University Place

The Jet Stream

[Tom Zinnen, Outreach Specialist, Biotechnology Center, University of Wisconsin-Madison]

Welcome, everyone, to Wednesday Nite @ the Lab! I’m Tom Zinnen. I work here at the U.W.-Madison Biotechnology Center. I also work for U.W.-Extension Cooperative Extension. And on behalf of those folks and our other co-organizers: Wisconsin Public Television, Wisconsin Alumni Association, and the U.W.-Madison Science Alliance, thanks again for coming to Wednesday Nite @ the Lab. We do this every Wednesday night, fifty times a year.

Tonight, it’s my pleasure to introduce to you Zac Handlos. He was born in Milwaukee, grew up in Manitowoc, graduated from Manitowoc Lincoln High School, came here and got all three of his degrees: his Bachelor’s, his Master’s, and just recently his PhD in the Department of Atmospheric and Oceanic Sciences.

He’s going to be talking to us about a river. Uh, but it’s a metaphorical river. It’s river called the jet stream. We’re pretty familiar with rivers and flowages here in Wisconsin. It’s a great thing to get to hear about how this river up in the sky affects the things that you and I get to do. Zac will be heading down to Northern Illinois University in a little bit to start his next stage of his career, so I’m looking forward to having him still within the area.

So, with that, I also want to acknowledge that we have the summer camp in meteorology for high school students here. This is an annual event at U.W.-Madison, and that’s why we have a meteorology speaker this time of year. Please join me in welcoming Zac Handlos to Wednesday Nite @ the Lab.

[applause]

[Zachary Handlos, Research Associate, Department of Atmospheric and Oceanic Sciences, University of Wisconsin-Madison]

Alright. Can everyone hear me okay with the – the microphone on? Alright.

So, my name is Zachary Handlos. I’m a research associate here at the University of Wisconsin-Madison. And yes, I recently just finished my PhD in the Department of Atmospheric and Oceanic Sciences. And today I’m very excited to talk to you about one of my favorite topics in meteorology. And that is of jet streams, which I like to call the rivers of the upper troposphere.

So, just to kind of outline what I’m going to be talking about this evening. First, I’ll talk about what the jet stream is. And also –

[slide titled – Outline – with the following list of subjects the lecture with cover – What is the Jet Stream? Why do we care? ; Types of Jet Streams and Why They Exist; Discovery of the Jet Stream; Jet Streaks and Weather Systems; Current Jet Stream Research]

– why do we even care about studying the jet stream? What’s so important about the jet stream? How is it relevant to our lives?

And then I’ll talk next about two primary types of jet streams that we study commonly in meteorology and then also the physics behind why these exist. And then I’ll transition next to talking about the discovery of the jet stream. And it turns out that actually the person who founded the Department of Atmospheric and Oceanic Sciences here played a prominent role in discovering the jet stream. It’s a really great story and I can’t wait to share it with you if you’ve never heard the story before.

Then Ill transition into talking about jet streaks. And these are the bursts of wind that are embedded inside the jet stream that have – have a role in the development of weather systems. And then finally, I’ll conclude with some current jet stream research that we’ve been working on here at the University of Wisconsin-Madison.

[the first bullet of the slide above – What is the Jet Stream? Why do we care? – becomes highlighted in bold text]

So, first let’s start with the basics.

[Zachary Handlos, on-camera]

What is the jet stream? So, the simple definition of the jet stream is that the jet stream is simply a fast, narrow current –

[slide titled – Jet Stream – and the points – Fast, narrow currents of moving air in the upper troposphere and What is wind? Why does air move? The slide also features an illustration of the Earth with two Jet Stream indicated by connecting arrows moving around the Artic region labelled, Polar Jet Stream, and colored blue and a second set of connecting arrows near the equator that is labelled, Subtropical Jet Stream, and is colored red]

– of moving air in the upper troposphere. And these are the two different primary jet streams that we’ll be focusing on this evening. And Ill talk more about the polar jet stream and the subtropical jet stream. But before we even get into that, it’s really important to understand what wind is and why air even moves in the atmosphere in the first place.

[slide titled – Why do we have wind? – featuring an illustration with the Sun on the right-hand side and half of the Earth on the left-hand side and the statement Curved Earth = Sun rays hit Earth at angles. Between the Sun and the Earth are three yellow arrows used to represent the Suns light, one at the equator marked 90 degrees and one above it and one below it that hit the Earth at 45-degree acute angles and noting that acute angles = less intense heat (cooler)]

And so, why do we have wind in the first place? So, wind is simply just air that moves around the atmosphere. The atmosphere is a gas, and it moves similar in a fluid motion much like water does. And the reason that we have wind has to do with differential heating by the Sun. And so, differential heating simply refers to uneven heating of the Earth from the Sun. So, the Sun sends electromagnetic radiation, or solar radiation, towards Earth, and certain regions of the Earth receive more direct solar radiation than others. So, for example, at the Equator, from a year-round average perspective, this is the warmest spot on Earth year-round because it receives the most direct incoming solar radiation. But because of the curvature of the Earth – the Earth is approximately a sphere – as you go to higher latitudes, you – Sun still emits solar radiation to these spots, but this solar radiation is distributed –

[Zachary Handlos, on-camera]

– over a greater area. And so, these regions receive less intense heat or less intense solar radiation per unit area relative to the Equator. And so, that’s why when you go higher to the North or South Poles, or go north or south respectively, it is colder on average year-round.

Now as a result of this, the atmosphere wants to be in a state of equilibrium, and it wants the temperature to be balanced everywhere throughout the atmosphere, but that’s never going to happen the way that the Earth is shaped and the way the Sun currently heats the Earth. So, as a response to this, the atmosphere has large-scale motions that –

[slide titled – Differential Heating induces atmospheric motion! – featuring the same illustration as the previous slide but now having an inset illustration on the lower right that shows air movement throughout the Earth. At the top of the Earth are two oval shaped connecting arrows to the right and the left of the North Pole that are labelled Polar Cells. To the right and left of these – heading toward the equator are two more oval shaped connecting arrows that are labelled Ferrel cells. Below these (on both sides) and just north and south of the equator are two more oval shaped connecting arrows that are labelled Hadley cells. In the lower half of the illustration these oval shaped connecting arrows repeat in mirror image to the South Pole. Also noted is that to the north of the Hadley cells the air is sinking, while to the south of the Hadley cells the air is rising. In the middle of the illustration between the Hadley Cells are red arrows that point to the west that are labelled Trade Winds. To the north and south of the Trade Winds are blue arrows that point up to the North pole and down to the South pole (between 30 and 60 degrees north and south) opposite of the Trade Winds and are labelled Westerlies. At the top and bottom (between 60 degrees north and south to the North and South poles respectively) below the Polar cells the arrows again head toward the left and are labelled Polar Easterlies]

– move air around and try to redistribute this temperature and balance everything out. So, for example, if you look at this bottom right diagram, there’s these Hadley cells, or these red circulation cells that extend from the Equator to about 30 degrees north or 30 degrees south latitude, depending on which hemisphere you’re in. And what these Hadley cells do is they take this excess warm air that’s sitting at the Equator, and this warm air rises, and then is transported polar aloft towards the higher latitudes. And then on the return flow of the Hadley cells circulation, cold air – cold air sinks and then returns from the higher latitudes Equator-ward towards the Equator. So, the goal of the Hadley cell is one way to try to balance out differences in temperature in the atmosphere.

Now as you go to higher latitudes, the Hadley cell sort of breaks down and other types of circulation systems are responsible for the transport of heat and cold air around the middle latitudes in the higher latitudes. And that’s where the Ferrel cells and Polar cells come in, respectively. And it turns out that the Ferrel cell region is a region where a lot of mid-latitude weather systems form and accomplish some of the heat transport. And we’ll investigate an example of one of those later on in this –

[new slide titled – Jet Stream – and indicating that – Jets reside in transition regions between large-scale, meridional overturning circulations. The slide also shows a 3-dimensional graph of the Jet Stream with the surface of the Earth from North Pole to the Equator on the x and z-axes and Altitude on the y-axis. From the left at the equator, the graph shows a Low-pressure Hadley cell pushing warm air up in the form of a cloud and circulating northward until 30 degrees latitude where it hits colder air and is pushed down by High pressure. At the top of this High pressure at 30 degrees north in the Tropopause area of the atmosphere is a blue arrow facing north labelled Subtropical jet. The Hadley cell then travels northward at a lower altitude until it hits the low-pressure Polar front at around 60 degrees latitude where the warm air is forced up into the higher altitude (again represented by a cloud) and at the top of the Tropopause is another blue arrow facing north labelled Polar jet]

-presentation.

Now the jet streams themselves reside in transition regions between these large-scale meridional overturning circulations. Where meridional is a word that refers to circulations that move in the north-south direction. So, for example, the subtropical jet stream, one of the two primary jets that we’ll be focusing on this evening, actually resides on the poleward edge of that Hadley cells circulation. And we’ll talk about why that is a little bit. The polar jet stream sits at a lower altitude but higher latitude relative to the polar – to the subtropical jet stream, and it sits sort of on the poleward edge of the theoretical Ferrel cell.

[new slide titled – Why do we care about the Jet Stream? – that features the following bulleted statements – Plays role in development and motion of weather systems; Airplanes can be affected by jet stream; Interact with other large-scale atmospheric phenomena]

So, why exactly do we care about the jet stream in the first place? Well, theres three primary reasons why we care. One is that the jet stream actually plays a prominent role in the development and motion of weather systems. And so, the weather systems specifically that we’ll be talking about are low-pressure systems. And maybe you’ve heard of these if you watched TV weather, or –

[Zachary Handlos, on-camera]

– weather.com or have gone to the Internet to look at weather maps. Low-pressure systems are usually these systems that are marked with an L on a weather map, and we’ll talk about how those relate to the jet in a little bit.

