths in the U.S. in 2008 probably also has a modest global warming component. Tornadoes are most common in the spring and early summer in weather systems moving across the U.S. that bring warm moist low-level air flowing from the Gulf of Mexico into the storms, while drier westerly winds aloft create wind shear that leads to rotation and thus tornadic thunderstorms. Because the Gulf air is warmer and moister than it would otherwise have been 30 or more years ago, the instability of these storms is enhanced. The effect is not measurable owing to the nature of tornado statistics which mainly reflect increasing numbers of people in more places."
Not sure what you have got but a few comments.
1) Statistics on tornadoes are unreliable and exhibit spurious upward trends that are known to correspond to more people being in more places to see them. We have comments on this in the IPCC report. There have been some attempts to adjust them but none are entirely satisfactory.
2) As Howie notes, the large-scale characteristics needed for tornadoes are reasonably known: strong buoyancy (high temps and plenty of moisture) at low levels, and wind shear that creates the vorticity for rotation. These come together in the US east of the Rockies more than any where else: warm moist air out to the Gulf (Southerly wind component), with westerlies aloft and air that is dry and which has come over the Rockies. So the wind shear is present, and so is instability in the atmosphere.
3) The juxtaposition of these 2 things varies a lot from year to year and storm to storm. If I recall correctly it was two winters ago in 2008 when these conditions became optimal and the US had an all time record number of tornadoes and deaths from tornadoes. It was La Nina conditions but the storm track was just right to hook up with the moisture in the Gulf. [Further south and you lose the wind shear; further north and you lose the connection to the moisture]
4) The effects of climate change are to raise the availability of moisture and buoyant air out of the Gulf, as sea surface temperatures rise. This was a factor in the big outbreaks. But the effects on wind shear and storm tracks are less clear.
5) Tornadoes are not resolved in climate models nor are the severe thunderstorms from which they emanate. There have been some studies on possible changes in tornadoes based on model environmental changes, mainly by David Karoly when he was at Oklahoma.
He had a paper with Marsh, and Brooks in ASL 2007 (abstract):
"Annual and seasonal cycles of convectively important atmospheric parameters for North America have been computed using the Community Climate System Model version 3 (CCSM3) Global Climate Model using a decade of CCSM3 data. Results for the spatial and temporal distributions of environments conducive to severe convective weather qualitatively agree with observational estimates from NCAR/NCEP global reanalyses, although the model underestimates the frequency of occurrence of severe weather environments. This result demonstrates the possibility for future studies aimed at determining possible changes in the distribution of severe weather environments associated with global climate change."
Hence the biases in climate models in reproducing the basic conditions ripe for tornadoes is not as well reproduced as one would like, and how one “corrects” for those biases (downscales) is subject to problems.
6) Rit Carbone at NCAR (a mesoscale and radar meteorologist expert) has raised the question about whether the environmental conditions might change in ways to greatly reduce the strong summer half year diurnal cycle and associated storms, by dealing with other details not resolved by climate models.
Chuck Doswell [ http://www.cimms.ou.edu/~doswell/ ], meteorologist, University of Oklahoma:
There have been some studies using numerical models that have attempted to assess the effect of climate change on tornadoes by looking at model forecast changes in some of the ingredients for severe weather – I’m sure Harold can help acquaint you with that work. But I believe the jury continues to be out regarding just what the effect of climate change will be on tornadoes. It’s not an easy issue to resolve – there are many steps between the global climate and the distribution of tornadoes in space and time, and important causal linkages along the way remain unknown….
Given the formidable challenges of detecting reliable signals in severe weather occurrence data – some of us have published a fair amount in the scientific literature about these challenges – I’d be extremely doubtful about deducing ANYTHING connecting tornado occurrences with climate change. The data have big problems and I think it would be correct to say that no one knows the TRUE distribution of significant tornadoes in space and time. To assert that we’re seeing the effects of climate change on that (unknown) distribution would be irresponsible science.
Tom Grazulis [ http://www.tornadoproject.com/tornproj/tornproj.htm ], a meteorologist, tornado historian and the author of “The Tornado: Nature’s Ultimate Windstorm [ http://books.google.com/books?id=3zEYILW2MJIC&printsec=frontcover&dq=grazulis+tornadoes&source=bl&ots=V6Yw8bbqNo&sig=rDdZQOyYWZW2kHUWRuehTfy-yoc&hl=en&ei=wby6Tcy5DtCUtwe68ZHbBQ&sa=X&oi=book_result&ct=result&resnum=5&ved=0CEEQ6AEwBA#v=onepage&q=grazulis%20tornadoes&f=false ]“:
I am well into a multi-year effort to improve and update the big green book — especially the path lengths of F3-F5 tornadoes back to 1880 (lots of travel needed). IMHO, F3-4-5 path length offers the only hope for finding trends in tornado activity that might link to overall climate change. F0, F1 and F2 events are completely useless for several reasons…. The EF-scale [Enhanced Fujita scale] has made it even more hopeless at the F2 level. Go to F3, for any F2 effort is doomed to failure.
