Why do sfc lows sometimes precede an upper trough?

Well, I have low confidence that anyone is going to be able to explain this to me in a way that I understand. Nevertheless, I challenge the educated amongst us to give it a go!

I notice, as I type this, that there is a sfc low positioned over NE MI. The high-amplitude upper trough, though, digs from MN southward to the GoM.

What caused the sfc low to precede the trough?

Also, is this seasonal phenomena? In the temperate seasons, sfc lows generally trail the migration of the troughs, do they not (which, of course, is partly how we get veering winds with height)?

Thanks!

Bob
 
This is a great question. One I had to answer on a test several years ago.

In deepening cyclones (not tropical cyclones) the surface low precedes the upper level low. You can get into a which came first argument here, but simply put its because the cold air at the surface comes before the upper level cold pocket. They are not vertically stacked.

Mature cyclones are vertically stacked. The upper level cold pocket and low sits directly over the surface low.
 
Hey, thanks, Bill! I think I actually sort of get that. Would you also say that the (unseasonably) warm temps in the upper Mid-West to New England of late explain the cyclone deepening mid-continent?
 
Originally posted by B Ozanne
This is a great question. One I had to answer on a test several years ago.

In deepening cyclones (not tropical cyclones) the surface low precedes the upper level low. You can get into a which came first argument here, but simply put its because the cold air at the surface comes before the upper level cold pocket. They are not vertically stacked.

Mature cyclones are vertically stacked. The upper level cold pocket and low sits directly over the surface low.

Ah, QG theory comes into play here...

Let's start with the upper-air pattern. When flow is straight, upper-level flow is nearly always in geostrophic equalibrium -- meaning that the two force balances are the pressure gradient force and coriolis force. The real wind will follow isobars / isohypses when this is the case. For curved flow, however, there is another force at play, and that one is courtesy of centripetal acceleration (giving rise to the fictitous force that is the centrifugal force_. When air "rounds" the base of a trough, the wind speed is a bit less than what geostrophic balance would suggest; when air rounds the top of a ridge, the wind is stronger than geo balance would suggest. We call these subgeostrophic flow (at the base of the trough) and supergeostrophic flow (at the top of the ridge). Now, let the ageostrophic wind be the difference between the real wind and the geostrophic wind. If we plot the ageostrophic wind vectors, we'd see that, for the northern hemisphere, the ageo wind vector points max westward at the base of the trough and max eastward at the top of the ridge. So, what about between the trough and the ridge? The ageo wind is southwesterly upsteam (near the base of the trough) and northeasterly downstream (near the top of the ridge) -- we have divergence! More specifically, the area downstream of a trough but upstream of a ridge is characterized by ageostrophic curvature divergence aloft. This creates upward motion, surface convergence, pressure falls at the surface, and an environment generally favorable for cyclogenesis.

Let's look at the other side of the trough -- the area downstream (east in the northern hemisphere) of a ridge and upstream (west) of a trough. Here, we have the opposite situation as that described above; this is an area of ageostrophic curvature CONvergence aloft. This creates large-scale subsidence, pressure rises, and an environment generally favorable for anticyclogenesis.

Of course, there are other sources of ageostrophic motion that can create synoptic-scale upward and downward motions. For example, for a straight upper-level jet streak, there is divergence in the right-entrance and left-exit regions, and convergence in the left-entrance and right-exit region of the jet streak. This creates something called the transverse circulation, where these is USUALLY upward motion in the left-exit and right-entrance regions, and downward motion in the left-entrance and right-exit regions. These motions result in a thermally direct circulation in the entrance region of the jet streak, since there is upward motion on the warm side of the jet and downward motion on the cold side of the jet (with a mean southerly flow between the two in the low-levels). In the exit region, there is a thermally indirect circulation -- upward motion on the cold side of the jet and downward motion on the warm side (with a mean southerly flow between the two). This is a very idealized explanation, but serves for a basic explanation.

Oh yes, can't forget about vorticity. In a trough, there can be significant cyclonic vorticity courtesy of the strong curvature. Throw in a strong jet (which can add significant shear vorticity) and you can see why you often see vort maxes along the trough axis, and vort mins in ridge axes. In the low-levels, wind speeds aren't usually very strong, so the shear-component of vorticity is usually relatively small. However, there is often strong curvature in the low-level flow field, yielding relatively strong vorticity from the curvature-component. In the upper-levels, we usually have the opposite situation -- large shear component and small curvature component. But what about in the mid-levels, like at 500mb? Here, we can still have significant shear vorticity (since we can have 60-90kt winds at 500mb), and we can still have significant curvature vorticity. This means that, ideally, vorticity is maximized in the mid-levels.

