GG 140: The Atmosphere, the Ocean, and Environmental Change

Lecture 16.?Frontal Cyclones

https://oyc.yale.edu/geology-and-geophysics/gg-140/lecture-16

Mid-latitude Frontal Cyclones [00:00:00]

We're finishing up a section on storms here, and we've talked about convective storms. That is, air mass thunderstorms, severe thunderstorms.?I've been calling these things severe thunderstorms. When you're reading the book, notice they have subdivided that category into three or four smaller categories. You've got your squall lines and your mesoscale convective systems, and so on.?And then the third category we had was hurricanes.?

Well then, the last category that we'll be doing is quite different. It's the mid-latitude frontal cyclone. It?gets its energy source from a different place. All the convective storms get their energy from the release of latent heat as water vapor condenses to form liquid or ice. But not these. These come from horizontal temperature gradients.

The point is that, if you've got cold air next to warm air, there is potential energy there. Because the warm air wants to rise, and the cold air wants to sink and spread underneath the warm air. If I had a wall, for example, separating the warm air and the cold air, and I suddenly remove that wall, things would start to change. The warm air would rise up. The cold air would slump and slide under. So there is energy available when you've got these lateral temperature gradients. And that is the source of energy for frontal cyclones.

So this name is a long one, but mid-latitude tells us where these things form. Frontal--a front is a boundary between cold and warm air. And, of course, this is a little bit redundant, because mid-latitudes is the region where you get the strongest contrast between cold air to the north and warm air to the south.?But then it's a cyclone.

It ends up being a counterclockwise circulation in the northern hemisphere, just like a hurricane. But the dynamics and the details are really quite different, even though they end up with a circulation around a low pressure center, just like a hurricane. They grow from a disturbance along the polar front.

The polar front is this boundary that goes around the earth in mid-latitudes, separating cold from warm air. And when a little kink develops in that boundary, as I'll show you, it then grows and develops a frontal cyclone. Because they occur in the belt of westerlies, as they evolve and grow they move from west to east--not always directly from west to east, maybe at some angle. But generally they move from west to east.?I don't think I've ever seen a frontal cyclone that went backwards, that moved towards the west as it developed. They all move east in some way.

And eventually--so these storms do have a life cycle. They don't last forever. They start from a little kink in the polar front. They grow. And as I will show, once the warm air has all been lifted upwards, then the energy source is gone, and these storms will begin to die. So they do have a natural life cycle. Typically, they last?five to seven days as they move across the landscape from west to east.

Here's a simple picture of one. It's got the classic features of the low pressure center, the cold front, and the warm front. And also scattered around the diagram are some of the indications of some of the types of local weather that you get. First of all you've got some rain showers along the cold front.

In this case you've got some freezing rain up here. Remember what causes that. That's that cold aloft warm and then cold near the surface that can supercool those raindrops before they hit the surface. And you've got the light snow here, heavy snow back in the cold air. So a lot of local types of weather are going along with this large scale storm.

Now in order to make any sense of frontal cyclones, you have to know the different definitions of the fronts. The word front by itself simply means a boundary between warm and cold air. A cold front, indicated by the filled triangles, is when the cold air is advancing. A warm front, indicated by the filled half-circles, is when the warm air is advancing. So those little symbols are telling you how these fronts are moving. The cold air is back here. The warm air is there.

So we know, because that's defined as a cold front, we know that it's moving in this direction. This one, the warm air is here, the cold air there. It's a warm front, so that means the warm air is advancing. It must be moving in that direction. So in a way, those are telling you a little bit about motion, how that front is moving with time.

So that’s a--a few other definitions--this little sector back here is called the warm sector. And of course it has warm air moving up, very often in New Haven Very often that air is coming from the Caribbean. And behind it you've got cold air. And usually that's coming down from Canada. So you end up with a contrast here, across the cold front between air masses of different sources--Caribbean area here, Canadian air here.

