Horizontal striations on storms

[Tim Bond]

Hullo,

A quick intro first, as I'm new here! I'm Tim, from the UK, so not terribly well located for frequent photogenic supercell storms. This is a shame, as from early childhood I've been fascinated by them, having seen a very otherworldly (by my standards) photo of a beautiful striated supercell in a junior-school textbook. Having finally finished college poverty, and got a job with a paycheque to enable transatlantic trips, I've spent a couple of weeks in each of the last two summers (bad timing to start chasing...) with SLT getting even more fascinated by severe weather - the 11th June '06 Scottsbluff storm got me completely hooked, if any additional addiction was needed. I'm now looking forward to 2008 as a newbie chaser heading west with a couple of chase-buddies and hoping for better conditions than 06/07 gave me. I'm curious to find out both more about storms and, in particular, more about the people who study and chase them, hence signing up here!

On to the question:

On the shelf-clouds (I hope this is the right term!) of multicell lines and on supercell updraughts there frequently seem to be strong striations, horizontal in appearance in the former case, and either horizontal (plate-stack style) or slightly angled (barber-pole style) in the latter.

My question: what's the physical background to why these striations form?

The only guess I have come up with is that it relates to some kind of vertical stratification of inflow, with variations in moisture content with height. This doesn't convince me entirely, in that whilst it roughly tallies with my mental picture of how the inflow is interacting with the line and the stack-of-plates supercell, it seems to falls down on the barberpole presentation.

Thanks in advance for explanations/pointers to literature!

Tim

 

[Shane Adams]

on supercell updraughts there frequently seem to be strong striations, horizontal in appearance in the former case, and either horizontal (plate-stack style) or slightly angled (barber-pole style) in the latter. My question: what's the physical background to why these striations form?

These striations are simply the physical manifestation of wind shear. Winds changing direction and speed with height create a horizontal 'spin' motion in the atmosphere (recently some chasers have stated they do not necessarily believe this is accurate, but it's the generally-accepted theory). When thunderstorms develop, the updraft tilts this area of horizontal spin into the vertical, creating the rotation you see as striations in the updraft.

That's a grossly-simplistic answer, but it's the gist of things. There are others with more experience/knowledge that may elaborate on this.

 

[Gene Moore]

On the shelf-clouds (I hope this is the right term!) of multicell lines and on supercell updrafts there frequently seem to be strong striations, horizontal in appearance in the former case, and either horizontal (plate-stack style) or slightly angled (barber-pole style) in the latter. My question: what's the physical background to why these striations form?

Tim, you're asking about striations, but your question covers two quite different situations. That is, the stacked plate banding as seen on outflow boundaries of multicellular (squall) lines vs striations on supercells. I'll try for a short answer and see if we're headed in the right direction. First, striations are caused by wind interacting with cloud material. It can interact in different ways, the higher speed wind can blow past (over) the cloud shearing it into a smooth surface; the wind can be behind the cloud pushing it into a smooth laminar surface, or the wind and the cloud are associated together. Each can mold or sculpt a somewhat different appearance. There are three different flow patterns with respect to the cloud, but all are mechanical, that is, there is little or no buoyancy. In the case of low level striations associated with gust fronts and stacked shelf clouds the condensation (cloud material) acted upon is quite likely neutral to negatively buoyant (colder than the surrounding air) so it has little of no motion of its own. Thus not being capable of rising it is easily acted upon (pushed around) and molded by the winds associated with the storm, or in the ambient atmosphere. In the case of the stacked shelf cloud it may be a combination of both. For example, if cold air pushes out of a storm and condenses a long outflow cloud it will first take the form of that pushing air. This is what happens in a no-flow tropical or summer regime. But for example, if that storm and shelf cloud move south across a regime of strong southwest winds (say at 5,000 to 10,000 feet AGL). Then the outflow cloud has two motions acting upon it. The cold air from the storm and (likely) warmer southwesterly flow at that level. The southwest winds will create convergence at that level causing the once shallow shelf cloud to "stack up" vertically. Exactly how this occurs is yet another subject, but it's this convergence/ lifting that causes the plate like appearance. Gust fronts tend to have surges from the down drafts of the storm. Each of these surges will likely create another stacked plate.