So, again, imagine that we are downstream of a trough (say a trough in the western US and ridge in the eastern US -- we're in the plains). As the vort max approaches, there is local positive vorticity advection across the plains. QG theory tells us that differential positive vorticity advection (or positive vorticity advection increasing with height) results in upward motion, which in turn tends to yield surface pressure falls. Note again that on the other side of the trough-ridge pattern, we have the opposite -- the vort max is moving away, so we can have different negative vorticity advection. Again, this is an idealized explanation, as it's not only positive vorticity advection increasing with height that causes upward motion, but there's also upward motion when there's negative vorticity advection decreasing with height.

Of course, we shouldn't forget thermal advection patterns. In terms of temperature/thermal advection, we often see the strongest advection occuring in the low-levels, with lesser advection in the upper-levels. Above the level of maximum warm air advection, there will be height rises; below the level of maximum warm air advection, there will be height / pressure falls. Warm-air advection that is maximized around 850mb yields surface pressure falls and height rises aloft (assuming the surface pressure isn't more than 850mb).

OK, so putting all of this together. Let's set up a west-coast trough and east-coast ridge situation, and let's place a jet streak at the base of the trough. Let's say that the jet streak is ejecting out of the trough axis, with the trough moving eastward. So, over the Plains, we can have several sources for surface pressure falls -- (1) ageostrophic curvature divergence aloft, (2) differential positive vorticity advection, and (3) left-exit region upper-level divergence. This makes the area between the upstream trough and the downstream ridge a mighty fine area of cyclogenesis, as we have numerous sources for surface pressure falls.

Perhaps the Plains example isn't the best, since the Plains has a very significant factor not really found in other places of the US (except for more local effects in the lee of the Appalachians and other mountain ranges)-- downsloping! As winds subside along the lee of the rockies, the air warms and expands, leading to surface pressure falls. Assuming a ridge across the eastern US, this lee trough creates an easterly pressure gradient and resultant southerly flow. This southerly flow tends to result in warm-air advection, further lowering the pressure across parts of the plains.

I've used a lot of simplifications, but this should describe, in a basic sense, why the area between a trough to the west and a ridge to the east is often favorable for cyclogenesis.

EDIT: One important thing to remember is that, when not saturated, air follows isentropic surfaces! This means that when air rises, for example in a warm-air advection regime, it follows a sloped ascent path. So, when you look at upward motion in the right-entrance region, remember that the air that is rising in the low-levels is NOT located immediately under the right-entrance region of the jet streak. If we take a jet streak in the westerlies, there may upward motion from the right-entrance region well south of that region of the jet streak. So, if there's a jet streak over OK, there may be low-level ascent associated with the right-entrance region that's well into southern TX.

Also realize that, when saturated, air follows moist isentropes (or theta-e surfaces), which are usually sloped more steeply than dry isentropes. This means that, in a saturated environment, the path of air parcels from the low-levels to the upper-levels is more erect. This also helps explain why precip in the warm-air advection zone can help deepen surface lows and amplify the upper-level pattern. With more focused and significant ascent along the more steeply-sloped theta-e surface (relative to theta, or dry isentropic, surface), stronger pressure falls may occur.

EDIT II: Fixed several grammatical and spelling errors.
 
I've been absent from the board for two weeks, and I immediately knew that Jeff would chime in somewhere on this thread when I saw its title. :p

Jeff the man...the legend...the forecasting machine! :lol:

Gabe
 
Yeah what Jeff said.

and to add, in his edit, this is for forced ascent, not convection obviously (or maybe not so obviously)

Excellent explanation Jeff. Too many people forget about the Ageostrophic wind equation. What an excellent tool for diagnosing vertical motion.

one thing you didnt directly hit on was WHY the different regions of jet streaks exhibit divergence/convergence. In an area of what to appears to be convergence aloft by just looking at the true wind vectors (ie faster winds blowing into slower winds, basically applying only QG theory), what you have is a slowing of the winds, and when this happens, you will get an ageostrophic wind vector to the right of the flow because of coriolis becoming stronger than the PGF for the slowing winds. this is what causes the divergence in the left exit region. the opposite happens in teh entrance side of a streak with winds speeding up, coriolis takes less of an effect and PGF becomes stronger directing an ageostrophic vector to the left near the entrance region.

That was semesters ago and I haven't kept in practice with trying to juggle work and getting a few more math classes out of the way before i graduate so something in that may not be dead on.
 
