RFD

[Justin Mace]

Earlier this year I was driving west with a large storm just to the north of me. I had blue skies above me. I then got into a lot of hail. It was odd driving down the BA in the bright sunshine with so much hail falling. I've seen some comments on here about hail being in the RFD. I researched it and found that hail can be kicked out of the backside of the updraft and falls into the RFD. I assume this is what I experienced. I also read that some theorize that evaporation from this ejected rain/hail causes the cool air that forms the RFD in the first place. What are some other possible causes for the RFD?

 

[Dean Baron]

I dont know if this is an actual possibility or not, but I've been thinking about it so here goes my theory. Please feel free to correct me if I'm wrong.
My theory is that the rotation of the updraft pulls part of the foward flank downdraft around the rotating updraft causing that part of the foward flank downdraft to be behind (on the rear of)the updraft/storm. Not sure if I explained that very well or not or if it is even possible but that's just my theory. As far as I know no one really knows what causes an RFD to form.

 

[Wes Carter]

From what I've read, the RFD can be warm or cool. If the RFD is warm, it can be drawn into the updraft and actually add to the buoyancy of the air and as it is drawn in it tends to tighten the rotation and due to the conservation of angular momentum the rotation gets faster. But if the RFD is cold it decreases the buoyancy of the updraft region and kills off any rotation that might be present.

Here is a link where a meteorologist explains it in terms that almost anyone can understand.

The American Meteorological Society has several articles about RFD. Articles 4 years old or older can be viewed for free. Here is a link to an article on RFD by Paul Markowski that is pretty informative.

 

[Matt Gingery]

RFD is a result of relatively warmer and drier air in the mid level colliding with the rotating updraft, then are funneled to the surface and fanned out the back side of the supercell as well as wrapped into the occlusion point where the inflow is pulled in. The updraft is essentially a blocking mechanism to the airflow aloft. The air collides with the updraft and is forced to the ground. I would say that the cause of warming would be the pressure rise as the air is forced down causing it to be warmed adiabatically and in the process becoming much drier. The temperature of the RFD would probably depend on what type of convergence you are dealing with. I would imagine the rogue hail could end up in th RFD as well do to the fanning effect that takes place at the rear flank of the cell.

That is my basic knowledge and round about definition of RFD from what I have learned. Very interesting phenominom and I believe that it is one key element that needs to be studied very closely in the cause of tornadogenisis. The Davies County cell last June 6th up in NW Missouri originated in SE Nebraska. Was a classic HP Supercell and wasn't a prolific tornado producer. RFD may have had some role in that. It ties into so many factors and we can get into many different subjects when discussing RFD such as FFD, Horizontal Vorticity, and Helicity.

 

[David Wolfson]

What Matt theorized is similar to what I've observed and thought, with some differences. RFDs are associated with deep, rotating updrafts that penetrate to the tropause. The inertia of the updraft interferes with the faster flow at upper levels, which is restrained by the tropause and reflects as lateral and downward movement. Another part of the complicated dynamics is the anticyclonic (left-split) component of the stretched vorticity tube which tends to concentrate some of the displaced mass downward in proximity to the updraft rather than spreading outward and away.

I don't think the RFD should be looked at like a wet or dry downburst traveling adiabatically from mid or upper levels; rather it's more of a push-down stack where the thermo- and hydrodynamic properties of the successive atmospheric layers morph downward. So by hypothesis strong, high Theta-E surface RFDs should correlate with higher speed shear and thermodynamic potential energy in the above-boundary 2k - 10k level. Dunno if it's true but it seems logical. FWIW.
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[Dave Kaplow]

What Matt theorized is similar to what I've observed and thought, with some differences. RFDs are associated with deep, rotating updrafts that penetrate to the tropause. The inertia of the updraft interferes with the faster flow at upper levels, which is restrained by the tropause and reflects as lateral and downward movement. Another part of the complicated dynamics is the anticyclonic (left-split) component of the stretched vorticity tube which tends to concentrate some of the displaced mass downward in proximity to the updraft rather than spreading outward and away.

I don't think the RFD should be looked at like a wet or dry downburst traveling adiabatically from mid or upper levels; rather it's more of a push-down stack where the thermo- and hydrodynamic properties of the successive atmospheric layers morph downward. So by hypothesis strong, high Theta-E surface RFDs should correlate with higher speed shear and thermodynamic potential energy in the above-boundary 2k - 10k level. Dunno if it's true but it seems logical. FWIW.
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I have long wondered and speculated on the causes of the RFD, and as far as I know there is no definitive answer, just various theories. The problem with a lot of these theories is that you can often find counterexamples. For instance, take the statement, " RFDs are associated with deep, rotating updrafts that penetrate to the tropause." Perhaps this is true in many instances, but take a look at one of my favorite RFD examples: the Marlin, TX tornadic LP updraft of 3/21/05. Aside from the fact that this thing is unbelievable beautiful, note that for most of its life this updraft was basically all RFD, with only a relatively thin U-shaped area forming the updraft base. Where is all that RFD coming from? This can hardly be called a deep tropopause-penetrating updraft, and as for precipitation being a factor, this cell essentially isn't producing any! Where does all that intense curling downward RFD come from? So far the only explanation that explains this cell's remarkable structure is the mid-level winds/updraft collision theory... But was there really all that much of a strong midlevel jet that day in that area? I simply don't know. There used to be a web page that went into the meteorological details of this unusual event, but it seems to have been taken down. Regardless, for me this particular storm, updraft, whatever you want to call it, stands as one of the ultimate examples of the true mystery of the RFD.

