September 1, 2013

For the Record

Filed under: Severe — Erin @ 7:06 pm

The El Reno tornado (2013) was, in the official records, downgraded from EF-5 to EF-3 on the basis that EF-5 damage was not found and “the Enhanced Fujita scale is a damage scale” (quotation my own).  Let me go on record right now as saying that I oppose this and all other instances where scientifically collected, calibrated wind speed data are ignored.  I oppose the practice of rating tornadoes based strictly on those factors that civil engineers deem important while throwing out data collected by meteorologists, and for several reasons.

  1. Estimates of wind speed that are derived ex post facto from damage are inherently less reliable than objective, instrumentally collected measurements.  This should not even be controversial.  Differences in materials, building practices (which can be very hard to determine in the event of total obliteration), and even environmental factors (e.g., temperature and humidity) prior to a tornado can affect at what wind speed the structure fails.  Surveys attempt to find out about such things, but it’s inherently impossible to cover all bases.  Measurements are always more reliable than estimates, even educated ones.
  2. The Enhanced Fujita scale was designed to be expanded.  In practice, vehicular and ground damage are now included as damage indicators in surveys, even though the official EF scale documents don’t (to my knowledge) list them.  There was also the intention, when the scale was formed, of leaving it open for actual wind data to be used in ratings.
  3. The Enhanced Fujita scale is a wind scale.  It is not just a way of rating the intensity of damage, which need not have anything to do with wind at all.  The EF scale is not used for rating damage caused by floods, hailstorms, or earthquakes; it is used for tornadoes, which are wind events.  Tornado surveys do not merely say that a tornado “has produced EF-3 damage.”  They also assign an estimated numerical wind speed to the storm.  This is apparently a subtle point for those who insist that the EF scale is a “damage scale,” but I really don’t think it’s all that hard to understand once you think about it.  Saying that the EF scale is a damage scale is like saying that, traditionally, the Celsius scale was a mercury expansion scale, not a temperature scale, because mercury thermometers were used to determine temperature.  That would obviously be ridiculous.  The EF scale is a wind scale.  Primarily it uses damage for the determination of wind speeds, but only because measurement data are not usually gathered.  That unfortunate circumstance is no reason to throw out valid data when they are available.
  4. Portable Doppler wind measurements can, in fact, be extrapolated to the surface in tornadoes.  The wind speeds near the ground level (i.e., damage level) of a tornado are likely to either match or even exceed those found at heights measured by portable Doppler radar (Wurman et al, Bulletin of the AMS, June 2013).  Though the researchers cited didn’t measure a tornado with winds this high, the research implies that, yes, 300 mph winds could occur at the surface if they were measured at portable Doppler level.  The cited research is another reason why I have gotten off the fence and decided that 320 mph or higher winds could also theoretically occur at the surface in subvortices of the most violent tornadoes, such as, perhaps, the Hackleburg, AL tornado of 4/27/2011.

It is becoming increasingly clear to meteorologists that, although the categories of the EF scale are probably accurate as regards the intensity of wind required to damage structures in specific ways, the scale is grossly inadequate for measuring the highest possible winds that a tornado could produce.  There is little question that the most powerful EF-5 tornadoes can generate winds well in excess of 200-210 mph at the surface, especially if they are multivortex.  Surface winds of 300 mph in subvortices are also a near-definite, and there is quite a difference between 210 and 310 mph.  The former will reduce a well-built house to its foundation but could be survivable; there are accounts of people who sat through Category 5 hurricanes, which could generate wind gusts of that intensity.  The latter will shred the debris into pellets and tear the human body to pieces (see, for example, the Jarrell, TX tornado of 1997, but get your Pepto and smelling salts if you read detailed accounts of that).  The former could be ridden out in an above-ground shelter (the kind of shelter, incidentally, that some non-meteorologists involved in the creation of the EF scale had a financial stake in selling—just saying).  The latter requires an in-ground storm cellar with guard rails to hold.  I regard it as, frankly, grossly irresponsible for the public not to be informed of the true intensity that EF-5 tornadoes can reach or what such incredible winds can do.

