Forecasting Negative Tilt Troughs

Hey All!
In anticipation of the season's first tornado outbreak, I checked out GHCC's sat imagery (looping both their basic GOES-10 & global composite water vapor) looking for some sign of negative tilting in the troughs ahead (i.e. those still out over the Pacific, around Aleutian longitude). I know analyzing troughs that far out in time & distance isn't the best idea, but I've been looking for some sort of pattern, perhaps atmospheric wave interaction that might induce a trough to tilt back. Now, I'm certainly no expert in fluid mechanics (or atmospheric thermodynamics, for that matter) but it seems to me that, given the necessary conditions in place like plentiful moisture & time of year (climatologically), perhaps the best 'first sign' of an oncoming tornado outbreak (setting aside shortwaves ejected from those Alaskan Gulf lows) lies in the orientation of a trough still over open sea. Just a thought. What are your ideas or opinions on this?
Good luck chasing everyone!
 
Here are the main causes of a negatively tilted trough (Haby Hints):

*Strong middle and upper level winds wrapping around the base of the trough;
*Strong jet streak near the base of a trough;
*A ridge to the east of the trough;
*Occlusion of low pressure.

Since any of these four primary factors can change at any time, I don't think it would be completely possible to forecast the angle of a trough from that far out. Many lows don't reach maturity or deepen until they arrive over the continental U.S., sometimes not until they arrive over the plains. And it also doesn't not come in contact with ridging to the east until it is well over the U.S. You may be able to forecast a potential setup that will result in a negatively tilted trough, but it's pretty dubious that it would be completely accurate all the time ... too many changeable factors.
 
Mike,
The four bullet points given above are better labeled 'rules of thumb' than "causes" of negatively-tilted troughs... If you really want to know what drives height falls and rises, and thus what drives trough orientation, shape, etc, type in "QG approximation" or QG theoryinto Google. Well, I don't know what that'll return, but ya'll are talking about the Quasigeostrophic theory... Let's take a look:

Actually, I don't quite have time to describe it. I'll just say that postive vorticity advection results in height falls; warm air advection results in height falls if above the height of maximum WAA (so, differential warm air advection). Given the thermal wind equation, the strongest mid-upper level winds will lie underneath the strongest low-level baroclinic zone (well, not entirely, since isentropic surface are sloped, so the max flow aloft will lie immediately north of the baroclinic zone). At any rate, just put simply, strong low-level cold-air advection on the back side of a system will cause strong height falls aloft, and thus "dig" the trough. Meanwhile, strong low-level warm-air advection will cause height rises aloft, and thus build the ridge. You can therefore get a negatively-tilted trough aloft by bringing the strong low-level cold air advection south of a low (if we assume we have a surface low in the favorable largescale ageostrophic curvature divergence zone between trough and ridge axis) and strong low-level warm-air advection norht of the low (for what it's worth, occlussion occurs as WAA wraps around to the northwest side of the low, and CAA wraps around to the southeast side of the low). This will cause strong height falls aloft to the south of the surface low, and strong height rises aloft to the north of the surface low. You're mid-level flow will back with time, eventually yielding a negatively-tilted trough. This neglects the effects of positive vorticity advection, which also causes height falls aloft (as negative vorticity advection cases height rises aloft), so the placement of vort maxes is important as well. In addition, this also neglects transverse circulation around jet streaks aloft, which can also play a role in advection patterns.

I brought up the thermal wind equation because, if you do have a strong upper-level jet 'diving down' the back side of a trough, then you also have a strong low-level baroclinic zone (thermal boundary) on the back side of said trough as well. Therefore, you have a much better opportunity for strong cold-air advection behind a surface low, since the grad(T) is relatively strong.

This discussion could talk up many and many paragraphs, so I may post more after my classes today.
 
Thanks Jeff! - Hope you'll have time to discuss it more a bit later. In the meantime I'll think about what you've said and do a little research on the subject. Hopefully I can get a clearer picture of the processes at play.
 
This is a nice discussion to get started - but keeping in mind the audience I want to avoid getting too technical. I think Jeff may have implied that you only get a negatively tilted system with an occluded surface cyclone, but this is misleading as occusion generally follows from negative tilt troughs, not the other way around.

More often, a negatively tilted trough results from a smaller scale intense disturbance riding around a larger scale trough. Below is a long-winded description of this process.

