Rating Tornado Intensity Based on Moblie Radar

[Dan Robinson]

There is a news article circulating around now that states that the NWS is 'reviewing' the EF5 rating based on the lack of EF5 damage found. I'm assuming this article is mistaken, given that it's already been determined that no EF5 damage occurred and that the rating was strictly derived from the mobile radar observations.

http://www.tulsaworld.com/article.a...view_NWS_official/20130607_11_a8_anatio567732

Anyone 'in the know' at liberty to comment?

 

[Shawn Schuman]

I've only seen this come from Greg Carbin. I haven't seen anything similar from OUN, and if they aren't interested in reexamining it then I can't imagine it will change. And if they are reconsidering it, then it makes absolutely no sense to have already publicly released the radar data and the EF5 rating.

 

[Jeff Snyder]

Why do most people use the EF-scale? I suspect that most people use the EF-5 as a proxy for tornado intensity. Yes, there are all sorts of caveats, but that's what it boils down to much of the time. If that's the case (otherwise, why would care about maps of tornado climatology based on EF-scale, etc.?), then I suspect any objective observations that can fill in the holes where obvious weaknesses in the EF-scale exist is valid for consideration. In this case, if there are no substantial DIs directly impacted by the highest winds, then using only damage will, by all admission, underestimate the intensity of the tornado substantially.

Of course, there are caveats in using radar data to look at near-surface winds. However, when we have not-far-from-ground (<150 ft) observations that show winds *WELL* in excess of not only the preliminary damage rating (in this case, EF3) but also of the EF4/EF5 threshold, it would be silly, in my opinion, to ignore such data. Sure, if mobile radars weren't in the area, it's possible this tornado may have ended up as an EF3. However, again if the EF-scale is used as a proxy for tornado intensity, this would be erroneous since we know that the tornado produced winds stronger than those associated with EF3. It would be leaving it up to complete chance with the rating depending entirely upon whether the small subvortices and perturbations hit or miss the sparse DIs out there; the "hit or miss" chance of being able to accurately estimate tornado intensity using the EF scale in this case seem to introduce more variability and error than using high-quality, high-resolution radar data.

 

[Jeff Duda]

Jeff, I have to ask.

Can I assume that the choice to rate the tornado as EF-5 was not based solely on the 295(6) MPH wind measurement at 500 ft AGL? I assume you measured >200 MPH winds much closer to the surface, and thus that was the leading cause of the tornado's rating, right?

 

[Jeff Snyder]

Jeff, I have to ask.

Can I assume that the choice to rate the tornado as EF-5 was not based solely on the 295(6) MPH wind measurement at 500 ft AGL? I assume you measured >200 MPH winds much closer to the surface, and thus that was the leading cause of the tornado's rating, right?

Jeff -- Yes. We have peak velocities >125 m/s on all almost scans between 0 deg and 5 deg elevation angle. It doesn't seem as though the max radial velocities are as sensitive to height as they are to time. The publicly-released max radial velocities are observed, essentially, as the subvortices or perturbations are sliding around the south or southeast side of the tornado straight towards the radar. From a geometric standpoint, we'd expect max winds to be observed at such a time. By the time the radar returns to 0 degree elevation angle again, the subvortices no longer are translating optimally to sample the strongest winds (i.e. part of the translation appears to be across the beam). The potentially-revised max that's currently under consideration is actually on a 1 deg scan. After accounting for non-zero pitch and roll angles and changes in ground elevation between the radar and the location of the max, the center of the beam actually would be below ground. There are trees between the tornado and the radar, so, obviously, the returned signal likely is only coming from the very top of the radar beam on account of partial beam blockage. Needless to say, these are close-to-the-ground winds. The "below 500 ft" observation that the NWS mentioned was because I hadn't had time to sit down and thoroughly examine the data, so I was most confident in the "below ~150 m ARL" qualifier. As we continue to analyze the data, we'll be able to pin down more details.

 

[Jeff Duda]

Thanks for the explanation.

So what about this tornado is being reviewed still?

 

[Shane Adams]

Part of the research behind the F-scale (and subsequent switch to EF-scale) was trying to decipher what the actual wind speeds needed to produce each level of damage were. Scientists thought enough of the wind speed estimates to back way down on the actual numbers when the EF-scale replaced the F-scale. IMO, if they went through the trouble to research that the F-scale wind estimates were too high, it would be foolish not to incorporate measured wind speeds into the ratings, now that we have the technology to get them.

