RFD

[David Wolfson]

Bill Purcell's photos are certainly great, and a striking example of shear dynamics. Without other evidence I just have to be skeptical, though, about Bill's conclusions that there's any RFD or, dare I say, a supercell. Strong shear can produce all of these phenomena, including the funnels, non-glaciated cumulus congestus-cum-minimal-nimbus, and a vigorously spinning cloud tower. FWIW.

Thanks for the Markowski paper link, rdale! Really interesting stuff.
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[Dave Kaplow]

Bill Purcell's photos are certainly great, and a striking example of shear dynamics. Without other evidence I just have to be skeptical, though, about Bill's conclusions that there's any RFD or, dare I say, a supercell. Strong shear can produce all of these phenomena, including the funnels, non-glaciated cumulus congestus-cum-minimal-nimbus, and a vigorously spinning cloud tower. FWIW.

Thanks for the Markowski paper link, rdale! Really interesting stuff.
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I knew somebody would say something along these lines, since that cell does seem to put the kibosh on any number of RFD formation theories... Now, I admit I'm not a highly trained meteorologist like some here, but in this case I apply Occam's Razor: it looks exactly like a true RFD, a textbook clear slot with tornado forming right where you'd expect it to... The simplest explanation by far is that this is a true RFD. To argue otherwise means that any picture of a perfect clear slot is now going to be held suspect unless someone was actually there to make a measurement. I'm sorry, until presented with evidence to the contrary I have to assume that this was exactly what it looks like.

What can I say, RFDs are just plain mysterious, they don't seem to like being confined to any particular theory. Perhaps that's why they fascinate me.


Edit: Just to be clear, the conclusion that there is a RFD/clear slot in those photos is mine, not Bill Purcell's. He took the photos - and we should all be thankful that he did - but I was the one pointing them out as an example of an RFD.

 

[Stephen Locke]

Stamford, Texas; Tornadic Supercell, May 1, 2009

Well I'm short on science but I take lots of pictures.
I'm reminded of this Stamford, TX
supercell that seemed to get wiped out / undercut
by a very strong (hot) RFD. Seems like
you can see a clear vertical shaft from the surface
on up. I just got a kick out of the
blue sky through the hole in the meso.

 

[David Wolfson]

I'm just a skeptic by nature, Dave! But I confess that my own eyeball application of Occam's Razor -- ouch! -- looks more like a highly sheared flanking line/inflow band with a highly sheared tower behind. The horseshoe could just be a matter of perspective. Not presuming to say that you can't have an RFD from such a small, low-top cell, but I'm having trouble seeing it here.
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[JF Massicotte]

There is another RFD under a low-topped storm.

This storm was less than 30 kft high and was cycling trought its 3rd tornado at this right moment (Boibriand, Québec, F1). The clear slot you see there is the storm's RFD. Two other tornadoes were confirmed earlier and documented by my partner Allan Theunissen. Busy at my brother's birthday, I was late at the show and got a funnel cloud under this same storm 30 min later.

 

[Greg Blumberg]

Upon speaking to Les Lemon on this, he told me to think about the supercell like it was an extratropical cyclone, with the dry slow similar to the RFD. I may be slightly off on this as our conversation happened a while ago, but the paper we were refernecing in our conversation is below:

Severe Thunderstorm Evolution and Mesocyclone Structure as Related to Tornadogenesis

I have a hard time seeing lower stratospheric/upper troposphere air 9 to 12 kilometers up in the atmosphere reaching all the way down to the surface. That would require a lot of energy to keep that air sinking down to the ground. I just don't know where you would get the energy to pull that off.

I wanted to toss this in there too. Gabe Garfield had a presentation that he gave to the Student Chapter of the AMS at OU. He mentioned that there was a possiblity that in the rear area of the supercell there is not one, but two downdrafts. One is the RFD and the other creates the interactions needed for tornadogenesis.

 

[Matthew Grzych]

The mid level (or upper level for that matter) tornadic RFD origin theory has been discounted for several years now considering that high theta-e and theta-v air is found in the RFD surrounding significant tornadoes. This may get a little technical for the non-meteorologist out there but equivalent potential temperature (theta-e) is conserved during adiabatic processes. With that said, if the RFD air originated from mid to upper levels on the backside of the updraft from ambient flow being forced downward and descending dry adiabatically all the way to the surface, it would be hot and dry with low theta-e values at the surface (like a heat burst). This is just simply not the case. Tornadic RFD's are warm and humid with theta-e values similar to the inflow air. This suggests that the tornadic RFD originates either from low levels or from updraft air. Since theta-e is conserved for the adiabatic process, if air in the updraft was dynamically forced downward (maybe in the core of the updraft at relatively low levels to minimize mixing of dry environmental air), you can get high theta-e air to the surface. I think the dynamic pressure-forced RFD is key for tornadogenesis (from rotation in the low level mesocyclone). A topic that I am really interested in is the LP supercell that goes on to produce a tornado. I've seen this several times on the High Plains where an LP storm is spinning like a top and a strong, small-scale RFD punches through the updraft and you get a mini-horse shoe updraft followed by tornadogenesis. In 2 of the cases I was directly under the updraft when the RFD surge developed and there was no rainfall in either of the cases. Anyways, just some thoughts on this interesting topic.