With regard to supercells, often there is a smooth laminar appearance in the cloud above the wall cloud/ mesocyclone. Again, the lowest level wind shear will sculpt the negatively buoyant air into the shape of the flow about the mesocyclone. This is due to the strong updraft above the wall cloud pulling the air aloft, even through it may not be buoyant. On rare occasions where the LFC (level of free convection) is very low we will see bubbles of condensation lifting off the side of the wall cloud (good sign). In this case there are less striations and more strong (through the layer) vertical motion. That is, the strong updraft will overwhelm the speed of the westerlies and the cloud motion will take that shape. It is quite common in supercells for the back (up wind) side of the updraft to be cumulaform and the downwind to be striated. The reasons for this are still up for conjecture and likely beyond this thread. Finally, some supercells are striated all the way up to the base of the anvil. These may occur with huge and violent mesocyclones that go deep into the storm. If lucky I get to see maybe one of these per year and it's a treat. On extreme cases I've seen cloud material rotating so fast that it tears off from the side of the CB updraft and dissipates as it falls behind the flow. I have never seen a supercell with this motion that it did not produce a large/ violent tornado, but I guess anything is possible. On most other occasions where striations extend up the side of the supercell it's more likely that the storm is being sculpted by winds flowing around the outside of the storm.* In this case the side of the cloud is not moving as fast as the ambient midlevel flow and there is no tearing off of cloud material. You'll need a clear view of the storm's updraft to anvil level to determine this visually. As for what exactly creates the 45 degree band spacing across the face of the updraft, that's more difficult. I believe there is some rotation across the flow as it rises allowing dry air to produce notches within the banding. Also the part of the band with the slower flow is more subject to dry air erosion. Finally, this pattern originates off the back side or updraft portion of the storm. It's possible that the banding is associated with the frequency of these sheared off updraft cells. Hopefully someone else may have another explanation for this pattern.

Let's see how you do with this answer, or if you have any questions before we go any further. Others (hopefully) will have comments to add, or they may disagree with me....that's what these discussion boards are for.

* This comment assumes that the storm is somewhat of a solid body to the upper winds and some of the flow goes around the cell. There are those in the scientific community that strongly object to this approach.

 

[Gene Moore]

These striations are simply the physical manifestation of wind shear. Winds changing direction and speed with height create a horizontal 'spin' motion in the atmosphere (recently some chasers have stated they do not necessarily believe this is accurate, but it's the generally-accepted theory). When thunderstorms develop, the updraft tilts this area of horizontal spin into the vertical, creating the rotation you see as striations in the updraft.

Hell Shane that works for me....didn't see your answer coming. Could have saved me 45 minutes

 

[Tim Bond]

Thanks, Gene and Shane, much appreciated.

You're right - I had assumed that both squall-line and supercell striations came from the same origin. I'm starting to get a better picture (thankyou!), and will come back with a few more questions.

In the case of the squall line, I hope I'm thinking on the right lines when I'm imagining a cold pool of storm outflow undercutting warm inflow and acting as a cold front forcing lifting of the inflow. Then once the inflowing (relatively warm, moist?) air has been lifted to condensation level, the condensation (which forms the cloud?) frees latent heat which sets the air parcel off up a wet adiabat and into strong buoyant convection. Feeding this back into your explanation of where the striations are coming from, I'm imagining pulsing outflow from the storm which temporarily increases the angle of the cold front and forces a discrete parcel of inflow upwards (giving a line of shelf cloud), followed by a line of weaker outflow (hence weaker forcing, hence less condensation). So, if I was watching a line, I'd see sequentially formed shelf clouds ascending into the body of the line. Am I imagining this correctly?

My slight confusion here is that you mention the shelf cloud is forming in neutral/negatively buoyant air, which sounds like outflow, whereas I'd imagined the cloud would be in the inflow and releasing latent heat into already buoyant air. Which of these is the shelf cloud actually in, or is it forming along the front itself between the airmasses?

(An added perplexity hitting me is that I'm not quite sure what you mean by an 'outflow' cloud - is this cloud forming in outflow air? Or condensation of moist inflow where it encounters outflow?)