But today, at least let me note that the official pre-1975 list of F2 tornadoes contain about 2000 events that are totally bogus…. overrated… Mostly the official “home unroofed” notes are merely missing shingles or the corner of the roof torn off ….. “House destroyed” is often “unanchored and house shifted.” These extra bogus F2’s were noted and graphed in my 1991 Symposium paper “Significant tornadoes over 110 years” page 470. NSSFC accepted my F4/F5 changes into the official data base, but would not go along with so many F2 corrections.
IMHO, the warming climate is reducing the overall risk to intense tornadoes in the United States, based on total path length of (F3-F4-F5) “intense” across the past 100 years. I have no mechanism, and the credibility of the data would be easy to rip apart… and very hard to defend.
Kerry Emanuel, a Massachusetts Institute of Technology meteorologist focused on climate change and hurricanes (but who has also studied tornadoes):
The question about trends tornadoes comes up frequently in discussions among insurers. You certainly contacted the right people about this… Chuck Doswell and Tom Grazulis. Way back in the 70’s I used to work on tornadoes and participated in a field project to get high quality film, chasing tornadoes around Oklahoma and Texas with Howie Bluestein.
I thoroughly concur with Chuck that detecting real trends is pretty much hopeless. Tom’s idea of looking at track length does bear some thought, but I suspect that it too is susceptible to changing technology for documenting storm damage and changing patterns of construction. Tornadoes are more likely to damage buildings than to leave much trace in an open plain or wheat field, so increasing suburban sprawl might increase the apparent path length of damage.
Climate trends in severe thunderstorms have not received much attention; Harold Brooks’ paper on this subject is the only one I know about. There is some chance of detecting real trends in the occurrence of hail, since hail is easily detected with radar, especially since the establishment of NEXRAD in 1987. There is a project underway to detect such trends, conducted by Travis Smith at NOAA.
Walker Ashley of Northern Illinois University:
I agree with the conclusions of Dr. Brooks regarding climate change and tornado frequency/severity linkage. A former graduate student and I have performed some research (in review at this time so I’m reluctant to provide the full results) that indicates that the environments that produce storms that have a greater propensity to generate tornadoes have not changed since at least the late 1970s in the U.S.
From my perspective, we are focusing on the wrong relationship. The heart of the matter lies with a growing and increasingly vulnerable population. That is what is driving “disasters”. In my opinion, a possible global climate change-induced increase of a percent or two here or there in the number of tornadoes/hurricanes/*enter your favorite hazard here* is orders of magnitude smaller (in terms of a problem) in comparison to vulnerability issues. I believe Dr. Roger Pielke Jr.’s research spells this out very well. It might be worth obtaining his perspective.
Ashley provided these links to papers on factors contributing to deadly outcomes in the tornado zone:
Ashley, W. S., 2007: Spatial and temporal analysis of tornado fatalities in the United States: 1880-2005. Weather and Forecasting, 22, 1214-1228. [PDF [ http://chubasco.niu.edu/pubs/Ashley%202007%20WAF.pdf ]] (Maps 7.c, 8.b, and panel 9 speak to mobile homes.)
Ashley, W. S., A. J. Krmenec, and R. Schwantes, 2008: Vulnerability due to nocturnal tornadoes. Weather and Forecasting, 23, 795-807. [PDF [ http://chubasco.niu.edu/pubs/Ashley%20et%20al.%202008%20WAF.pdf ]] (I speak to the mobile home issue at night on page 805 — last paragraph of results.)
[1:27 p.m. | Updated A reader, Brad Barrett, pointed out that the word "unknowable" in my headline clashes [ http://community.nytimes.com/comments/dotearth.blogs.nytimes.com/2011/04/29/killer-tornadoes-horrible-and-still-unknowable/?permid=2#comment2 ] with the greatly increased skill forecasters have shown in identifying zones ripe for outbreaks of powerful tornadoes, and he's right. I was reflecting on the lack of overarching links between climate trends and tornado behavior.]