We can reduce this down to considering jet stream level dynamic features only. I found this image on Steve Ackerman's web site:

[Broken External Image]:http://mapmaker.meteor.wisc.edu/~jbrunner/ackerman/upperair/upair5.gif

The lines here could represent a few of the height contours typically seen on a 300 mb surface map. Geostrophic wind speed is the air speed that you would expect based on the how close together the contours are - so the closer the line contours - the faster the wind speed. If you were to look at the temperatures at 500 mb - you'd find a relative cold pocket of air in the trough and a relative warm pocket of air in the ridge. Now, because of the effect of the Earth's rotation on air motions, air flowing through a trough (the "dip") is slowed (subgeostrophic), whereas air flowing through the ridge is accelerated (supergeostrophic) relative to the geostrophic flow - and this leads areas of divergence and convergence aloft. The surface low is below the region of divergence aloft (where are is being removed from the air column) and the surface high pressure is below the convergence region (where air is piling up). The tilt of the trough shown above is neutral - as in if you were to draw a line down the axis of lowest heights, it would be up/down as viewed on your screen. The "normal" orientation of upper troughs is positive tilt - meaning the line drawn down the axis of lowest heights would lean from SW-NE as viewed on a weather map in the northern hemisphere. With positive tilt troughs, the amount of curvature leading to divergence is less - so weaker forcing for a surface system. If we could get a more negative tilt (SE-NW orientation), the amount of curvature is increased and subsequently so is the amount of upper level divergence and forcing for rapid intensification of a surface cyclone.


So, how can we get the "normal" positive tilt trough to become negatively tilted? We need help from a smaller scale disturbance riding around the upper trough. The larger the scale of a disturbance - the slower it moves, so the trough/ridge pattern we talked about above moves very slowly (it actually it is quickly retrograding against the mean flow). Shortwaves - which we often look for as vortcity maxima on 500 mb charts - are much smaller in size and therefore retrograde much slower, and so move through the upper level trough ridge pattern acting to distort the upper ridge trough. This shortwave can often be observed as a jet streak embedded in the ridge/trough pattern. So, to get our negatively tilted trough - let's start with a jet streak (shortwave) on the west side of the upper trough (where convergence is occuring owing to the curvature effect). Shortwaves/jet streaks cause imbalances in the forces that leads to areas of rising/sinking motion. I like the jet streak model - so let's look at that one:

[Broken External Image]:http://www.ems.psu.edu/Courses/Meteo200/lesson6-2/graphics/jetstrk.gif

The above image is from a Penn State web site. It's busy - but let's just focus on what's important. The upper panel shows an elongated bullseye of strong winds - with the maxima in the center. the top right corner is labeled "left exit" and there air is divergergent at upper levels forcing upward air motion (see bottom panel). Back to the top left is the entrance region where air converges at upper levels causing sinking air and pressure rises (again see bottom panel). When air rises - it cools due to expansion as it finds itself in progressively lower pressure (recall pressure decreases with increasing height). Conversely, air sinking warms due to compression. Recall that the upper trough is collocated with a mass of relatively cold air aloft - so the localized lifting caused by the jet streak in effect adds a smaller "bulge" of cold air as it slips around the larger scale trough.

Ok, now comes the difficult part. Again, as we started above, let's begin with our shortwave on the west side of the upper trough. As the left front edge jet streak approaches the base of the trough (bottom right edge if we are viewing it on our screen, rotate the jet streak model 90 degrees clockwise), it is lifting air in the base of the upper trough - and this leads to further cooling (height falls) in the base of the trough and the trough subsequently expands southward. Since the jet streak is smaller in scale, it continues to move around the larger scale upper trough until the left exit region of our jet streak is colocated with the upper level divergence owing to curvature. Here, things get interesting as the bulge from cooling has slipped around to the east side of the upper trough, and with the phased together upper level divergence - the low-level cyclone intensifies rapidly owing to potent height falls which in turn leads to a marked increase in warm air advection (often called the warm conveyor belt). Warm air advection leads to height rises - and in effect leads to ridge building ahead of the region of upper divergence - causing an indentation in the upper height contours back to the NW. So - adding together our cool lump on the southeast edge of the upper trough with our warm indentation to the north of the upper divergence core we have created a negatively tilted upper trough.

