RFD= Additional Energy source?

[Lanny Dean]

On the discussion of the "warm" RFD, I believe that it's an inappropriate term to describe the difference between the different RFD characteristics. In the papers that describe this difference, the idea of warm and cold are relative terms, similar to what rdale meant by "warmer." I've had discussions with people involved in the tornadogenesis-RFD link field and they've all told me that the perception of warm and cold in RFDs is often misunderstood.

In the Markowski paper Jeff referenced, it mentions relative warmness and coldness. The paper also uses surrogate thermodynamic variables such as Theta-E and virtual potential temperature to describe the properties of RFDs.

I would argue that comparing such variables to personal observations is a real stretch, mostly because we as humans are not able to estimate accurately the Theta-E or virtual potential temperature of certain types of air. Also, I wouldn't fully trust just "how I feel the temperature is in the RFD" observations. From the Markowski paper, I'm roughly calculating a range of 64-70 F in the RFDs observed (just by taking the mobile mesonet observations in the figures.) Add that to an assumed completely saturated air and high winds generated by the RFD, and you've got some pretty messed up human-based measurements (think about the heat index and wind chill calculations.)

I think this is dead on with the only caveat being that 80-85% of said "warm" and "cold" experiences were not only felt by either myself and or whomever I was with, but also fully meassured with the Davies. I can recall during the March 17, 2003 event near Gotebo, OK at one point we had dewpoints in the 30's during the crash of the RFD. I have included video below but it does not have the RFD included with it. I think I even shot video of me taking a reading of temps and dew points from this event and I will try to post if I can find it.
Watch video >

.... In relation to the supercell studies, it would appear that actual tornado production is related to the temperature of the RFD (is their a magic number?) as being one that is at least characteristic of having enough low-level humidity and CAPE to allow for more bouyancy so that the RFD can more readily coalesce with the rotating updraft and be stretched vertically, enhancing near-storm helicity.

I would be interested, as I am sure we all would, if there was such a correct number. As far as the temp of the RFD, I would also be interested to know if speed and temp both have an effect on tornadogenesis...most particularly if one has a relationship with the other.
By the way, Jesse, I was not trying to hammer your thought process on the RFD...I was simply pointing out the fact that we still don't know much about the RFD or its real role in tornadogenesis as a whole. This includes temp, speed, width and the like.

 

[Skip Talbot]

I don't know that I have ever felt a warm moist RFD. What is the temperature requirement for a cold dry RFD versus a warm moist RFD?

Right, that's why I said warmer, and even that felt awkward to type since I've never been in an RFD that felt warm. Maybe not as cold would have been better. I just meant that RFD's that are warmer than others are more bouyant. A significant portion of the air a supercellular tornado is sucking up is from the RFD. Tornadoes thrive on that warm, moist, unstable air. It would be a detriment to the circulation if had to drag up cold, dry, stable air. Its such a disappointing feeling standing in front of a rain free base, feeling some modest inflow and then getting blasted with bone chilling RFD. You know that storm probably isn't going to produce.

I might also add that at no time during the Edison tornado did I feel/experience any RFD nor could I make out a discernible clear slot. It was not until the supercell moved north towards the Bird City area that I was able to make out a clear slot. Here the tornado was doing F-3 damage and was very large.

There was definitely a distinct RFD/clear slot behind that tornado. The updraft base had bowed into a large horseshoe shape, larger than you often see with a tornado producing storm to the point that it was starting to look like a gust front, but it was definitely an RFD behind the tornado that was driving that bowing base. I've labeled a few photos from that day to better illustrate what I observed that day:

You're definitely right on there not being a good lowering underneath the updraft base for the first few tornadoes. The first few tornadoes appeared to form on the apex or south of the apex of the bowing updraft base, however, whereas you'd normally expect them on the northern curling end of that horseshoe base. You can see there was some wet RFD pouring out of that storm during this tornado, which was heavily rain wrapped.

This image better illustrates where the clear slot was. From our viewing positions, it did not appear to be a neatly cut hole in the base of the storm, since the base of the storm had already bowed out quite a bit. But the brighter area behind the updraft base is definitely the RFD clear slot. You can see where the Edson tornado formed, a non descript portion of the RFB to the south of where you'd normally expect the action. In fact there was a wall cloud forming at that this time, and I believe it was that portion of the storm that went on to form the Bird City wedge.