I think the same confusion is puzzling me with the supercell case - I'd always envisaged cloud droplets as being in the warm, positively buoyant air, and evaporating in the cooler, negatively buoyant air, hence I think I've got hold of the wrong end of the stick with your explanation (sorry!). In the supercell case, I'd envisaged the negatively buoyant air being the RFD/FFD and cold pool, both of which I'd thought were characterised by having quite a bit of precipitation falling through them but being regions of evaporation rather than condensation, hence cloud-free.

You explanation certainly clicks in my mind thinking about video where I think I've seen striated clouds appearing to rotate around the storm at low level without much ascent, but now I'm puzzled as to which airmass those clouds are in.

As an additional question arising: now I'm thinking along the lines of cloud sculpting from environmental winds, would it be the case that where you get a really smooth, curving hodograph you don't get striated storms, as the effect of wind on the storm would just change smoothly with height? Thus, where you get quite sharp, vertical changes in wind speed and/or direction, would you get more striation? I suspect this is too vast a simplification to be valid..

Thanks again,

Tim

 

[Gene Moore]

Thanks, Gene and Shane, much appreciated.
My slight confusion here is that you mention the shelf cloud is forming in neutral/negatively buoyant air, which sounds like outflow, whereas I'd imagined the cloud would be in the inflow and releasing latent heat into already buoyant air. Which of these is the shelf cloud actually in, or is it forming along the front itself between the air masses?

First let me touch on the over used term "inflow." This was originally derived from "inflow jet" into a mesocyclone of a supercell. It's reached a point today where all SE winds pouring into a storm are called inflow. This is not totally correct, what's actually going on is convergence as opposed to a channeled inflow (into a mesocyclone). All storms have convergence or there would not be a storm, but not all storms have inflow, or an inflow jet. In the case of a squall line there is most likely a broad field of convergence as opposed to inflow. Of course if air can outflow out of a storm why can't it inflow into the storm....hence the problem with the semantics here. If we are trying to be correct we should think in terms of convergence as opposed to specifically inflow, especially in a squall line.

(An added perplexity hitting me is that I'm not quite sure what you mean by an 'outflow' cloud - is this cloud forming in outflow air? Or condensation of moist inflow where it encounters outflow?)

The shelf cloud or arcus gust front cloud on the leading edge of a line is cold (near saturation) outflow. It is for that reason I referred to it as negatively buoyant. It is not going to rise on its own unless acted upon. Thus enter the convergence that helps form the stacked plates, both work together. Now with that in mind go back and reread what I wrote about how the outflow turns into the stacked plates. I think (hope) it will make more sense this time. Stacked plates are stable, if a new convective buildup starts to form along the leading edge of the outflow boundary then the moist converging air is being lifted. Your original question was for stacked plates though, the not buoyant outflow case.....follow?

I think the same confusion is puzzling me with the supercell case - I'd always envisaged cloud droplets as being in the warm, positively buoyant air, and evaporating in the cooler, negatively buoyant air, hence I think I've got hold of the wrong end of the stick with your explanation (sorry!). In the supercell case, I'd envisaged the negatively buoyant air being the RFD/FFD and cold pool, both of which I'd thought were characterized by having quite a bit of precipitation falling through them but being regions of evaporation rather than condensation, hence cloud-free.

Correct, but let's take that a step further. The cooler air mixes with and is pulled into the updraft/ developing mesocyclone...it is entrained into the updraft flow. That in turn lowers the LCL (lifted condensation level) and forms the wall cloud. So, yes there is moist buoyant warm air being lifted in the meso updraft, BUT it's getting mixed with the cooler more saturated outflow air from the FFD. When you see a wall cloud which side is usually lower? Bet you it's the side nearest the storm core. If all the air that is being lifted is buoyant then there will be few striations other than what comes from the high speed rotation of the wall cloud itself. But, they likely won't have that laminar striated appearance you originally asked about. That usually occurs with entrained air that is neutral to negatively buoyant.....hope this is beginning to makes sense. That's why all storms don't look exactly alike, the temperature of the rising air is somewhat different than its surrounding environment. Think of it this way, updraft clouds are buoyant (cumulus), stratus cloud are not. The clouds in a storm that get spun up or sculpted into smooth glass looking configurations are likely neutral or negatively buoyant. Only in a case where strong to violent rotation is in progress is this buoyancy or cumulaform appearance overcome and smeared out into a smooth laminar appearance. And when this happens generally if you look at the storm closely you'll see that turbulent tearing away of the side of the updraft. After awhile you'll get a trained eye and you will be able to tell the difference in how the striations are formed. It's not all the same process depending on the height in the storm, the temperatures in the rising air and the shear. It's these subtle differences that give us drastic configurations in supercells. Sometimes supercells form arching clouds that wrap around the updraft. These have a stable look about them and generally are laminar in appearance. I think they form when the air outside or surrounding the updraft is lifted. These clouds sometimes attach to the side of the updraft adding to the spun glass or spun pottery look to some supercells.