Another reason is that we have airplanes or aircraft that fly at very high altitudes, and they commonly fly at altitudes that are the same height as the jet stream. Now if an airplane is trying to fly against the jet stream, if the wind speeds within the jet stream are fast enough it can be very difficult for an airplane to fly against that wind speed. Or it can be convenient if the plane is flying along the jet stream along a fast current of air. So, this is of interest to people that fly airplanes.

And then lastly, the jet stream also interacts not only with the weather systems but with other large-scale atmospheric phenomena. And we’ll talk about an example of that later in this talk as well.

So, that’s sort of the basics about the jet stream. Now let’s talk specifically about the two different types of jet streams that I’ve introduced and specifically why they exist on Earth.

So again, the two different jet stream –

[slide titled – Jet Stream Types – featuring the previous illustration of the Earth with the blue circular connecting arrows toward the North pole labelled Polar jet stream and the red circular connecting arrows towards the equator labelled Subtropical jet stream]

– types that were going to focus on are the polar jet stream and the subtropical jet stream. And specifically –

[new slide titled – Subtropical Jet Stream – featuring the same illustration as the previous slide but now with these two bullet points – Resides within the subtropical latitudes; Exists is accord with angular momentum balance]

– let’s start with the subtropical jet stream. So, the subtropical jet is this red arrow that circumnavigates the globe. And there’s actually a subtropical jet stream both in the Northern Hemisphere as well as the Southern Hemisphere. And it’s called the subtropical jet stream simply because it resides within the subtropical latitudes. So, were thinking about 30 degrees north latitude. So, for example, the Sahara Desert lies approximately in this latitude, and the southwestern continental United States, as two examples.

And this jet stream exists approximately in accord with something known as angular momentum balance.

[new slide titled – Angular Momentum Balance – that features the following equation – Angular Momentum = (Mass) x (Velocity) x (Radius from axis of rotation)] As an example, the slide features an illustration of a figure skater with her arms up and one leg out getting ready to do a spin. At the top of the illustration is a dotted line that extends from the top of her head to the bottom of her planted foot which is labelled Axis of Rotation. Another double-sided green arrows points from her outstretched fingertips to the bottom of her neck which is labelled Radius/Distance]

And so, maybe angular momentum balance sounds sort of as an intimidating sort of scientific term, but you’ve probably seen an example of this often, especially if you’re a fan of the Winter Olympics. And so, what angular momentum is is this quantity that is conserved for objects that are on Earth, and Earth is a rotating object. And it’s really just a product of an object’s mass, velocity, and the distance, or radius, from the axis of rotation in which an object is rotating about.

And so, a classic example with respect to angular momentum balance is that of a figure skater. So, maybe you’ve watched the Winter Olympics before –

[Zachary Handlos, on-camera]

– or ice skating on TV, and figure skaters will commonly do tricks where they have their arms expanded out and they’ll sort of spin in a circle around a theoretical axis of rotation. And from time to time, they’ll bring in their arms from the outside and bring them in. And you notice that the speed that they rotate changes. That’s actually tied to this angular momentum balance idea. And so, just for reference, when we’re saying radiance or distance, we’re talking about the distance sort of from the mid-torso of the ice-skater all the way out to the extension of their arms, or their fingertips.

And so, let’s say we have a figure skater and theyre – and they’re doing some sort of spin trick with their arms extended outwards. Now what happens to the speed of the ice skater if she brings her arms inward? How can you show this with angular momentum balance? So, here is our equation again. Angular momentum is yet a product of the skater’s mass –

[slide again under the title – Angular Momentum Balance – featuring the same illustration of the figure skater as the previous slide and again the equation Angular Momentum = M x V x R. Additionally, there are now factors for the four variables – Angular Momentum must stay constant; Mass stay the same; Radius decreases (as the skater pulls her arms in); Velocity will increase]

– velocity, and its distance from its axis of rotation. Now throughout this exercise the angular momentum remains constant the whole time and so does the mass of the skater, unless we decide to give the skater a backpack or something to wear in the middle of the trick, but that would be very difficult to do considering that they’re spinning. So, we’ll say that their mass remains approximately the same. Now as the skater brings her arms from the outside inward, that distance between the axis of rotation and where her arms are extended is going to decrease, right? Because we’re bringing that arm span inward and wrapping it around her body. So, as a result, since we want angular momentum to remain conserved the only thing that can happen to the velocity of the skater is that it has to increase in order –

[Zachary Handlos, on-camera]

– for the left-hand side of this equation to remain constant. And that’s exactly what happens. So, this picture is supposed to represent, since it’s a still image, the skater spinning around faster with her arms inward. And so again, since angular momentum is constant, as you decrease that radius the velocity or the rotational speed of the skater is going to –

[new slide under the same title – Angular Momentum Balance – now featuring the illustration of the figure skater with her arms held inward decreasing the Radius and blurred to show the increase in speed. The slide still features the Angular Momentum equation – AM = M x V x R and the outcomes of the various variables – Angular Momentum is constant; Mass stays the same; Radius decreases; Velocity increases. And the statement that since Angular Momentum (AM) is constant, Velocity must increase!]]

– increase as a process of this. And the opposite happens too: if the skater brings her arms outward throughout this process, she’s increasing that radius, and so by definition the velocity has to decrease, and she’ll actually slow down her rotational speed as she spins around this axis of rotation.

[new slide titled – Angular Momentum Balance – Air on Earth – featuring an illustration of the Earth tilted on its axis with green circular arrows indicating rotation at the north and south poles as well as at the equator]

So, you are probably wondering: How does figure skating have anything to do with the jet stream on Earth? We’re talking about two completely different things, right? It turns out there are actually some similarities with respect to this. So, even air – air has mass on Earth, and therefore it has angular momentum, and this is approximately conserved on Earth.

So-

[new slide titled – Consider an object at rest at equator – featuring the same illustration of the tilted Earth with the green circular arrows at the north and south poles but now the green circular arrow at the equator has a red dot indicating an object at rest at the equator]

– as an example, let’s consider some sort of object. We’ll say we have this huge red ball that’s just sitting at the equator at rest. Now while were on Earth and we see a ball at rest at the Equator, it may not be appearing to be moving, however, this object is still rotating around Earth’s axis of rotation. Everyone in this room right now is actually accelerating around Earth’s axis of rotation. We don’t feel it because in our frame of reference, we’re on the rotating Earth –

[Zachary Handlos, on-camera]

– but if someone was watching this from space and they could actually see this room right now, they would see us rotating around Earth’s axis of rotation. That would be a different frame of reference. I’m not exactly sure how they would see through the building, but that’s just a theoretical example. So –

[laughter]

So, we have this object at rest at the equator. But what happens if we take this object, and we decide we’re going to move it very far north? So, lets say were gonna – we’re gonna to kick this kickball, for example, we’re gonna kick it 30 degrees north latitude. Again, that’s a very far kick and a far distance, but it’s just a theoretical idea.

So, what happens as this object moves poleward? Well, as it moves poleward – again, remember –

[slide titled – Distance from Earths axis of rotation decreases – featuring the same illustration as the previous slide but now with a yellow arrow pointing northward on the red dot on the equators circular green arrow]

– Earth is approximately a sphere. Now the distance between the Earth’s axis of rotation and the Equator is much greater than any distance between the Earth’s axis of rotation and a point farther poleward. So, as the ball or the object moves poleward, the distance between the object and the Earth’s axis of rotation decreases throughout this process.

[new slide with the same illustration as the previous slide but now with the equation for Angular Momentum at the top – AM = M x V x R and the results of the variables in the upper right – Mass stays the same; Radius decreases; Velocity =? ; Angular Momentum is constant]

So, this sounds very similar to the figure skater idea, right? With a figure skater example, the radius of rotation decreased. And what we saw there is the velocity had to increase. It turns out that the same sort of idea can be applied here too for this object. The angular momentum has to be conserved. The mass of the object stays the same, we’re not changing the mass of the object. And the radius is decreasing with respect to the Earth’s axis of rotation. So as a result, and by definition, mathematically, the velocity has to increase –

[the slide animates on increases to replace the ? as to the question of Velocity]

– during this process.

Now what does that look like or how does that translate in terms of the motion we see on Earth?

[the slide animates out the Angular Momentum equation and variables and replaces them with the statement – Objects speed around Earths axis of rotation increases – moves eastward at a faster speed!]

Now, by increasing the velocity that means this object is going to spin – when it gets to that higher latitude – is going to spin faster around Earth’s axis of rotation relative to the Earth itself.

[Zachary Handlos, on-camera]

So, how we would see it on Earth is that wed appear – the ball would appear to have right-ward deflection in its path. It would appear to move west to east over time and accelerate in the eastward direction. This is also referred to as the Coriolis effect. So, if you’ve ever learned about Coriolis force or Coriolis effect, but this is essentially what I’m explaining as well too. So, as we decrease that axis of rotation, the object moves west to east at a faster speed.

How does this relate to the jet? Well, if you notice the subtropical jet stream –

[slide featuring a new illustrated Earth with colors representing the regions of the Earth. The Tropical Region is in pink and is from the equator to approximately 30 degrees north and south latitude of the equator. The Temporate Region is in olive and is from approximately 30 degrees to 60 degrees north and south latitude. The Polar Region is in blue and is approximately from 60 degrees north and south latitudes to the poles. Also indicated on the map are the two Polar jet Streams as connected circular arrows at the top and bottom of the illustration (60 degrees) and the two Subtropical Jet Streams as connected circular arrows at approximately 30 degrees latitude north and south of the equator. Additionally, the slide has the following two statements – Air transported upward and poleward by Hadley Cell; Increases speed in the west-east direction (westerly wind)]

– sits at about 30 degrees north and south latitude. If you remember what that Hadley cell circulation, that red circulation arrow I showed earlier, we have warm air that’s rising at the Equator and then it moves poleward aloft. So, just think about the Northern Hemisphere for example, that air is moving from the Equator northward aloft. Its angular momentum is going to remain conserved and its radius or distance from that axis of rotation is also going to decrease. So, that air is going to accelerate in the eastward direction over time due to this effect. And it turns out that the maximum limit in which this occurs is where the subtropical jet stream resides. So, at about 30 degrees north latitude the air will eventually have a speed of about, well over 100 mph sometimes over 100 knots – or sorry – or sometimes over 150 to 200 knots in some cases –

[Zachary Handlos, on-camera]

– depending on where you are. And that’s – thats exactly where the subtropical jet stream sits. So, at the northern edge of the Hadley cell or the southern edge in the Southern Hemisphere that’s where air is deflected and has west-to-east motion, and that’s why we have subtropical jet stream. So, the Subtropical jet stream is more or less existent due to this angular momentum balance idea.

