- Welcome, everyone, to Wednesday Nite @ the Lab! I'm Tom Zinnen. I work here at the UW-Madison Biotechnology Center. I also work for UW-Extension Cooperative Extension. And on behalf of those folks and our other co-organizers: Wisconsin Public Television, Wisconsin Alumni Association, and UW-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, 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 UW-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]  - All right. Can everyone hear me okay with the microphone on? All right. My name is Zachary Handlos. I'm a research associate here at the University of Wisconsin- Madison. Yes, I recently just finished my PhD in the Department of Atmospheric and Oceanic Sciences. 

Today I'm very excited to talk to you about one of my favorite topics in meteorology. And that is 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 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 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. 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 I will transition into talking about jet streaks. These are the bursts of wind that are embedded inside the jet stream that have a role in the development of weather systems. 

Finally, I'll conclude with some current jet stream research that we've been working on here at the University of Wisconsin-Madison. First let's start with the basics. What is the jet stream? So, the simple definition of the jet stream is that the jet stream is simply a fast, narrow current 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 I will 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. Why do we have wind in the first place? 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. The reason that we have wind has to do with differential heating by the Sun. So differential heating simply refers to uneven heating of the Earth from the Sun. 

The Sun sends electromagnetic radiation, or solar radiation, toward Earth, and certain regions of the Earth receive more direct solar radiation than others. 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, the sun still admits solar radiation to these spots, but this solar radiation area is distributed over a greater area. So these regions receive less intense heat or less intense solar radiation per unit area relative to the Equator. 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. 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 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 move air around and try to redistribute this temperature and balance everything out. 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° north or 30° south latitude, depending on which hemisphere you're in. 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. Then on the return flow of the Hadley cells circulation, 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, Hadley cells 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. That's where the Ferrel cells and Polar cells come in, respectively. 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 presentation. The jet streams themselves reside in transition regions between these large-scale meridional overturning circulations. Meridional is a word that refers to circulations that move in the north-south direction. For example, the subtropical jet stream, one of the two primary jets that we'll be focusing on this evening, actually resides on the polar edge of that Hadley cells circulation. 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 subtropical jet stream, and it sits sort of on the polar edge of the theoretical Ferrel cells. 

Why exactly do we care about the jet stream in the first place? There are 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. Maybe you've heard of these if you watched TV weather, or 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. 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 lastly, the jet stream also interacts not only with the weather systems but with other large-scale atmospheric phenomena. 

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. Again the two different jet stream sites that were going to focus on are the polar jet stream and subtropical jet stream. Specifically, let's start with the subtropical jet stream. 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. We're thinking about 30° north latitude. So, for example, the Sahara Desert lies approximately in this latitude, and the southwestern continental United States, as two examples. 

This jet stream exists approximately in accord with something known as "angular momentum balance."  Maybe angular momentum balance sounds sort of like 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. What angular momentum is is this quantity that is conserved for objects that are on Earth, and Earth is a rotating object. 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.  A classic example with respect to angular momentum balance is that of a figure skater. So maybe you've watched the Winter Olympics before 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. 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. 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. Let's say we have a figure skater and they're doing some sort of spin trick with their arms extended outward. 

What happens to the speed of the ice skater if she brings her arms inward? How can you show this with angular momentum balance? Here is our equation again. Angular momentum is a product of the skater's mass, velocity, and its distance from its axis of rotation. 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, and 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. 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 for the left-hand side of this equation to remain constant. And that's exactly what happens. This picture is supposed to represent, since it's a still image, the skater spinning around faster with her arms inward. Again, since angular momentum is constant, as you decrease that radius the velocity or the rotational speed of the skater is going to 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. 

