Synoptic Review

We're good? Okay well what we're going to do is take a look at the review of, this
is sort of an overview of synoptic meteorology, but primarily QG theory,
frontogenesis, some of the stuff that you will see and ends up being a
building block for trying to make the forecast. So again, we'll go over
this and then we'll, in our break, we can take any questions. This is going to be
the easiest version of the QG height tendency equation you've ever seen
because that's it. We're just going to wave the magic wand and we don't have
any of the other nasty details, so it's just essentially two terms once we make all
of our assumptions and simplifications. The nasty version of it of term one
is geostrophic advection of vorticity by the geostrophic wind. Now we should know
that the wind is not perfectly geostrophic in the real atmosphere, so,
it's a pretty good first order approximation though, so you're not going
around, if you were to calculate this using numerical model output or
something like that, you would want to use the geostrophic wind, but the point
is, if you're looking at weather maps, we plot the real wind so it's the total
wind and all the mess that's involved with that. It's usually a good enough
approximation and there ways to tell if the flow is highly ageostrophic. Point
here is that cyclonic vorticity advection corresponds to height falls, so
where the vorticity is increasing with time, the heights are falling, that's
generally or you would expect a trough to move. The other one's just the thermal
advection term, which in this case it's the change in height of thermal
advection. We're looking for a max or a min somewhere in the column. So in the
case here, if we have differential cold advection, there's a level at which cold
advection is maximized somewhere in the atmosphere, the column in terms of
thickness shrinks around that maximum cooling. If you're below that, you'll see
height rises, and if you're above that, you'll see height falls.
So again, differential thermal advection for height change and just cyclonic
vorticity advection. So that's for the QG height tendency equation. Again, you just,
we can summarize this in text and most of this the same stuff that Ariel
already talked about, he went through in the derivations. You're just looking for
maxima in thermal advection in the column if you're looking for height falls
or height rises, and then you're looking for cyclonic or anticyclonic vorticity
advection. So it works the same way and when we get to the Omega equation, it's
literally just flip that around. It's just thermal advection and differential
vorticity advection, so it's really the main thing to remember. Here's a
real-world example of what this looks like.
We've got here, we're assuming the maximum in cold advection, that would be
this area up here along the California coast which if I can get the mouse back
where, there we go. This area here, this is a maximum in cold advection. We're just
assuming that that's the maximum in the column, it's weaker above and below that.
And so we would expect if there's a maximum in cold advection there, there
should be height falls above that. So say 500 millibars, you should see the heights
fall in relation to this maximum in cold advection, and we also have to consider
the vorticity advection term. Now this is one where I don't have the actual
advection on there, but if you look at where the contours cross, this is the
height contours cross the vorticity isopleths, somewhere along roughly
San Francisco into maybe a little bit of the Central Valley and then eventually as
you go east, the contours are parallel, so the advection is strongest somewhere in
here. So some combination of the cold advection up here, vorticity advection
down here, you combine the two, now this is what has happened in the previous 12
hours. You see that the heights of the 500 millibar pressure surface have
fallen the most in a result to the combination of cyclonic vorticity
advection and differential cold advection or max in cold advection
somewhere below that level. So that's how you would use that information to
determine where is the synoptic wave going. Is it going to amplify? Which way
should it move? Okay, the Omega equation. Again, simplest version you'll ever see
and it's just kind of a flipped around version of what we saw for the height
tendency equation. So in this case, we've got differential advection of absolute
geostrophic vorticity by the geostrophic wind blah blah, it's primarily going to be
just differential vorticity affection is what we're looking for.