© 2011 The New York Times Company
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(items linked in) http://investorshub.advfn.com/boards/read_msg.aspx?message_id=62621956 and preceding (and future following)
By Andrew Freedman
Published: April 29th, 2011, Last Updated: April 30th, 2011
With at least 340 dead from dozens of tornadoes [ http://news.yahoo.com/s/ap/us_severe_weather;_ylt=AuSrvk_IdkZe8YeNhOwD0a2s0NUE ], including at least 249 deaths in Alabama alone, the tornado outbreak that tore across the South on Wednesday was certainly one for the record-books. Storm surveys are still going on, but it appears likely that this outbreak spawned several violent tornadoes, of the EF-4 to EF-5 category on the Enhanced Fujita Scale [ http://www.spc.noaa.gov/faq/tornado/ef-scale.html ]. Already one tornado has been designated an EF-5 [ http://news.yahoo.com/s/ap/20110429/ap_on_re_us/us_severe_weather_tornado_rating ]. It destroyed much of the town of Smithville, Mississippi, killing at least 14. The tornado that decimated parts of Tuscaloosa and Birmingham, Ala. is likely to be rated at least an EF-4 [ http://forecast.weather.gov/product.php?site=BMX&product=PNS&issuedby=BMX ], with maximum winds of at least 165 mph.
Such tornadoes are capable of ripping a home off its foundation and leaving just a concrete slab behind. The 1974 "Super Outbreak", which was the most recent outbreak of a similar magnitude, killed 330 people and featured a whopping 24 F4 and F5 tornadoes, including the infamous Xenia, Ohio F5 tornado [ http://www.ohiohistory.org/etcetera/exhibits/swio/pages/albums/1974_tornado/1974_tornado_albumPage02.html ], which destroyed most of that Ohio town.
The images of the tornadoes and the destruction they have wrought have sparked plenty of questions: why did this outbreak occur, and why has April been such an active month for tornadoes? And, might global climate change be making tornadoes stronger or more frequent, or perhaps be shifting Tornado Alley out of the Great Plains and into more heavily populated areas of the South?
Those of us who write about climate change are often accused of attempting to link every unusual weather event to climate change, as if increasing air and ocean temperatures can explain everything from hurricanes to snowstorms. In this case, with the second-deadliest tornado outbreak in US history, and with the most tornadoes for any April since records began in the early 1950s, it's important to understand that the scientific evidence indicates that climate change probably played a very small role, if any, in stirring up this violent weather. This might disappoint some advocates [ http://www.huffingtonpost.com/peter-h-gleick/tornadoes-2011_b_855032.html ] who are already using this to highlight the risks of climate change-related extreme weather.
As I wrote at washingtonpost.com [ http://www.washingtonpost.com/blogs/capital-weather-gang/post/tornado-outbreak-for-the-record-books-how-did-deadly-destructive-event-happen-and-what-does-it-mean/2011/04/28/AFLQ942E_blog.html ], tornadoes form when a combination of ingredients are present in just the right amounts. These include: abundant moisture, winds that change speed and/or direction with height (also known as "wind shear"), and a trigger to force air to rise into towering thunderstorms — such as a cold front. All of these ingredients were present in abundance on Wednesday, and in fact they have come together again and again this month across the South, Southeast, and Carolinas. In fact, so much wind shear was present on Wednesday that nearly every thunderstorm that formed in Alabama and Mississippi showed signs of rotation, meaning they were capable of producing tornadoes, and aircraft approaching Atlanta's Hartsfield International Airport — the world's busiest — had to break off their landings [ http://www.washingtonpost.com/local/at-atlanta-airport-storms-create-harrowing-tests-for-pilots-and-controllers/2011/04/28/AFRNxX8E_story.html ] because of tailwinds that quickly turned into headwinds and back again, altering their airspeed and posing a serious risk of a serious accident.
A major factor in spawning all of the massive tornadoes was unusually powerful jet stream winds, which both helped trigger the storms and gave them a supercharged dose of wind shear to work with. The airflow in the upper atmosphere has steered numerous dips or "troughs" in the jet stream far to the south, helping to bring warm, moist air on a collision course with cold and dry air to the north.
