Vorticity and Impulses

[David Williams]

Hi All,

I am trying to create a mental model for positive vorticity advections role and purpose it plays in wave impulses and storm development. So, I was hoping to type out my thinking and get some confirmation or correction from some ST friends out there.

As a trough moves into the western U.S., we see a jet streak within the trough's jet stream moving into the left side of the trough. With time, the jet max will "strengthen" the trough as it rounds the base of the trough. With the strengthening and potential deepening of the trough we have corresponding positive vorticity advection. As fast moving wind speeds pass perpendicularly through high values of vorticity, positive vorticity advection is strengthened. On the leading edge of positive vorticity advection we have the slowing of wind speeds and divergence that will create lift in the atmosphere. Additionally, high values of vorticity downstream within the jet stream can create short wave impulses or short waves within the larger trough. These impulses or short waves can be localized areas of ascent for storm development due to the same principles that take place within the larger trough, PVA, and divergence interplay. As the jet max/streak moves off to the right of the trough, possibly causing the trough to take on a negative tilt, it will move to the right of the main low/trough. The low/trough will then weaken with time. Identifying short waves or impulses in water vapor imagery via the identification of vort maxes and dry slots can pinpoint areas of potential storm development. Also, identifying areas of positive vorticity in 500 mb and 700 mb model data can help pinpoint areas of potential storm development.

Does that sound right? I used the word "impulse" in there because I hear that word thrown around a lot, but I only assume it exclusively has to do with short waves within the main trough. I looked it up on NOAA's site and it's defined as "Alternate term for Upper Level System and Shortwave; a general term for any large-scale or mesoscale disturbance capable of producing upward motion (lift) in the middle or upper parts of the atmosphere." So, I'm not sure if there is another use for or implied meaning for the word "impulse" out there.

Anyway, please give me your feedback. Thanks

 

[Jeff Duda]

You're on the right track, but unfortunately you kinda missed out on the real fundamentals of cyclogenesis.

You should look up the Norwegian Cyclone model of cyclogenesis.

Just some notes
-Using quasigeostrophic theory, the only role vorticity advection actually plays is propagation of the trough. It actually does nothing to deepen a trough. Advection of planetary vorticity (-v*dBeta/dy) actually takes care of some of the deepening
-There is no requirement of a pronounced jet streak in cyclogenesis. In fact, you can still get surface cyclogenesis with a weakness in the wind velocities in the base of a trough. Actual winds are typically sub-geostrophic at the base of a trough, and it's the acceleration and resulting divergence along the lee side of the trough that promotes air removal from the column.
-Arguably the most important result that falls out of QG theory is that differential vorticity advection is associated with vertical motion in the atmosphere. The "differential" refers to a vertical derivative. In other words, upward motion will only occur if the magnitude of vorticity advection is increasing with height. Strictly speaking, simply having PVA at a fixed level (e.g., 500 mb) is not sufficient to grant upward motion and cyclogenesis. However, in most situations, it does tend to be the case that DPVA occurs along with PVA at a level in the mid-troposphere.
-There are a lot of phrases meteorologists use to describe the pocket of vorticity that is associated with surface cyclogenesis: "energy", "vort max", "shortwave", "upper level system", "disturbance", and "trough" all generally mean the same thing (colloquially). You may incite flames of hatred from some meteorologists from using some of these terms, though.

 

[David Williams]

Jeff I'm glad you mentioned DPVA's vertical derivative. Even though I've read about DPVA many times, I don't think I've heard it stated as clearly as you stated it. Knowing that if the magnitude of vorticity advection increases with height it will produce upward motion was the precise principle that I needed to understand. Thanks for explaining that.

With that said, we would therefore need to see impulses or kinks in the trough at multiple heights in addition to being associated with DPVA to infer that convection can take place in said "kinks" "impulses" or "shortwaves" (assuming all other storm ingredients are present)? And if that is correct, we can use WV for identifying the upper level impulses, right? So, I would use the models to identify the the placement and associated DPVA of the impulse and WV to find its current position for potential storm development.

