100 millabar pressure drop

[Jared Farrer]

As most of you know on the 24th of June 2003 Tim Samara's would successfully intercept the Manchester storm. When he would lay down a series of probes and had one pick up a 100 millibar pressure drop. Now to the point of my post. It is difficult to convey what a 100 millibar pressure drop means to me, but what I want to know is what it means to you?

I am not a meteorologist far from it in fact so my knowledge is limited. Their are alot of smart chasers out there with years of experience, and alot of knowledge when it comes to storm dynamics and behavior. That is beside the point anyway.

I guess what I am asking is what did the information obtained on that day mean to you guys? And I am asking everybody. Hopefully to generate some insight from everybody. And feel free to deviate from the topic a little bit but not much, as long as it is related to the subject anyway.

 

[Paul Austin]

Hi Jared.

Not an expert here, either, but perhaps a few thoughts would help. The Saffir-Simpson Scale for hurricane categories (the second table here) give Cat 1 hurricanes a barometric pressure typically >980 mb, wind speeds 74-95 mph and Cat 5 hurricanes typically <920.1 mb, >155 mph respectively. These typical values obviously represent less than a 100 mb spread. The danger with this comparison is that windspeeds are not directly related to barometric pressure, but rather to barometric pressure gradients. That is, the change in barometric pressure over a given spacial distance. Other factors can come into play, but the pressure gradient is the short answer. With hurricanes, the pressure change occurs over relatively large distances: 10's to 100's of miles. With Samaras's messurement, the pressure change occured over only 10's to 100's of feet. (Sorry, I don't actually know the exact specs. Tim would have to answer that.) This incredibly tight pressure gradient speaks to the violent nature of tornados in general, and certainly to the Manchester event.

Anyone more qualified, feel free to correct or add to the above.

 

[rdale]

As a chaser it really doesn't affect the chase strategy, as a forecaster it doesn't impact much in that realm either. Great video - but in the big picture it doesn't do much...

 

[Robert Dewey]

Great video - but in the big picture it doesn't do much...

Is this from a chase perspective, or a meteorological perspective? This finding did two things: 1) it verified the pressure fall, and 2) it proved that a probe can successfully be placed within the path of a tornado to collect data.

Personally, I think that does a lot for both the small and big picture.

 

[Paul Austin]

I'm with Dewey. I think the measurement is much more significant than just some great camera footage, athough it certainly is fantastic footage.

 

[Justin Turcotte]

We already have the equations for pressure perturbations in cyclostophic flow but it is always fun to see verification of the numbers work in the real world.

 

[rdale]

Is this from a chase perspective

Yep - as a chase forum discussion, I don't think this changes chasing in "the big picture".

 

[Robert Dewey]

You forgot the "weather" ... it's "Weather and Chasing" - feel free to discuss the meteorological impact as well

 

[rdale]

Good catch ;>

I haven't seen anything revolutionary that came out of the measurement to be quite honest, and I don't do any work with the microscale to know what it might be able to do meteorologically... I was speaking re: chasing & forecasting and in those areas it really didn't change anything except give us some awesome video!

 

[Glen Romine]

First, Tim Samaras is a board member and can undoubtedly offer more insight on how the data is being used. From my perspective, I think this was a crucial first step to gaining a better understanding of the dynamics at play within a tornado. It also demonstrates a probe concept that can provide some useful measurements. Does it have implications for forecasting? Probably not, but for researchers interested in tornado dynamics this is some pretty good stuff.

There are a number of suggested models for describing the wind field and pressure relationship for a tornado - (not as simplistic as cyclostrophic flow), and gathering real world measurements gives us a better starting point for understanding the dynamics at play, which can then be used in engineering studies, etc... There are also subtler features in the measurement overlain on the larger signal that could have significant implications - but are poorly understood. So, answering some questions and creating more is a quite useful observation. The observations on the date mentioned are not accompanied by video in the probe (though there was video at close range to the tornado by several individuals this day) - I think you are mixing this event with the one from IA a few years later with the probe with cameras. Anyhow, the downside to the measurement is that it is made very low to the ground - and there is little understood about what is happening between ~ 6 inches off the ground to 100 ft or more off the ground (remotely sensed by a radar). The video probes may help some in this regard by enabling photogrammetry to look at wind profile proxies along with pressure measurements near the ground, as well as providing a better understanding of the breadth of the tornado circulation extent outside of the visible condensation funnel. Again, I'm sure Tim Samaras could offer a lot more information in this regard, but in summary I think he's making considerable strides in collecting in situ (measurements from inside the tornado instead of those remotely sensed) tornado observations.

 

[Brian Stertz]

I was a real life probe of sorts 7 years ago...and the changes that occur as a tornado approaches happen so rapidly that it was both stunning and scary. Pressure changes were incrredible and our ears did pop bigtime as we rode out the half mile wide F3 tornado in the basement. The farmhouse was on the south edge of the tornado, so it certainly would have been even worse in the central core of the tornado...where damage was nearly F4 in spots.

 

[Gene Moore]

It would be great to get some confirmed benchmarks for wind and pressure loading that the building industry could use. Hopefully this work will go on until we're sure of the data. And, there is the still debated topic of "trailers exploding" from pressure. I thought that was settled and it was overpressuring of inner walls from high wind coming inside though broken doors and windows, but I still read it discussed with pressure.

I despise this assumption that "it's a tornado, nothing can survive." I'm convinced we can build tornado proof houses for most events, especially since 69 percent of tornadoes (NWS statistic) are non-violent and only 2-3 percent are EF-4/5. Now we have some numbers and hopefully more in the future.

As for pressure measurements helping with forecasting, it's generally accepted that surface pressure has little or nothing to do with tornado formation. I raised this topic on a more technical forum and got ripped to pieces. For me, it's still one of the most difficult findings to accept.

As mentioned already, the change in wind speed with height off the ground is important. Certainly the surface of the earth with wind reducing turbulence and the height of a two story house will be drastically different. How do you get a 20-30ft off the ground instrument measurement, tough. As for photogrammetry, that's difficult too since some, if not quite a bit of damage, may be caused by localized vortices, or suction spots. Getting a large enough arc to do tangential velocity measurements from video would be a huge challange. To do this one must do a survey of the path and the area to be analyzed. From say a mile away this is not difficult, but getting down to hundreds of meters would be. Any object tracked must be referenced to a ground distance. The object tracked in the measurement is always in an arc and could have radial changes. The best situation is to get an object that does a complete 360 back to the origin of the measurement. That too is difficult as most objects on the small scale fly away not to be seen again. It is considered bad science to track any object of condensation because it's formaing and dissipating with time....but I do stray from the original topic.

Looks like Tim needs to develop a "tall probe" package....hummm, any plans?

Gene Moore