One climate cycle that is being eyed as a contributor to the deadly April weather is La Niña, which is a natural cycle of climate variability that is characterized by cooler than average waters in the Pacific Ocean. Here's how I described La Niña's influence on tornado season over at the Capital Weather Gang blog:
Like its sibling El Niño, La Niña can influence weather patterns far from the Pacific Ocean. A study that examined the relationship between sea surface temperatures in the Pacific and the number of tornadoes in the U.S. found a weak correlation between La Niña and a greater number of tornadoes. Another study [ http://pielkeclimatesci.wordpress.com/2011/04/24/la-nina-and-tornado-outbreaks-in-the-usa/ ] found tornadoes during La Niña years had longer than average track lengths, more violent tornadoes, and a good probability of having an outbreak of 40 or more tornadoes. Brooks points out that both the Palm Sunday tornado outbreak in 1965 [ http://www.crh.noaa.gov/dtx/palmsunday/ ] and the Super Outbreak in 1974 [ http://www.april31974.com/ ] occurred during La Niña years.
Another broader factor that may be aiding and abetting the destructive weather is a very warm Gulf of Mexico, where sea surface temperatures have been between 1 and 2.5°C above average for this time of year. This is important because it means there is more moisture flowing northward off the Gulf, and a humid environment is necessary for severe thunderstorms to form. As I wrote earlier this week:
“Tornado Alley” owes its existence in part to humid air transported northward from the Gulf of Mexico, since without this moisture, you would not have the combustible clash of warm, humid air and cool, dry air masses in the Plains. Farther east, into “Dixie Alley [ http://voices.washingtonpost.com/capitalweathergang/2011/03/re-defining_tornado_alley.html ]”, the Gulf also serves as the fuel for many tornadic storms.
How Might Climate Change Affect Tornadoes?
Climate change is already changing the environment in which severe thunderstorms and their associated tornadoes form, and it's bound to have some sort of influence on tornado frequency or strength. But as of now, no discernible trend has been detected in the observational data, and studies of how tornadoes will fare in a warmer world show somewhat conflicting results.
While overall tornado counts have been increasing in the U.S. in recent years, the number of strong tornadoes has actually been going down, leading researchers to conclude that they are catching more of the weaker tornadoes that are far more common than the mile-wide "wedge" tornadoes that decimated communities like Tuscaloosa, Ala. this week. The trends in tornado counts is likely a result of improvements in observational technology, such as Doppler radar networks and a national network of trained storm spotters. Also, historical tornado data is not considered very reliable, since it's thought that many tornadoes — particularly weaker ones — were missed in the early decades of record-keeping, and construction methods have also changed with time. This is important since the strength of a tornado is determined by a damage survey, rather than actual instrument measurements of tornadic winds (no wind-recording instrument has ever survived a direct hit by a tornado).
As I reported for the Capital Weather Gang:
As for global warming, this is an active area of scientific research, with some conflicting projections so far about whether a warming atmosphere will make it more or less likely that tornadoes will form. Since more moisture gets added to the atmosphere as the climate warms, additional water vapor may help severe thunderstorms and tornadoes to form. On the other hand, wind shear is expected to decline due to climate change, which would argue against an increase in tornado numbers.
According to some studies, though, by the end of the present century, the added water vapor will be enough to overcome the lower wind shear, and create more opportunities for severe thunderstorms to form.
Tornadoes are a bigger wild card for climate scientists than other types of extreme weather and climate events, such as heat waves and flooding. (Studies have consistently found that both of these hazards will occur more frequently and severely as the world warms.)
Online Resources:
Studies: A 2008 paper [ http://www.agu.org/pubs/crossref/2008/2008EO530001.shtml ] in EOS Transactions, a publication of the American Geophysical Union, discussed challenges involved in analyzing tornado trends and climate change.
This 2009 paper [ http://www.stanford.edu/~omramom/Trapp_GRL_09.pdf ] in Geophysical Research Letters found that climate change may make many parts of the U.S. more susceptible to severe thunderstorms, and possibly to tornadoes as well.
This 2007 study [ http://www.pnas.org/content/104/50/19719.short ] in Proceedings of the National Academy of Sciences found that severe thunderstorms may become more frequent as the climate warms, particularly in areas close to the Gulf of Mexico and along the Atlantic Seaboard.
Weather Underground chief meteorologist Jeff Masters detailed these studies and others in a 2008 post [ http://www.wunderground.com/blog/JeffMasters/comment.html?entrynum=910&tstamp=200802 ] on this topic.
New York Times blogger Andy Revkin has extended comments [ http://dotearth.blogs.nytimes.com/2011/04/29/killer-tornadoes-horrible-and-still-unknowable/ (below)] from some of the top researchers studying climate change and tornadoes.