Sent from my SAMSUNG-SGH-I337 using Stormtrack mobile app

 

[Jeff Duda]

With that said, we would therefore need to see impulses or kinks in the trough at multiple heights in addition to being associated with DPVA to infer that convection can take place in said "kinks" "impulses" or "shortwaves" (assuming all other storm ingredients are present)? And if that is correct, we can use WV for identifying the upper level impulses, right? So, I would use the models to identify the the placement and associated DPVA of the impulse and WV to find its current position for potential storm development.

I'm not totally sure what you're saying here. Could you clarify? I'll take a shot - generally if there's trough present at one level in the troposphere, it's going to also be present at other nearby levels, but not necessarily in the same location or with the same height/pressure gradient. In fact, in the classic Norwegian Cyclone model, a developing/intensifying cyclone should have a rearward/upstream tilt with height. In other words, you should find the trough axis moving generally westward as you go up from 850-700-500-300 mb etc.

 

[David Williams]

I'm not totally sure what you're saying here. Could you clarify? I'll take a shot - generally if there's trough present at one level in the troposphere, it's going to also be present at other nearby levels, but not necessarily in the same location or with the same height/pressure gradient. In fact, in the classic Norwegian Cyclone model, a developing/intensifying cyclone should have a rearward/upstream tilt with height. In other words, you should find the trough axis moving generally westward as you go up from 850-700-500-300 mb etc.

Right, let me explain that better. In that second paragraph I was transitioning from talking about the... Holy crap! I just noticed that you have your signature updated with Ph.D. Meteorology, University of Oklahoma, 2016. I assume that means you just graduated! That's awesome! Congratulations!... okay, back on track. I was transitioning from talking about the theory and my mental construct of PVA, DPVA, impulses, etc. to the nature in which I should apply that information to forecasting severe weather. The second paragraph of my previous post is my question about whether I'm developing a good forecasting process. I'm doing my best to apply theory to actual forecasting.

So, in general is the following correct? When forecasting, I should look through multiple layers of the troposphere to identify the trough placement at each level. I look at vorticity in each layer to identify where vorticity is strong and to identify that differential positive vorticity advection is evident in many layers and increasing with height. But, and this is one of the major points I'm trying to clarify in my mental model and forecast process, this DPVA and associated lee cyclogenesis, in addition to the divergence taking place on the right or downstream side of the trough, is not the only possible location for forced assent in the trough. There may be additional areas of DPVA in other locations of the trough. These would show up on models of say 500 mb vorticity with higher values along another location of the trough. That's the thing I'm trying to get a handle on. Can these other locations be localized areas for ascent? So, for example, if I'm going chasing and I notice the models have an impulse or little wave in multiple layers (assuming the model is valid and error free ; ) associated with DPVA over Amarillo at 21Z and all other severe weather ingredients are in place, can assume I have a shot at storms initiating there... even though the main DPVA and expected area of forced ascent is associated with the divergent winds and DPVA over Kansas or wherever?

I know there are a bunch of holes in that because I'm not giving you all the ingredients and values, but is that the general idea? If it is the general idea, then taking my previous example, while I'm getting into position for storms to fire, would I be able to look at WV to try and identify the location of the impulse on it's way to the TX panhandle in order to know where I need to set up to chase? Or at least could it give me more assistance in helping me identify a chase target.
Thanks for your time and patience Jeff : )

 

[Jeff Duda]