So, now you’re probably wondering what about the Polar Jet Stream? Is this –

[slide titled – Polar Jet Stream – featuring the previous illustration of the Earth with the Polar jet stream indicated by the connected blue circular arrows (and the Subtropical jet stream indicated by the connected red circular arrows) and with the following bulleted points – Resides within the middle to polar latitudes; Exists in accord with thermal wind balance]

– also a function of angular momentum balance? Not exactly. It actually turns out that this angular momentum balance or sort of theoretical approximation doesn’t necessarily hold up once you get to the middle latitudes or higher latitudes. Angular momentum conservation breaks down a little bit. And the polar jet stream actually exists due to a complete – for a different reason. It’s actually tied to differences in temperature at the surface, and it’s tied to this concept called thermal wind balance.

[new slide titled – Thermal Wind – with the following two bulleted statements – Change in wind speed with height due to horizontal temperature differences; If you are standing with colder air to your left (in the Northern Hemisphere), wind speed increases with height flowing in the direction that you are facing]

Thermal wind simply refers to the change in wind speed with height due to horizontal temperature differences. This is kind of a weird concept to understand. So, thermal wind is not actually a quantity that you can physically measure outside with a weather instrument, but it’s something you can theoretically calculate if you know how temperature changes over a distance and how wind speed is changing from the surface as you go higher aloft into the upper troposphere.

Another way to think about it is that if you’re standing with colder air to your left, assuming you’re in the Northern Hemisphere, the wind speed is going to increase with height flowing in the direction that you are facing. So, let’s –

[new slide titled – Thermal Wind – Example – featuring a bar graph with the surface of the Earth on the x-axis with the equator far left and the North Pole far right and the y-axis is Altitude. One red bar near the equator is labelled Warm Air and it extends high into the atmosphere, the other blue bar is near the North Pole and is labelled Cold Air and it is about half the length of the Warm Air bar]

– look at this in a more pictorial form as an example. So, let’s say I’m standing right here and the Equator in this case is going to be to my right side or to your left side, and cold air is going to be to my left side or your right side. So, in this situation –

[the slide animates four different lines radiating from just above the North Pole each at differing wind pressures – at the bottom at a slight angle is 850 mb, at a slightly higher angle is 700 mb, at a slightly higher angle still is 500 mb, and at the top is at slightly more than a 45-degree angle is 300 mb. Between the two bars from bottom to top are a circle with a dot in the middle with double arrows through the center that increase in size at higher altitudes. On the Warm Air side, the arrow is labelled CF and on the Cold Air side the arrow is labelled PGF. Also noted is – Wind is a function of strength of pressure gradient force (PGF), which is proportional to slope of pressure surfaces]

– there are variations in pressure as you go higher up in altitude. Pressure decreases with height because pressure, air pressure, is simply the amount of weight of air pushing down on a unit area. But if you go higher up in altitude, there’s less air molecules forcing their weight down on you, so the pressure decreases over time. But it turns out that in a warm air column such as this red rectangular region, what happens is air expands and it actually push these pressure surfaces to higher altitudes. And with a cold air column this air is more compressed, and these pressure surfaces are at lower altitudes. And it turns out, what happens is you get a gradient in these pressure surfaces that are gradients in the height of where these pressure surfaces are located in the atmosphere. And as a result, the stronger, the – the bigger the slope is of that pressure surface, the faster the wind goes at these altitudes. So, as you see the slope of these pressure lines, or these isobars, gets more and more slanted as you go up in higher altitudes, and as a result the wind speed actually increases as you increase altitude. And it’s all tied to this –

[Zachary Handlos, on-camera]

– horizontal temperature difference.

So, the dots in this picture represent the wind that’s flowing out of the slide toward you right now. So, if I’m facing this way, in my situation the cold air is to the left, so the wind is increasing speed with height flowing –

[return to the – Thermal Wind – Example – slide above]

– in the direction that I’m looking right now. If I flip this way and the cold air was to my left, then the wind speed would be increasing in the direction towards the wall that I am looking at. That’s, essentially, wind balance.

[the slide animates on two more bullet points – Via thermal wind, geostrophic wind speed increases with height; Wind speed strongest in upper troposphere above regions of strong horizontal temperature contrast]

[new slide titled – Polar Jet Stream and Thermal Wind Balance – featuring the statement – Polar Jet Stream resides over regions of strong horizontal temperature contrast in the lower troposphere. Additionally, the slide has two thermal images side-by-side showing the Polar Jet Stream at 300mb Wind Speed and at 850mb Temperature where a strong, horizonal temperature contrast is shown by isobars that are wide apart near isobars that are close together]

Now there’s more mathematics to it, and we’ll get some – to some of that later, because that plays a prominent role actually in the discovery of the jet stream. But here is a weather map example to kind of show again how thermal wind balance works and ties directly to the polar jet stream. So, what I have here on the left-hand side is a plot showing 300 millibar wind speed. And basically, where you see cyan, green, yellow colors, these are faster wind speeds, so the wind speeds are shaded by color. And so, you can kind of see this narrow stream of warmer colors, and that represents the polar jet stream in this case. And I boxed a portion, or an example, of that polar jet stream within that black rectangle on the left-hand side.

Now if you notice on the right-hand side, we have a plot of 850 millibar temperatures. So, this is temperature about a mile above the surface. And regions which correspond – which correlates pretty well with temperature at the surface. The only difference is that when we use these maps it’s not affected by the diurnal cycle or changes in the temperate due –

[Zachary Handlos, on-camera]

– to the Sun rising and the Sun setting. So, but it still gives you a really good idea of where warm and cold air masses are in the atmosphere.

And so, regions where you see yellow, orange, or red colors represent warm air regions. And then when you go to the greens, and blues, and purple colors, these are regions of colder air. So, regions where you see a really strong change in color over a short distance, this would be an example of a strong, horizontal temperature contrast. And so, I boxed an example –

[return to the previous – Polar Jet Stream and Thermal Wind Balance – slide with the thermal images]

– of that on the right-hand side. And if you notice, here again is a region of strong horizontal temperature contrast. If you look to the left, this is exactly where our polar jet stream resides. So, the polar jet, this maximum wind speed in the upper troposphere, resides exactly over the region of strong horizontal temperature contrast and thats all due to thermal wind balance. And it’s kind of cool. If you look around the rest of the plot – so, this polar jet is circumnavigating the globe around the upper – the higher latitudes of the Northern Hemisphere. You notice that sort of the transition of where the colors change from blue to red that the polar jet stream follows right along this transition. And again, that’s due to thermal wind balance.

[return to the – Outline – slide with the bullet point – Discovery of the Jet Stream highlighted in bold]

So, how does this – how does this thermal wind balance idea tie to the discovery of the jet stream?

[slide titled – Discovery of the Jet Stream]

So, again, the discovery of the –

[the slide animates on the bullet point – AOS Department Founder Professor Reid Bryson – along with a photograph of Professor Bryson in his lab]

– jet stream was actually – this story is based on a calculation performed by the – the founder of the Atmospheric and Oceanic Sciences Department here, Professor Reid Bryson. And so –

[new slide still under the title – Discovery of the Jet Stream – featuring a photo of a U.S. Air Force F4U Corsair World War II fighter plane and the following info list – Setting – World War II – 1940s; Preparing for daytime bombing mission to Japan; Aircraft to fly at 30,000 to 35,000 feet; Calculate how fast winds would be]

– what he did is, at the time he was working in the Armed Forces during World War II in the 1940s. And he was helping out in preparation for a daytime bombing mission, actually, to Japan. So, he was working with some commanding officer or commanding general, and another colleague to basically do weather analysis and weather forecasting for the military during World War II.

And so, for daytime bombing mission –

[Zachary Handlos, on-camera]

– at the time, these aircrafts in the 1940s when they were flying to Japan over the Pacific, would fly at very, very high altitudes in order to avoid anti-aircraft guns or any sort of defense mechanism that the Japanese would have upon arrival. Ans so, they figured if they fly – if they flew high enough in the atmosphere, they could sort of avoid this danger as they were flying.

And so, their job for this particular mission was they were going to calculate how fast the winds were going to be in this particular – for this particular day. And they were actually calculating a 20-hour forecast, using the data that they had at the time. This was a very hard task in the 1940s. In the 21st-century atmospheric scientists like myself are spoiled with lots of satellite data, observational data, balloon launches, etc. We have all sorts of data that we can pull up on a computer and immediately perform calculations and figure out a forecast within seconds or minutes. Back then there was no satellites yet. And the funny thing is that actually the first weather satellite was actually invented here at the University of Wisconsin-Madison. So, we have a lot of good things going in our department here historically, which is great. But at the time, this was not here yet and so they had limited data to work with. However, they had just enough limited –

[slide still under the title – Discovery of the Jet Stream – with the points – Limited data; What Professor Bryson and colleague knew – cold, dry air to north of Japan; warm, moist air to the south of Japan; Thermal Wind Theory! The slide also features a triangle formed by Lower-Level Wind as a blue arrow pointing up into the atmosphere; Upper Level Wind as a red arrow pointing east from the end of the Lower Level Wind arrow; and a black arrow forming the hypotenuse between the start of the blue arrow and the end of the red arrow labelled Thermal Wind with the mathematical formula for its calculation below it]

– data to figure out a pretty accurate calculation of what the wind speed was going to be at the level of the jet stream.

