I received many thoughtful and passionate responses to my previous post regarding the upgrade of the El Reno / Piedmont /Guthrie tornado in Oklahoma to EF-5 based, in part on radar observations of 60 m AGL wind speeds. As I noted there/then, the EF scale, as was the F-scale before it is a damage scale, not a wind speed scale. Some have argued that, for this reason, actual wind speed measurements should have no bearing on the EF-scale rating, while others have argued that we should try to incorportate wind speed measurements in EF-scale ratings whenever they are available.
Let’s climb into our “way back machine” and go back to 1971. (Granted, this precedes my own birth by nearly a decade, but I digress.) Dr. Ted Fujita was motivated by the question, “How fast are tornado winds?” Doppler weather radar was still in its infancy, photogrammetry was only possible with high-quality, well-documented film, and in situ measurements of the winds were, logistically, all but impossible to collect (despite valiant attempts to do so). The way I see it, Dr. Fujita asked, “What evidence for wind speeds do tornadoes most consistently leave behind?” His answer: Damage. In 1971, in a paper proposing the Fujita scale, he writes,
“…one may be able to make extremely rough estimates of wind-speed ranges through on-the-spot inspection of storm damage. For instance, the patterns of damage caused by 50 mph and 250-mph winds are so different that even a casual observer can recognize the differences immediately. The logic involved is that the higher the estimate accuracy the longer the time required to make the estimate. Thus a few weeks of time necessary for an estimate with 5-mph accuracy can be reduced drastically to a few seconds if only a 100 mph accuracy is permissible in order to obtain a large number of estimates with considerably less accuracies… high wind-speed ranges result in characteristic damage patterns which can be distinguished by trained individuals with the help of damage specifications…”
Fujita clearly spells out his rationale for the scale; his strategy was to use damage as a proxy for wind speeds in the absence of near-surface wind speed measurements. Forty years later, thanks to innovations like miniaturized, in situ probes and mobile Doppler radar, obtaining near-surface wind speeds in tornadoes is not so far-fetched. Because only a handful of such instruments exist, and deployments are challenging (the presence of a mere tree or building between the radar and tornado can compromise the measurements), we are still not collecting near-surface wind speed measurements in tornadoes with any consistency. And, we are finding that the wind speed bins don’t always match up with the damage indicators in the EF scale.
In my opinion, this means the scale needs to be made more flexible, or possibly supplemented by a wind measurement-based alternative (i.e. two ratings, one damage based and one measurement-based). One could envision expanding the EF-scale into a second dimension (i.e. an EF matrix), the second dimension only expanded if reliable wind measurements (M) are available, and collapsed if they are not. The El Reno / Piedmont / Guthrie tornado would, for example, be rated EF-4 based on its damage, but M-5 based on the RaXPol wind measurements extrapolated to the surface via an objective method.
What I outline above is merely my own half-baked idea, and I am eager to hear other suggestions from people closer to the subject area. I am not a tornado damage expert; I am an observationalist. Keeping the damage-based scale certainly has its merit, primarily in the interest of maintaining consistency with the last 40 years of records (fraught with uncertainty though it may be; see Doswell and Burgess 1988). However, a blanket disregard for reliable remote or in situ wind measurements seems unwise, when obtaining tornado wind speeds was precisely Dr. Fujita’s objective.
- Fujita, T.T., 1971: Proposed characterization of tornadoes and hurricanes by area and intensity. Satellite and Mesometeorology Research Project Report 91, The University of Chicago, 42 pp.
- Doswell, C.A. III, and D.W. Burgess, 1988: On some issues of Unites States tornado climatology. Mon. Wea. Rev., 116, 495-501
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For the most part, if there were such a matrix, the M-dimension is going to be pretty sparsely populated for the foreseeable future! I think Prof. Fujita was unduly optimistic about how easy it would be to tell the differences between wind speeds based only on damage. It likely will be some time before measurements (even extrapolated to 3 m from measurements at heights of several 10s of meters) will be common enough to use for the process of estimating tornado intensity, so I believe we must continue to refine the F-scale and its successors as we learn more about the wind speed-damage relationship. I would hope no one is advocating that we disregard wind measurement – I’m certainly not.
Obtaining detailed space-time measurements of tornado wind speeds surely is an ultimate goal, irrespective of what might have motivated Prof. Fujita to develop his scale back in 1971. The Richter scale (and its more recent successors) replaced an earlier earthquake intensity scale based on damage. With the development of diagnosis tools using seismometer data and a greater understanding of earthquakes, it became possible to estimate the energy released in earthquakes, which was of vastly more value than any estimates based only on damage. We can only hope that someday, the F-scale concept of estimating wind speeds based on damage will be replaced similarly by something much more meaningful. In the meantime, we must do the best we can with the crude tools we have and continue to make incremental improvements.
The EF-scale provides a SYSTEMATIC method for estimating atmospheric truth from damage. Presently (& obviously) not enough appropriate radar exist to sample U.S. storms, nor have extrapolation methods to the ground surface been perfected for such a purpose. So, herein lies the problem: either we start to go down this road and do it right or officially, or let’s not. But, throwing in a volumetric radar-observation from a single storm out of left field in the absence of top-ranked damage surprised me greatly. That’s simply not scientific, and even though a lot of people, academics included, have been quiet about this, it simply would not fly, not even at a thesis-proposal meeting.
I really am not opposed to the idea of using different methodologies to estimate a 10 m wind speed strength in a tornado. Perhaps the radar-based method is still immature but not for long. Now we’ve got more anemometers that can survive a relatively debris-free wind. And that’s not all. In the near future we will likely have another methodology of using tree-fall patterns to estimate tornado strength. You can look up Beck and Dotzek in the AMS journals to see this methodology.
If you look at it in this way the one thing that’s clear to me is that we have to return back to a wind speed scale. We cannot keep the EF-scale as THE tornado rating system when it represents one of multiple methodologies. When we go back to estimating wind strength then it’s quite clear that the source of the estimate must accompany it. Even within damage ratings, there are different qualities of surveys that makes it imperative that require the source to accompany the estimate.
I agree fully with Jim. The thing that is most meaningful in a broader scientific context is the wind speed. I see the concept of Damage Indicators (DIs) and Degrees of Damage (DODs), that forms the basis of the EF-scale, as a mere tool to arrive at a best guess of the wind speeds. Although it is a helpful method it the wind speeds currently associated with certain DIs and their respective DODs should not be cast in stone.
In Europe we have yet to develop such a system, but it has no use from a European perspective to project DOD/DI combinations onto a scale that is an adjusted version of a pure windspeed scale (as Robin pointed out) to account for alleged past rating errors in the USA. The EF-scale should be open to adjustments regarding the wind speed that is associated with a particular DI/DOD combination as more measured data becomes available in order to continue to be a helpful tool. Probably it is wise to keep track of individual observations of damage for past wind damage events, so that they can be adjusted when one finds that for a DOD/DI combination measurements become available where the best we had until then was an expert elicitation. But what matters in the end is that be obtain the best estimate we can of the wind speed.