Copyright © 2011 Climate Central
http://www.climatecentral.org/blogs/tornado-outbreak-raises-climate-change-questions/ [no comments yet]
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Killer Tornadoes, Horrible and Still Unknowable
By ANDREW C. REVKIN
April 29, 2011, 10:27 am
The awesome power of great tempests is bad enough. But the terrifying destructiveness of the hundreds of roaring funnels that have ripped apart dozens of communities [ http://www.nytimes.com/interactive/2011/04/28/us/map-of-the-tornadoes-across-the-south.html ] this year in the worst tornado season in decades is unimaginable except for those who’ve experienced it. *
My heart goes out, particularly, to people who, out of poverty or lack of awareness, live without access to a basement or shelter — particularly those who live in trailer parks. As Walker Ashley, a meteorologist at Northern Illinois University, found in 2008, 44 percent of deaths in tornadoes [ http://www.niu.edu/PubAffairs/RELEASES/2008/feb/tornado.shtml ] occur in mobile homes [1:27 p.m. | Updated He just told me it's up to about 50 percent now.]
And, as Kirk Johnson discusses in an excellent overview of the mix of hazard and vulnerability in Tornado Alley, the fast growth in many parts of the South has generated precisely the wrong kind of housing in the wrong kinds of places [ http://www.nytimes.com/2011/04/29/us/29tornadoes.html ].
Johnson’s piece, sifting for a cause for this terrible storm season, explores the climate-change question and finds no easy answer. (The story I reported in 2002 from the wreckage left by an F4* [ http://www.nytimes.com/2002/05/07/science/finding-the-what-when-where-and-why-of-the-supertwister.html ] tornado in La Plata, Md., has more of the basic science.) While there’s evidence that increasing greenhouse heating of the planet is exacerbating hot spells and extreme downpours, and may be related to hurricane intensity (but not frequency), a combination of imprecise records and deep complexity in the mix of forces that generate killer tornadoes has clouded any link to global warming. Anyone implying such a connection is in the spin zone [ http://humanitariannews.org/20110428/peter-h-gleick-cost-denying-climate-change-accelerating-climate-disruptions-death-destructi ].
There’s certainly no reliable trend in the categories of tornadoes that matter — those designed 2 to 5 on the Fujita scale of twister ferocity. The graph below, of data from the National Severe Storms Laboratory in Norman, Okla., shows trends over the last 60 years. The big increases in weak tornadoes and slight dip in the F2 class are thought to be mainly the result of changes in reporting methods and also a far greater reporting rate as the South has grown more populous and radar has helped identify tornadic storms.
Trends in reported tornadoes by intensity on the Fujita scale (F0 weakest; F5 worst).
NOAA
Last January, when there was a rare winter tornado outbreak, and some talk of human-driven global warming playing a role, I consulted a batch of meteorologists and climate scientists who have studied trends in the categories of tornadoes that kill people, which are those designated F2 through F5 on the five-step Fujita scale of intensity (gauged by the amount and type of damage that is wrought).
I’ve strung their thoughts at the end of this post for those wishing to dig deeper.
Despite a lot of new technology and intensive research, including flying sturdy aircraft in and around potent thunderstorms (I got to go on one such flight long ago [ http://lh5.googleusercontent.com/_gmR8fkmAnjw/Tbq1GbLhhqI/AAAAAAAAD4M/7ni8_Us3Wzw/RevkinStormFlight.jpg ]), the mix of conditions that produce these monstrous funnels remain beyond human capacity to understand.
The bottom line is very similar to that in earthquake country. There are regions of clear risk, and practices — like building trailer parks or shoddy houses in known danger zones — that are guaranteed to produce high losses. But knowing when a particular quake or killer funnel strikes remains impossible.
[3:32 p.m. | Updated Roger Pielke, Jr. [ http://rogerpielkejr.blogspot.com/2011/04/bill-hooke-on-tornadoes.html ], drew my attention to an important recent post by Bill Hooke [ http://www.livingontherealworld.org/?p=257 ], the director of the American Meteorological Society's policy program, on the steady rise in vulnerability in America's tornado hot zone. The takeaway line?
Tornadoes hitting downtown areas in the past? Rare – almost unheard of. But tornadoes hitting downtown areas in the future? Increasingly common.]
The biggest risk, for the moment, still lies in mobile home parks. That means that the imperative in tornado country is to get far more serious about toughening standards for shelters and, particularly, trailer parks. I’m trying to find out whether “tie downs” for mobile homes — straps that secure them to the ground — have mad much of a difference in survivability.
Please post any info on research along these lines. For the moment, trailer park residents seem quite concerned [ http://www.topix.com/forum/city/greenville-ky/TP6M48NVJSSVPIUUR ] about trailers lacking this added protection.