Hey Jeff, not sure your explanation above might not go over the heads of a few folks, but I'm also not sure you answered part of the original question - namely the role of baroclinicity in the horizontal displacement between the surface and upper level cyclones. Not suggesting a potential vorticity explanation, but maybe something akin to that type of positive feedback interaction aspect that might shine some light on the relationships between surface and aloft energy sources. Maybe you could add that in since you are already on a roll.

Glen
 
Here is something that may be of interest:

Quasi-Geostrophic Theory: A Review of Basic Concepts
http://www.eas.slu.edu/CIPS/Presentations/.../Q-G_theory.ppt

ISENTROPIC ANALYSIS RESEARCH
http://www.eas.slu.edu/People/JTMoore/isen.html

PV Intro - Structure and Evolution of Baroclinic Waves and Fronts: Isentropic Potential Vorticity
http://www.comet.ucar.edu/class/aes_canada...ocs/PVintro.pdf

Vorticity Advection and Vertical Motion by Chuck Doswell
http://www.cimms.ou.edu/~doswell/PVAdisc/PVA.html

IPV Physical Discussion by C.A. Doswell III
http://www.cimms.ou.edu/~doswell/ipvdisc/IPV.html

Summary of Quasigeostrophic Theory
http://www.personal.psu.edu/users/k/m/kmg2...c.html#qgtheory

QG Intro - Structure and Evolution of Baroclinic Waves and Fronts: Quasi-Geostrophic Theory and its Practical Applications
http://www.comet.ucar.edu/class/aes_canada...ocs/QGintro.pdf

A few links that may help, sure they are more out there.

Mike
 
Originally posted by Blake Michaleski

one thing you didnt directly hit on was WHY the different regions of jet streaks exhibit divergence/convergence.

Yeah, my wrist was getting tired, so I opted not to explain that. It's a little easier to explain transverse circulation by using a graphic / figure... I strongly suggest this site if you are interested in learning more about jet streak circulations: http://www.meted.ucar.edu/norlat/jetstreaks/ .

Regarding the vertical tilt of low-pressure systems... Let's start with a stationary boundary oriented west to east, and let's assume this all takes place in the northern hemisphere. Cold air lies to the north; warm air lies to the south. Cold air is more dense, and thus is associated with low mid and upperlevel heights (assume cold is air deeper the farther north you go). Conversely, warm air is less dense and is associated with higher mid and upper-level heights (assume warm air is deeper the farther south you go). The thermal wind formula indicates that the mid and upperlevel flow strength and orientation is relative to low-level baroclinity (actually, thickness, but that's relative to low-level thermal fields). More specifically, the thermal wind relates thermal gradients to the change in geostrophic wind with height. Therefore, strong flow aloft will often be associated with a strong baroclinic boundary in the low-levels. I'm getting sidetracked..

Let's go back to our stationary front... Let's impart a "ripple" along that front (this'll ring of the wave-cyclone model). Warm-air occurs downstream (east and northeast) of the developing low-pressure system and cold-air advection occurs west and southwest of this low. The height field will respond to this thermal advection pattern. Remember that the preferred area for cyclogenesis is often downstream of a trough (near the trough-ridge inflection point), where we have large-scale ascent due to the reasons given in my previous post. In the developing cyclone, forcing terms can be quite significant. As the warm-air advection increases ahead of the low, upper-level heights rise; as cold-air advection increases behind the low, heights aloft fall. This increases the curvature of the flow aloft (trough deepening and ridge strengthening), increasing the strength of the vorticity embedded in the trough. DPVA downstream of this trough / vort max may then lead to enhanced upward motion and resultant surface pressure falls, deepening the low. This, in turn, increases the low-level pressure gradient, which may then increase the thermal advection in the low-levels, and the feedback continues.

his'll be a tad cliche, but the cold front can 'catch up' with the warm front, particularly with more intense cyclones (where advection patterns are more significant). By this time, the cyclone is more barotropic, meaning that there are reduced thermal gradients and less baroclinicity near the cyclone. Actually the better term may be something along the lines of equivalent barotropic, where there are temperature gradients on a constant pressure surface, but there is very little thermal advection (similar to tropical cyclones -- there's a warm core and thus thermal gradients, but there is very little/no thermal advection). Note that there are other factors that are involved in the developing of a relatively homogeneous thermal environment near an occluding surface low.... For example, widespread precipitation in the warm-air advection regime often leads to cooling (through evaporation). In addition, strong cold-air advection leads to subsidence behind the cold front (remember -- non-saturated air follow isentropic surfaces, which bend downward as one approaches the cold front from behind since the deeper, colder air well behind the front has lower potential temperature). So, in strong cold-air advection, there is also strong subsidence, which is a negative feedback that acts to warm the cold sector. Since the low-level thermal pattern is more uniform, and the coldest air is likely located very near the center of the low, the lowest height will likely be juxtaposed with this feature. These occluded lows tends to move more slowly as they, concurrently, slowly weaken.
 