 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. It turns out there are actually some similarities with respect to this. Even air, air has mass on Earth, and therefore it has angular momentum, and this is approximately conserved on Earth. 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. While we are on Earth and we see a ball at rest at the Equator, it may not appear 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, 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 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? We're going to kick this kickball, for example, we're going to kick it 40° 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 pole-ward? Well, as it moves pole-ward... again, remember 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 pole-ward. As the ball or the object moves pole-ward, the distance between the object and the Earth's axis of rotation decreases throughout this process. This sounds very similar to the figure skater idea, right? With a figure skater example the radius of rotation decreased. 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 during this process. What does that look like or how does that translate in terms of the motion we see on Earth? 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. How we would see it on Earth is that 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. As we decrease that axis of rotation, the object moves west to east at a faster speed. 

How does this relate to the jet? If you notice, the subtropical jet stream sits at about 30° north and south latitude. If you remember that Hadley cell circulation, that red circulation area I showed earlier, we have warm air that's rising at the Equator and then it moves pole-ward aloft. So just think about the Northern Hemisphere for example, that air is moving from the Equator northward aloft. It's 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. At about 30° north latitude the air will eventually have a speed of about, well over 100 mph sometimes over 100 knots -- sorry-- sometimes over 150 to 200 knots in some cases, depending on where you are. And that's exactly where the subtropical jet stream sits. So the northern edge of the Hadley cell or the southern edge of the Southern Hemisphere that's where air is deflected and has west-to-east motion, and that's why we have subtropical jet stream. The Subtropical jet stream is more or less existent due to this angular momentum balance idea. 

You're probably wondering what about the polar jet stream? Is this 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 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. 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. 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. Let's look at this in a more pictorial form as an example. 

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. In this situation 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. If you go higher up in altitude, there's less air molecules forcing that weight down on you, so the pressure decreases over time. But it turns out that in a warm air column such as this, the red rectangular region, what happens is air expands and it actually pushes this pressure surfaces to higher altitudes. And with a cold air column this air is more compressed and these pressure surfaces are at lower altitudes. 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. As a result, the stronger, the bigger the slope is of that pressure surface, the faster the wind goes at these altitudes. So 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. It's all tied to this horizontal temperature difference. 

The dots in this picture represent the wind that's flowing out of the slide toward you right now. 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 in the direction that I'm looking right now. If I flip this way and the cold air is 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. There's more mathematics to it, and we'll get to some of that later, because that played a prominent role actually in the discovery of the jet stream. Here is a weather map example to kind of show again how thermal wind balance works and ties directly to the polar jet stream. What I have here on the left-hand side is a plot showing 300 mb wind speed. Basically, where you see cyan, green, yellow colors, these are faster wind speeds, so the wind speeds are shaded by color. You can kind of see this narrow stream of warmer colors, and that represents the polar jet stream in this case. I boxed a portion, or an example, of that polar jet stream within that black rectangle on the left-hand side. 

If you notice on the right-hand side we have a plot of 850 mb temperatures. This is temperature about a mile above the surface. Regions which correspond -- which correlate 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 to the Sun rising and the Sun setting. It still gives you a really good idea of where warm and cold air masses are in the atmosphere. So regions where you see yellow, orange, or red colors represent warm air regions. When you go to the greens, and blues, and purple colors, these are regions of colder air. 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. So I boxed an example of that on the right-hand side. 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. The polar jet, this maximum wind speed in the upper troposphere, resides exactly over the region of strong horizontal temperature contrast. That is all due to thermal wind balance. It's kind of cool. If you look around the rest of the plot, this polar jet is circumnavigating the globe around 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. Again, that's due to thermal wind balance.  How does this thermal wind balance idea tie to the discovery of the jet stream? Again, the discovery of the jet stream was actually-- this story is based on a calculation performed by the founder of the Atmospheric and Oceanic Sciences Department here, Professor Reid Bryson. What he did is, at the time he was working in the Armed Forces during World War II in the 1940s. He was helping out in preparation for a daytime bombing mission, actually, to Japan. 