For weather and climate geeks, here’s that roundup of meteorologists on tornadoes and climate change (jargon alert):
Howard Bluestein [ http://som.ou.edu/peopleDetails.php?facID=25 ], a meteorologist at the University of Oklahoma and author of “Tornado Alley [ http://books.google.com/books?id=jajQ6YsesYoC&pg=PR6&lpg=PR6&dq=bluestein+tornadoes+fax&source=bl&ots=ZOq8YSNAxS&sig=G6xct5foQ2LtjK73f9kic3xnSWE&hl=en&ei=OLq6TZzyIJS4tweDzoHDBQ&sa=X&oi=book_result&ct=result&resnum=2&ved=0CCAQ6AEwAQ#v=onepage&q&f=false ]”:
We simply do not understand the relationship between tornado intensity and the environment. We do know that supercells, which spawn the most intense tornadoes, need strong vertical wind shear and adequate buoyant energy. I think that Jeff Trapp (at Purdue) has done some studies showing how vertical shear and buoyant energy (CAPE) may vary in the future based on climate models, but even if the climate models were correct, we simply don’t know how the change in the environment would affect tornado intensity. I suggested doing such a study a number of years ago, but my colleague Kevin Trenberth questioned the ability of climate models to deal well enough with moisture, so that the estimates of buoyant energy would be seriously questioned.
I asked Harold Brooks [ http://www.nssl.noaa.gov/users/brooks/public_html/ ] of the National Severe Storms Laboratory (who provided the data on tornado trends) to explain the changes in tornado reporting over the decades that produce the distortions in the graph above:
The primary changes appear to occur ~1975, most likely as a result of the retrospective rating process that assigned ratings to tornadoes prior to the near-real-time ratings that began when the [National Weather Service] adopted the F-scale operationally in the mid-1970s, and ~2000, for reasons that aren’t completely clear, but are likely due to an increased emphasis on examining construction details and policies that changed the nature in how the ratings are created for the strongest tornadoes. Both have lead to a decrease in probability of a tornado being very strong, given that it’s strong. It’s possible that there’s a meteorological component, but the reporting practice changes are large enough that I don’t think we can pull a physical signal out, even if it’s fairly large.
Here’s Brooks on climate change:
As far as climate change and tornadoes go, there’s no clear expectation. The two most important large-scale variables severe thunderstorms are convective available potential energy (CAPE-a thermodynamic measure) and the vertical wind shear (magnitude of the difference between the horizontal winds near the surface and aloft, say, near 6 km above the ground.) A zeroth order expectation for the changes in the mean behavior of those two is that the favorable thermodynamics will increase as surface temperature increases and the favorable wind shear will decrease, as the equator-to-pole temperature gradient decreases. The models indicate that this will occur over the US, although there are sub-continental scale regional variations, and the signal in the thermodynamics is weak in many other regions of the globe. The models support the notion that the increase in thermodynamics will be greater than the decrease in shear and that the frequency of favorable conditions for severe thunderstorms will increase over the US, east of the Rockies.
There are important caveats, however. First, it is possible that initiation of thunderstorms won’t follow that trend (we can’t model it well at this point) and that the atmosphere won’t ‘take advantage’ of the favorable conditions at the same rate as it does now. For tornadoes, a second important issue is that tornadoes are much more dependent on wind shear that non-tornadic severe storms. As a result, the increased thermo/decreased shear implies that the fraction of storms that are tornadic could decrease. How that compares to a increase in the base rate of severe storms is unknown, but does imply tornado frequency is less likely to increase than non-severe storms.
As far as intensity goes, there’s only been a little work done and even less published. In the US, the vertical wind shear is a strong factor in estimating intensity, with stronger tornadoes associated with higher wind shear. It would appear, then, that the distribution of tornadoes by intensity could shift slightly to less intensity at the high end. It’s not clear that the differences would be large enough to detect without very large datasets and a stable reporting system.
There’s been some work done on hail in other countries (China, Italy, France) that suggests that hail occurrence has decreased in China because of the raising of the level in the atmosphere at which freezing temperatures are reached and that the size distribution of hail has shifted slightly to larger hail over the last 25-40 years (Italy, France.) A possible consistent explanation is that the CAPE has increased, allowing larger hail to form, but the raising of the freezing level allows for longer melting, with the smallest stones, that fall more slowly, preferentially melting the most.