I don’t blame the meteorologists at Norman for what happened.  They wanted to use hard data in the rating of the El Reno tornado, obviously.  There must have been pressure exerted from some other source.  I do hope, however, that weather scientists are soon able to force an official change in the procedure of rating tornadoes when calibrated, scientifically valid wind data are available.  One way to bring this change about more quickly is to increase funding for university meteorology departments so that they can send out chase teams equipped with portable Doppler.  Disregarding one or two sets of data, all from one small region, can apparently be done by the “powers that be.”  Disregarding data from all over Tornado Alley might not be doable.

June 7, 2013

Thoughts on Instrumental Measurements in Tornado Ratings

Filed under: Severe — Erin @ 10:32 pm

It’s been a while since I blogged anything.  I’ve decided that I do not really want to be a forecaster, but instead, a research meteorologist, and the war for funding is so intense that I’d much rather publish research in a scientific journal than on my blog.  However, this post is not research; it is commentary and speculation.  The opinions in it are no one’s but my own.

A controversy in meteorology has developed about the use of mobile Doppler wind data to rate tornadoes.  It flared up initially in 2011 when a tornado in El Reno, OK was rated EF5 purportedly because of mobile Doppler measurements.  However, it later came to light that the tornado had produced EF5 damage indicators along its path as well, including the hurling of very heavy oil tankers, the moving of equipment weighing a million pounds, and the intense scouring of dirt.  The controversy has arisen again, though.  At least two tornadoes in May 2013 had their ratings increased (rather significantly, I should add) strictly because of wind measurements.  The May 31 El Reno, OK tornado was increased from EF3 (from damage indicators) to EF5 because of a mobile Doppler measurement of 296 mph at 500 feet above ground level.  The Rozel, KS tornado was increased from EF2 to EF4 because of a wind measurement.

Some people seriously object to the use of instrumental readings in tornado ratings.  “The EF scale is a damage scale!” they say.  And, to an extent, it is.  However, that’s not all that it is.  In surveys, tornadoes are not simply said to have produced damage of a particular category.  Attached to each of the six ratings is a range of wind speeds that were determined, via engineering analysis, to produce such damage.  Surveys include an estimate of the wind speed of the tornado as well, and these wind speed estimates are often very specific.  I have seen surveys of EF4 tornadoes, for example, that distinguish between 170 and 190 mph winds.  Since the EF scale does not simply classify the level of damage produced by the tornado, but also includes numerical wind speeds for the tornado itself, I therefore have to come down on the side of those who use mobile Doppler and other calibrated, accurate forms of measurement to rate tornadoes.

However, there is a caveat.  I’m concerned about the use of mobile Doppler in areas like the Oklahoma City metro area resulting in a skewed picture of the distribution of EF4 and EF5 tornadoes.  They also are documented in areas that don’t happen to house the Storm Prediction Center, University of Oklahoma meteorology department, Norman OK National Weather Service Office, and National Severe Storms Laboratory.  However, if measurements of these tornadoes are never taken because of a lack of resources, they can be misrated.  The May 31 El Reno tornado was initially rated an EF3 from damage.  One cannot help but wonder how many tornadoes outside this Mecca of meteorology are misrated because there may not be a massive pool of storm chasers with state-of-the-art instruments.  Nevertheless, the proper course of action to correct for this is to fund more tornado research and wind-measuring equipment, not to sacrifice scientific accuracy on those occasions when we can obtain it.

The 296-mph winds in the El Reno tornado (at 500 ft.) were detected in a mesovortex.  This fact would also explain why, perhaps, some tornadoes are underrated; such small vortices might not strike anything if the path of the tornado is primarily unpopulated.  The outer funnel of the El Reno tornado had winds in the EF4 range, though again, at 500 feet.  Winds at the surface in the outer funnel may in fact have only been in the EF3 range, as the damage indicated.  However, this brings up several interesting points.

First, some meteorologists objected to the EF scale because they knew that the winds in EF5 tornadoes could reach speeds much faster than 200-210 mph, the range given in every damage survey for an EF5 tornado until the Joplin tornado.  They knew it from hard observations, including the mobile Doppler measurement of 300 +- 20 mph in the Bridge Creek tornado of 1999 and a measurement of 284 mph in the Red Rock tornado.  Now it seems that this was not just a pair of flukes; such extreme wind speeds may occur much more frequently in multivortex tornadoes than previously imagined, and not just those officially rated EF5.  The Red Rock tornado was rated F4 rather than F5 because wind measurements did not count in the old Fujita scale, and the 2013 El Reno tornado apparently didn’t produce demonstrable EF5 damage.