So, what they – what he – what Professor Bryson and his colleague knew were that there was some sort of cold front feature that was going through Japan. And a cold front is an interface – is a boundary that separates two different types of air masses. So, to the north there was very cold and dry air over northwestern Japan and northwest of that region. And then to the southeast there was warm, moist air present. And then secondly, and most importantly, they had thermal wind theory to work with them. And so, this is – just for fun I put up the calculation of thermal wind here on the bottom of this plot here. But they could essentially use this sort of mathematical equation along with their observations to figure out not only how the wind –

[Zachary Handlos, on-camera]

– was going to change with height, but also what the wind speed was going to be approximately at 35,000 feet above sea level.

So, what they did is they calculated the change in wind with height, using this temperature data because, again, if they know something about the horizontal temperature contrast, they can figure out how the wind is going to change with height. And they did that, and they had enough data to figure out the surface wind so they could easily calculate or at least estimate the wind speed in the upper troposphere at 35,000 feet. And the wind speed that they calculated at the time was around 168 knots –

[slide still under the title – Discovery of the Jet Stream – now with a photo of palm trees blowing sideways in a hurricane and the following bulleted points – Bryson and Plumley calculated the change in wind with height using their data; Wind speed calculated at 35,000 feet – 168 knots from the west (approximately 193.3 miles per hour)]

– from the west, which is about 193.3 miles an hour.

This is a very impressive number. And it turns out it was so great of a wind speed that –

[new slide still under the banner – Discovery of the Jet Stream – with the following list – Airplanes flew at a speed of 180-250 knots max; Told General about situation; General said to calculate again; New calculation -]

– no one believed them when they made this calculation. So, what they were concerned about is that airplanes only fly at about a speed of about 180 to 250 knots max at the time. And so, if these airplanes are flying into a jet that’s moving the same speed against them, they’re just going to look like they’re sitting in the air stationary, not moving at all. And that’s going to be a problem because they’re not going to make it to Japan or they’re going to run out of fuel.

So, they told their commanding general about the situation, and the general said, Theres – there’s no way this is true. That – that – that wind speed is too fast; we’ve never observed this to our knowledge. You got to do the calculation again and see what you get. So, they did the calculation again. And, you know, it’s legitimate to recalculate. It’s always good to check your work in science, and sometimes people make calculation mistakes. So, they did it again, and this time they got –

[the slide animates on – 168 knots from the west (approximately 193.3 miles per hour) – under the New calculation bullet]

– 168 knots from the west, or 193.3 miles an hour. So, they got the same number again –

[Zachary Handlos, on-camera]

– with the theory, so they must’ve done their calculations either right or something was wrong about the theory.

So, the commanding general thought something was wrong with the theory and said, There’s just no way this is right. We’re going to carry out the mission anyway. And we’re going to prove that you’re wrong. And we’re actually going to measure the wind speed and come back and tell you what the wind speed is gonna be when we fly at 35,000 feet. Well, the mission never actually worked out because the airplanes could not fly against the jet because the jet was moving quite fast at that time and the – the mission was a complete bust. And the commanding general actually came back after the mission and said – no, he actually apologized to Professor Bryson and his colleague and said: I’m sorry. You were – you were correct. We actually measured the wind speed and couldn’t carry out the mission. The actual wind speed was 170 knots from the west. It was actually faster than they calculated with the theory which –

[slide still under the title – Discovery of the Jet Stream – featuring a black and white weather chart map of Japan with isobars and the subsequent points – Decided to carry out the mission anyway (calculation must be wrong!); Actual observed wind speed – 170 knots from the west (approximately 195.6 miles per hour)]

– is pretty crazy. So, the wind was moving at about 195.6 miles an hour, which is equivalent to a very strong tornado, maybe an EF2, EF3 type of tornado or a very strong typhoon in the West Pacific. So, this was happening in the upper troposphere when they tried to accomplish their mission. And heres an example of a weather chart from the 40s sort of showing how fast the West Pacific jet stream was at that time. And it turns out that this region, the West Pacific jet itself, is actually one of the reg – the regions with the strongest jet stream on average –

[Zachary Handlos, on-camera]

– especially during wintertime, around the world. So, theory is very powerful. Even with limited observations and theory they could figure out what the wind speed was fairly accurately, which is quite impressive, considering there were no computers yet and there were no satellite data yet and very limited data.

And actually, a funny story is that this sort of fast current of air was later coined the name “jet stream” by Reid Bryson’s PhD advisor at the University of Chicago, Carl-Gustaf Rossby, who named it the jet stream because at the time he was doing a lot of work with dynamics with waterflows and jets in the water and thought this air current seems very similar to a waterjet, so we’re going to call it the jet stream. And forever on – forever on, now it’s called the jet stream in the atmosphere. So, it truly is a river of the upper troposphere in that matter.

So, the next thing Im going to talk about are – let’s get a little more into the specifics or the dynamics of jet streams and talk about specifically jet streaks and how these are associated with weather systems or weather that affects our day-to-day lives here in the mid-latitudes.

This is a map here showing –

[slide titled – Jet Streaks – featuring the following points over a thermal photo of winds speeds over the Earth with greens and blues in the middle and upper and lower regions and magentas and purples and reds along the jet streams with two very curved areas of one of the jet streams circled. The bulleted points are – Jet Streak = Local maximum wind speed embedded within the jet stream; Tied to formation of extratropical cyclones]

– wind speed all over the globe. And regions where you see the warmer colors such as the greens, yellows, reds, and especially purples, these are your fastest winds, and everything is flowing in this map approximately from west to east. Of course, with meandering winds going north and south. Now a jet streak specifically is defined as a local maximum in wind speed that’s actually embedded with inside the jet stream. So, the jet stream flows a general pattern, but every once in a while, the wind can speed up or slow down, accelerate or de-accelerate, and these regions of acceleration and de-acceleration of the winds and very fast wind bursts are called jet streaks.

And it turns out that these jet streaks are actually tied to the formation of extratropical cyclones or low-pressure systems. So, how exactly does that work? So, what we’re going to do is I’m going to show you a theoretical picture of what a jet streak looks like and I’ll show you why –

[new slide titled – Jet Streak Model – featuring an illustration of three nested ovals with the center oval labelled, Wind Speed Max and its borders labelled 140 knots, the next ovals border is labelled 120 knots, and the outer ovals border labelled 100 knots. In the lower right corner is a compass indicating that the top of the slide is north. Additionally, there is the statement – Consider the above idealized jet streak at 300 millibars, assume Northern Hemisphere]

– these tie to the formation or strengthening of extratropical cyclones.

So, here’s our basic jet streak model again. It’s not as exciting as the weather map I showed on the last page or some of the other weather maps, but this will work perfect – perfect for our theoretical purposes. So, here what you see are – the black lines represent isotachs, or lines of constant wind speed. And we’re going to assume in this case – and I put a compass down in the bottom right – that the wind is just flowing west to east in this picture. But when you – when the wind gets to regions where the wind speed max is, the wind is speeding up as it enters that region and it’s slowing down as it leaves that region. So, Ill –

[the slide animates on six arrows, three on the left that get longer as they move towards the wind speed max, and three on the right that get shorter as they leave the wind speed max, and that cross through the center of the ovals east to west, and a key indicating that the arrows = Wind Vectors. Additionally, there are two dots on the right and left of the 120 knots oval where the wind vectors intersect that oval]

– put some arrows on here to help illustrate that idea. So, the wind is flowing west to east. The wind vector, the bigger the wind vector the faster the wind speed is moving on this picture. So, as you notice, on the western side of the jet streak, the wind is increasing as it goes into the jet streak max. And then on the eastern side wind is decelerating, or decreasing in speed as you go eastward.

So, another way we can represent that is by drawing wind acceleration vectors.

[the slide animates off the six wind vector arrows and replaces them with blue arrows that equal wind acceleration vectors – one on the left that is pointed from the left-hand dot to the wind speed max area and one on the right that is pointed from the right-hand dot also towards the wind speed max area – so in opposing directions]

So, the blue vector here represents wind acceleration. And where the wind is accelerating toward the jet streak core, or the center of the jet streak, that blue arrow is pointing towards that direction. So again, on the western edge, we saw those vectors getting bigger as we got toward the jet streak, so our wind is accelerating west to east into the jet core. Now, on the other hand, we saw the wind decelerating as it left the jet streak core from the east, which means that there’s a negative acceleration, which means the wind acceleration vector points in the opposite direction towards the wind speed maximum.

[Zachary Handlos, on-camera]

Now, why is this relevant to the formation of extratropical cyclones? Well, it turns out that because of the way the wind accelerates and de-accelerates into and out of a jet streak core, that vertical motions are actually generated from this. I’m not going to go through all the detailed theory on this –

[return to the – Jet Streak Model – slide now with the blue arrows gone and two dotted lines intersecting the ovals in the middle both north to south and east to west. The upper left quadrant is labelled, Left Jet Entrance Region and it is noted that there is a downward vertical motion here; the lower left quadrant is labelled, Right Jet Entrance Region and it is noted that there is a upward vertical motion here; the upper right quadrant is labelled, Left Jet Exit Region, and it is noted that there is upward vertical motion here; the lower right quadrant is labelled, Right Jet Exit Region and it is noted that there is a downward vertical motion here]

– but the idea is that because the wind accelerates into these – through the entrance region into the jets – the wind speed maximum of the jet streak, we actually have upward vertical motion that exist in what’s called the right jet entrance region. So, what we call this is a classic four-quadrant model in atmospheric sciences, where you can kind of divide the jet streak into four parts, as denoted by the dashed lines, where if you draw that first line north-to-south this case, the left side or the west side these are referred to as the jet entrance regions because the air is entering the jet streak on that side. And then the jet exit regions are on the east side because the air is leaving that region. Even though it’s decelerating, it’s still leaving that region over time.

