Dissecting the Bowdle Mesocyclone

[Skip Talbot]

In the pic with the radar screenshot, is that GRLv3 or is it something else?

Its custom software I'm developing. See:
http://www.stormtrack.org/forum/showthread.php?26200-2010-Post-Chase-Radar-and-GPS-Analysis

 

[Rob H]

I was hoping that I caught something else, but that inflow feature sure looks like my image from much further out:

What are your thoughts on why that's an inflow feature as opposed to a tornado? Your picture makes it look even more like the real thing. It's cylindrical, and it's scraping the ground, RFD is cutting across it and it's on the back side of the RFB. I'd almost be tempted to say it was an old tornado that's occluding.

 

[Skip Talbot]

I was hoping that I caught something else, but that inflow feature sure looks like my image from much further out:

What are your thoughts on why that's an inflow feature as opposed to a tornado? Your picture makes it look even more like the real thing. It's cylindrical, and it's scraping the ground, RFD is cutting across it and it's on the back side of the RFB. I'd almost be tempted to say it was an old tornado that's occluding.

What's the time stamp on your image? Your shot is probably a different feature from the one in my shot. My shot was before the first tornado and a good 20 minutes before the wedge I believe. The shot I've labeled is also from underneath the RFD clear slot/rain free base, as we were driving underneath the storm at this point. Its a little confusing not being able to see the whole sky, but if you were to look straight up at that point you'd be looking into the RFD clear slot and to your right you'd see more of the rain free base where the tornado formed.

The "inflow feature" is being obscured by terrain. Its not in contact with the ground, and its not rotating (or at least not strongly). Its on the very back edge of the storm, probably caught in the wake of the outflow, an area where you'd see a lot of rising scud usually. The shape of it suggests its a wall cloud like feature, sucking in rain cooled air from the forward flank as that's the direction its pointing.

 

[Rob H]

Yeah, completely different times - but possibly the same type of feature. My picture is from 7:17pm. I haven't seen many of these weird clumps of scud on the back of a storm before, so while I'm fairly certain it's not a tornado, I'm not sure what processes caused it. They're certainly some of the more convincing lookalikes that I've seen.

 

[Joel Taylor]

Skip - I really enjoy the timelapse. One thing I noticed, is when the first tornado lifts around the 5:12 mark, you can follow the eddy all the way around the wall cloud and it looks like it becomes the first tornado to "re-touchdown" at the 5:17 mark once it gets back to the area of the wall cloud where it initially lifted. (does that make sense?)

Robert - that inflow feature forms when some of the rain cooled/moistened air from the hail/rain core gets sucked into the wall cloud/updraft. Since the temp and dewpoint are closer together (smaller dewpoint depression), the LCL (lifted condensation level) will be lower. Essentially part of the same idea as to why the wall cloud forms: mixing air from the "hot" moist inflow region, warm moist rfd region, and cool moist ffd regions, thus lowering the LCL. There's more that goes into it, but this is part of the reason.

Alot of times it can be extremely difficult to tell what you're looking at when it is at a distance. Something that looks like it is scraping the ground could easily be a couple of thousand feet up.

 

[Jeremy Den Hartog]

Wow this thread has a ton of great info for me to learn from

 

[Rob H]

Thanks for the explanation Skip and Joel. I think I wasn't understanding the diagram with the interaction of the FFD and RFD. If the RFD is cutting in front of it, then it can't be on the back side of the storm, and it looks to be closer to the viewer than the precip anyways. If I understand correctly this is near, or part of, the wall cloud, and towards the very back of the updraft/downdraft interface and right next to the rain-cooled air from the precip core? The FFD wouldn't be part of this process then, right? I added a red square to the following diagram where I *think* this feature existed. Please correct me if I'm wrong:

I've seen scud rising into bases before, but never anything as organized and 'funnel-like' as this. What causes it to be so tightly concentrated and in only one place?

Finally, Skip answered the other question I was getting at - with the storm-relative position of this feature I guess you need to be close enough to see the ground and observe any (or lack of) rotation before being able to tell the difference between this and the real deal. That's what I get for sleeping too long in Ft. Pierre that day.

 

[Skip Talbot]

Skip - I really enjoy the timelapse. One thing I noticed, is when the first tornado lifts around the 5:12 mark, you can follow the eddy all the way around the wall cloud and it looks like it becomes the first tornado to "re-touchdown" at the 5:17 mark once it gets back to the area of the wall cloud where it initially lifted. (does that make sense?)

Time timelapses are great for picking out these motions. I see stuff I didn't see when I was actually out there, and every time I watch these videos I usually pick out something new. Watching that circulation orbit its parent circulation was really cool. It seemed like that entire parent circulation is what eventually became the Bowdle EF4, and the first few tornadoes were almost like sub vortices of it. As that second tornado forms you can see the whole mass converging, like its squeezing out that second tornado too. We were parked right next to you guys at the time, and it seemed like such a blur at the time trying to figure out what the storm was doing and keep ahead of it, while managing everything else in the chase. It all starts to make sense when you see it in motion and label the images.