Second, I would bet that the usage of the EF scale, however accurate it is for below-EF5 winds, has resulted in some extremely inaccurate official wind estimates for EF5 tornadoes in surveys.  210 mph for the Smithville, MS and Hackleburg, AL tornadoes?  I do not believe that for a minute.  Now, I know that it is apparently not possible to distinguish between 200 and 250 mph on residential home damage alone, but if that much uncertainty exists, and if we know that tornadoes do indeed produce 250 mph winds at times, then I think damage surveys should not attempt to estimate a precise wind speed for an EF5 tornado from damage.  To do so implies a level of accuracy and surety that does not actually exist.

Finally, it is worth noting again that the 2013 El Reno tornado did not, apparently, produce demonstrable EF5 wind speeds in its outer funnel or its damage path, but a mesovortex inside the tornado nevertheless reached 296 mph.  This raises some serious questions about just how strong those mesovortices can become.  Now, many damage surveys for EF5 tornadoes note that the swath of EF5 damage was very small, a fact that indicates a mesovortex as the probable culprit.  One can be particularly confident in this if video exists of multiple vortices and the tornado’s path crossed over a developed area, as was tragically the case for the 2013 Moore, OK tornado.  However, what does that suggest for tornadoes that do produce wide swaths of EF5 damage along their paths, swaths too large to have been created by transient mesovortices and that were probably generated by the main funnel itself?  If the El Reno tornado generated an inner vortex spinning 110 mph faster than its main funnel, then I would be inclined to say that some multivortex (E)F5s that were rated on damage may in fact have generated “F6″-range winds (319+ mph) in their inner vortices.  (I say this with some trepidation, because there are few things more controversial and inflammatory in severe storms meteorology than the use of the term “F6.”)  I’m looking at the Hackleburg-Phil Campbell tornado and the Kemper-Philadelphia tornado (both of the April 27, 2011 super outbreak) in particular for this.  The former tornado had an uncommonly large path of EF5 damage, indicating that the main funnel may have reached EF5 levels; the latter had a small region in which the dirt was dug out of the ground to a depth of 2 feet, indicating the possibility of an inner vortex of truly incredible intensity.

I’ve personally been on the fence for a long time about whether such winds can occur on Earth–but this information about the El Reno tornado is edging me off that fence.  I doubt it could happen very often, of course.  I’m not suggesting that every EF4 or EF5 tornado is harboring an inner funnel with 330 mph winds at the surface.  This most assuredly is not the case.  Most EF5s earn their ratings not because of an EF5 damage swath attributable to the outer funnel, but because they do tend to be multivortex, and something had the misfortune of being struck by an inner vortex with EF5 winds.  But do I think 319+ mph winds could occur in a tornado that did have EF5 winds in its outer funnel?  Do I think they may have occurred before?  Honestly, at this point, I’m inclined to give a tentative yes.

December 30, 2011

Was the Joplin Tornado the Deadliest We Can Expect?

Filed under: Severe — Erin @ 10:58 pm

Meteorologists and weather-watchers are bidding the year 2011 a less-than-fond farewell.  While it was certainly a banner year from the point of view of storm chasing—6 EF-5 tornadoes, 17 EF-4s, and many of them highly photogenic, as the dozens of home videos on Youtube illustrate—it was a catastrophe in terms of the human impact.  With 552 fatalities, this year is tied for the second-deadliest tornado year in the U.S.  The death toll is an order of magnitude greater than even most of the “bad years” of the 1975-2010 period.  Two events are primarily responsible for this:  the April 27 Dixie Super Outbreak, which killed over 300 people (breaking the 1974 Ohio Valley Super Outbreak’s grim record by a hair), and the Joplin, MO EF-5 tornado, with approximately 160 fatalities.

With the 2011 Super Outbreak, meteorologists are starting to work out an approximate historical return period for these large-magnitude events.  Before the 1974 event, the last comparable event occurred in 1936, with an outbreak popularly known as the Tupelo-Gainesville outbreak for the violent tornadoes that occurred in Mississippi and Georgia.  It seems that these huge events occur approximately every 35-40 years.  Obviously, a comparable event could occur next spring, but statistically, it seems that they are a 35- to 40-year event.  And, given that the 1974 Super Outbreak and 2011 Super Outbreak saw comparable death tolls, I think we can also estimate what the human toll for such an event will unfortunately be as long as the affected communities have unsuitable safety options for EF-4 and EF-5 tornadoes.