So if cyclonic vorticity advection increases with height, that's a
contribution to vertical motion or ascent. And then the thermal advection
term is straightforward. Warm advection is ascent, and I'll show you what that
looks like in an isentropic surface, and differential CVA is also a contributing
factor for ascent. Now the other thing to remember with the QG omega equation
if you remember, there's a, the sigma term which is 1 over sigma that's a static
stability scale. Steeper lapse rates amplify the response. So if you have an
unstable profile, the vertical motion response will be smaller scale and
stronger than you would get in a statically stable environment. Okay, so the
vertical motion. Again, you can just summarize this and the key thing is just
remember, you're looking for differential CVA and thermal advection for Omega and
you're looking for CVA and differential thermal advection, you just flip it
around, for height change. So it's not too hard or hopefully not too hard to keep
up with that. That's the part you're going to use day-to-day whether trying
to make a forecast or judge the quality of the numerical model forecast.
Okay, here's a real-world example of what it looks, two different ways to look at the
thermal advection term. That gaudy color, this yellow area here, this is a maximum
in geostrophic thermal advection on the 850 millibar pressure surface, so it's
maximized over Maryland, Pennsylvania. I mentioned isentropic is another way to
look at this. An isentropic surface is we're just assuming that that we, it's
adiabatic the flow, so technically there wouldn't be any condensation, which might
be violating cases with precipitation. But here, if you look at the way I've got
the pressure surfaces labeled, this is 800 millibars, 850, 900, if you look at the
direction of the flow on that theta surface, it's going from higher pressure
to lower pressure. That's upglide. It's literally, think of it as going up a ramp.
So when we go back to the previous slide, this one, the flow is not constrained by
the pressure surface. It's not, you know, the air is not flowing at 850 millibars
and staying there. It's literally can be moving through the pressure surface. But
when we go to the isentropic surface, the air more or less, it's a better
approximation that it's staying on this isentropic surface. So in this case, warm
advection is isentropic upglide. They tell you the exact same thing, it's just
two different ways to look at it. Alright, the other term in here, so this is
the the same case we were looking at for thermal advection. Here this is
differential cyclonic vorticity advection in blue, this is just straight
from the SPC Mesoanalysis page. There's differential CVA downstream of this
shortwave trough, no big surprise there. Same area we have low-level warm
advection maximized and you see the precipitation echoes are in the zone
where the warm advection and the differential cyclonic vorticity
advection overlap, and this is pretty typical. Now in storm cases, it will
probably look a lot different than this because when you have large buoyancy and
low static stability, you don't tend to see these big stratiform areas of
precipitation. It will contract way down into the intense convection and it
probably wouldn't look like this, but the idea is to understand the background
processes first. Okay, frontogenesis. It may have been mentioned, I don't
think you guys have talked about it a whole lot, but it's another thing that we
would want to consider here. Point is the atmosphere doesn't like changes in the
temperature gradient. If it strengthens, the atmosphere will try to weaken the
gradient. If it's weakening, it will try to strengthen it. So and the point of
frontogenesis, if you're strengthening the temperature gradient, how would you
weaken it? You need to cool the warm side and you need to warm the cool side, which
would spread the gradient out.That's what the vertical motion field does, so
you get ascent on the warm side of a strengthening front and subsidence on
the cold side of a strengthening front. And these are zones that also, you have
enhanced advection. You've got a tighter gradient to begin with, so all of the QG
processes are tend to be maximized along these enhanced thermal gradients.
They're corridors for cyclogenesis and generally we're talking about thermal
gradients in the lower troposphere something you'd see at the surface up
through about 850 or 700 millibars. So again, these are all, the whole point here
is this stuff is all tied together. You know, the corridors of cyclogenesis,
frontogenesis, the QG forcing for ascent, height change, they all tend to be
related to where the gradients are strongest, so these are, you know, kind of
a big surprise, it's with the strongest synoptic waves and the strongest fronts.
Okay, we can illustrate this with a jet streak. I believe y'all talk about it a
little bit. Well this is just the frontogenesis explanation for the vertical
motion that you get with a straight jet streak, which is a pretty unrealistic one.
You don't see very many straight jet streaks. But just in this case, we have my
fancy animation air parcel moving through. What it's doing is as you're
coming toward the jet core, you're encountering a stronger gradient in
temperature. That's, from the parcels perspective, that's frontogenesis.
As you exit the jet core, you're encountering a weaker thermal gradient.
