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.

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