Great thread, guys. I've always been confused about all of this also. I've got a question to add: are negative-tilt troughs and acompanying surface lows stronger than positive-tilt systems?
 
Negative-tilt Trough - An upper level system which is tilted to the west with increasing latitude (i.e., with an axis from southeast to northwest). A negative-tilt trough often is a sign of a developing or intensifying system.

Positive-tilt Trough - An upper level system which is tilted to the east with increasing latitude (i.e., from southwest to northeast). A positive-tilt trough often is a sign of a weakening weather system, and generally is less likely to result in severe weather than a negative-tilt trough if all other factors are equal.

Source:http://www.srh.noaa.gov/oun/severewx/glossary3.php

WHAT IS A "NEGATIVELY TILTED TROUGH"?
http://www.theweatherprediction.com/habyhints/127/

TILTING OF TROUGHS ON A HORIZONTAL PLANE
http://www.theweatherprediction.com/habyhints/58/

Mike
 
Originally posted by Michael Auker
Great thread, guys. I've always been confused about all of this also. I've got a question to add: are negative-tilt troughs and acompanying surface lows stronger than positive-tilt systems?

Typically, yes. There are two components that make up relative vorticity -- curvature and shear. With a negatively-tilted trough, the curvature term can become very significant, leading to a potent vorticity maximum. With a stronger vort max, more intense DPVA and associated upward motion and surface pressure falls may occur. In addition, this strongly-curved flow enhances ageostrophic circulations which can provide further ascent and surface pressure falls. For example, a highly-curved flow will result in stronger ageostrophic curvature divergence aloft ahead of the trough (relative to less amplified flow). Furthermore, negatively-tilted troughs are often associated with strong cold-air advection on the back side of the trough and a jet streak that's diving down the back side. As this jet streak rounds the base of the trough, the wind profiles ahead of the trough may become very favorable (e.g. with lee troughing, think southerly or southeasterly surface flow under strong westerly or southwesterly mid/upperlevel flow).

With positively-tilted troughs, curvature vorticity tends to be much weaker,. Surface cyclones associated with positively-tilted troughs tend to be weaker, and low-level cold advection tends to be rather weak in this situation. In my experience, this often results in a veered surface wind flow ahead of the cold front as the surface low moves northeastward.
 
Excellent explanations Jeff...I couldn't have asked for it any better than that. Simple enough so I can understand it at my level but advanced enough so I'm still learning quite a bit.

Muchos gracias. :D

...Alex Lamers...
 
Great explanations! Can a negative-tilt trough/low move east-northeast, instead of a more northerly track, and still bomb out? Well, I'd better explain further...what is behind all my questions about negative-tilt troughs is the low that spawned the Tri-State Tornado. This low moved east-northeast and bombed from 29.60 or so in Northern Arkansas to below 29.00 in Indiana. The low moved generally east-northeastward which suggests positive-tilt, but conversely, underwent intense bombogenesis, which suggests negative-tilt...
 
Originally posted by Michael Auker
Great explanations! Can a negative-tilt trough/low move east-northeast, instead of a more northerly track, and still bomb out? Well, I'd better explain further...what is behind all my questions about negative-tilt troughs is the low that spawned the Tri-State Tornado. This low moved east-northeast and bombed from 29.60 or so in Northern Arkansas to below 29.00 in Indiana. The low moved generally east-northeastward which suggests positive-tilt, but conversely, underwent intense bombogenesis, which suggests negative-tilt...

{Apologies for hogging this thread... I'll take a break to give others a chance after this post}

Actually, I'm not sure I'd recommend concluding that the movement of the low precluded it from being produced by a negatively-tilted upper-level trough. With a strong baroclinic boundary, the most intense warm-air advection (and resultant pressure falls, which directly influence the movement of the low) will often occur to the east or northeast of the low... If the primary baroclinic boundary is located in a WNW / ESE fashion (i.e. the warm front lies to the ESE of the low) the max WAA will likely occur to the east or ene of the surface low. Add in a couple other factors (such as the vort max swinging through the base of the trough), and I'd say that the ENE motion of the surface low could very well have fit in with a typical scenario of a neg-tilted trough aloft.