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. For daytime bombing mission 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. They figured if they flew high enough in the atmosphere they could sort of avoid this danger as they were flying. Their job for this particular mission was they were going to calculate how fast the winds were going to be for this particular day. 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 pool by 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. We have a lot of good things going in our department here historically, which is great. 

At the time, this was not here yet and so they had limited data to work with. However, they had just enough limited data to figure out a pretty accurate calculation of what the wind speed was going to be at the level of the jet stream. What Professor Bryson and his colleague knew were that there was some sort of cold front feature that was going through Japan. A cold front is a boundary that separates two different types of air masses. To the north there was very cold and dry air over northwestern Japan and northwest of that region. And to the southeast there was warm, moist air present. Secondly, and most importantly, they had thermal wind theory to work with them. Just for fun I put up a calculation of thermal wind here on the bottom in this spot here. They could essentially use this sort of mathematical equation along with their observations to figure out not only how the wind was going to change with height, but also what the wind speed was going to be approximately at 35,000 feet above sea level. What they did is they calculated the change in wind with height, using the 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. 

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. The wind speed that they calculated at the time was around 168 knots from the west, which is about 193.3 miles/hour. This is a very impressive number. It turns out it was so great of a wind speed that no one believed them when they made this calculation. What they were concerned about is that airplanes only fly at about a speed of about 180 to 250 knots max at the time. 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. They told their commanding general about the situation, and the general said there's no way this is true. 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. 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 168 knots from the west, or 193.3 miles per hour. They got the same number again 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. We're actually going to measure the wind speed and come back and tell you what the wind speed is 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 mission was a complete bust. 

The commanding general actually came back after the mission and said-- he actually apologized to Professor Bryson and his colleague and said: I'm sorry. You were correct. We 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 is pretty crazy. 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. This was happening in the upper troposphere when they tried to accomplish their mission. Here's an example of a weather chart from the 40s showing how fast the West Pacific jet stream was at that time. It turns out that this region, the West Pacific jet itself, is actually one of the regions with the strongest jet stream on average, especially during winter time, 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. 

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, now it's called the jet stream in the atmosphere. It truly is a river of the upper troposphere for that matter. Today's things to talk about are -- let's get a little more into the specifics or the dynamics of jet stream and talk about specifically jet streaks and how these are associated with weather systems or whether that affects our day-to-day lives here in the mid-latitudes. This is a map here showing 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. Everything is flowing in this map approximately from west to east. Of course, there are meandering winds going north and south. Jet streak specifically is defined as a local maximum in wind speed that's actually embedded inside the jet stream. The jet streak 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. 

It turns out that these jet streaks are actually tied to the formation of extratropical cyclones or low-pressure systems. How exactly does that work? What we're going to do, I'm going to show you a theoretical picture of what a jet streak looks like and I'll show you why these tie to the formation or strengthening of extratropical cyclones. Here's our basic jet streak model again. It's not as exciting as the weather map on the last page or some of the other weather maps, but this will work perfect for our theoretical purposes. Here what you see are-- the black lines represent isotachs, or lines of constant wind speed. We're going to assume in this case-- I put a compass down in the bottom right-- that the wind is just flowing west to east in this picture. 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. I put some arrows on here to help illustrate that idea. 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. Another way we can represent that is by drawing wind acceleration vectors. 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 the wind is accelerating west to east into the jet core. 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. Why is this relevant to the formation of extratropical cyclones? Well, it turns out that because of the way the wind accelerates and decelerates into and out of the jet streak core, that vertical motions are actually generated from this. I'm not going to go through all the detailed theory on this, but the idea is that because the wind accelerates into these -- through the entrance region into the wind speed maximum of the jet streak, we actually have upward vertical motion that exist what's called the right jet entrance region. 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. 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. 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 hard to see from a bird's-eye view, but if you took a cross-section and rotated this picture you'd actually see the upward and downward motion. I'll show an example of that in a few slides. How does this tie to extratropical cyclones? Again, an extratropical cyclone, if this is sort of a new term for you, refers to a low-pressure system. We also sometimes refer to these as weather systems because they generate all sorts of weather and interesting types of weather with them. 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 1000 km. 