Glen
 
Guys,
Thanks for the quick replies! Jeff & Mike, thanks for your point-for-point explanations. I really enjoy reading about & conceptualizing the general factors that influence the potential severity and character of such systems. Jeff, I'll also do that search you suggested & see if I can find some detailed diagrams & get a better understanding of the processes you touched on. Would love to read more on this! Please, post more info whenever you get the time.
Thanks!
 
Hey Glen,
Thanks for your detailed input too :)! Sorry, by the time I posted my last message I guess you had already posted yours. Thanks again for all the great info & explanations! Now we have quite a bit to munch on for the impending season, before mother nature throws us her first real curveball! :wink:
 
This is a nice discussion to get started - but keeping in mind the audience I want to avoid getting too technical. I think Jeff may have implied that you only get a negatively tilted system with an occluded surface cyclone, but this is misleading as occusion generally follows from negative tilt troughs, not the other way around.

Glen,
You are correct... I was basing my description more on the semi-typical east-coast negatively-tilted trough with surface low occlussion, which seems to quite common this time of year. This is a different process than what causes neg-tilted troughs in the plains, as you stated. However, I'm not so sure than you can say that surface cyclone occlussion general follows negatively tilted troughs... One of the main process by which heights rise/fall is via thermal advection patterns, particuarly in the low-levels. It's kind of a chicken or egg deal, since we could ask if the low-level WAA causes height rises or if the height rises (caused by other processes such as NVA) cause low-level WAA.

Shortwaves - which we often look for as vortcity maxima on 500 mb charts - are much smaller in size and therefore retrograde much slower, and so move through the upper level trough ridge pattern acting to distort the upper ridge trough.

Shortwaves propagate courtesy of the advection of relative vorticity still, while longwaves propagate based on the advection patters of earth vorticity (owing to their large-scale nature). I suppose it depends upon how one defines, in terms of the wavelength, "shortwave" or "longwave"... Regardless, shortwaves are typically of the scale that relative vorticity advection determines their propagation, while the advection earth vorticity determines the propagation (which is retrograding) of longwaves.
 
Shortwaves propagate courtesy of the advection of relative vorticity still, while longwaves propagate based on the advection patters of earth vorticity (owing to their large-scale nature). I suppose it depends upon how one defines, in terms of the wavelength, "shortwave" or "longwave"... Regardless, shortwaves are typically of the scale that relative vorticity advection determines their propagation, while the advection earth vorticity determines the propagation (which is retrograding) of longwaves.

Vorticity advection is compartmentalized in met courses - but the atmosphere has no appreciation for such distinctions. There is still a planetary vorticity contribution in shortwaves, it's just much smaller. Anyhow - seems I did a poor job of explaining how jet streaks propagate - as in why they don't move at the same speed as the mean flow they are embedded in. As discussed above, for a jet streak moving through a trough there is rising in the left front and sinking in the right front. This leads to an increase in the temperature gradient - and hence increase in the height gradient (~ vertical integration of the temp gradient). The opposite happens in the entrance region of the jet streak - such that the vertical circulations act to weaken the temp/height gradients - and so the jet streak "moves" along this pattern of increasing gradient and away from the decreasing gradient. A gross rule of thumb is that shortwaves move at ~ 1/2 the speed of the flow they are embedded within. Haven't looked, but this is probably stuff discussed in Tim's forecasting book. For the mets, look at Bluestein Vol II.

Glen
 
LOL - well this topic started about as high over my head as the boundary layer, but now I think it has officially progressed into tropospheric altitudes. I don't think I'm quite ready to read Bluestein yet - wish I was ... :)

Still interesting to read, though -
 
LOL - well this topic started about as high over my head as the boundary layer, but now I think it has officially progressed into tropospheric altitudes. I don't think I'm quite ready to read Bluestein yet - wish I was ... :)

Still interesting to read, though -

Sorry Mike, I was trying not to make it that way - but I wanted to avoid too much handwaving in explaining things. Oh well. If specific points have you hung up - please ask and maybe Jeff, I or someone else can pipe in with a better explanation.

Glen
 
oh, no apology necessary ... it's been great to read the posts ... just think it's time for me to duck out of the exchange! - I really appreciate the time and thought that go into many of the posts by the qualified mets and students among us. It helps us all continue to make some progress.
 
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