As an additional question arising: now I'm thinking along the lines of cloud sculpting from environmental winds, would it be the case that where you get a really smooth, curving hodograph you don't get striated storms, as the effect of wind on the storm would just change smoothly with height? Thus, where you get quite sharp, vertical changes in wind speed and/or direction, would you get more striation? I suspect this is too vast a simplification to be valid..

Environmental shear alone is only half the process, remember we have to take into account the temperature of the air being lifted. Sometimes I can look at a sounding/ hodo and assume "twisted updrafts today," but it doesn't always work. I will say this, some of the most drastically sculpted clouds (supercells) I've seen were south moving storms in NW flow aloft. If the sounding shows a nice smooth transition from southeast winds at the surface to NW winds aloft there is tremendous directional shear with height. This often makes for very photogenic (twisted) supercells.

Gene Moore

 

[David Wolfson]

Likely Gene has already said more eloquently and accurately what I'm about to opine simplistically, but here goes....

I read it that one of Tim's questions is why there are multiple striations, i.e. why the structure is sometimes prominently banded. I'd puzzled over that question myself and come to some conclusions in my mind's eye.

Unless I'm mistaken cloud striations in general mark the ascent and apex areas of shear rolls (and gravity waves) whereas the cloud-free areas between the striations mark the descending and counter-flow areas.

The boundary layer (0-3k feet) in the storm environment is mainly affected by the local scale influences of friction, outflow, FFDs and RFDs, and the storm-generated inflow, etc. Above that mesoscale influences become significant -- where the LLJ is found, etc. There's significant velocity shear between the boundary and the layer above and stability conditions that are neutral or inverted enough so as not to immediately mix up the boundary. One's intuition wants to think that whole rather violently dynamic environment is a big blender bottom to top, but I think that is not the case.

Shear rolls naturally develop between these lower layers. Sometimes I've seen these cited (incorrectly I believe) as the horizontal rolls that may become "twisted" to the vertical by the storm updraft and play a significant role in tornado formation. Anyway... most of the time the thermodynamics aren't right to put a condensation "hat" on these rolls as they slide toward the main updraft.

As they reach and slide successively into the main updraft they're sliced off and ascend, whereupon the formerly invisible rolls may become visible due to the adiabatic forcing. This is my explanation for the barber-pole striation structure somewhat bell-shaped at low-mid levels. FWIW.

 

[Gene Moore]

Unless I'm mistaken cloud striations in general mark the ascent and apex areas of shear rolls (and gravity waves) whereas the cloud-free areas between the striations mark the descending and counter-flow areas.

The boundary layer (0-3k feet) in the storm environment is mainly affected by the local scale influences of friction, outflow, FFDs and RFDs, and the storm-generated inflow, etc. Above that mesoscale influences become significant -- where the LLJ is found, etc. There's significant velocity shear between the boundary and the layer above and stability conditions that are neutral or inverted enough so as not to immediately mix up the boundary. One's intuition wants to think that whole rather violently dynamic environment is a big blender bottom to top, but I think that is not the case.

Shear rolls naturally develop between these lower layers. Sometimes I've seen these cited (incorrectly I believe) as the horizontal rolls that may become "twisted" to the vertical by the storm updraft and play a significant role in tornado formation. Anyway... most of the time the thermodynamics aren't right to put a condensation "hat" on these rolls as they slide toward the main updraft......................

Nice David, thanks for the help. I was hoping someone would take this the rest of the way. I got too caught up in trying to explain how these features develop, like gust fronts...before I got into the stacking.

