Kyle Brittain
EF1
Hey folks,
First of all, I'm new to this forum and am excited to be a part of it. To my knowledge, we don't have anything like this in Canada (and am not sure how many Canadians use it), so I'm stoked to be able to discuss severe weather here and hopefully learn some things from you folks, especially from experienced chasers and meteorologists.
*Be advised that this post is LONG, so please, read and respond at your discretion
I understand that the topic of how tornadoes form is still in many ways poorly understood, however my attempt here is to solidify the current knowledge we do possess on the subject today. Some questions remain outstanding for me (and maybe they do for everyone). Four sources of info that I have been referencing lately represent both a meteorological perspective and a storm chaser/spotter's perspective, and include:
How to Make a Tornado, by Markowski and Richardson
- http://www.weatherwise.org/Archives/Back Issues/2013/July-August 2013/torando_full.html
What we know and don't know about tornado formation, by Markowski and Richardson
- http://scitation.aip.org/content/aip/magazine/physicstoday/article/67/9/10.1063/PT.3.2514
Skip Talbot's youtube video "Storm Spotting Secrets"
-
Advanced Storm Spotter Training Webinar, from NWS Norman
-
1) With reference to the scholarly articles, the gist of their message is that environments conducive to strong tornadoes tend to have two things:
- Enhanced low level shear, which increases the strength of the mid level meso, which thereby increases low level "suction," that of course is crucial for low level vorticity stretching
- Increased low level moisture, which leads to less evaporative cooling from downdrafts, which leads to relatively warmer downdrafts (though still possessing baroclinicity), that aren't so negatively buoyant that low level vorticity cannot be ingested into the updraft
However, no specific mention is made of an "RFD" as opposed to an "FFD." In fact, from the images that are used, it almost seems to be implied that the low level vorticity that is generated along the interface of rain-cooled downdraft air and ambient (inflow) air originating from the FFD is responsible for tornadogenesis. This is evidenced by the fact that it shows air descending down from right to left from the main precipitation cascade to beneath the wall cloud. If this is the case, what, if any, role does the RFD play in tornadogenesis?
In other conceptual models I have seen for supercell wind fields, it looks like tornadoes form at the occlusion point between the RFD (wrapping into the hook) and the inflow air.
2) Skip's excellent resource for storm chasers provides a way to "landmark" some of these processes in being able to more precisely locate where a tornado may form in a supercell thunderstorm. He does make mention of the RFD as being a process that "feeds and drives a tornado," and shows that the RFD punches into the updraft base, creating a "horseshoe" shape, characterized by a clear slot that often forms from RFD air. (See 5:50-10:50).
On a reflectivity scan of a supercell, the "hook echo" is the visual manifestation of the RFD, is it not? How then can it be representative of drier, subsiding air? I know that there can be dry and wet RFD's (the latter of which often being associated with HPs), and that supercell mode can change within the same storm, however is there always precipitation reaching the ground beneath the hook echo? One would think that precip falling into drier air would then have a greater evaporative cooling potential, thereby increasing the negative buoyancy of that air, potentially causing tornadogenesis failure. But this clearly doesn't follow in many cases.
3) In the NWS resource, from 1:02:30 on, the origin of the RFD and mechanisms for tornadogenesis in general seem vague (perhaps because an unpacking of this concept isn't the intent of this resource), but some claims are made that seem to be contrary to the above. For example, at 1:03:54, the speaker says "you really need a warm, unstable RFD to stretch that vorticity in the low levels," which is clearly at odds with the idea that the upward directed PGF from the strength of the mid level meso is what is responsible for vorticity stretching. Moreover, in their hypothesis on the origin of the RFD, they claim that there is relative high pressure in the mid levels and strong low pressure in the low levels, that is responsible for a sort of "downward directed" pressure gradient force, that is in turn responsible for subsidence of RFD air. Of course, this too seems to be at odds with the above theory. (I have heard alternate explanations that RFD air is FFD air that has been wrapped around the mesocyclone and modified by mid and upper level air that has been forced to descend after colliding with the back of the storm).
The resource also claims that RFD air causes the clear slot, and that it is "hot and unstable" (though presumably not so "hot" as to not contribute to baroclinicity).
Is there any generally agreed upon hypothesis as to the origin of the air the RFD? And if it's properties are different from those of FFD air, are there in fact 3 different air masses within the storm scale vicinity of a tornadic supercell (ambient/inflow, FFD, and RFD)?
Congrats if you've made it through this and have seen my 3 general questions through all the chatter. Again, I realize that there may be no agreed upon explanations of the questions I've presented - though I do seek clarification for apparent inconsistencies in current scientific knowledge. The questions were, again, in sum:
1) To what extent does the RFD (as opposed to FFD) play a role in tornadogenesis?
2) Is there always precip reaching the ground beneath a hook echo, and can an RFD associated with an echo be simultaneously associated with a clear slot?
3) What is the origin of RFD air, and are there three distinct air masses in the vicinity of most tornadic supercells?