Kevin Trenberth [ http://www.cgd.ucar.edu/cas/trenbert.html ], National Center for Atmospheric Research:
In Congressional testimony is 2009 I wrote:
"The record breaking numbers of tornadoes and deaths in the U.S. in 2008 probably also has a modest global warming component. Tornadoes are most common in the spring and early summer in weather systems moving across the U.S. that bring warm moist low-level air flowing from the Gulf of Mexico into the storms, while drier westerly winds aloft create wind shear that leads to rotation and thus tornadic thunderstorms. Because the Gulf air is warmer and moister than it would otherwise have been 30 or more years ago, the instability of these storms is enhanced. The effect is not measurable owing to the nature of tornado statistics which mainly reflect increasing numbers of people in more places."
Not sure what you have got but a few comments.
1) Statistics on tornadoes are unreliable and exhibit spurious upward trends that are known to correspond to more people being in more places to see them. We have comments on this in the IPCC report. There have been some attempts to adjust them but none are entirely satisfactory.
2) As Howie notes, the large-scale characteristics needed for tornadoes are reasonably known: strong buoyancy (high temps and plenty of moisture) at low levels, and wind shear that creates the vorticity for rotation. These come together in the US east of the Rockies more than any where else: warm moist air out to the Gulf (Southerly wind component), with westerlies aloft and air that is dry and which has come over the Rockies. So the wind shear is present, and so is instability in the atmosphere.
3) The juxtaposition of these 2 things varies a lot from year to year and storm to storm. If I recall correctly it was two winters ago in 2008 when these conditions became optimal and the US had an all time record number of tornadoes and deaths from tornadoes. It was La Nina conditions but the storm track was just right to hook up with the moisture in the Gulf. [Further south and you lose the wind shear; further north and you lose the connection to the moisture]
4) The effects of climate change are to raise the availability of moisture and buoyant air out of the Gulf, as sea surface temperatures rise. This was a factor in the big outbreaks. But the effects on wind shear and storm tracks are less clear.
5) Tornadoes are not resolved in climate models nor are the severe thunderstorms from which they emanate. There have been some studies on possible changes in tornadoes based on model environmental changes, mainly by David Karoly when he was at Oklahoma.
He had a paper with Marsh, and Brooks in ASL 2007 (abstract):
"Annual and seasonal cycles of convectively important atmospheric parameters for North America have been computed using the Community Climate System Model version 3 (CCSM3) Global Climate Model using a decade of CCSM3 data. Results for the spatial and temporal distributions of environments conducive to severe convective weather qualitatively agree with observational estimates from NCAR/NCEP global reanalyses, although the model underestimates the frequency of occurrence of severe weather environments. This result demonstrates the possibility for future studies aimed at determining possible changes in the distribution of severe weather environments associated with global climate change."
Hence the biases in climate models in reproducing the basic conditions ripe for tornadoes is not as well reproduced as one would like, and how one “corrects” for those biases (downscales) is subject to problems.
6) Rit Carbone at NCAR (a mesoscale and radar meteorologist expert) has raised the question about whether the environmental conditions might change in ways to greatly reduce the strong summer half year diurnal cycle and associated storms, by dealing with other details not resolved by climate models.
Chuck Doswell [ http://www.cimms.ou.edu/~doswell/ ], meteorologist, University of Oklahoma:
There have been some studies using numerical models that have attempted to assess the effect of climate change on tornadoes by looking at model forecast changes in some of the ingredients for severe weather – I’m sure Harold can help acquaint you with that work. But I believe the jury continues to be out regarding just what the effect of climate change will be on tornadoes. It’s not an easy issue to resolve – there are many steps between the global climate and the distribution of tornadoes in space and time, and important causal linkages along the way remain unknown….
Given the formidable challenges of detecting reliable signals in severe weather occurrence data – some of us have published a fair amount in the scientific literature about these challenges – I’d be extremely doubtful about deducing ANYTHING connecting tornado occurrences with climate change. The data have big problems and I think it would be correct to say that no one knows the TRUE distribution of significant tornadoes in space and time. To assert that we’re seeing the effects of climate change on that (unknown) distribution would be irresponsible science.
Tom Grazulis [ http://www.tornadoproject.com/tornproj/tornproj.htm ], a meteorologist, tornado historian and the author of “The Tornado: Nature’s Ultimate Windstorm [ http://books.google.com/books?id=3zEYILW2MJIC&printsec=frontcover&dq=grazulis+tornadoes&source=bl&ots=V6Yw8bbqNo&sig=rDdZQOyYWZW2kHUWRuehTfy-yoc&hl=en&ei=wby6Tcy5DtCUtwe68ZHbBQ&sa=X&oi=book_result&ct=result&resnum=5&ved=0CEEQ6AEwBA#v=onepage&q=grazulis%20tornadoes&f=false ]“:
I am well into a multi-year effort to improve and update the big green book — especially the path lengths of F3-F5 tornadoes back to 1880 (lots of travel needed). IMHO, F3-4-5 path length offers the only hope for finding trends in tornado activity that might link to overall climate change. F0, F1 and F2 events are completely useless for several reasons…. The EF-scale [Enhanced Fujita scale] has made it even more hopeless at the F2 level. Go to F3, for any F2 effort is doomed to failure.
