Tuesday, August 21, 2018

The tornado wind measurement of 19 August 2018 in Inola, OK

Here's an interesting observation the Inola, OK mesonet site recorded after a tornado passed near or overhead.  The 2 meter, 3 second wind gust observation wound up being higher than that located at10 m.  The tornado responsible for this wind observation developed just southwest of the mesonet site, then struck a farm a mile to the northeast before moving on several more miles before dissipating.  Overall, this tornado was rated EF1 based on the damage to some barns and trees.

An infographic of the tornado track and the reported mesonet wind gusts from the Inola tornado of 2018 August 19. (courtesy of NWS Tulsa)
This event would seem to confirm that tornadoes can produce stronger winds very close to the ground and shows that tornado wind environments don't adhere to the log normal vertical wind speed profile assumed during larger, straight line severe wind storms.  While this rare measurement at two levels seem to confirm that tornadoes produce stronger winds below 10 m, there are some things I heard from several knowledgeable people in the mesonet program that may suggest the difference in wind gusts may be due to differences in instrumentation.   I contacted Chris Febrich, James Hocker, and Cindy Luttrell at the Oklahoma Climate Survey, the organization in charge of the Oklahoma Mesonet.  They mentioned that the 2 m anemometer is a standard wind cup while the one at 10 m is an RM Young propeller vane anemometer.  While not immediately obvious, their hypothesis suggested that the RM Young 10 m anemometer was not directed into the wind when the strongest gust arrived.  A possible reason for this was that the wind shifted rapidly with time and that the RM Young at 10m may not have turned into the wind in time to sample the full speed of the peak gust. A propeller vane anemometer isn't likely to sample the wind at full speed unless pointed directly into the wind.  And some time is needed for the vane to respond to a new wind direction.  Conversely a cup anemometer isn't sensitive to changes in horizontal wind direction.  Chris sent these few wind direction observations to highlight his concern.

171 degrees at 20:36
267 degrees at 20:37(moment of peak gust)
289 degrees at 20:38

Now, a vane anemometer is designed to completely reorient into a new wind direction in a much faster time interval than implied by these measurements.  But we are talking about a small tornado and so the changes in direction may have happened much more quickly.

Chris also pointed out that the mesonet record contains no instances where the 2m wind gust exceeded the 10m at the same place and time.  Within that period of record were four other mesonet sites struck by tornadoes.

So this doesn't put cold water on the possibility that stronger winds occurred at 2m vs 10m but it may mean we cannot use this case to show otherwise.  Still, these are interesting observations which align well with the observed damage from this tornado.  The link below gives you a drone-based view of the tornado path from where it passed through the mesonet site and then to the northeast to a farm residence.  
The peak wind speed at 2m altitude is quite consistent with the damage incurred to the farm that can be seen in the drone footage and also witnessed by Scott Peake that you can see in his video link below.

These kinds of observations don't come by very often, and when they do, it's most likely courtesy of a field project such as TWIRL (Tornadic Winds: In-situ and Radar observations at low Levels) operated by the Center for Severe Weather Research.  The teams in this project deployed portable surface stations into tornadoes to measure winds at 1m above the ground  while mobile radar teams measured the winds overhead.   The TWIRL project successfully deployed surface stations into two tornadoes, the 2018 May 9 Sulphur, OK tornado and the 2018 May 24 Dodge City area tornadoes.  Neither tornadoes had measured winds at 1m exceeded the wind speeds measured by nearby radar.  However, efforts to simultaneously measure winds at low and high levels in previous field programs, like VORTEX2, netted examples where winds measured at 10 m above ground exceeded the radar observations at higher altitudes.  And another event occurred where a serendipitus, and somewhat scary, encounter by a CSWR team produced very high resolution profile of winds where the maximum measured was only 3.5 m AGL.

So why the fuss about whether the strongest winds are at low vs higher levels?  It's because engineers design buildings to withstand a certain level of winds  tuned to a reference level of 10m (the kind measured at most airports) over 3 second durations and then assume the winds are weaker below that level and stronger above.  This wind profile is called a log normal wind profile.  So for example, engineers would typically construct a building able to withstand a 3 second long gust of 90 mph at 10 m above ground where they would assume the wind at 3 m above ground would be perhaps 75 mph, and a wind at 100m above ground would be stronger.  But the evidence shows tornadoes produce wind profiles that don't follow the log normal profile. And the result could be stresses on buildings that greatly exceed that produced by a standard log normal profile.  But to what level tornadoes deviate from the log normal profile is something we don't know.  Wouldn't it be nice if we did?  Then we could have a better idea of how to design buildings with improved tornado resistance.  The only way to find out is with more observations in a tornado boundary layer.  That's why even serendipitus observations are important.

BTW, I am a Chair of a standards committee for the American Society for Civil Engineering (ASCE) that seeks to put the guidance in wind speed estimates into a document that includes not only anemometers and radars but also wind speed estimates from the EF Scale, tree-fall patterns, remote aerial measurements and building forensics.  Hopefully soon we'll also include a section on photogrammetry such as what could be derived from Scott's video above.