As a cyclone matures and begins to occlude, you'll often see it move more northerly as the warm air advections wraps all the way to the north or northwest of the surface low. This happens quite a bit with occluding coastal storms in the eastern US. As the low fills, it may continue to move toward the area of max low-level warm-air advection, which may indeed be to the north or northwest even.

Because of the intensity of surface lows often produced by negatively-tilted troughs, in addition to the favorable shear profiles often present in such an upper-level pattern, some of the biggest and baddest tornado and severe weather outbreaks have been associated with negatively-tilted troughs. It's certainly a good feature to be able to recognize, and you'll probably read a lot of AFDs from various NWS offices mention either positively- or negatively-tilted troughs in terms of unfavorable or favorable severe weather / tornado setups. Of course, a pos. tilted trough doesn't mean that a setup will be garbage, just as a negatively-tilted trough doesn't guarantee an outbreak.

EDIT: I see where a potential flaw exists in my reasoning... A deep, negatively-tilted trough is characterized by relatively high heights to the north of the vort max (since the trough is oriented in some degree from the NW to the SE, heights indeed increase to the north of the trough axis, moreso towards the base of the trough). High heights, in turn, are related to warm mean thermal profiles... This would indicate that the primary warm front would probably not be located east of the low, but more to the northeast or even more to the north of the low. The max WAA-causing-max-surface-pressure-falls process would then occur more to the NE or NNE of the sfc low, not so much to the ENE. I would say, then, that DPVA provided significant pressure falls to move the surface low east-northeastward. In addition, was there widespread precip to E or NE of the low as it developed and moved? Latent heat release can contribute to surface pressure falls, which may have also aided in moving the cyclone to the ENE....

I haven't looked at an upper-air map from that day, so this is an educated guess. I probably should before I say much more, in case the upper-air map doesn't look anything like what I'm imagining in my head. For example, the presence of a couplet jet feature may have proven significant (which it often does in "bombogenesis").. Maybe Glen could chime in, since there may be more complex processes at play that I don't know too well (e.g. I'll be learning IPV in grad. synoptic class this spring).
 
Originally posted by Michael Auker
Great explanations! Can a negative-tilt trough/low move east-northeast, instead of a more northerly track, and still bomb out? Well, I'd better explain further...what is behind all my questions about negative-tilt troughs is the low that spawned the Tri-State Tornado. This low moved east-northeast and bombed from 29.60 or so in Northern Arkansas to below 29.00 in Indiana. The low moved generally east-northeastward which suggests positive-tilt, but conversely, underwent intense bombogenesis, which suggests negative-tilt...


Weather balloon launches were not regular yet around the time of the tri-state, so we don't know for sure what the upper air features were on that event, but there were enough surface observations to at least construct some idea of what the event must have looked like. Using the original study done back in 1925, I reconstructed a cartoon of the surface features below:

tri-state.png


The storm that produced the tri-state tornado was just ENE of the surface low, tracking along the overlap of the warm front and pre-frontal trough at about 60 mph. Based on the orientation of the front, there almost certainly was a positive tilt to the upper trough, at least during this early phase as cyclogenesis was just starting to crank up, and there was most likely a strong jet streak/shortwave embedded in this upper trough that was the primary forcing for movement of the surface cyclone along the front. There may also have been a cold front aloft, or at least some feature driving a pre-frontal trough, as the winds were strongly veered ahead of the cold front. Typically, there would be surface dry air there then, with the moist transport (shown in green fill above) most likely ahead of the pre-frontal trough. There were other long-track tornadic storms that day, and the tracks suggest these storms were probably developing along the warm front and/or pre-frontal trough.

Glen
 
Ok I am being serious here when I say I am willing to pay someone to explain this to me in person or over the phone b/c this thread was why I struggled so in my atm. dynamics class last semester. I will be taking that class again next fall. So I have until then to learn this stuff but the sooner I can understand it the better prepared I will be.

A lot of the terms Jeff mentioned I am familiar w/ from the dynamics course last semester but yet I do not fully understand them. I would really really really appreciate someones time and patience in explaining this to me. Being a visuall "learner" for lack of a better word, 3D graphics always help. Just email me if anyones willing. Since the topic of this thread is crucial to the development or potential development of severe weather in the Plains, Id be stupid to be posting topics and threads on ST w/out knowing this stuff. Call it a pride factor.
 
Thanks, Glenn Romine and Jeff Snyder, for your great explanations. This thread should be condensed and pinned for a while...
 
Back
Top