And it turns out that upward vertical motion occurs in the right jet entrance region and left jet exit region based on how and wind accelerates and de-accelerates through the jet streak. And then in the other two quadrants – the left jet entrance region and the right jet exit region – there’s actually downward vertical motions that exist in these columns of air. So, even though this jet streak we’re looking at is a two-dimensional picture, there’s three-dimensional motions associated with this. We also have up-down motions, which are sometimes hard to see from a bird’s-eye view, but if you took a –

[Zachary Handlos, on-camera]

– cross-section and rotated this picture, you’d actually see the upward and downward motions. And I’ll show an example of that in a few slides.

So, how does this tie to extratropical cyclones? So, again, an extratropical cyclone, if this is sort of a new term for you –

[slide titled – Extratropical Cyclones – featuring the following info on top of a satellite image of a cyclone – Low pressure systems – Lifetime = approximately 5-7 days, Horizontal Size is approximately 1,000 kilometers, Vertical size – all the way up to the tropopause! ; Also noted is the statement – Cyclonic wind circulation at surface – counterclockwise in Northern Hemisphere]

– refers to a low-pressure system. And we also sometimes refer to these as weather systems because they generate all sorts of weather and interesting types of weather with them. And so, a lifetime of these systems is about 5 to 7 days, and their horizontal sides or how much area they take up over a certain region can exceed 1,000 kilometers. So, here’s a classic example in the background of the slide of an extratropical cyclone. Notice just the beautiful comma cloud structure. This is actually a cyclone during October 2010 that broke records for minimum in sea level pressure over the upper Midwest. I believe in Duluth the min – the minimum sea level pressure was about 955 millibars, which is the equivalent of Category 2 or 3 hurricane, which is pretty rare for that to happen with a low-pressure system over land like this. So, very impressive storm.

Also, even though there’s a horizontal scale list, there’s also a vertical scale of these systems as well. These can extend all the way from the surface –

[Zachary Handlos, on-camera]

– up to the tropopause. And if you look at the cloud structure in this image, regions where you see darker cloud shading, those –

[return to the – Extratropical Cyclones – slide above]

– are clouds that are closer to the surface. And the brighter colored clouds are cloud tops that are higher up in the atmosphere. So, you can sort of sense the three-dimensional structure a little bit just from looking at this satellite picture as well too.

And these systems have what’s called a cyclonic wind circulation associated with them, meaning that in the Northern Hemisphere winds spin counterclockwise around the center of the low-pressure system.

[Zachary Handlos, on-camera]

Actually, it turns out that in the Southern Hemisphere the winds spin clockwise around the cyclonic disturbances for reasons that we can discuss another time.

So, here’s another example showing the horizontal cyclone structure.

[slide titled – Horizontal Cyclone Structure – featuring an illustrated weather map of the United States with a Low-Pressure system over Illinois. The slide notes that to the northeast of the system is Cooler Air, to the southeast of the system is a Warm Sector and to the southwest is the Coldest Air]

So, the L represents the region of minimum sea-level pressure associated with the cyclone. So, at this point the pressure is lower relative to everything around its surroundings associated with that weather system. And these weather systems have fronts associated with them: where you see that blue line extending from southern Ohio all the way into the Gulf of Mexico represents a cold front. And that separates a region of warm air, which we denote as the warm sector. And then to the west of that is a region of cold dry air which we call sort of the coldest air of the system. And then there is also a warm front structure. And this example it extends from South Central Ohio all the way to the East Coast. From this we separate warm air to the southeast from slightly cooler air and slightly drier air that exists in the north of this warm front boundary.

[new slide titled – Horizontal Cyclone Structure – IR Image – that features an infrared satellite image of the previous weather map showing the low-pressure over Illinois, the cold front from Ohio to the Gulf and the warm front from Ohio to the east coast. There appear to be storms east of the cold front along the Atlantic shoreline from Florida to the bottom of the warm front. The slide also notes that – Darker colors = warmer surfaces (i.e., surface, low clouds); Whiter colors = colder surfaces (i.e., cloud tops); Green = coldest surfaces (e.g., high cloud tops associated with deep could structures, more intense precipitation)]

Another way you can kind of consider looking at this air is by looking at what is called an infrared image. And what this is showing – this is a satellite image that is detecting cloud-top temperatures, or temperatures emitted from surfaces in the infrared radiation spectrum – and regions where you see whiter colors or greener colors represent really high cloud tops and deeper cloud structures. So, that’s probably a region of thunderstorms or pretty – or possibly severe weather activity. And regions where you see darker colored clouds or shades of clouds, that represents clouds that are lower and shallower to the ground. And regions where you see just dark spots, such as over Mississippi and Alabama, these are cloudless regions. So, what the satellite is actually detecting is the ground, and since the ground is closer – is at the ground – and these cloud tops are higher up, the surface shows them as dark because the ground is warmer than the cloud tops themselves.

And so, if you look in the warm sector, we see this is where our region of showers and thunderstorms are. In this case we have deeper clouds. And in the cooler air regions and coldest air regions we have shallower clouds, if not no clouds at all cause this air is cooler and drier over time.

[new slide titled – How do jet streaks affect extratropical cyclones? – featuring an illustration marked A and two graphs marked B. The A illustration is another representation of a jet streak from above with the Jet represented as an oval with the entrance region and exit regions split into quadrants and noting that the lower-left and upper-right quadrants – marked as DIV – are regions of upward vertical motion. The B graphs are of Direct Circulation and Indirect Circulation showing that in the Direct Circulation graph (entrance region) the winds move counter-clockwise from warm to div to conv to cold and in the Indirect Circulation (exit region) the winds move clockwise from warm to cold to div to conv]

Now, how exactly does this tie to jet streaks? How do jet streaks influence the – the genesis or strengthening of extratropical cyclones? So, this image is actually taken from a paper by Bjerknes, 1951 and also Uccelini, 1990. And Louis Uccelini was actually another graduate of this – of the Department of Atmospheric and Oceanic Sciences and also plays a role as the director of the National Weather Service. So again, another big name that came from the University of Wisconsin-Madison. And what he was showing in this paper, in this review paper, was sort of how the three – three-dimensional circulation ties to the – to the genesis of extratropical cyclones.

And here on this picture since the terminology is a little different, I just tried to circle some of the highlight spots that we talked about earlier. Regions where I’ve circled as DIV section represent the regions of upward vertical motion in this picture. So again, when we’re looking at Panel A, we’re assuming the flow here is flowing west to east in this picture, and where it says JET that sort of our wind speed maximum, kind of like our idealized model we looked at earlier. And then Panel B is actually showing the vertical circulation associated with these dotted cross-section lines A through A’, and B through B’. So, here, for example, where we have the divergence, here is our vertical motion in the orange box, or in this region we actually have air rising. And then in the convergence region here, this is our downward vertical motion region, that’s this down arrow that I’m representing with my mouse here. Then you actually get the opposite sent circulation in the jet exit region.

[the slide animates on two low-pressure Ls on the A illustration were the DIV spots were on the illustration, and the slide also animates on the statement – Suppose a cyclone (low-pressure system) was located at the surface under the circled regions (the Ls in the illustration)]

And so, let’s suppose that there were low-pressure systems located in the regions where upward vertical motions is occurring. So, how does this play a role in strengthening a surface cycle or surface low-pressure system?

[the slide animates on an illustration with a line denoting the surface under a low-pressure L between to circles, one with a dot in to the left of the L and one with an X through it to the right of the L. An arrow also points upwards from the low-pressure L to denote that – Air rises upwards from the low-pressure system center]

So, heres a sort of a diagram showing a cross-section of this low-pressure systems. So, altitude increases upward, as you follow my mouse pointer. And this dotted X symbol represents the cyclonic or counterclockwise circulation associated with the lows. So, the X means the flow is going into the slide and the dot means the flow is going outside of the slide. So, you can kind of imagine this counterclockwise circulation associated with the low at the surface. Now this dark black UP arrow represents upward vertical motion associated with the low because air is rising in these regions where the Ls are labeled in Panel A.

[the slide animates on a new line above the upwards moving low-pressure air that is labelled as Tropopause, and as the air from the low-pressure hits the Tropopause there are two more arrows that face in opposite directions indicating that the air rising from the low-pressure system moves in opposing directions once it hits the Tropopause]

And as this air rises eventually it gets to the upper troposphere at high enough that it hits sort of the lid of the troposphere called the tropopause. Now above the tropopause is the next layer of the atmosphere and that’s called the stratosphere. This air is very stable and –

[Zachary Handlos, on-camera]

– it’s very difficult for air to rise to the tropopause and penetrate into the lower stratosphere. So, sort of like a lid, what happens is this air rises, hits the top of the tropopause, and then diverges outward. And it’s actually due to a principle known as mass continuity. So, the air rises, and the air diverges aloft.

Now, as a result of this, how we determine how strong a low-pressure is that the lower the pressure is at the center of the cyclone, the stronger the low-pressure is. Now if we’re taking air mass, and it’s rising up, and diverging outward away from the cyclone, we’re taking weight of error outside that column where the low-pressure center is. And so, you’re going to reduce the sea level pressure associated with the low-pressure system center. And by reducing that you’re only going to act to make that cyclone stronger over time. And so, as a result, you have stronger circulation. So, I tried to represent that by making bigger –

[return to the – How do jet streaks affect extratropical cyclones? – slide now with the arrows in the low-pressure illustration marked in bold to signify increased intensity and the statement – Decreases surface pressure and low gets stronger!!!]