The Joplin tornado is a different beast.  We do not have a comparable modern event.  Individual tornadoes in 1953 killed over 100 people in Waco, TX and Flint, MI, but that year was something of a catalyst of public outrage, for a third tornado in Worcester, MA killed 94 people.  Public sentiment that year was essentially, “DO something so that this never happens again!”  And for 57 years, no single tornado in the U.S. did kill over 100 people.  Then… it happened again.

Was the Joplin event a worst-case scenario?  Is this the deadliest (give or take) that a single tornado can actually be now?

I think the answer to the first question is a guarded “yes,” at least for the specific case of a tornado striking a city.  The tornado was about as strong as they come; its winds were estimated to be up to 250 mph.  They can get more intense than that, but it doesn’t make a lot of difference in terms of structural damage.  The tornado rapidly intensified precisely as it entered the heavily populated regions of Joplin, and it passed right through residential and commercial shopping areas—the worst areas it could strike.  Examination of the track shows that there was also a pretty large corridor of EF-4 and EF-5 tornado damage, which would be expected for a wedge tornado.  Sometimes the area of violent damage is comparatively small, but this was not the case with this tornado.  Storm cellars were rare in this area, making survival above ground mostly a matter of good luck.  The tornado was also rain-wrapped for much of its existence.  In terms of the storm’s power and the location of impact, you can’t get much worse than this.  However, I should note that it occurred on a Sunday.  Some have argued that if it had happened at the same time of day on a work day, it could have been worse.  We don’t know for sure, and let’s hope we don’t find out.  I tend to think it probably would not have been much worse, given that residential areas (not a likely area for commuters to be stranded) and the shopping district (which probably would get more foot traffic on weekends than work-week afternoons) were such a large part of the damage zone.  In my opinion, the Joplin tornado was essentially a worst-case scenario for a tornado striking an urban area.  A comparable tornado striking an urban area probably would have a comparable human toll.

Unfortunately, the second question—is the death toll of ~160 the highest we could see for a single tornado in the modern era—has a different answer.  There are two ways that a single tornado could kill a lot more people than that.

One is the possibility of a weak, poorly-built or dilapidated high rise building taking a direct hit from a violent tornado and collapsing with a lot of people inside it.  Generally, these buildings are not supposed to collapse even in EF-5 events.  Images of collapsed high rises on hurricane landfall sites are misleading; these buildings mostly had shallow foundations and were undermined by the storm surge.  They were not blown over by wind alone, and storm surge is obviously not a factor for tornadoes.  The St. John’s Hospital building in Joplin took a direct hit from the tornado when it was at EF-4 intensity and it did not collapse.  However, a poorly-constructed or dilapidated one could.  (As an aside, one does have to wonder about the possibility of a tornado tearing up ground several feet deep, as happened in the EF-5 tornado on April 27 in central Mississippi. This could definitely undermine a slab foundation on a house, resulting in the foundation being ripped from the ground—the supposed hypothetical “F6 intensity” signature that one heard bandied about prior to the adoption of the Enhanced Fujita Scale.  However, high-rise buildings have much deeper foundations than residential homes.)

The other possibility is that of a violent tornado striking a crowded spectator event, such as a sports game, a fairground, a speedway, etc.  This possibility has been discussed at length by meteorologists such as Dr. Roger Edwards of the Storm Prediction Center.  It’s almost happened before, in fact; in 2008 an EF-2 tornado in Atlanta, GA struck the Georgia Dome while a basketball game (involving my college team) was going on.  It had gone into overtime, so people were not milling around outside.  Still, there are videos from that event of pieces of the roof collapsing and falling to the floor while the spectators were left to fend for themselves in the stands.  A stronger tornado could very easily have taken that roof off.

So yes, although the Joplin tornado was very likely a worst-case event for a tornado strike on a city, thereby representing an approximate limit on fatalities for that type of disaster, the potential exists for individual tornadoes to kill far more people than that in a different sort of disaster.  Let us hope that we can deal with the infrastructure and the safety considerations of large venues so that these greater disasters do not occur, either in 2012 or years to come.

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