Rear Flank Downdraft

[Andrew Stoller]

This is the best visual reasoning I can provide. Early in the video the first 10 sec. or so you can see the first formation of a "wall cloud" and just to the left of it, the tiniest "cut". Then the video skips ahead (no more than 5 min real time) and that tiny cut is now a large RFD/Clear Slot, you can see the sun shining through. Just a classic example of how an RFD impacts tornadogenisis...
http://www.youtube.com/watch?v=Byym59xE13k

In this video, you can see the downword motion of the RFD interacting with the upward motion of the updraft. That is cool!

 

[Andrew Stoller]

I believe one reason the NWS is adding emphasis to identifying the RFD is to help spotters to be more accurate when identifying wall clouds and tornadoes. Over the years I have seen many false reports where someone thought a scud tag or shelf cloud was a wall cloud or funnel. If a chaser or spotter understand what an RFD slot looks like they are more likely to accurately identify tornadic features.

Many times in my young career as a storm chaser, I end up focusing on the RFD gust front too much, thinking it's part of the updraft base. Identifying the RFD clear slow in relation to the updraft has helped me focus more on the action spot of supercells.

May 23rd, 2008 Wakeeny, KS evening supercell was a classic example. That storm cycled and produced a new mesocyclone with a big clearslot. It was beautiful because the sunset was allowing all kinds of bluish/orange light to come through the RFD, but I kept focusing on that and the RFD gust front, only to look a bit further north to see the actual meso!

 

[Janette Bontempo]

Thank you so much everyone for all of your posts and information!! Sorry I didn't get back sooner....I was mia for surgery and in the hospital....its looking like it might be busy for the reg's the later part of this upcoming weekend and early part of next week. Stay Safe!!!

 

[Stephen Locke]

Cold vs Warm RFD

Someone earlier stated that RFD is cold air that wraps around the meso, and while that can be true, often RFDs that promote tornadogenesis contain relatively warm, boyant air that literally wants to rise back into the meso.

If it is generally agreed that RFD contains (cold) air from aloft why are so many RFD's reported to be warm. Does the air mix out with warm air at the surface or does the air actually heat up due to compression when the cascading vertical
column strikes the surface. And, would this compression be a viable energy source to help kick start tornadogenisis.

-Stephen

 

[Paul Knightley]

The fact that RFDs around tornadic storms often seem to be 'warm' is a major area of research at the moment. Some theories centre around Fujita's original 're-cycling' theory, whereby air which has already entered the storm from being lifted along the forward-flank downdraught then descends in the RFD and re-cycles into the tornado, perhaps bringing increased vorticity with it. How/why this air is warmer is not really known, although some theories suggest that low LCLS/high RH in the boundary layer prevents too much evaporative cooling of in the RFD (as precip evaporates) and thus the RFD reaches the ground with some CAPE.

 

[Stephen Locke]

RFD-warm or dry?

The fact that RFDs around tornadic storms often seem to be 'warm' is a major area of research at the moment. Some theories centre around Fujita's original 're-cycling' theory, whereby air which has already entered the storm from being lifted along the forward-flank downdraught then descends in the RFD and re-cycles into the tornado, perhaps bringing increased vorticity with it. How/why this air is warmer is not really known, although some theories suggest that low LCLS/high RH in the boundary layer prevents too much evaporative cooling of in the RFD (as precip evaporates) and thus the RFD reaches the ground with some CAPE.

Are the cold RFDs typically dry? Does a warm RFD feel moist?

-stephen

 

[Paul Knightley]

Are the cold RFDs typically dry? Does a warm RFD feel moist?

-stephen

Do you mean dewpoint-wise? A cold RFD can have quite high relative humidity. It's the amount of CAPE contained within the RFD which seems to be key, thus a warm, moist one is more conducive than a cold one, whether it's wet or dry.