Here's a classic example in the background of the slide of an extratropical cyclone. Notice 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 minimum sea level pressure was about 955 mb, 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. 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 up to the tropopause. If you look at the cloud structure in this image, regions where you see darker cloud shading, those are clouds that are closer to the surface. And the brighter colored clouds are cloud tops that are higher up in the atmosphere. You can sort of sense the three-dimensional structure a little bit just from looking at this satellite picture as well. 

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. 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. Here's another example showing the horizontal cyclone structure. The L represents the region of minimum sea-level pressure associated with the cyclone. At this point the pressure is lower relative to everything around its surroundings associated with that weather system. 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. That separates a region of warm air, denoted as the warm sector. And then to the west of that is a region of cold dry air which we call then coldest air of the system. There is also a warm front structure. And this example 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. Another way you can kind of consider looking at this air is by looking at what is called an infrared image. What this is showing is a satellite image that is detecting cloud-top images, 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. That's probably a region of thunderstorms or possibly severe weather activity. Regions where you see darker colored clouds or shades of clouds, that represents clouds that are lower and shallower to the ground. 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 these cloud tops are higher up, the surface shows them as dark because the ground is warmer than the cloud tops themselves. 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. 

In the cooler air regions and coldest air regions we have shallower clouds, if not no clouds at all because the air is cooler and drier over time. How exactly does this tie to jet streaks? How do jet streaks influence the genesis or strengthening of extratropical cyclones? This image is actually taken from a paper by Bjerknes, 1951 and also Uccelini, 1990. Louis Uccelini was actually another graduate 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. What he was showing in this paper, in this review paper, was sort of how the three-dimensional circulation ties to the genesis of extratropical cyclones. 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. The 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'. 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.  Let's suppose that there were low-pressure systems located in the regions where upward vertical motions is occurring. How does this play a role in strengthening a surface cycle or surface low-pressure system? Here is sort of a diagram showing the cross-section of this low-pressure system. 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 the upward vertical motion associated with the low because air is rising in these regions where the L's are labeled in Panel A. 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 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. 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. 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. So you're going to reduce the sea level pressure associated with the low-pressure system center. By reducing that you're only going to act to make that cyclone stronger over time. As a result, you have stronger circulation. 

I tried to represent that body making bigger wind symbols on the diagram. The 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. 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. Actually if the pressure decreases at a fast enough rate, we call it explosive cyclogenesis. Or another fun word is bombogenesis, as referred to in the snappy meteorological world. I have a few weather maps here to show an example of a strengthening of a low-pressure system right in a left-jet exit region, which is a region associated with the jet streak where we'd expect a low-pressure system to strengthen. Just so you can get your bearings straight on this map here, 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 blue or purple colors mean cooler air. At 300 mb, 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 speed. 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. 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. The bottom left that's where the low-pressure center is with respect to that feature over New Mexico and Texas. 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. You can sort of compare the upper-level flow with what's going on at the surface. This is 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 6 AM local time in Madison. 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 mb, which was just below mean or average sea level pressure around in the mid-latitudes. 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 two more of a green or lighter blue. This is actually an example of a just streak just forming right over the southern Central US. That L is sitting right in the exit region and pretty close to the left jet exit region. So we'd expect that during the next 12 hours we'd expect air to be rising out of the low-pressure center and then diverging aloft. We should see that cyclone strengthening in our surface map over time. One way we can tell if a 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. Again, this is at 6 PM on December 19, which is in Zulu time UTC 20th of December 2012. Let's go 12 hours ahead to 6 AM on December 20. Notice-- I'll flip back one more time-- notice how there's more of these blue isobar lines are also tighter together on this new image. 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. 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. That air's been rising, taking that air out of the low-pressure center column. 

