Tornado Size and Intensity Estimates

The 05/31/2013 El Reno, OK tornado was given an Enhanced Fujita (EF)-5 rating and declared the widest tornado ever recorded at 2.6 miles wide.

How do we get this information?

I spent a lot of time in my M.S. program at Purdue being quizzed by my committee and other scientists about tornado size. It’s a tricky subject. Thankfully my research, at least as I last left it, was not dependent on any distance parameters, so my defense didn’t involve much beating around the tornado bush on size questions. But I was working with DOW data, and Josh, Karen, and company have to deal with this question a lot.

First of all, the EF scale was developed to expand on the original Fujita scale, which connected the Beaufort and Mach scales.

Fujita Scale

The Fujita scale lacked the detailed damage indicators that surveyors use to estimate Enhanced Fujita scale ratings today. If you compare the numbers, significant differences don’t appear until the higher (E)F numbers, for the “significant tornadoes.” The Fujita scale relates wind speeds to damage, while the Enhanced Fujita scale relates damage to wind speeds.

There are problems with the (E)F scale, but it is often the best information that we have to use when estimating tornado intensity. This method really falls apart when there are not enough damage indicators to use to rate the tornado–not because the storm was incapable of producing damage, but because it occurred in one of the oh-so-common wide open areas in middle America.

For years now we have been attempting to relate Doppler radar intensity estimates to the damage estimates we obtain after-the-fact. Since the development of mobile radars, there has been an increasing number of events whose EF-Scale ratings have been increased due to mobile radar data. These include:

The Calumet-El Reno-Piedmont-Guthrie Tornado of May 24, 2011

PRELIMINARY DATA...
EVENT DATE: MAY 24, 2011
EVENT TYPE: TORNADO
EF RATING: EF-5
ESTIMATED PEAK WINDS (MPH): GREATER THAN 210 MPH
INJURIES/FATALITIES: UNKNOWN/9
EVENT START LOCATION AND TIME: 4 ESE HINTON 3:50 PM CDT
EVENT END LOCATION AND TIME: 4 NE GUTHRIE 5:35 PM CDT
DAMAGE PATH LENGTH (IN MILES): 65 MILES
DAMAGE WIDTH: TO BE DETERMINED
NOTE: RATING BASED ON UNIVERSITY OF OKLAHOMA MOBILE DOPPLER RADAR
MEASUREMENTS.
---------------------------------------------------------------------------------------------

June 5 – Goshen County Tornado Given Official Rating of EF2

The Goshen County, Wyoming tornado of June 5th, 2009 has been given an official rating of EF2 on the Enhanced Fujita (EF) Scale, according to Meteorologist-in-Charge, John Eise of the National Weather Service Office in Cheyenne, Wyoming.
Preliminary wind measurements provided by the Verification of the Origins of Rotation in Tornadoes Experiment 2 (VORTEX2) were added to the damage reports from the storm assessments on June 6th, 2009.  These two factors were considered in the new rating.

———————————————————————————————————————————–

And of course, I am biased towards using all data available to make a tornado estimate. I think it is silly to limit ourselves to the same methods used in the 1970s and ignore new technology when trying to advance the science. I do agree with others that suggest that mobile radar-based intensity estimates should begin to form their own database separate from the current damage database.

Now what about size?

WITH THIS INVESTIGATION... THE TORNADO HAS BEEN UPGRADED TO AN EF5
TORNADO BASED ON VELOCITY DATA FROM THE RESEARCH MOBILE RADAR DATA
FROM THE UNIVERSITY OF OKLAHOMA RAXPOL RADAR AND THE DOPPLER ON
WHEELS RADARS FROM THE CENTER FOR SEVERE WEATHER RESEARCH. IN
ADDITION... THE WIDTH OF TORNADO WAS MEASURED BY THE MOBILE RADAR
DATA TO BE 2.6 MILES AFTER THE TORNADO PASSED EAST OF US HIGHWAY 81
SOUTH OF EL RENO. THIS WIDTH IS THE WIDTH OF THE TORNADO ITSELF AND
DOES NOT INCLUDE THE DAMAGING STRAIGHT-LINE WINDS NEAR THE TORNADO AS
DETERMINED BY THE HIGH-RESOLUTION MOBILE RADAR DATA. THE 2.6 MILE
TORNADO PATH WIDTH IS BELIEVED TO BE THE WIDEST TORNADO ON RECORD
IN THE UNITED STATES.

Shouldn’t size be pretty straightforward? I mean, you can see the tornado.

Wakimoto et al.

Contoured Doppler velocity values overlaid on visual of the tornado at that time.

Whoa. Ok stay with me.

You can see the tornado here, right? It’s the tornado-looking thing in the middle, covered in squiggly lines. The squiggly lines are the mobile Doppler radar velocity estimates. That’s right, the same ones used to make the tornado intensity estimates. Anywhere you see those lines there is wind.

So what counts as the tornado winds? Well, so far it seems that the NWS has not been very consistent with its criteria for measuring size through mobile radar data. The El Reno statement from 2013 states that the “width is the width of the tornado itself,” which, I’ve heard from other sources, they used the EF-1 tornado minimum wind speed as the cut-off. Put in another way, how far out do the winds that are at least strong enough to count as an EF-1 go?

Should the NWS use only the EF-5 wind field for the El Reno tornado to estimate size? Probably not. People who lose property at a greater radius from the tornado center don’t care that they were “only” in the EF-4 or EF-3 winds. Should the cutoff be EF-3 winds? Does a tornado smaller than an EF-3 even get counted then? Even though we have better radar data than ever, there are still smoothing and averaging issues–how much variation are we missing at various levels of the tornado?

The El Reno size and intensity ratings sparked a lot of debate in the community, which has predictably died down over the past week. I know my colleagues still active in the research on this event will be presenting interesting data over the next few years on this storm, and I expect that this may cause the CSWR crew to pull out some size comparisons from their previous events.

And once someone figures out the “correct” answer to the tornado size question, let me know. I’ve been grasping at straws for a few years now.

Advertisements

3 thoughts on “Tornado Size and Intensity Estimates

  1. Pingback: Top Search Terms That Brought You To This Blog | The Casual Conversationalist

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google+ photo

You are commenting using your Google+ account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s