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RFD Origin As It Relates To Tornadogenesis

Here's some thoughts I had on tornadogenesis and the RFD as posted to wx-chase. Feedback Welcome.

Note that this is by no means a scientific study, but instead some thoughts and hypothesis I wanted
to share regarding RFD's and Tornadogenesis. As far as I know this concept as I explain / propose it
has not been proposed before although various individual aspects of it have. This is my attempt at a
bit of a 'unifying' of various individual concepts and ideas already documented or proposed. Perhaps
some of what I propose through a layman's perspective even though it may not be entirely correct or
accurate may help 'turn the light on' for some to then delve deeper into the true innermost workings
of the processes at work in supercells.

After reading "An Examination Of The 12 June 2004 Mulvane, Kansas Tornado" http://www.targetarea.net/MulvaneSESSS.pdf by Scott Blair and Eric
Nguyen, I am reminded once again of many of the factors which have been hypothesized to be related
to tornadogenesis in supercell storms. From my studies, my current understanding is that the state
of the art is still somewhat mystified by this process and the mechanism(s) of tornadogenesis. It
seems various sources have hinted at and show relationships between Rear Flank Downdraft (RFD) winds
and the later somewhat often simultaneous development of a tornado. It is my understanding however
that it has been documented that supercell tornadoes sometimes do form in the absence of an RFD, but
that in the majority of cases RFD influence seems to be present. As noted in the article mentioned
above and other sources, the radar indicated BWER has also been observed as often collapsing at a
time just prior to tornado development. Additionally I personally have also witnessed supercell
overshooting tops collapsing just prior to tornado development. I have also been witness to at least
one grand visual example of an obvious RFD pushing down the back side of a supercell and pulling /
forming cloud material down below (behind) updraft cloud base, which quickly led to the formation of
a tornado.

In my mind I have pushed these ideas around and tried to make them fit together to try and
personally make sense of what is happening in the storm. It is my understanding that in addition to
the process of tornadogenesis not being totally understood, it is also not clearly understood what
the origin of the RFD is, and the process that is involved to form it. Additionally the mechanism
whereby the RFD contributes to tornadogenesis is not clearly understood either even though it
appears it does have a role to play.

After considering these mechanisms together I'd like to hypothesize that the observation of lowered
BWER height, collapsing overshooting top, and development / formation of the RFD are related, and
that the RFD is a significant contributor to tornadogenesis. The real question of course is how are
they related and what factors precipitate this process?

There could be a number of scenarios suggested that might partly explain how this interaction could
result to create the RFD. Intuitively, I'm thinking that the updraft behaves somewhat similarly to a
large fountain such that water is forced upward to the point where energy and momentum is exhausted,
and there the water somewhat pools and attempts to back up and fall down. In a storm situation
however the parcel of air would rise very quickly until it reached a level probably somewhat above
it's equilibrium level and parcels would then begin to sink and back up. Many if not most of these
parcels would be swept downwind and into the anvil; however I believe some would attempt to fall out
the backside such as in the case of a back sheared anvil and hail stones that cycle in the cell by
first traveling out the back side of the storm cell. Probably this would lead to a type of
circulation at the high levels. At some point, it would appear that something happens to disrupt
this cycle. I'm thinking somehow the updraft organization is disrupted, and the strength of the
updraft falters for a small period of time. When this occurs the overshooting top collapses, and the
BWER height falls. At this point there is chaotic backwash which occurs as the parcels being held
aloft develop momentum falling behind (upstream) the updraft. Probably the 'spillage' of the updraft
would occur on all sides to some degree but downwind system flow may become congested because it is
already at max flow rate for the previous state of the system. The backwash then comes down in the
form of the RFD, and upon reaching the ground spreads out and a large amount is then re-ingested
into the updraft.

I'm thinking at this point somehow the properties of the RFD are such to enhance tornado formation.
Part of this may be that it enhances vertical vorticity as the RFD winds spread out around the
tornado but are then pulled in such as a loop and tightened as it enters the updraft thereby
tightening up into a tornadic circulation. Additionally I would think the bouyancy of the RFD such
as temperature and dewpoint being of slightly different composition to what was described as the
BSSI (Base Storm State Inflow) is also a factor. The aforementioned article referenced that RFD's
associated with tornadic supercells often contain surface equivalent potential temperature, virtual
potential temperature, and convective available potential energy values equal to or slightly less
than the BSSI. This 'warm and buoyant' source assists in maintaining or enhancing tornado potential.

Previously I mentioned that somehow the updraft organization and strength is disrupted which leads
to this cycle. One of the things in my opinion that could possibly temporarily interfere with the
updraft in a supercell storm could be cell ingestion and maturation of flanking storm cells, or
perhaps ingestion of an outflow boundary. Another could be cell mergers with other cells. As I
understand it mergers usually enhance the tornadic development however in the case of Mulvane it was
a left split storm which have been documented to produce a disorganizing effect in the merger of
left and right moving supercells. Perhaps if the Mulvane cell had merged with another cyclonically
rotating cell the mesocyclone would have been strengthened. Regardless what I am getting at is
external interaction with the updraft which leads to a 'pulsed' flow momentarily.
Bill, I found the following article on this topic quite useful and scrutable:


My most interesting take was the notable differences in RFD properties such as CAPE between tornadic and non-tornadic systems. Specifically, CAPE values of the RFD were higher (relative to surrounding storm environment) in the storms with lower LCL heights, perhaps contributing to the vorticity necessary for the low level "spin up", through the principle of conservation of angular momentum.
Great post Bill, I love reading stuff like this :)

I remember having read somewhere that dry middle to upper tropospheric air can aid in producing an RFD that is anomalously warmer when compared to the surrounding environment. I'm not well read on the topic but I believe it has to do with some sort of adiabatic process. Of course there was another study that said too much mid-level dry air can cause the downdraft to become too strong and undercut the circulation.

I was wondering if you knew anything about this. I'm still browsing some papers right now trying to find something on the warming of the RFD.

...Alex Lamers...
There really are atleast two papers everyone should make sure to read when diving in to RFD processes:

Markowski, P. M., 2002: Hook echoes and rear-flank downdrafts: A review. Mon. Wea. Rev., 130, 852-876. [/url]

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.
RFD in tornadogenesis

This thing about the RFD not being particularly cold sounds familiar to me. I think that I did read that even although the air in an RFD is "outflow", it still often has non-trivial CAPE in supercells that are producing tornadoes. I would guess that typicaly the air in the Forward Flank Downdraft (FFD) and the main precipitation core is usually so cold that it would contain no CAPE at all.