That's frontolysis. So the response of the atmosphere as the parcel moves in is
to offset the frontogenesis through vertical motion, and the way to do that
is cooling on the warm side, which is the right-rear quadrant or the right
entrance region of the jet core, in this case would be the a prime, down here is
the right entrance region so there would be ascent to try to cool the profile,
and there would be descent in the left entrance region to try to warm. So what
it's trying to do is offset the frontogenesis. The exact opposite happens in
the exit region. Now it's encountering a weaker gradient.
So remember, air parcels move through the jet core. This isn't, it's not a chunk of
air moving along by itself. They're accelerating through the tight gradient
and so frontogenesis can describe the jet core vertical motion quadrants, the
same thing can be done and I believe y'all did it with the QG Omega equation
with differential vorticity advection alone gives you the exact same
explanation, so I'd encourage you to go back and look at that. There's multiple
ways to explain these processes on the large scale. Okay, so the jet streaks,
again, they're with stronger temperature gradients, the air moves through, and it's
the same thing that I just summarized and the key is the response to it and as long as
you understand how these processes work, there are multiple ways to get the same
answer. It's not like you say well, jet street dynamics the only thing I can
apply in this case. That's usually not true. You could apply QG omega, anything
else, any of these other processes, you could look at warm advection, you could
look at flow on an isentropic surface, you tend to get the same answer which is
a good thing. These are not competing theories, they are actually just
different ways to state the same thing. Okay, the thermal wind. Now this is one,
this starts getting into changes in the geostrophic wind with height. It's not a
real wind, but it's a way to explain the response in the atmosphere to changes in
thermal advection with height or changes in the gradient with height. The
important thing here is that a veering wind with height, of veering geostrophic
wind with height, infers warm advection, which is related to vertical motion.
So now you don't have to look just at a pressure surface or an isentropic
surface, you can simply look at a wind profile and if the flow is largely
geostrophic and you have a veering wind with height, you've got thermal advection.
Cold advection is just the exact opposite, so it's a backing wind with height.
We're interested for storm purposes, the warm advection profile is the one that
we're primarily interested in. And again, if you have a thermal gradient, the
vertical shear, there tends to be a shear in the vertical wind which is related to
the thermal wind, the stronger gradients in the low troposphere, the strongest
flow tends to be sitting above those in the middle and upper troposphere.
So low-level fronts are important. You find where they are in the lower troposphere,
you can actually predict where the jet core will likely be aloft before you
even look at the data. And the reason you might say well why would I do that? Why not
just look at the stuff aloft? Point here is, first of all I'm going to give
you exercises coming up. Not today, but you're going to be asked to make
assumptions based on limited data and you need to have these conceptual models.
And I can tell you from personal experience, the easier it is for you to
piece this stuff together conceptually, the less time you're going to waste on
the mechanics of going through the meteorology in a forecast and you're
going to spend that time wisely on thinking about what it means for the
forecast. So if you don't have the mechanics down,
you get lost in all the details and you don't even think about the forecast, and
that's what happens to a lot of budding meteorologists as they get tied up in
the details oh why would I look at this? And you need to get that part down.
So we're not talking about this just, I'm just some windbag and I want to say hey,
this is, someone's twisting my arm and making me talk about this. I use
this stuff weekly. So it's very helpful and if you can get to where you
understand it in a basic sense, you can really help yourself as a forecaster.