First of all, I'm new to this forum and am excited to be a part of it. To my knowledge, we don't have anything like this in Canada (and am not sure how many Canadians use it), so I'm stoked to be able to discuss severe weather here and hopefully learn some things from you folks, especially from experienced chasers and meteorologists.
*Be advised that this post is LONG, so please, read and respond at your discretion
I understand that the topic of how tornadoes form is still in many ways poorly understood, however my attempt here is to solidify the current knowledge we do possess on the subject today. Some questions remain outstanding for me (and maybe they do for everyone). Four sources of info that I have been referencing lately represent both a meteorological perspective and a storm chaser/spotter's perspective, and include:
How to Make a Tornado, by Markowski and Richardson
- http://www.weatherwise.org/Archives/Back Issues/2013/July-August 2013/torando_full.html
What we know and don't know about tornado formation, by Markowski and Richardson
- http://scitation.aip.org/content/aip/magazine/physicstoday/article/67/9/10.1063/PT.3.2514
Skip Talbot's youtube video "Storm Spotting Secrets"
-
Advanced Storm Spotter Training Webinar, from NWS Norman
-
1) With reference to the scholarly articles, the gist of their message is that environments conducive to strong tornadoes tend to have two things:
- Enhanced low level shear, which increases the strength of the mid level meso, which thereby increases low level "suction," that of course is crucial for low level vorticity stretching
- Increased low level moisture, which leads to less evaporative cooling from downdrafts, which leads to relatively warmer downdrafts (though still possessing baroclinicity), that aren't so negatively buoyant that low level vorticity cannot be ingested into the updraft
However, no specific mention is made of an "RFD" as opposed to an "FFD." In fact, from the images that are used, it almost seems to be implied that the low level vorticity that is generated along the interface of rain-cooled downdraft air and ambient (inflow) air originating from the FFD is responsible for tornadogenesis. This is evidenced by the fact that it shows air descending down from right to left from the main precipitation cascade to beneath the wall cloud. If this is the case, what, if any, role does the RFD play in tornadogenesis?
In other conceptual models I have seen for supercell wind fields, it looks like tornadoes form at the occlusion point between the RFD (wrapping into the hook) and the inflow air.
2) Skip's excellent resource for storm chasers provides a way to "landmark" some of these processes in being able to more precisely locate where a tornado may form in a supercell thunderstorm. He does make mention of the RFD as being a process that "feeds and drives a tornado," and shows that the RFD punches into the updraft base, creating a "horseshoe" shape, characterized by a clear slot that often forms from RFD air. (See 5:50-10:50).
On a reflectivity scan of a supercell, the "hook echo" is the visual manifestation of the RFD, is it not? How then can it be representative of drier, subsiding air? I know that there can be dry and wet RFD's (the latter of which often being associated with HPs), and that supercell mode can change within the same storm, however is there always precipitation reaching the ground beneath the hook echo? One would think that precip falling into drier air would then have a greater evaporative cooling potential, thereby increasing the negative buoyancy of that air, potentially causing tornadogenesis failure. But this clearly doesn't follow in many cases.
3) In the NWS resource, from 1:02:30 on, the origin of the RFD and mechanisms for tornadogenesis in general seem vague (perhaps because an unpacking of this concept isn't the intent of this resource), but some claims are made that seem to be contrary to the above. For example, at 1:03:54, the speaker says "you really need a warm, unstable RFD to stretch that vorticity in the low levels," which is clearly at odds with the idea that the upward directed PGF from the strength of the mid level meso is what is responsible for vorticity stretching. Moreover, in their hypothesis on the origin of the RFD, they claim that there is relative high pressure in the mid levels and strong low pressure in the low levels, that is responsible for a sort of "downward directed" pressure gradient force, that is in turn responsible for subsidence of RFD air. Of course, this too seems to be at odds with the above theory. (I have heard alternate explanations that RFD air is FFD air that has been wrapped around the mesocyclone and modified by mid and upper level air that has been forced to descend after colliding with the back of the storm).
The resource also claims that RFD air causes the clear slot, and that it is "hot and unstable" (though presumably not so "hot" as to not contribute to baroclinicity).
Is there any generally agreed upon hypothesis as to the origin of the air the RFD? And if it's properties are different from those of FFD air, are there in fact 3 different air masses within the storm scale vicinity of a tornadic supercell (ambient/inflow, FFD, and RFD)?
Congrats if you've made it through this and have seen my 3 general questions through all the chatter. Again, I realize that there may be no agreed upon explanations of the questions I've presented - though I do seek clarification for apparent inconsistencies in current scientific knowledge. The questions were, again, in sum:
1) To what extent does the RFD (as opposed to FFD) play a role in tornadogenesis?
2) Is there always precip reaching the ground beneath a hook echo, and can an RFD associated with an echo be simultaneously associated with a clear slot?
3) What is the origin of RFD air, and are there three distinct air masses in the vicinity of most tornadic supercells?