 

[Matt Gingery]

Thanks for that link Dave! That is truly amazing. . .There is absolutely no precip on that cell at all. Beautiful LP low topped supercell. Would love to research the dynamics that day. That cell definately did not penetrate the tropopause. Alot of midlevel influence going on that day.

 

[rdale]

the OST punching the Tropopause and redirecting the upper flow downward, hence RFD.

That doesn't explain how non-trop punching storms have RFD's though. A great primer on RFDs and possible theories and connections to tornadogenesis is freely available at :

http://ams.allenpress.com/perlserv/?request=get-document&doi=10.1175%2F1520-0493%282002%29130%3C0852%3AHEARFD%3E2.0.CO%3B2

 

[rdale]

What are some other possible causes for the RFD?

Section 3e and on of the Markowski link is probably still the latest and greatest available summary. Or at least let me rephrase it - it's the latest and most complete that I know of.

As to which theory is correct? Or correctest? I don't know. I'm not sure that anyone has a be-all / end-all RFD development model yet.

It doesn't sit well that someone will disagree with me on a post, and not be able to back it up in own words.

I think several people have pointed out that RFDs can come out of storms that don't have overshooting tops. So in my mind, if your theory says "OSTs are the cause of RFDs" and several here point out observations otherwise - that is a reason to disagree. Markowski section 4 talks extensively about "occlusion downdrafts" which begin at the low levels and then extend upward, which clearly would not be connected to OSTs.

 

[Stephen Locke]

6-17-09 Aurora, NE

Coincidentally I am working today on a short time-lapse of the Aurora, NE RFD. I can testify that this particular RFD was hot. Very hot.

http://vimeo.com/7974667

 

[rdale]

Thanks -- Nice imagery!

 

[John Peters]

RFD is a result of relatively warmer and drier air in the mid level colliding with the rotating updraft, then are funneled to the surface and fanned out the back side of the supercell as well as wrapped into the occlusion point where the inflow is pulled in. The updraft is essentially a blocking mechanism to the airflow aloft. The air collides with the updraft and is forced to the ground. I would say that the cause of warming would be the pressure rise as the air is forced down causing it to be warmed adiabatically and in the process becoming much drier. The temperature of the RFD would probably depend on what type of convergence you are dealing with. I would imagine the rogue hail could end up in th RFD as well do to the fanning effect that takes place at the rear flank of the cell.

That is my basic knowledge and round about definition of RFD from what I have learned. Very interesting phenominom and I believe that it is one key element that needs to be studied very closely in the cause of tornadogenisis. The Davies County cell last June 6th up in NW Missouri originated in SE Nebraska. Was a classic HP Supercell and wasn't a prolific tornado producer. RFD may have had some role in that. It ties into so many factors and we can get into many different subjects when discussing RFD such as FFD, Horizontal Vorticity, and Helicity.

I buy the dry mid-level air intrusion theory - there are different mechanisms that would likely result in the descent of these air parcels.

Evaporational cooling within the dry air parcels as they encounter high liquid water content air within the updraft would likely cause descent. As the air descends through a vertically sheered environment it looses momentum and becomes slower than the mean storm motion vector, hence wrapping around and reaching the ground to the SE of the main updraft.

 

[Justin Mace]

Ok, I'm the "layman" in this discussion. You guys have given me some great info to help with my understanding of RFDs. I'm curious about the reasons for hot and cold RFD. Thoughts?

 

[Justin Mace]

I have long wondered and speculated on the causes of the RFD, and as far as I know there is no definitive answer, just various theories. The problem with a lot of these theories is that you can often find counterexamples. For instance, take the statement, " RFDs are associated with deep, rotating updrafts that penetrate to the tropause." Perhaps this is true in many instances, but take a look at one of my favorite RFD examples: the Marlin, TX tornadic LP updraft of 3/21/05. Aside from the fact that this thing is unbelievable beautiful, note that for most of its life this updraft was basically all RFD, with only a relatively thin U-shaped area forming the updraft base. Where is all that RFD coming from? This can hardly be called a deep tropopause-penetrating updraft, and as for precipitation being a factor, this cell essentially isn't producing any! Where does all that intense curling downward RFD come from? So far the only explanation that explains this cell's remarkable structure is the mid-level winds/updraft collision theory... But was there really all that much of a strong midlevel jet that day in that area? I simply don't know. There used to be a web page that went into the meteorological details of this unusual event, but it seems to have been taken down. Regardless, for me this particular storm, updraft, whatever you want to call it, stands as one of the ultimate examples of the true mystery of the RFD.

So if you have an extremely sheared updraft tower with RFD that is the signature of RFD caused by mid-level winds?

 

[rdale]

I'm curious about the reasons for hot and cold RFD. Thoughts?

If you search on "warm RFD" here you'll get previous threads where we've discussed it. The likely cause for a warm (use the term relative though - it's "warmer" than a cool RFD, it's not necessarily "warm") RFD is higher theta-e air being ingested into the updraft.


Markowski, P. M., J. M. Straka, and E. N. Rasmussen, 2002: Direct surface thermodynamic observations within the rear-flank downdrafts of non-tornadic and tornadic supercells. Mon. Wea. Rev., 130, 1692-1721.