That's why we see that pressure decrease and also see an increase in the winds over time. It was about this time as well as this next time 12 hours later that Madison got absolutely hammered with a lot of snow. 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. I think schools got canceled pretty much everywhere. The buses might even have shut down, which is a very rare event Madison, Wisconsin. 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 the cyclone was tied to those jet streak dynamics associated with the polar jet stream, that weather system got stronger. 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 and warn the public, and we feel confident when we think a storm is going to strengthen and impact our area. 

All right. 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. 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, way to you see what happens when the two merge together into one jet stream. We're first going to start -- we're just going to look and quickly review the three-dimensional structure of the polar and subtropical jet streams because it turns out that a jet superposition is simply a merger of the two, and sometimes looking at a cross-section of a jet will better reveal if you see an jet superposition versus the two jets separately. As a basic example here: this is a map of the polar jet stream and subtropical jet stream on April 27, 2010. I labeled vectors on there to show where the fastest wind speeds were, so too with the jet. The next image I'm going to show is a cross-section A through A'. So basically, what were going to do is take this image and rotate it 90° so that our surface is going to be A to A', and then our axis on the Y axis on the left is going to be altitude increasing over that distance. 

So here's that cross-section. Again, A-- if we go back to this image for a second-- represents sort of the pole-ward side of the cross-section, and A' represents the Equator-ward side of that cross-section. What you see here-- there's a lot of lines going on here-- what you see here if you look at the red line 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. The wind speed maxima labels are circles with Xs in this case. With this cross-section, what you're seeing is those Xs represent the flow that's going into the the slide in this case, so it's more or less going west to east in this picture, given how we took that cross-section. 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. 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 to merge together? This is what we call a vertical jet superposition event. 

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 merge together like this. It's kind of a fun hand motion to do as well too. What does this look like, though, in a cross-section? Here is an example of a jet superposition event on October 24, 2010, at 0 UTC. 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. Now let's consider this cross-section B through B' through the jet core. And wow! This is a much different picture than what we saw in the last cross-section, right? One thing you might notice 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. 

Notice these red lines, these isotachs, or lines of constant wind speed. There's more of them as you go towards the center of the jet. This would mean that the wind speed is much faster in this jet relative to the polar jet or subtropical jet stream. Also notice too 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. 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, and we identify each with the dots, we call that a vertical jet superposition event in our database. Another thing I will note too is notice that there are these three black lines that sort of extend and then drop off and then continue further to the left. This is actually the height of the tropopause. 

It turns out that if we go back a couple images, we noticed there were sort of three steps in the tropopause here. The tropopause height varies depending upon the temperature below it. In the classic model there are three steps, but when we have a 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 a gradual three-step situation. Okay. That's pretty cool but why do we care about studying these superposition events? Is it just cool because the two jets come together and that 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 I will give you two examples this evening. One is that there is a superposition event that was associated with a significant flood event on the first and second of May 2010. 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. 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. 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. On this day there were almost 200 tornadoes reported. I think it was 187 approximately or maybe even more after further analysis. These are some really interesting pictures. Also, I really like that picture on the right because that was actually a Skycam image taken during live news of the 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. 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 happened there extended all the way to the Eastern United States and helped to induce this event. What happens in other parts of the world can affect what happens here in the US, 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 groups here at the University of Wisconsin-Madison that started to investigate where these events occur and how they work. 

This is what's called a climatology from a Master's thesis done by Croix E. Christenson that shows how often these events happen per month in fall, winter, and spring. 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. They don't really happen too much in the summer. In the wintertime these two jets are more in a position to merge or vertically align together to form a superposition event. 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. Even looking at these three regions together, it's clear that the West Pacific is a hotspot for these jet superposition events. It's really interesting to consider what makes this a special spot for jet superposition events. This is a question that we are considering in our lab group. 