Notice also, this meets the criteria that I laid out a day or two ago about needing to transport heat northwards. So here's northward-moving warm air, southward-moving cold air. Probably the same amount of air altogether is moving, but there's a net transport of heat. Because you're pushing warm air northward and cold air southward, there's a net amount of heat being carried northward.?And that helps to balance the excess solar radiation near the equator, and the deficit of solar radiation and emitted radiation near the pole. So this plays a big role in the general circulation as well.?

Lifecycle of a Mid-latitude Frontal Cyclone [00:13:01]

So here?is the history of one. It starts as a little dimple in the polar front. And already you can see the characteristics. The cold air begins to advance, so we can label that as a cold front. The warm air begins to push northward, so it can be labeled as a warm front. But it's very weak. A day or two later it'll look more like??a fully developed, mature cyclone--low pressure center, cold front, warm front, warm sector, cold air from behind.

Then it goes to this stage of?early occlusion. And the word occlusion refers to the fact that now the cold fronts and the warm fronts, at least in the northern part--not here but up here--they're beginning to come together.?These fronts are drawn where they intersect the surface of the Earth. But they're not vertical boundaries in the atmosphere. They are tilted boundaries between cold and warm air.?When they come together like this, now they are occluding. And notice that the warm air, which was in the middle, has been pinched off the ground. Now the warm air is up, and you've got cold air and cold air that have touched each other at the surface of the Earth.

So that's the meaning of the word occlusion. It's where the cold front and the warm front have come together, pinching the warm air upwards. Warm air is buoyant. It wants to be up. So as soon as the occlusion begins, then the energy source to keep this storm going begins to weaken. This is the early occlusion. And now we can say, cold front, warm front, and occluded front.

They put the warm front and the cold front together on the same side of the line, indicating that's now an occluded front. And here, it's occluded all the way down to there. And you've only got a little bit of the original cold and warm front. This one, they've labeled it late occlusion. This storm is dying at this point, it's weakening. Due to friction and so on, the energy source is gone. Friction is acting to decay it.

Typically, it'll take five to seven days to go from this stage to that stage. And by that time, it's probably traveled 5,000 or 10,000 kilometers from west to east. This storm may have begun to form east of the Rocky Mountains, and it may finally occlude as it's approaching Greenland, or even England. So these things travel a long distance while they are going through this kind of a life cycle.

So this is another cartoon. But this one shows a little bit of that tilted structure. So here's the map view--north, south, east, west. There's the cold front, the warm front, the warm sector. Now if I do a vertical slice of the atmosphere here, it's drawn at the bottom. There's the sloping cold front. And where it hits the ground is where it's marked on this map. There's the sloping warm front. And where it hits the ground is marked on the map there.

And there are some characteristic cloud patterns that are connected with these fronts. Here--cirrus, cirrostratus, altostratus, nimbostratus. Nimbo means rain, and you see it's raining there. And these are generally stratus clouds out ahead of the warm front. The cold front--because you're lifting warmer air with more moisture in it, air that's in the warm sector, possibly air that’s come up from the Caribbean--you tend to get more instability.

That's a little cumulonimbus cloud there. Or maybe in some cases it's stratus--nimbostratus and altocumulus. But these tend to be more unstable near the position of the cold front. And then cold air back here, warm and cold again here.

If we back out now and look at a larger piece of the Northern Hemisphere on a typical day, anytime between now and next April, when you're getting strong temperature gradients north to south, this is typically what it looks like.

So, generally, you've got the polar front wrapping all the way around the globe in mid-latitudes, with a fairly strong temperature gradient across it. And then, at different places it'll be doing different things. So on this little diagram you've got an occluded frontal cyclone here. You've got a mature one here and one that's just starting out here.

A?stationary front is a definite cold-warm contrast, but it's not moving. Whereas an occluded front is a cold front and a warm front that have come together. Therefore, you've got cold air on both sides. And the warm air has been lifted aloft. So there is a big difference.?Notice that they are drawn differently. This has the symbols on opposite sides, where here the symbols are on the same side.

So this is the big weather producer throughout the fall, winter, and spring months in mid-latitudes. These things just come at us, one after the other. Generally they control the cycle of weather in mid-latitudes. You'll have a couple days of warm air blowing from the South. Some precipitation will follow. Then you'll have a couple days with cold air coming down from the North.?Imagine this whole thing moving west to east while you are stationary. It's almost as if you are moving in the other direction--experiencing this, and then this, and then this--as these things move over you. Unfortunately they're not periodic. They don't happen with great periodicity.