There is a localized pressure gradient flow that's a reflection of the strengthening mesocyclone aloft. The midlevel rotation causes lifting pressure gradients and new stronger updrafts. Its this dynamically induced pressure gradient (in the meso) with height that takes the storm updrafts beyond classic thermodynamic velocities. Would this vertical pressure reduction extend to the edge of the visible updraft, thus contributing to the horizontal banding along the outside of the updraft at midlevels, seems so to me, but you appear to have a better grip on this?

 

[Tim Bond]

Many thanks, Gene and David. You're answering questions I didn't know I had, with information I didn't know to ask for - great!

The distinction of converging air masses vs. inflow is certainly not something I'd appreciated the semantic pitfalls of. I think that's clearer in my mind now, and particularly useful too is the clarification of what a shelf cloud is - I'd thought before that it was forming in the airmass converging with the outflow rather than in moist outflow air.

I might have got hold of the wrong end of the stick again, but have I understood correctly that where you see a shelf cloud at the leading edge of a squall line you're seeing condensation within lifted outflow, the clear air in front of the shelf cloud is the southeasterly flow converging with the cold outflow front, possibly warm and buoyant but not necessarily so, and if there is clear air behind the shelf cloud it will be subsiding or vertically-static outflow?

I'd also completely misinterpreted what I was seeing in supercell shelf clouds. If I've now understood correctly, the beautiful smooth, striated low-level cloudforms are forming from moist, cool outflow lifted by the updraught from around the updraught base and smoothed by environmental winds. Am I on the right lines now?

Trying an extension of this idea - would it be the case that you'd be likely to see a shelf-cloud extending most/all of the way around the updraught before a strong RFD develops, but once a strong RFD is present it would block the updraught from raising a shelf cloud on the rear flank of the storm, though would develop a squall-line style shelf cloud along a flanking line?

Many thanks again for your time and patience - very much appreciated!

Tim

 

[David Wolfson]

For sure there's a whole lot of stuff going on in and around thunderstorms most of which I'll happily leave to Gene and the gurus to try to account for. In general though I think it's fair to say that smooth clouds usually characterize laminar, smooth flow whereas rough, scuddy clouds indicate turbulent, mixed flow.

For example smooth shelf clouds in general indicate laminar forced lifting of warmer/moister environmental air. When there is more air locally forced upward than the storm can accommodate through local free convection the excess must go somewhere. The disposal of that local excess is characterized by turbulence such as scuddy rotor stuff separating one or more shelf clouds from the outflow front.

There are different mechanisms by which the forced lifting may occur. Two that are I think especially relevant to this discussion are 1) downdraft outflow and 2) convergence (e.g. "storm-relative motion"). The first is characterized by lower level shelf clouds associated with a gust front. The second is characterized by the sort of more elevated "shelf clouds" associated with supercells (annular) , bows (bowed -- duh), and squall lines (linear).

The healthy supercell is at best a well-oiled machine where the vorticity and potential energy of the environment is ingested in mostly laminar flow -- more efficient that way -- and forced to instability only as it associates closely with the highly turbulent main updraft. Under the right conditions the laminar cloud features are thus particularly well-exposed in comparison to less organized storm structures.

FWIW. Note: I am not a meteorologist, but am currently staying at a Holiday Inn Express in RST.

 

[Gene Moore]

Many thanks, Gene and David. You're answering questions I didn't know I had, with information I didn't know to ask for - great!
Tim, we're not trying to take this off the deep end, it's just hard to answer some of the questions without getting a basis for what's going on first. For example, the stacked shelf cloud... it's difficult to address that without back tracking to what the storm is doing. Getting to that point needs some background of where the air is coming from and why. I certainly did better on that part than I did the banding

I would like to see a Stormtrack section where all of us could ask questions like yours, but this is a good start.

 

[Tim Bond]

Gene - I *like* that you are adding in more information. Far too often answers are a bit hand-wavy and just assume the reader knows all the background, and leave lots of misunderstandings lurking in the background. Your answers and patience didn't, and did a great job of shoring up my somewhat shaky background knowledge to the point that I now feel happy that I really have the beginnings of an understanding of what's going on, rather than just knowing a quick answer without really getting to grips with *why* it's the answer.

I don't mind in the slightest diving in deeper than initially intended - it's fascinating to learn more and I hope that I get the chance soon to put the knowledge into field practise. If this kind of question isn't so askable in the main forum I hope I remain a junior member for a while as it is very interesting!

Thanks again

Tim