And usually when you start doing your research project you look at past literature to try to see what have other people looked like with respect to these events and what do we know about  the West Pacific and these sort of West Pacific superposition events. We actually did that to start the project. This is just a highlight that the West Pacific is a particularly interesting region. 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 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. We did some investigation. After reading all sorts of papers, or really I should say 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. We are talking a year--we didn't even have satellite data yet. 

That was the only. There was a scientist in Japan who 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. 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 study in the atmosphere. Based on that motivation, 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 is specifically tied to West Pacific jet superpositions? I showed you a couple examples of extreme weather 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. The methodology that we used is we each first tried to identify several cases in this West Pacific region where superposition events occurred, and there were 44 cases that caught our eye that we considered for our analysis. 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 a lot of cases to look at by eye and do an analysis with. You could do it, but it would take a long time, and as a grad student, you have to eventually graduate. 

You have to find some faster ways to think about strategies for looking at multiple cases at the same time. That's where this technique called a composite analysis comes into play. How a composite analysis works 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 is what's called a composite in an average sense. 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 the Event A and Event B. Here's Event A and Event B. This is just showing windspeed in the upper troposphere where, again, the warmer the color, 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 it by 2 to get the average of that data and look at how this event evolves as a composite. The composite map is shown on the bottom here, which you can kind of see it looks like an average of Events A and B. We did the same sort of idea, but we did this with 44 cases rather than 2 cases because you want to have at least a big enough sample size, otherwise scientists won't be interested if you only have one or two cases. 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, but I think this will give the general idea of what we're investigating with respect to these West Pacific superposition events. Anywhere where you see on this map-- this is centered over the Pacific Ocean-- and anywhere where you see red lines represent isotachs again, and the further you go inward toward the jet core near Japan, the faster the wind speed goes. Then there are four regions 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. 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. 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. 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 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. 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. This is actually really interesting to us because there are these cold surge events that happen and are associated with the phenomenon known as the East Asian Winter Monsoon. The East Asian Winter Monsoon -- maybe you've heard of the word monsoon before and you think lots of rain. Actually, a monsoon really just refers to a large-scale shift in the wind pattern. In the winter in over East Asia the Siberian Mongolian high-pressure system really revs up, and this thing has a large clockwise circulation. At the eastern edge of that clockwise circulation you have winds going north to south. In Siberia there's very cold air in the winter, sometimes colder than what we see in the US or even in Canada. The northerly winds of this high can take this cold air and bring it all the way southward into the East and South China Sea regions. Sometimes we know what's called colder outbreak events as far south as Hong Kong, which can actually be devastating for people who live in these areas because they're not used to that cold, unlike we are in Wisconsin. 

How does this tie into the jet superposition idea? 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 coming down from Siberia. What we observed in our composite analysis is that this cold air overtime starts somewhere up in Siberia or northeastern China and moves southward towards lower latitudes near Indonesia. In this region, it's very common in the winter that there are some sort of tropical thunderstorms or what we call a tropical convection that's present at this time. 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. Rising air continues to fuel these tropical thunderstorms in this region. When this air rises, remember, it's eventually going to hit the tropopause and it's going to have to go outward and go somewhere. This air will hit the tropopause and eventually start to move towards the Pole. Pole-ward of this air is where the jet stream lies in time before it's superposed. So this air moves pole-ward and we get a situation like this where you get an interesting three-dimensional cross-section where this tropical air, which originated from a tropical thunderstorm, actually manifests itself and gets stuck or stored in the subtropical jet layer at the same altitude where the subtropical jet resides. 

It turns out by putting that air there it actually has the 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. This thermal wind is more confined to the upper troposphere versus looking at horizontal temperature gradients at the surface. At the same time there's stuff going on on the pole-ward 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. It turns out when we have had very stable air on the pole-ward side along with this unstable air or tropical convection air on the Equator-ward 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. So that's sort of the basics. We learned all this through our composite analysis, but our next steps 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 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 UW AOS 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. Though 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. Thanks for being here this evening. And I will take any questions. [applause]