– wind symbols on this diagram. So, that cy – that cyclonic, or counterclockwise circulation, associated with that low-pressure system becomes enhanced over time, and that low-pressure system has an even lower pressure at its minimum in the center of the cyclone. So, if you get a cyclone that sets up in one of these two entrance or exit regions at just the right time, they can actually further strengthen over time and become a monstrous extratropical cyclone. And actually, if the pressure decrease – decreases at a fast enough rate, we call this explosive cyclogenesis. Or another sort of fun word is bombogenesis, as referred to –

[Zachary Handlos, on-camera]

– in the snappy meteorological world, so.

Now, I actually, I brought so, I showed – I have a few weather maps here to show an example of strengthening of a low-pressure system right in a left-jet exit region, which is a region again associated with the jet streak where we’d expect a low-pressure system to strengthen. So, just so you can get your bearings straight on this map here –

[slide titled – Example – December 2012 Snowstorm 1200 UTC 19 December 2012, minimum pressure = 1,000 millibars – and featuring three maps. In the upper left is a temperature map of the United States on December 19, 2012, showing a low-pressure system over New Mexico with very cold temperatures in the northern Plains and very hot temperatures in central Texas. The second map is a secnd temperature map showing the polar jet stream at 300 millibars showing the fastest wind speeds in the lower Midwest and Great Lakes. The third map is the air pressure map for this system showing a very large area of pressure around the low]

– this left plot here, again, is showing temperatures about a mile above the surface. So, it shows you just sort of the thermal distribution associated with a low-pressure system. Again, orange and red colors mean warmer air and then the blue or purple colors mean cooler air. At 300 millibars, that’s the map at the top right, this is the level at which the polar jet stream is observed. In this case, where you see the colored pattern, that’s faster wind speeds. So, the region where you see the green, yellow, and orange color that represents a faster wind and that’s where the jet stream resides. So that’s the polar jet stream. Then, in the bottom left we have a sea level pressure map. So, the blue lines are isobars, or lines of constant sea level pressure. And the L on every map represents where the low-pressure center is at the surface.

So, its – so, and the bottom left that’s where the low-pressure center is with respect to that feature over New Mexico and Texas. But, just to help you sort of imagine the three-dimensional structure, I also labeled that same L on the other two plots on the top. So, you can sort of compare the upper-level flow with what’s going on at the surface.

So, this is a – a snowstorm that actually hit Madison in December 2012, and actually led to the cancellation of finals on campus, which is a very rare event at the University of Wisconsin-Madison. So, what this is showing is – these are maps on December 19, 2012, at 12 UTC, which translates to about 6 AM local time here in Madison. And so, I am going to arrow through and it’s going to jump 12 hours forward. So, the minimum sea level pressure associated with this was around 1000 millibars, which was just below mean or average sea level pressure around in the mid-latitudes. And as we arrow forward, 12 hours later the minimum pressure does decrease a little bit, but what I really want to point out is in the top right plot notice there is this streak of yellow color that’s sitting right over New Mexico and Texas. It’s only a short distance that it lasts, and where the L actually is in that diagram, the color turns to more of a green or lighter blue. And so, this is actually an example of a just streak just forming right over the southern Central U.S. And that L is sitting right in the exit region and pretty close to the left jet exit region.

So, what we’d expect is that during this – the next hour – 12 hours we’d expect air to be rising out of the low-pressure center and then diverging aloft. And we should see that cyclone strengthening in our surface map over time. And one way we can tell if the cyclone is strengthening is that we’ll see more of these blue isobar lines appear and also be tighter, their gradient will be tighter, which means the wind speeds are getting faster as well. So, let’s jump ahead. So again, this is at 6 PM on December 19, which is in Zulu time zero UTC 20th of December 2012.

So, let’s go 12 hours ahead to 6 AM on December 20. And notice as we – I’ll flip back one more time – notice how there’s more of these blue isobar lines and theyre also tighter together on this new image. And our minimum sea level pressure has dropped now to 989 mb, which is pretty impressive for a winter storm at this time of year. Maybe not the most impressive but it’s getting up there. And so, also, if you look at our jet streak image on the top right, notice that our jet streak is now sort of curved. It’s going sort of west to east, then it starts to flow northward. And that L is sitting right continuously in that left jet exit region. So, that air’s been rising, taking that air out of the – the low-pressure center column. That’s why we see that pressure decrease and also see an increase in the winds over time. And its about this time as well as this next time 12 hours later that Madison got absolutely hammered with a lot of snow. And I believe we actually got 18 inches of snow during this timeframe, which I think was actually a record at the time and that’s why classes got canceled here on campus. And I think schools got canceled pretty much everywhere. The buses might even have shut down, which is a very rare event Madison, Wisconsin, so.

So, here’s one more image showing that that L is still in the left jet exit region. We still have a very strong cyclone; it actually weakened a little bit over the 12 hours. All because that – this cyclone was tied to those jet streak dynamics associated with the polar jet stream, that weather system got stronger. So, as a forecaster, if you see something like this in a forecast map, you’re concerned because you know this is going to strengthen and can lead to a snowstorm. But having this theory now settled in science, we can use this information in forecasts to warn –

[Zachary Handlos, on-camera]

– the public, and we feel confident when we think a storm is going to strengthen and impact our area, so.

Alright. Finally, I’m going to conclude with some current jet stream research that I’ve been working on. Were going to talk about a phenomenon that’s relatively new in the atmospheric science field. And it’s the idea of something called a jet superposition event. So, if you thought the polar jet stream and the subtropical jet streams by themselves were pretty cool, wait til you see what happens when the two merge together into one jet stream.

So, where we’re first going to start is – is we’re just going to look –

[slide titled – 3D Structure of Polar, Subtropical Jet Streams – which features a map of the Pacific Ocean with the west coast of the United States on the right-hand side. On the left-hand side of the map is an illustration of the Polar Jet Stream with primary colors near the center and cooler colors on the outsides. On the bottom right of the map is the Subtropical Jet Stream near the Baja peninsula and flowing toward the Gulf of Mexico with the same color scheme. North of the Polar Jet is a large A that is on top of a vertical line that ends to the southern edge of the Subtropical Jet. There is an A prime near the end of this line]

– and quickly review the three-dimensional structure of the polar and subtropical jet streams because turns out that a jet superposition is simply a merger of the two, and sometimes looking at a cross-section of the jet will better reveal if you see an jet superposition versus the two jets separately.

So, as a basic example here: this is a map of the polar jet stream and subtropical jet stream on April 27, 2010, and I labeled vectors on there to show where the fastest wind speeds were, associated with the jet. And the next image I’m going to show is a cross-section A through A prime. So, its basically – what were going to do is take this image and rotate it ninety degrees so that our surface is going to be A to A prime. And then our axis on the Y axis or the left is going to be altitude increasing over that distance.

[slide titled – Cross-Section of Jets – featuring a graph of the two jet streams from the previous slide in cross-section with the line A to A prime on the x-axis, altitude, and pressure on the two y-axes. The Polar jet appears on the left with its low-pressure center marked by a circle with an x in it and pressure isotachs (lines of constant wind speed) around it. The Subtropical Jet appears on the right also with its low-pressure center marked with a circle with an x through it along with its isotachs. On the top of the graph is a squiggly line in black that appears as slowly larger mountains and it is labelled Tropopause. Underneath the Tropopause are squiggly lines that represent potential temperature. The slide also notes that – Jet Streams reside where gradient of tropopause height is largest (between tropopause steps), and the Subtropical Jet is at a higher altitude than the Polar Jet. Additionally, there is a blue line that intersects the center of the Polar jet vertically from the top to the bottom of the graph and a red line that intersects the center of the Subtropical Jet vertically from the top to the bottom of the graph]

So, here’s that cross-section. So again, A – if we go back to this image –

[return to the – 3D Structure of Polar, Subtropical Jet Streams – slide with the map above]

– for a second – represents sort of the poleward side of the cross-section, and A prime represents the Equator-ward side of that cross-section.

[return to the – Cross-Section of Jets – slide described above]

And so, what you see here – there’s a lot of lines going on here – but what you see here if you look at the red lines specifically, these are isotachs, or again lines of constant wind speed. And the further you go inward toward those red lines, the faster the wind speed is. And the wind speed maxima are labeled with these circles with Xs in this case. So, with this cross-section, what you’re seeing – those Xs represent the flow that’s going into the slide in this case, so it’s more or less going west to east in this picture, given how we took – how we took that cross-section.

And so, what we see with the red line and the black dots represents the subtropical jet stream, and the blue line with the black dot represents the polar jet stream. And this is similar to what we saw way back in the beginning with that three-dimensional cross-section, where we saw that the subtropical jet sits at a higher altitude but a lower latitude relative to our polar jet stream feature. So, this is consistent with what we expect earlier.

Now what happens when the two merge together?

[new slide titled – Vertical Jet Superposition Event – with the definition as – vertical alignment of the polar and subtropical jets into a single, strong jet. The slide features a map of the eastern Pacific Ocean with parts of Alaska and the west coast of the continental United States and Canada visible. In the Pacific is an illustration of a Vertical Jet Superposition that is color-coded with high wind speeds in the middle as white, pinks, reds and oranges, and slower wind speeds at the edge in yellow, green, and blue. A large bold arrow pointing east is in the center of the jet and a straight vertical line is drawn from the bottom of Alaska to the southern area of the Pacific. The top of this line (near Alaska) is labelled B, and the bottom of this line is labelled B prime]

This is what we call a vertical jet superposition event. So, a vertical jet superposition we define as really the vertical alignment of the polar and subtropical jets into single strong jet. If you imagine – here’s your polar jet here, so here’s the Equator on this side of me, your subtropical jet sits somewhere at a high altitude, your polar jet sits somewhere at a slightly lower altitude, but closer to the North Pole. These two can actually vertically align and form one very strong jet together and sort of merge together like this, so. It’s kind of a fun hand motion to do as well too.

