Rapid-Scan Mobile Radar Observations of Tornadogenesis (ground-up tornadogenesis paper) [Paywalled]

Dan Robinson

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Historically, the generally accepted hypothesis surrounding tornadogenesis in supercell thunderstorms is referred to as the 'top-down' hypothesis. According to this theory, strong rotation develops above the ground first, followed by progressively strengthening rotation at lower heights until the vertically rotating column intersects the ground, contracts, and forms a tornado. This paper provides evidence from a novel rapid-scanning mobile Doppler radar (RaXPol) that this hypothesis is likely incorrect. Instead, it appears that in many cases, tornadic-strength rotation develops either at near-ground levels first, or contemporaneously throughout the depth of the tornado-bearing layer.
This is the paper presenting the "bottom-up" theory of tornadogenesis. Of note is the idea that the RFD is a consequence of the tornado cyclone, not the producer of it.
 

Dan Robinson

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So, I've had some questions about this theory that pertain to behavior that many of us have observed in the field. Namely, that RFD seems to "operate" independent of a low-level mesocyclone. That is, visually you can have a clear slot with very little in the way of motion in the cloud base nor a couplet on radar. There are many cases where the RFD simply overpowers everything - the clear slot cuts in and the updraft is blown apart. Or, the RFD and FFD gust fronts merge and together destroy any hint of a low level meso. The tornadoes seem to be most likely when the RFD and inflow are well-balanced. The fact that they are so often out of balance is what is confusing.

I'm just having a hard time wrapping my head around the RFD being a consequence of the low-level circulation when so many real-world cases seem to suggest otherwise. The RFD appears to be an independent feature that is not well-coupled to the low-level meso or even the midlevel circulation.

It's not unsurprising that once a strong low-level meso develops, it would start influencing the RFD's behavior during the more advanced stages. It's just the idea that a pre-existing circulation is responsible for the RFD from its earliest stages is hard to grasp.
 
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Very late to this post (my bad) but I was perusing some late night “reading material” to get my eyes sleepy and I came across this article.

I see Dan R. has made a previous thread and wanted to chime in.
@Dan Robinson any updates on this proposed theory? I couldn’t find any additional information. Looks like Anton and Jeff Snyder were on board...would love to see any updates if you know of any.

Im struggling with the proposal of the RFD in general.
 

Jeff Duda

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So, I've had some questions about this theory that pertain to behavior that many of us have observed in the field. Namely, that RFD seems to "operate" independent of a low-level mesocyclone. That is, visually you can have a clear slot with very little in the way of motion in the cloud base nor a couplet on radar. There are many cases where the RFD simply overpowers everything - the clear slot cuts in and the updraft is blown apart. Or, the RFD and FFD gust fronts merge and together destroy any hint of a low level meso. The tornadoes seem to be most likely when the RFD and inflow are well-balanced. The fact that they are so often out of balance is what is confusing.

I'm just having a hard time wrapping my head around the RFD being a consequence of the low-level circulation when so many real-world cases seem to suggest otherwise. The RFD appears to be an independent feature that is not well-coupled to the low-level meso or even the midlevel circulation.

It's not unsurprising that once a strong low-level meso develops, it would start influencing the RFD's behavior during the more advanced stages. It's just the idea that a pre-existing circulation is responsible for the RFD from its earliest stages is hard to grasp.
I think the real connection is between the mid-level mesocyclone and the RFD. I suspect that the RFD and any low-level mesos in most storms truly do operate independently of one another.

The seminars (many hosted by authors on that abstract) I attended while at OU all suggested there is not one unified theory for how all tornadoes form...even supercellular ones. There are many possible mechanisms. So in that regard I'm a little surprised to hear the confidence of that wording "incorrect."
 
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