 

[Skip Talbot]

Thanks for the explanation Skip and Joel. I think I wasn't understanding the diagram with the interaction of the FFD and RFD. If the RFD is cutting in front of it, then it can't be on the back side of the storm, and it looks to be closer to the viewer than the precip anyways. If I understand correctly this is near, or part of, the wall cloud, and towards the very back of the updraft/downdraft interface and right next to the rain-cooled air from the precip core? The FFD wouldn't be part of this process then, right? I added a red square to the following diagram where I *think* this feature existed. Please correct me if I'm wrong:

I've seen scud rising into bases before, but never anything as organized and 'funnel-like' as this. What causes it to be so tightly concentrated and in only one place?

Finally, Skip answered the other question I was getting at - with the storm-relative position of this feature I guess you need to be close enough to see the ground and observe any (or lack of) rotation before being able to tell the difference between this and the real deal. That's what I get for sleeping too long in Ft. Pierre that day.

This is in the context of the images I've labeled right?

In terms of the "inflow feature" I've labeled, your red dot is fairly close, but I would actually put it more left, inside the green core of the storm. The core of the storm there was not as fat as is shown in that diagram, and is more like the skinny part of the hook at the bottom of the image. This "inflow feature" is probably a wall cloud. Its not THE wall cloud though. Keep in mind that the dashed region in the schematic is where you're going to see the visible updraft base and wall cloud. The main wall cloud (or wall clouds as the storm was cycling) and tornado did form from there. I believe there was a second wall cloud, inside of the core, towards the west edge of the storm. This would be that inflow feature. My position at the time of the photos was pretty close to your red dot. The tornado formed off to my ESE, and the inflow feature was WNW of me. The top two blue lines are probably feeding this embedded wall cloud with rain cooled air from the FFD, while portions of the RFD are coming down ahead and around it. Its not pulling in the juicy air like the main wall cloud/updraft base is, and its not wrapping up where the RFD and FFD occlude like on the schematic. So its probably not a rotating feature, or at least not tornadic like the main updraft base is. Visually, to me, it looks striking and got my attention at the time, but it does not look like a tornado to me. The shape of the feature and the orientation of it, make me think more of a lowered base with a tail cloud. That point is acting like the tail cloud, where the rain cooled air is condensing well below the rest of the base. A funnel or tornado would have a more vertical orientation in relation to the base and wouldn't be the leading edge of lowering (usually that is, I've seen leading edge funnels point out ahead of the parent lowering). You can see the point on that inflow feature flows pretty neatly into the rest of the lowering. It says wall cloud to me, and even a kind of shelfy wall cloud as its mostly outflow responsible for this feature. I hope some of this makes sense. I'll see if I can draw something on that schematic when I get home.

 

[Joel Taylor]

Thanks for the explanation Skip and Joel. I think I wasn't understanding the diagram with the interaction of the FFD and RFD. If the RFD is cutting in front of it, then it can't be on the back side of the storm, and it looks to be closer to the viewer than the precip anyways. If I understand correctly this is near, or part of, the wall cloud, and towards the very back of the updraft/downdraft interface and right next to the rain-cooled air from the precip core? The FFD wouldn't be part of this process then, right? I added a red square to the following diagram where I *think* this feature existed. Please correct me if I'm wrong:

I've seen scud rising into bases before, but never anything as organized and 'funnel-like' as this. What causes it to be so tightly concentrated and in only one place?

Finally, Skip answered the other question I was getting at - with the storm-relative position of this feature I guess you need to be close enough to see the ground and observe any (or lack of) rotation before being able to tell the difference between this and the real deal. That's what I get for sleeping too long in Ft. Pierre that day.

I would say rotate the image 90 degrees clockwise to orient it similar to the Bowdle storm but overall looks pretty accurate. The red dot could be moved farther down toward the hook, but there could easily be the same type of feature in that location. Supercells are all different, and if there's one thing you'll learn it's that you can't say any rule applies 100% of the time.

Positioning on a storm can completely change your perspective. Distance, sun angle, and location with respect to the storm itself will all give you a completely different view. You can watch videos from the same tornado at the same time shot in different locations and would have absolutely no idea that it was the same storm.

 

[Skip Talbot]

Took a stab at labeling a radar shot from about the time of the "inflow feature"/secondary wall cloud:

The location of the inflow feature is an approximation. It should be somewhere towards the middle of that shaded region I'm guessing . The actual lowering associated with that feature was probably a lot smaller than the shaded region depicts.

 

[Rob H]

Skip - that is an awesome diagram! It almost looks like the start of a second echo appendage, which makes a lot of sense when we discuss two wall clouds. When looking at the scan or two before it, was that a new feature being formed from divergent RFD, or something older that was occluding and making way for the larger appendage?