So, what does this look like, though, in a cross-section? So, here is an example of a jet superposition event on October 24, 2010, at 0 UTC. And so, notice that there’s sort of a white circular spot in this because the wind speed associated with it is very, very fast compared to either the subtropical jet stream by itself or the polar jet stream by itself.

And now let’s consider this cross-section B through B prime –

[the slide animates on a cross-section graph of the Vertical Jet Superposition with B to B prime (left to right) on the x-axis, and altitude and pressure on the y-axes. The low-pressure, high-speed center of the jet is marked again by a circle with an X through it and red isotachs appear circling around the center but closer together (because of higher wind speeds). The Tropopause is again shown as a three black mountain-like lines in a shape which gets higher at higher altitudes and there is a green area of wavy lines above and below the tropopause that represents potential temperature. Additionally, there is a straight vertical line from the top of the graph to the bottom of the graph that intersects the center of the jet]

– through the jet core. And wow! This is a much different picture than what we saw in the last cross-section, right? So, one thing you might notice right away is that there’s only one jet. Now this makes sense because we just took a cross-section through a single jet but not two. But it’s really obvious when we look at this cross-section. So, notice these red lines, or these isotachs, or lines of constant wind speed. There’s more of them as you go towards the center of the jet. This means that the wind speed is much faster in this jet relative to the polar jet or subtropical jet stream. Also notice too that those two black dots we had – we had one that marked the subtropical jet and one that marked the polar jet – are now aligned in the same vertical grid column. It’s just like that hand motion thing I was doing – we have both of our jets in the same vertical grid column now. And the reason we use dots is that we’ve come up with this identification scheme where given at a certain altitude if a certain wind speed threshold is met for each of the two jets and they happen in the same grid column –

[Zachary Handlos, on-camera]

– and we identify each with the dots, we call that a vertical jet superposition event in our – in our database.

And so, another thing I will note too is notice that there are these – these three black lines that sort of extend and then drop off –

[return to the – Vertical Jet Superposition Event – slide described above]

– and then continue further to the left. This is actually the height of the tropopause. And it turns out that if we go back a couple images –

[return to the – Cross-Sections of Jets – slide described previously above]

– we noticed there were sort of three steps in the tropopause here. So, the tropopause height varies depending upon the temperature below it. And in the classic model theres three steps, but when we have a –

[return to the – Vertical Jet Superposition Event – slide described previously above]

– super-posed jet case that middle step actually gets eliminated as the two jets come together, which is kind of interesting. So, we actually go from a high step all the way down to a lower step in height versus having sort of a gradual three-step situation.

[new slide titled – Why do we care about studying jet superposition events?]

So, okay, that’s pretty cool but why – why do we care about studying these superposition events? Is it just cool because the two jets come –

[Zachary Handlos, on-camera]

– together and that said it looks nice on a map? There’s actually more to it than just how neat it looks on a weather map. One big thing is that these are associated with very extreme weather events, and Ill give you two examples this evening.

[return to the – Why do we care about studying jet superposition events? – slide now with the bulleted answer – Associated with extreme weather events. The slide also features two photos, both of a flooding event that happened in Nashville, Tennessee on the first and second of May 2010. One is of downtown Nashville flooded and the other is of flooding on the outskirts of Nashville]

One is that there is a superposition event that was associated with a significant flood event on the first and second of May 2010. And one of my colleagues, Dr. Andrew Winters, and our advisor actually looked at this case in detail and showed that this superposition played a big role in forming a lot of the rain that was associated with this flood event. And I think some regions actually got almost if not over 20 inches of rain during this event. So, very intense rain events in Nashville, Tennessee.

[the slide animates out the photos of a flooded Nashville and replaces them with two photos of very large tornados in Tuscaloosa Alabama with the caption – April 2011, Alabama Tornado Outbreak]

Well, this could also lead to other interesting events too, such as there was actually a jet superposition event associated with the April 2011 Alabama tornado outbreak. So, on this day there were almost 200 tornadoes reported. I think it was 187 approximately or maybe even more after further analysis. And these are some really interesting pictures. And also, I really like that picture on the right because that was actually a Sky Cam image taken during live news of this tornado going through Tuscaloosa, which is sort of a scary situation, especially if you are a newscaster and trying to explain what’s going on. But nevertheless, this event was associated with a jet superposition event, and even more interestingly – remember this is in Alabama – it turns out that the jet superposition event actually started in the West Pacific and whatever –

[Zachary Handlos, on-camera]

– happened there extended all the way to the Eastern United States and helped to induce this event. So, what happens in other parts of the world can affect what happens here in the U.S., which is very interesting to think about.

The other thing, too, is that despite the association with extreme weather events, jet superposition events are not actually that well understood. And so, recently there’s been more and more research, especially in our lab –

[slide titled – Where do superposition events most often occur? – and featuring three world maps from a top view with the North Pole at the center and the Northern Hemisphere in the center; they are labelled, Fall, Winter, and Spring and have green areas on these white maps that show where and when superposition events occur. The Spring map has the fewest areas of green, the Fall map has a larger area of green but with less intensity, and the Winter map has the most areas of green and also has the most intensity from around Japan to the Pacific Ocean]

– groups here at the University of Wisconsin-Madison that started to investigate where these events occur and how they work. And so, this is a – what’s called a climatology from a masters thesis done by Croix E. Christenson that shows how often these events happen per month in fall, winter, and spring. Now, if you look at the winter plot it’s clear if you see the darker green colors that these events happen more often in the winter versus fall and spring. And they don’t really happen too much in the summer. So in the wintertime these two jets are more in a position to merge or vertically align together to form a superposition event.

[the slide animates on three boxes on the Winter map, one in aforementioned Pacific Ocean area, the second in the south-central United States, and the last one in North Africa extending towards Saudi Arabia]

And there’s really three hotspot regions in the winter in which these events occur. There’s the West Pacific region, and then there’s the Southern United States region, and North Africa. But even looking at these three regions together, it’s clear that the West Pacific is a hotspot for these jet superposition events. So, it’s really interesting to consider what makes this a special spot for jet superposition events.

Now, this is a question that we are considering in our lab group. And usually when you start doing your research project –

[Zachary Handlos, on-camera]

– you look at past literature to try to see what have other people looked at with respect to these events and what do we know about the West Pacific and these sort of West Pacific superposition events.

So, we actually did that to start the project. Oh, and this is just a highlight that the West Pacific is a particularly interesting region. So, one thing I was going to talk about, though, is after doing some research, after looking at some of these research studies – I was going to ask you. Maybe – maybe you know. How many research studies have investigated these events in the West Pacific? Because, again, this is sort of a new idea, and again, it could be in the literature about we weren’t sure. So, we did some investigation. And it turns out that after reading all sorts of papers, or really, I should say doing all sorts of digging through papers and looking for any sort of paper that talked about jet superposition events at all, whether in the West Pacific, specifically in the West Pacific, I should say actually. There are some in the Atlantic, but in terms of the West Pacific, there is only one research paper that ever looked at jet superposition events in the West Pacific. That was in the year 1953. So, were talking a year – we didn’t even have satellite data yet – and that was the only – there was a scientist in Japan that did a case study of a superposition event, had a beautiful conceptual model or diagram to show how it worked, and no one has touched it ever since – until we have here at the University of Wisconsin-Madison.

And I find that kind of surprising because this is associated with extreme weather, and extreme weather is a big concern to society. So, to me it seems like this is a very important phenomenon to – phenomenon to study in the atmosphere. So, some of the questions – based on that motivation –

[slide featuring three questions that Zacharys lab group was hoping to answer – 1. Why do superposition events most often occur in the West Pacific? 2. What large-scale atmospheric phenomenon induce superposition occurrence? 3. What types of extreme weather events are tied to superpositions?]

– some of the questions that we’ve been considering recently are: First, why do these superposition events happen most often in the West Pacific? What makes this spot or this region of the globe so special? Secondly, what large-scale atmospheric phenomena induce or cause these superpositions to occur in this region? And then finally, what sort of extreme weather events are specifically tied to West Pacific jet superpositions? Ive showed you a couple examples of extreme weather –

[Zachary Handlos, on-camera]

– that are a tied to events that happened in the United States, but we don’t know too much specifically about any sort of events that happened in the West Pacific directly associated with West Pacific jet stream superposition events.

So, the methodology that we used is we first tried to identify several cases in this West Pacific region –

[slide titled – Methodology – with the bullet point – Identify superposition events in the West Pacific (44 cases). The slide also features a larger version of the Winter northern hemisphere world map from the previous slide showing the incidences of vertical superposition jet events with the area of the West Pacific highlighted in a box]

– where superposition events occurred, and there were 44 cases that caught our eye that we considered for our analysis.

Now one approach in analyzing these cases is that you could look at each individual case and do a very rigorous case study analysis or data analysis and determine what’s going on in each case. But there’s 44 of them. That’s sort of a lot of cases to look at by eye and do sort of an analysis with. You could do it, but its gonna take a long time, and as a grad student, you have to eventually graduate. So, you have to find some faster ways to –

[laughter]

– think about strategies for looking at multiple cases at the same time.

[new slide still under the – Methodology – heading with the second bullet point – 2. Average data for all cases together and analyze as a composite event. Example – Average upper tropospheric wind from two cases together. The slide also features three graphs with longitude and latitude on the x and y axes with the Pacific Ocean as the area studied, and featuring two different superposition events there, one labelled Event A and one labelled Event B and the third graph labelled Average of Event A and B (Composite)]

And that’s where this technique called a composite analysis comes into play. So, how a composite analysis works is what we did is we took all 44 cases and the data for those 44 cases, and we just averaged them together and looked at how over time the cases evolved as what’s called a composite or in an average sense.