Here's the Edson tornado. Again, no wall cloud as you mentioned. But I believe the reason for this is that the tornado was actually anticyclonic. Jerry Funfsinn noted clockwise rotation over the radio. It makes sense if you think about the structure on the storm at the time, as it was the southern portion of the bowing horeshoe shaped updraft base that was producing, and this is an area in which you'd most suspect anticyclonic tornadoes to form. Note the area of cyclonic rotation is becoming rain wrapped to the north. Had that been the area producing at this time, we would have probably been out of luck at the time, and too far south. With the lightning back lighting later on, however, we had a nice view of the Bird City wedge that probably formed in that region of the storm. Note the clear area behind the tornado is indeed where the RFD is originating from, and although its not a neatly shaped hole, that whole area could be considered the clear slot, with the forward flanking downdraft well off to the right there in the darker portion of the photo.

One of Jerry's video caps of the Bird City wedge and a secondary tornado. This image doesn't tell much about the RFD/clear slot, but its nice to see with the rest of the structure analysis shots. The second tornado might have also been anticyclonic, but by only catching momentary glimpses of it through backlit lightning flashes, it was impossible to tell.

 

[Jeff Duda]

I also can't proclaim to be an expert here, but I probably should have stated that I was referring more to studies that involved true supercell tornadoes. I think there's still quite a bit to be learned about tornadogenesis, and there are likely still concepts to be fully understood, especially with these non-traditional events like CC setups. In relation to the supercell studies, it would appear that actual tornado production is related to the temperature of the RFD (is their a magic number?) as being one that is at least characteristic of having enough low-level humidity and CAPE to allow for more bouyancy so that the RFD can more readily coalesce with the rotating updraft and be stretched vertically, enhancing near-storm helicity.

Right, that's why I said warmer, and even that felt awkward to type since I've never been an RFD that felt warm. Maybe not as cold would have been better. I just meant that RFD's that are warmer than others are more bouyant. A significant portion of the air a supercellular tornado is sucking up is from the RFD. Tornadoes thrive on that warm, moist, unstable air. It would be a detriment to the circulation if had to drag up cold, dry, stable air. Its such a disappointing feeling standing in front of a rain free base, feeling some modest inflow and then getting blasted with bone chilling RFD. You know that storm probably isn't going to produce.

I'm glad someone finally mentioned buoyancy here. "Warm/cold" and "warmer/colder" still don't mean as much as what Jeff and Greg have mentioned with theta-e and theta-v perturbations. Those things indicate one of the more important ways in which RFDs affect tornadogenesis: buoyancy. If an RFD advects negative buoyancy perturbations or causes microphysical cooling (i.e., evaporation/melting/sublimation of hydrometeors or even evaporation of cloud water droplets) into the region near the mesocyclone, then air parcels in that region will have a hard time rising to generate vertical vorticity from horizontal vorticity, whereas RFDs that bring in positive buoyancy perturbations will result in parcels near/under the mesocyclone being more likely to freely rise due to buoyant forces and thus generate more vertical vorticity.

I would like to add one thing I haven't seen mentioned yet that RFDs do, and that is cause extreme low-level convergence where the RFD gust front boundary meets the outflow boundary from the FFD. This usually helps to get parcels moving upward as well, buoyant or not. That's probably a good chunk of the source of the generalization that a tornado frequently occurs near the interface between the updraft and downdraft of the storm.

 

[Greg Blumberg]

Jeff, thanks for putting the buoyancy term so nicely. That part about the RFD gust front boundary and outflow boundary from the FFD is also a source of vorticity generated by storm-scale baroclinic zones. From what I've heard, theories about the source of vorticity for tornadogenesis have multiple sources.

- RFD/low level inflow generating baroclinic-generated horizontal vorticity
- horizontal vorticity generated along the RFD-FFD gust front interaction that you mentioned
- horizontal vorticity generated via differential heating from the anvil cloud

So I guess to connect the topic of RFD as a so-called "energy source", I guess it can be said that the RFD is part of the conversion of potential energy into kinetic energy for tornadogenesis via thermodynamic and mechanical processes (which in the case of baroclinic-generated vorticity it's both.) This is how I've understood it so far, and I could be wrong...I'll know once I finish my Mesoscale Meteorology course this semester (which I have no idea whenever I'll get to take my second class because of the snow and the AMS annual meeting.)