But today, at least let me note that the official pre-1975 list of F2 tornadoes contain about 2000 events that are totally bogus…. overrated… Mostly the official “home unroofed” notes are merely missing shingles or the corner of the roof torn off ….. “House destroyed” is often “unanchored and house shifted.” These extra bogus F2’s were noted and graphed in my 1991 Symposium paper “Significant tornadoes over 110 years” page 470. NSSFC accepted my F4/F5 changes into the official data base, but would not go along with so many F2 corrections.
IMHO, the warming climate is reducing the overall risk to intense tornadoes in the United States, based on total path length of (F3-F4-F5) “intense” across the past 100 years. I have no mechanism, and the credibility of the data would be easy to rip apart… and very hard to defend.
Kerry Emanuel, a Massachusetts Institute of Technology meteorologist focused on climate change and hurricanes (but who has also studied tornadoes):
The question about trends tornadoes comes up frequently in discussions among insurers. You certainly contacted the right people about this… Chuck Doswell and Tom Grazulis. Way back in the 70’s I used to work on tornadoes and participated in a field project to get high quality film, chasing tornadoes around Oklahoma and Texas with Howie Bluestein.
I thoroughly concur with Chuck that detecting real trends is pretty much hopeless. Tom’s idea of looking at track length does bear some thought, but I suspect that it too is susceptible to changing technology for documenting storm damage and changing patterns of construction. Tornadoes are more likely to damage buildings than to leave much trace in an open plain or wheat field, so increasing suburban sprawl might increase the apparent path length of damage.
Climate trends in severe thunderstorms have not received much attention; Harold Brooks’ paper on this subject is the only one I know about. There is some chance of detecting real trends in the occurrence of hail, since hail is easily detected with radar, especially since the establishment of NEXRAD in 1987. There is a project underway to detect such trends, conducted by Travis Smith at NOAA.
Walker Ashley of Northern Illinois University:
I agree with the conclusions of Dr. Brooks regarding climate change and tornado frequency/severity linkage. A former graduate student and I have performed some research (in review at this time so I’m reluctant to provide the full results) that indicates that the environments that produce storms that have a greater propensity to generate tornadoes have not changed since at least the late 1970s in the U.S.
From my perspective, we are focusing on the wrong relationship. The heart of the matter lies with a growing and increasingly vulnerable population. That is what is driving “disasters”. In my opinion, a possible global climate change-induced increase of a percent or two here or there in the number of tornadoes/hurricanes/*enter your favorite hazard here* is orders of magnitude smaller (in terms of a problem) in comparison to vulnerability issues. I believe Dr. Roger Pielke Jr.’s research spells this out very well. It might be worth obtaining his perspective.
Ashley provided these links to papers on factors contributing to deadly outcomes in the tornado zone:
Ashley, W. S., 2007: Spatial and temporal analysis of tornado fatalities in the United States: 1880-2005. Weather and Forecasting, 22, 1214-1228. [PDF [ http://chubasco.niu.edu/pubs/Ashley%202007%20WAF.pdf ]] (Maps 7.c, 8.b, and panel 9 speak to mobile homes.)
Ashley, W. S., A. J. Krmenec, and R. Schwantes, 2008: Vulnerability due to nocturnal tornadoes. Weather and Forecasting, 23, 795-807. [PDF [ http://chubasco.niu.edu/pubs/Ashley%20et%20al.%202008%20WAF.pdf ]] (I speak to the mobile home issue at night on page 805 — last paragraph of results.)
[1:27 p.m. | Updated A reader, Brad Barrett, pointed out that the word "unknowable" in my headline clashes [ http://community.nytimes.com/comments/dotearth.blogs.nytimes.com/2011/04/29/killer-tornadoes-horrible-and-still-unknowable/?permid=2#comment2 ] with the greatly increased skill forecasters have shown in identifying zones ripe for outbreaks of powerful tornadoes, and he's right. I was reflecting on the lack of overarching links between climate trends and tornado behavior.]
© 2011 The New York Times Company
http://dotearth.blogs.nytimes.com/2011/04/29/killer-tornadoes-horrible-and-still-unknowable/ [with comments]
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