So, to give you more of a visual example of how we do this, let’s say that we’re going to do a composite analysis of two jet stream events that we’re going to call Event A and Event B. So, here’s Event A, Event B. This is just showing windspeed in the upper troposphere where, again, the – the warmer the colors, the higher the wind speed.

Now we could do an analysis of both Event A and Event B separate, but if we want to save some time and get a sort of a general idea of what’s going on, we can take A plus B and divide by 2 to get the average of that data and look at how this event evolves as a composite. And so, the composite map is shown on the bottom here, which you can kind of see it looks sort of like an average of Events A and B.

So, we did the same sort of idea, but we did this with 44 cases rather than 2 cases –

[Zachary Handlos, on-camera]

– cause you want to have at least a big enough sample size, otherwise scientists won’t be as interested if you only have one or two cases.

So – so, here’s an example of some results. I’m not going to show you all sorts of results because it can get a little complex –

[slide titled – Results – noting that the sample size was 44 cases, the composite wind speed was 250 meters per second. The slide shows four graphs of composite results at differing altitudes and wind speeds. The graphs show areas in yellow that have values that are greater than the climatological mean and areas in blue that have values less than the climatological mean. The first is 250 hPa anomalous Wind Speed showing greater wind speeds in the core of the jet than the mean and areas on the edge of the jet that have wind speeds less than the mean. The second is 250 hPa anomalous Geographic Height showing height greater northwest and south of the jet and lesser height slightly north of the jet center. The next map is 500 hPa anomalous Geographic Height and showing greater height than expected again northwest of the jet and less than expected on the northwestern edge of the jet. The last map is of 925 hPa anomalous temperature and showing higher area of temperature again northwest of the jet and also in the middle of the jet and less than expected temperatures on the trailing edge of the jet]

– but I think this will give the general idea of what we’re – what were investigating with respect to these West Pacific superposition events. So, anywhere where you see on this map – so, again, this is centered over the Pacific Ocean – and anywhere where you see red lines represent isotachs again, and the further you go inward towards the jet core near Japan, the faster the wind speed goes. And then there are four regions –

[the slide animates in black circles around the yellow area in the center of the 250 hPa Wind Speed map, the yellow area south of the jet in the 250 hPa Geographic Height map, the blue area on the northwest edge of the 500 hPa Geographic Height map, and the blue area on the trailing edge of the map on the 925 hPa Temperature map]

– that I want to highlight specifically associated with this composite jet. And the shading in these cases represents whether the variable I am highlighting is either greater than what you’d expect on a day-to-day average basis, or what we call the climatology. And the blue regions are where this variable is less than we expect on a day-to-day basis, or climatology.

So, for example, if you look at the top left panel here you see the sort of green/yellow/orange blob inside the jet core, right? What this means is that the wind speeds and the jet core at this location are faster than we’d expect on an average basis with respect to any jet stream that resides here. So, that’s good news because we’re compositing superposition events together. We expect that the wind speeds going to be faster because, remember, I showed you that cross-section where we saw the wind speed is faster in a superposition event compared to either the polar jet by itself or the subtropical jet by itself.

And then there are some other features of interest, such as on the top right panel there’s this interesting clockwise circulation that’s on the south side of the jet. There’s an interesting counterclockwise circulation, or what we call a trough-like feature, on the – on what’s called the northern side of the jet. And then on the bottom right – this is really interesting – this blue region represents anomalous cold air. So, that means when these events occur in a composite sense, we expect very cold air relative to average to be occurring in the East and South China Sea regions. And this was actually really interesting to us because there are these cold surge events that happen and are associated with the phenomenon –

[new slide titled – East Asian Winter Monsoon (EAWM) – that has the definition of a EAWM – Large scale boreal winter event based on shift of cold air associated with the SMH (Siberian-Mongolian High). The slide also features a map of the region of Japan and the South China Sea with a High-Pressure system over China which is pushing air towards the sea]

– known as the East Asian Winter Monsoon.

Now the East Asian Winter Monsoon – maybe you’ve heard of the word monsoon before, and you think lots of rain. But actually, a monsoon really just refers to a large-scale shift in a wind pattern. And in the – in the winter in over East Asia the Siberian-Mongolian high-pressure system really revs up, and this thing has a large clockwise circulation. And on the eastern edge of that clockwise circulation, you have winds going north to south.

Now in Siberia there’s very cold air in the winter, sometimes colder than what we see in the U.S. or even in Canada. And the northerly winds with this high can take that cold air and bring it all the way southward into the East and South China Sea regions. And sometimes lead to whats called colder outbreak events as far south as Hong Kong, which can actually be devastating for people who live in these areas cause they’re not used to that cold, unlike we are in Wisconsin.

[new slide titled – Conceptual Model – featuring a map of the Pacific Ocean with Russia, China, and Southeast Asia on the left and Alaska and the west coast of Canada and the U.S. on the right and having the Siberian-Mongolian High-Pressure system marked as two clockwise rotating green arrows over Mongolia. To the right (east) of the Siberian-Mongolian High-Pressure system is another circle shaped area shaded in dark blue representing the cold air mass associated with it and with two bolded purple arrows pointing down from it indicating strong wind flow pushing south from the high-pressure system into the South China Sea. The jet stream is indicated in the middle of the Pacific by a circled, bolded arrow pointing west to east and labelled Jet. The slide also notes – Cold air moves equatorward via the Siberian-Mongolian High-Pressure System and the purple arrows represent Northerly flow associated with the cold surge]

So, how does this tie into the jet superposition idea? So, that same high-pressure system I showed you, that’s marked with green arrows here on the left side of this diagram. And that sort of oval or circular dark blue blob represents theoretical cold air thats coming down from Siberia. What we observed in our composite analysis is that this cold air over time –

[The map animates off the Siberian-Mongolian High-Pressure system and moves the circle of cold air represented by the dark blue south into the East China Sea so that the dark blue circle occupies the same area of the map that has the purple arrows just to the left (east) of the Jet. Additionally, the slide has little cloud figures over Indonesia to indicate tropical thunderstorms. The slide also notes – Pre-existing WP convection strengthens due to air converging]

– starts somewhere up in Siberia or northeastern China and moves southward towards lower latitudes near Indonesia. Now in this region, it’s very common in the winter that there are some sort of tropical thunderstorms or some sort of what we call a tropical convection that’s present at this time. And actually, when this cold air interacts with that tropical convection it can enhance what’s called convergence, or more air starts piling up at the center of this convection, and that forces this air to rise even faster over time.

[The slide animates off the cold air features and now draws a red circle around the tropical thunderstorms in Indonesia and adds a large green arrow that points to a new high-pressure system that forms just below the Jet. The slide notes that – Air within convection rises and moves poleward and eastward. Additionally, air manifests within the Subtropical Jet layer in an anticyclonic flow]

And rising air continues to fuel these tropical thunderstorms in this region.

And when this air rises, if you remember, it’s eventually going to hit this tropopause and it’s going to have to go outward and go somewhere. And this air will hit the tropopause and eventually start to move towards the Pole. Now poleward of this air is where the jet stream lies in time before it’s superposed. So, this air moves poleward –

[the slide animates on a 3D map of the area titled – Convection/Jet Interaction – with the Pacific Subtropical Jet Stream at the top of the atmosphere and to its left the tropical air from 3D thunderstorms pushes air upwards toward the Jet which increases wind speeds in the Jet]

– and we get a situation like this where you get an interesting three-dimensional cross-section where this tropical air, which originated – originated from a tropical thunderstorm, actually manifests itself or gets stuck or stored in the subtropical jet layer at the same altitude where the subtropical jet resides. And it turns out by putting that air there it actually has an effect on increasing the wind speed associated with the jet in that region. It actually ties to the thermal wind relationship because it creates sort of a temperature difference or pressure difference in this case, and you actually get an increase in wind speed associated with that via thermal wind balance. Though this thermal wind is more confined to the upper troposphere versus looking at horizontal temperature gradients at the surface.

[the slide animates on an orange circle with an orange arrow facing downwards indicating the air from the poleward side sinking down towards the Jet and combining with the subtropical jet to create a superposition jet]

At the same time there’s stuff going on – on the poleward side as well too. We have air on the polar side that’s actually sinking down and it’s taking stratospheric air, which is very stable air, and brings it down into the upper troposphere. And it turns out when we have had very stable air on the poleward side along with this unstable air or tropical convection air on the Equatorward side of the jet, this also, in combination, helps to rev up the jet wind speed and brings sort of polar jet characteristics together with any sort of subtropical jet present in this case and induces what we call a jet superposition. And so, that’s sort of the basics and we learned all this from our composite analysis –

[Zachary Handlos, on-camera]

– but one of our next steps now is we’re going to look at cases individually – some of the cases – and see What are some of the differences between the cases? and How do they match up compared to our conceptual model for our composite case?

So just to summarize, what we learned is that jet streams are these fast –

[slide titled – Summary – with the following bulleted list – Jet streams are fast, narrow currents of air that have impact on weather systems, large-scale circulation; Discovered by scientists that founded U.W.-A.O.S. department! ; Jet superpositions are associated with extreme weather events, large-scale phenomena that affect society]

– narrow currents of air that have an impact on weather systems and are also related to large-scale circulation such as the East Asian Winter Monsoon. It was discovered by a scientist that helped found the U.W. A.O.S. Department, which is a pretty great story. And finally, jet superpositions or the vertical alignment of our polar and subtropical jet stream features are associated with extreme weather events and large-scale phenomena that affect society. So, this warrants further investigation, and that’s something were going to be looking at not only through the summer, but beyond in the lab group here at the University of Wisconsin-Madison.

So, I thank you for your attention.

[new slide titled – Thanks for attending! – that features an illustrated Bucky Badger holding a lightning bolt in his right hand]

Thanks for being here this evening. And I will take any questions.

[wide shot of audience]

[applause]