 

[Joshua Nall]

In general I think its better to look at the apparent clear slot/rfd and the tightening of the lowlevel meso/occasional tornado, as both being part of a larger process. Since this is the educational section, I think the proper perspective for those new to the topic would be to consider the storm as a whole, and look at both the formation of the rfd and formation of any tornado, as the result of how the storm evolves in its environment. With all the talk about the different characteristics of the rfd, temp, buoyancy, etc... and what was observed as far as whether or not there was a tornado involved...someone new to the topic might be led to believe that the rfd plays a much larger role than it really does. Its good to remember that rather than a cause, the rfd, along with the tornado, is a result of something more significant going on with the parent storm, even though they may interact with each other and effect each other.

I personally tend to look at the formation of a lowlevel meso as the result of the overall evolution and strengthening of a storm in a good environment, and it is simply this that ultimately leads to tornado formation. I really can't think of a more simple way to look at it... just like over time, water going down a drain spins faster and tighter, until there is a tiny vortex all the way into the drain... even though in reality I know it's more complicated than that. And it is this same process that causes the observed updraft downdraft interaction in a storm, the rfd, etc... In other words, the same process that leads to formation of a tornado also leads to intense updraft/downdraft interactions. I realize though I may be wrong and there may be more truth to what others have concluded. Again, rather than focus too much on rfd however, for someone learning, a more correct perspective may be to view it all as simply the process of storm evolution, rather than one effect causing the other. That is just my take on it, without getting into the science and terminology I don't fully understand.

 

[Paul Knightley]

I think Jeff has made the research material quite clear: it's the relative temperature of the RFD when compared to the environment, and not really how it subjectively 'feels' to us. Most wind feels cool to us due to the evaporative cooling of our skin - only direct measurements of temp and dewpoint can ascertain how the RFD is in relation to the inflow airmass.
My interpretation is that the RFD causes numerous small vortices to form along its leading edge, mainly down to the mechanics rather than thermodynamics. The inflow then wraps these into the updraught area, increasing vorticity beneath it. Now, this is where RFD thermodynamics probably kick in. It's clearly easier for the updraught to lift air parcels which are warm as opposed to cold. However, the updraught is already likely to be very powerful, so lifting a relatively shallow layer of air, be it warm or cold, might seem to be not that difficult.
I wonder whether 'cold' RFDs surge forward too rapidly and prevent many of the leading-edge vortices making it to the area beneath the updraught, whereas 'warmer' RFD perhaps keep pace with the updraught area better - thus, more low-level vorticity can be fed into the updraught area? Just some thoughts, anyway.

 

[Greg Blumberg]

Paul, I would add that the RFD's effects can be generated by a mixture of thermodynamics and mechanics. It's a complex idea because often in theoretical meteorology thermodynamics and dynamics are very intertwined when it comes to describing phenomena. The vorticity equation's baroclinic term utilizes thermodynamic variables, so I'd say it's the thermodynamics that causes the RFD small vortices. Likewise, the downward motion of the RFD can cause the same vortices by mechanical shear. I think it's tough to pinpoint at what point the mechanics and the thermodynamics play a role. I think your idea of the 'colder' RFDs surging too far forward is on the right track, but I'm not sure about the idea of the 'warmer' RFD.

There's a theory that the source of low level vorticity (as a precursor to the tornado) is caused by the baroclinic effects of the RFD air interacting with the outside air. Think of it as two balls with different weights. If you kick both of them, one is going to go further than the other and thus have a "shear" type effect. With this, in order for you to create more spin you need to have a stronger density difference in the low-level environmental air and the RFD air. Considering a warm low-level inflow, you'd think that you would want your RFD to be as cold as possible to generate as much spin as your can for your tornado. The problem is your spinning air needs to be tilted vertically and stretched in order to generate the tornado. Buoyant low level air can help stretch and tilt the spinning air which will make it spin faster (think the ice skater pulling in her arms.) I believe there needs to be a balance in the RFD for successful tornadogenesis.

 

[Andrew Pritchard]

I've seen a couple Markowski publications posted in this thread and would recommend the following paper as well from Paul, Jerry Straka, and Erik Rasmussen on 'Tornadogenesis Resulting from the Transport of Circulation by a Downdraft: Idealized Numerical Simulations'

Schematic generalizing the model results. In this illustration, a downdraft spirals down the periphery of the updraft, approximately 1 km from its center. Upon reaching the ground, relatively cold downdraft parcels spread mostly away from the center, and convergence of angular momentum beneath the updraft is weak, resulting in a short-lived tornado or no tornado at all. But if the downdraft parcels are relatively warm, they more readily spiral toward the center upon reaching the ground, converging angular momentum to a small radius and forming a long-lived, significant tornado. The differences in the degree of angular momentum concentration owe to differences in low-level stability as well as differences in centrifugal forces