So it becomes rather difficult to predict the weather. But as you get into the cycle--if you begin to watch the weather every day, and occasionally take a peek at a weather map, you'll begin to understand this cycle of events that is coming at you day after day, week after week, throughout the cool season of the year.?

So these are storms, and we need to know what they do. To a large extent they are beneficial because they bring us the rain that we need. And the snow up in the mountains that will then melt in the spring and summer and give us water in the rivers. But, on the other hand, they can also bring damaging winds.?

Sometimes you get strong winds in the warm sector, but usually the strongest winds are back here, behind the cyclone, coming down from the Northwest. And of course the rain, while it's needed, can sometimes be very heavy. Or the snow can be very heavy, which can cause flooding or deep, impenetrable snow drifts.

Because these storms do bring cold air down from the north, often they can cause very severe wind chill episodes, with strong wind, sub-zero temperatures, and quite dangerous to be out in, in this kind of a storm. And then over the ocean, those strong winds mentioned here, winds passing over the ocean generate ocean waves. The stronger the winds, the bigger the waves. And they can be quite damaging to ships at sea. So these things have benefits but also dangers.?

Nor’Easter [00:26:02]

Meteorologists like to put names on things. I think every field likes to put names on things. These are four typical storm tracks, directions where mid-latitude frontal cyclones tend to move fairly frequently. Ones that come in off the west coast that were generated in the vicinity of Hawaii are sometimes referred to meteorologists as the Pineapple Express.?

Ones that come down from Canada are generated up in here and then come down towards the Midwest are often referred to as Alberta Clippers. We get a lot of storms in the west that were generated just east of the Rocky Mountains. We call those Rocky Mountain Storms because of where they generated.

But the most famous one from New England that you may have heard about is the Nor'easter. So this is just another mid-latitude cyclone--frontal cyclone--but it's called a Nor'easter. And its characteristic is that it's normally generated in the northern part of the Gulf of Mexico, or more typically here along the coast, the East Coast of the United States. And then it moves up to New England from the South.?

If you are a New England fishermen or a farmer and you're fishing offshore here, and the wind begins to pick up from the northeast, that is a sign that a storm is coming. And you would quickly head back to a sheltered cove or back to land to get your ship out of the storm.?So a Nor'easter is named Nor'easter because the first sign of it in New England is the development of a northeasterly wind, an wind coming in from the northeast. You're in the belt of westerlies here. So the winds are normally coming from west to east. So when a wind begins to go from the northeast, well that's a little bit different.

And why is that a sign of a storm to come? Well, if a cyclone is coming at you from the South--remember the winds are coming around it this way--and so the first sign of it you will feel as it moves towards you is that easterly wind. You add a little bit of friction into that, and it's going to be a northeasterly wind. So the term Nor'easter comes from the wind direction that is the first indication that this storm is approaching you.?

Down in the frictional boundary layer, where the winds aren't quite geostrophic, there's a little bit of a turning. They tend to move in, still generally going parallel to the isobars but with a little bit of cross-isobar flow as well. So here you’ve got some northeasterly winds.?Now as this storm moves up in this direction, this pattern just translates. And so the first thing a fisherman up here feels is the wind blowing from the northeast, which is a sign that that's--so it's not the fronts that I'm talking about. It's the winds around that low pressure center that gives rise to the northeasterly winds as the first sign that this thing is approaching, as it moves up the coast.?

Southern Hemisphere Cyclones [00:33:02]

Now you get the same thing in the Southern Hemisphere.?In Southern Hemisphere winter you get a big north-south temperature gradient. And you get the mid-latitude frontal cyclones. They rotate in the opposite direction. Just like hurricanes rotate oppositely in the Southern Hemisphere, so do mid-latitude frontal cyclones. The winds spin clockwise around frontal cyclones in the Southern Hemisphere.?And they are very strong down there, perhaps even stronger than in the Northern Hemisphere, which has given rise to these amusing terms for these latitude belts.

