Quick question on RFD

[Pete Kustra]

Okay, forgive my ignorance, (I've just recently become a storm enthusiast), but how does the RFD play a role in tornadogenesis? I've scanned the web and haven't found anything explaining how exactly the RFD is linked to the actual formation of the tornado.

 

[NealRasmussen]

https://en.wikipedia.org/wiki/Tornadogenesis#Supercellular_tornadoes

Don't ask me. I have several issues with that first paragraph, so...

It seems stated in there. Right. Like we know...

To me, the RFD is half the mesocyclone. The large scale. The descending air in the RFD is sucked around the updraft, or perhaps more accurately, into the updraft, as it nearly bisects it. The RFD is the Yin to the updraft's Yang.

The RFD used to be called the "clear slot", (not dry slot), as for a long long decades ago, folks noticed the clear slot wrapping around the updraft. Now refined to the RFD. And now we know the theta-e, or "potential engergy", of the air at the surface from an RFD dictates the life and perhaps strength of the tornado. Cold or dry RFD will choke off the tornado, IF the tornado forms. Whereas a warm moist RFD with as high a theta-e as the warm sector can get you the EF3+.

A tornado is simply a vertical wave of entrainment. We are much like CA surfers, but their waves are water and break horizontally.

Now if we could only predict 1) the resultant theta-e of the RFD and 2) the shape of the RFD, say about 12 hrs ahead of time...

 

[JamesCaruso]

....

A tornado is simply a vertical wave of entrainment. We are much like CA surfers, but their waves are water and break horizontally.

Now if we could only predict 1) the resultant theta-e of the RFD and 2) the shape of the RFD, say about 12 hrs ahead of time...

Really like the surfing analogy, never thought of it that way!

Of course the 12-hour prediction would be great for society. But if we ever get there, it would take 80% of the enjoyment of chasing away. Wonder if others feel the same way? But that's a topic for another thread, maybe I will start one someday...

 

[Mike Marz]

Here are just a few links that may be helpful for you, or at the very least, interesting to explore.

https://stormtrack.org/community/threads/tornadogenesis-and-the-role-of-the-rfd.28766/

http://www.spc.noaa.gov/publications/kis/drc-role.pdf

https://stormtrack.org/community/threads/rfd-and-tornadogenesis-visuals.27873/

http://tornado.sfsu.edu/geosciences/classes/m835/pdfs/HookEcho.pdf

http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/svr/torn/dgr/evsch.rxml

http://www.meteo.psu.edu/~pmm116/pubs/2009/MR09ATMOSRES.pdf

 

[Kyle Brittain]

Hey Pete, check out my recent thread "Tornadogenesis and the role of the RFD" over in Advanced Wx and Chasing. A few folks have contributed some valuable insight and resources there.

 

[Caleb Routt]

RFD plays a vital role in tornadogenisis. The way that RFD comes about is that as you get closer and closer to the ground the overall rotation of the storm gets faster and faster and that drives a pressure perturbation. Basically there is lower pressure near the base of the meso than aloft. So air from above the ground gets pulled downward. On days with dry air aloft, you may see what is called a "clear slot" because that dry air is pulled downward. HP supercells tend to have rainy RFD because there is not good storm ventilation going on. Therefore rain/hail from aloft gets pulled downward. Remember the RFD is a representation of what the air is like above the surface.

As for tornadogenisis its somewhat still unknown! Its not just RFD that helps tornadogenisis as FFD ( Foward Flank Downdraft) plays a role as well. On the leading edge of any downdraft there is a temperature gradient between the warmer air and the cooler air. This creates what we call a "Bariclinic Zone". That zone has a horizontal spin to it because as it hits the ground the air moves back upward (Kind of hard to explain). So on the gustfront of the RFD and FFD there is a horizontal spin and that spin can be flipped upward by the main updraft and tilted at low-levels to produce a tornado. Not to mention that along a downdraft (Especially the FFD) there is little eddies that can form called "Misocyclones" that can add vorticity to a mesocyclone if not a tornado. RFD and FFD also re-oreients vorticity. But thats getting too complex for now. Hopefully the diagram below will help!

Remember if the RFD or FFD is too negatively bouyent, then the updraft cannot tilt that vorticity. You have to have a balence. Its like the pourage fary tale, it has to be "just right"

I recommend these videos to better understand the role of RFD and tornadogenisis. Its a bit much to take in at once!