Okay, I'll just show a real-world example. This is the 850 millibarb analysis,
somebody might recognize the date. This is 00z April 28, 2011. There was a little
bit of weather that day in the southeast, 200 tornadoes and it's basically the out,
tornado outbreak of a generation was ongoing at this time, but if you note
where we have the strong thermal gradient, I've sketched in the front here
850 millibars, it's across Mississippi. We note what happens with height. There's a
tilt to the system and this is, this is what we mean by a baroclinic
wave, It tilts to the northwest with height. So the gradient in the low-levels
tends to slope upward. So again a front is not a, on a pressure surface, it's not
a two-dimensional object. It's actually a sloping three-dimensional surface is
what you're really thinking of, and that's what an isotropic chart will show
you, is if you pick the right data surface, you can actually see the slope
of the front and instead of being gradients and temperatures, it would be
gradient and pressure whould show you that slope. Okay, so notice what's sitting
right above the tight low-level thermal gradient, the jet core So there it's no
accident. This is essentially just a real-world demonstration of the thermal
wind concepts. Okay, baroclinic system, I just mentioned that.The importance of
a baroclinic system is that you have advections going on. You have a system
where there's thermal advection, vorticity advection, you can lead to
vertical motion, system movement, system amplification, and then also as I
mentioned here the last bullet differential advection in a
thermodynamic sense can lead to destabilization. And again, you will tend
to see veering wind profiles in the warm sectors of baroclinic waves
because the atmosphere, it's not fully modified
and that gives you a large clockwise hodograph. We'll talk about that with
the supercell tornadoes. It's just a little bit relevant,
so baroclinic waves are what you're looking for. And then of course jet
streaks are associated with vertical shear and I know that's another big shocker
that a max in the wind aloft would come in and there'd be vertical shear increases,
but point is you need to look for the path of these baroclinic waves because
it is relevant. So again, the stronger gradient at the ground, this is the case we
just showed you, you notice that 500 millibars it's slopes back to the
northwest. This is just a classic baroclinic structure with a slope usually to
the northwest or to the west or south west with height, and that's typical in
westerly flow. Now equivalent barotropic, a little less interesting and this is
sort of a specialized case. We've got all the thermal contours or parallel to the
height contours, there's really no advection and you tend to not have the
same vertical shear and you don't have the same advections, you don't get the
amplification of the system, you don't get the motion, and that's what these
systems look like, these closed lows cut off from the main belt of flow. And then
if you notice here, this is 850 millibars, at 500 it's vertically stacked and you
notice all the flow, it just looks circular. It's all just and there's little
advection of any kind so it's hard, a system like this from a QG perspective
has no excuse to move anywhere. If there's no thermal advection, no
differential vorticity advection, it's not going to go anywhere, it's not
going to amplify, if anything it's just slow spin down process and then that
also says something about the vertical shear profiles. You don't have these big,
cyclonically curved hodographs in the warm sector of these closed lows, you
don't tend to see that. So for severe storm cases, baroclinic waves are much
more interesting than equivalent barotropic. Okay, so the key here in all
this, the whole point I've been trying to get to through all of this and all of
the stuff you went through with the derivations and you know, I don't,
I looked over that and it's like I don't envy you, I remember doing it myself,
you want to have solid conceptual models for what you, how you deal with the
overall synoptic environment. You want to know why you should expect ascent. I mean,
your other option is you can just let a numerical model do it for you and you
have no job because that can be automated right now. So you want to have
something to say okay, why should I expect there to be ascent, because the
numerical model is much more complicated than a quasi-geostrophic framework, but a
lot of times there are details in a model output that are going to mask the
important large scale processes. Again, stronger gradients, stronger advections,
stronger ascent, stronger height falls. It's all interrelated and the important
thing here is that we want you to be able to apply these concepts to figure
out how do I set the stage for severe thunderstorm development and maintenance.
It's not just an exercise and where's this trough going to go or is it going
to amplify, where will it be favorable for severe thunderstorms.
So we've already given you some of the basic ingredients and you know, I'll
follow up with some more stuff that I think is a little bit more specific, but
the idea here is tie it all together, have an understanding of large-scale.
As soon as you do that, you can shed a lot of these wasting time with the mechanics
and you're actually thinking about the process and I can tell you that just,
these simple concepts can help you put out a superior forecast at times
compared to the most sophisticated numerical model. Take any version of the
HRRR you want, any run, whatever your favorite is, the four kilometer NSSL ARW
that's run for us at SPC, whichever one you like.
Pick that and I guarantee you can outperform those at times using these
simple large-scale processes, just the conceptual model to represent them. Okay.

Synoptic Review

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