The winds down here are westerlies, just as they are in the Northern Hemisphere mid-latitudes. So if you wanted to go to Australia you'd come down here and then go east.?Here you've got to go around Cape Horn. And especially Cape Horn is famous for all the shipwrecks that have occurred as ships have tried to go around or through this belt of very intense and frequent mid-latitude frontal cyclones.

Weather Forecasting [00:35:29]

Now?weather forecasting. You could take a whole course in this. But I want to say a few words about how one forecasts all these different types of storms, especially the mid-latitude frontal cyclones. It's pretty simple but it's a big enterprise.

The way you do it is first describe the initial state of the atmosphere--that is, temperature, pressure, wind, humidity--everywhere, globally and vertically, at one time. You use weather balloons for that, surface stations, and satellites to get that state of the atmosphere described. Then you can use that as the initial condition on a giant integration of the equations of motion.

These complicated equations, which you have seen simplified versions of in this course--like the perfect gas law, the hydrostatic equation, cloud processes, and so on--are all programmed into these equations. And then they're integrated forward. This is a huge job and usually the largest supercomputers that exist in the world are dedicated to this problem.

In this country it's the National Weather Service that does all of that--collects the data and puts them into the equation and runs—puts them into the computer and integrates those equations forward. And you get your forecasts directly from the Weather Service if you want, but then there's a big private-sector industry that lies on top of that. They get the raw results from the computer output and then process it to make it useful for the consumer. They make pretty graphics, nice maps, predictions of what's going to happen hour by hour, build nice websites.

Now here's a diagram put out by the National Weather Service, so it may be a little bit biased. But, no, it's very statistical. They define a skill score. It's some kind of a measure of the quality of the forecast--how accurate the forecast was, compared to what actually happened. And it's plotted on a scale from 0 to 80. But 100 would be a perfect forecast, perfect in all respects.

This happens to be a forecast for the 500 millibar level. So it's not every aspect of the weather that's being presented here, just the shape of the isobaric patterns at 500 millibars. And starts in 1955 when weather forecasting first began, using computers, and goes up to almost the present day.?That's a pretty good improvement over the time. It's caused by two things, improvements in the numerical models and improvements in measuring that initial state.?

The further out you go in the forecast the worse is going to be the quality of your forecast. But that, too, has been improving. So today the 72-hour forecast is about as good as a 36-hour forecast was in the early '80s.?And of course, people are trying to compute out still further in time. If you go into the Weather Channel you'll find forecasts out seven, eight, even nine days in advance. I don't usually trust those. But they're fun to look at and speculate about what might be. Questions on this?

So here's my off-the-cuff summary about how well we're doing with various types of storms. With severe thunderstorms we predict the probability pretty well. In other words, you'll hear a forecast saying something like 80% chance of thunderstorms tomorrow afternoon. We do that pretty well.

But in terms of predicting when and where an individual storm is going to occur, we have almost no skill at that. We don't know whether it's going to occur over Hartford or Stamford. We just can't do that kind of local forecasting with this kind of storm because they're too chaotic, they're too random, in what triggers them.

Hurricanes?and the tornadoes which come out of those, we're very poor at predicting when and where a tornado will arise. So we use what's called nowcasting.?You could give a few minutes' warning to a community where that tornado was approaching. But in terms of being able to predict something like that from a numerical model, no. We have no skill at being able to predict from first principles when and where a tornado will occur.

Moving onto hurricanes, genesis means when you first create the hurricane. We're very poor at that. Once the hurricane is formed, we're quite good at predicting where its track is going to be. For example, Hurricane Irene was a good example of that. They really had that track nailed, even out three or four days in advance. They had a very good track forecast. As it changes its strength however as it moves along its track it may weaken or strengthen. We're not very good at that. So certain things we do well.

Frontal cyclones we're pretty good at. These wintertime storms we often have good skill out to five days. Occasionally we have a real bust. The weather service will fail to predict the big storm genesis event or will forecast one that didn't occur. But those busts are increasingly rare. And I'm amazed sometimes at how skillful the prediction of these storms can be, out even several days in advance.?