The one argument I keep seeing against using mobile radar measurements is "it's not consistent with the methods used to rate tornadoes in the past." IMO this argument is invalid. We created a damage scale based on estimated wind speeds because we had no way to measure actual wind speeds. Now that we do, should that not be a top-tier indicator when assigning ratings? "But violent tornadoes in the past that weren't sampled and didn't hit anything always got F0 ratings. That's messing with the data." Well, science is a fluid endeavor, and as we learn, things change. If those past tornadoes had been sampled, they'd be handed the rating based on actual wind speeds...but they weren't. That this is "inconsistent" with today's measured ratings is simply a circumstance. We didn't have the means to assign a rating based on wind speed measurements so we went with damage indicators and estimates....because we had no other options. Do we now ignore this new technology that allows us to better-sample actual tornado wind speeds and give more truthful ratings just so we can "remain consistent" with how we used to do it?

That seems a step backwards.

 

[Kevin Walters]

Unless tornadoes are being sampled with mobile radars in North Dakota, Alabama, Wisconsin, etc maybe an "asterisk" should be placed next to certain tornadoes. Just like some want done with baseball records.

 

[Rob H]

The arguments for seem to be that if we have these accurate wind speeds measured, why shouldn't we use them? Well, the biggest problems are that we don't really know how accurate speeds measured at 150m are compared to the surface, and we have no rules for applying them. What if they were measured at 250m? 500m? I personally think it's a good idea to try and capture the true strength of a tornado when possible, but I'm sure some of the sticky points being argued right now resemble the following:

1. There needs to be a consistent and easily applied method to incorporating these measurements into tornado rating
2. Part of this consistent method will require deep analysis of what it means at the surface when 300mph winds are measured at 150m. Or 50m, or 500m. That last 100m or so to the ground still seems to be unknown in many ways, and is consequently an area of heavy focus.
3. There needs to be some way to denote when EF-3 damage is found, but EF-5 winds are measured, and to put an asterisk next to that entry. Even though previous StormData has tons of errors, and is affected by urban sprawl, etc. at least you knew there was a consistently applied scale being used and you knew the limitations of it.

On a related note, I assume the survey teams strive to be unbiased, but when you have something like the Moore tornado and you find such a relatively small area of EF-5 damage - I wonder if there was pressure creeping in from politicians, friends, family, peers, and internally. A lot of people really, really wanted Moore to be an EF-5 because it's unthinkable that anything but the strongest tornado could have killed so many, just like how no one wants to think a "lowly" EF-3 took the lives of Tim, Paul, and Carl. Maybe they looked harder at Moore than they would have Bowdle, Langley, or Wadena? Even the possibility is a strike against DIs and a win for measured speeds if we can figure out how to use them appropriately.

 

[rdale]

Well, the biggest problems are that we don't really know how accurate speeds measured at 150m are compared to the surface, and we have no rules for applying them. What if they were measured at 250m? 500m?

Is that claim based on research? I'm under the understanding that winds were coordinated with TIV measurements a few years ago and deemed fairly accurate. Are you saying that's not true?

 

[rdale]

The NWS Director sent out a memo stating that mobile Doppler data CANNOT be used in changing EF-scale ratings...

http://cadiiitalk.blogspot.com/2013/06/the-ef-scale-ratings-brouhaha.html

 

[Dan Robinson]

It's getting interesting.

http://www2.ucar.edu/atmosnews/opinion/9696/terrible-tornado

The consensus document that guided creation of the enhanced Fujita scale (see PDF) gives the green light to use radar data in this way. It states: “The technology of portable Doppler radar should also be a part of the EF Scale process, either as a direct measurement, when available, or as a means of validating the wind speeds estimated by the experts.”

I can see Doswell's point. If a tornado produces measurable EF5 winds, it's at least *capable* of EF5 damage even if it doesn't hit anything. It seems like knowing the true strength of a tornado in a given environment can be used as a basis for forecasting future events in similar environments, or at least assessing the potential threat.

 

[Rob H]

Is that claim based on research? I'm under the understanding that winds were coordinated with TIV measurements a few years ago and deemed fairly accurate. Are you saying that's not true?

I don't know if it's true or not - I haven't seen anything counter to that assumption, and we still have research teams claiming that they need to find out what's happening at the surface for tornadoes, dropping probes in the path. "Fairly accurate" doesn't seem good enough for scientific progress, although the EF scale admittedly throws accuracy right out the window with its damage estimations.

edit: VVV haha, good enough for me.

 

[rdale]

"Fairly accurate" was my paraphrasing of stuff I've seen from DOW and OU people. I've not seen the research myself, but if it's good enough for Howie it's good enough for me

 

[Jeff Duda]

The NWS Director sent out a memo stating that mobile Doppler data CANNOT be used in changing EF-scale ratings...

http://cadiiitalk.blogspot.com/2013/06/the-ef-scale-ratings-brouhaha.html

Okay...if this is true and wasn't JUST written in the last week or so, then there will need to be a number of changes made to some tornadoes from as far back as two years ago. I seem to recall the El Reno tornado of 24 May 2011 was rated EF5 also based on mobile Doppler radar measurements. Did this not come up then (memory isn't great, but I don't recall it happening)? Seems like quite the oversight on the part of local WFOs or a strike back from the headquarters in light of the debate this has sparked.