So, in general is the following correct? When forecasting, I should look through multiple layers of the troposphere to identify the trough placement at each level. I look at vorticity in each layer to identify where vorticity is strong and to identify that differential positive vorticity advection is evident in many layers and increasing with height. But, and this is one of the major points I'm trying to clarify in my mental model and forecast process, this DPVA and associated lee cyclogenesis, in addition to the divergence taking place on the right or downstream side of the trough, is not the only possible location for forced assent in the trough. There may be additional areas of DPVA in other locations of the trough. These would show up on models of say 500 mb vorticity with higher values along another location of the trough. That's the thing I'm trying to get a handle on. Can these other locations be localized areas for ascent? So, for example, if I'm going chasing and I notice the models have an impulse or little wave in multiple layers (assuming the model is valid and error free ; ) associated with DPVA over Amarillo at 21Z and all other severe weather ingredients are in place, can assume I have a shot at storms initiating there... even though the main DPVA and expected area of forced ascent is associated with the divergent winds and DPVA over Kansas or wherever

I think the difficulty here is that you're combining separate topics into one. To be honest, I just use 500 mb heights and vorticity to diagnose where surface cyclogenesis will take place and what shape the surface pressure pattern will have. As I said before, that's not strictly accurate, but it is typically good enough for weather forecasting. So you don't need to do a thorough analysis of vorticity at all levels. Furthermore, QG theory is very complex and you cannot 'eyeball' magnitudes of DPVA and WAA very well. There are a lot of situations in which DPVA occurs in the same area as CAA. Those factors result in conflicting signs of vertical motion, but you can't glance at the advections and determine which one is overwhelming the other. You'd need to calculate things numerically.

Yes, there are other factors besides vorticity advection that indicate forced ascent on the large scale or the mesoscale. Low-level temperature advection is another such source of forced ascent. Orography is another one. There are also two ways vorticity can be present within an air flow. One is called 'curvature vorticity' which is assocated with gradient wind flow (i.e., approximately geostrophic wind flow) around curved height contours (e.g., the base of a trough). The other is 'shear vorticity', which is the result of horizontal wind shear. Whereas curvature vorticity is concentrated where the height contours bend the most sharply (typically at the apex of a well formed trough), shear vorticity can be large anywhere, including areas outside of a trough even. If there is strong enough shear vorticity outside of the area where curvature vorticity is greatest, then you can get forced ascent in other areas besides near the base of a trough. In my experience, it's not that uncommon for shear vorticity to be a leading factor in the evolution of a cyclone. Typically shear vorticity appears on maps as ribbons or long streaks of stronger vorticity, and advection of those ribbons tends to lead to formation of sweeping cold fronts. But, that's not always the case. You kind of have to analyze each one on a case-by-case basis.

Finally, while lee cyclogenesis is common in the central US because of the placement of the sloping Great Plains and Rocky Mountains, lee cyclogenesis is not part of the Norwegian Cyclone model. Lee cyclogenesis is more mesoscale driven. It is the result of compressional warming as air parcels ride down the slopes of the Rockies and into the plains. Compression -> warming -> decreased air density -> reduced pressure. So try to separate lee cyclogenesis from the more typical/textbook cyclogenesis model in your head. What's typically occurring in the U.S. Great Plains is a superposition of those two events. You can get a lee trough days ahead of the arrival of an upper-level trough. If the Rockies weren't there, however, you would not tend to see a lee trough form so far out ahead of an upper-level trough. Again, evidence that the two processes are separate from each other.

Finally, keep in mind that the vorticity advection argument applies ONLY on the synoptic scale. It does not apply on the mesoscale. You cannot assume that a tiny little ball/pocket of vorticity over a small region like the Texas panhandle is going to promote lift if it's not part of a larger vort max. However, as you said in previous posts, you can typically infer whether lift is occurring using water vapor satellite. All I'm saying is there's no guarantee any lift you see occurring via WV satellite is being caused by vorticity advection. It may be caused by other factors. On chase days where forcing is less certain, I use water vapor satellite myself to try to pick one area over another as far as where storms might fire. However, on days with stronger large-scale forcing, you typically don't need to worry about looking for small pockets of lift on WV satellite because initiation is generally going to be preferred at the surface boundaries (dryline, cold front, warm front etc.).