Real Meteorology EAS-135 Wind: Global Systems. Real Meteorology EAS-135 Chapter 10 Overview...

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Wind: Global Systems


Chapter 10 Overview– General Circulation of the Atmosphere– Jet Streams– Atmosphere-Ocean

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General Circulation of the Atmosphere

– We learned in the chapter on temperatures that, averaged over the entire earth, incoming solar radiation is roughly equal to the outgoing earth’s radiation.

– In that discussion it was shown that there was an uneven heating of the earth.

• The energy balance is not maintained for each latitude as there is a net gain of energy at the tropics and a net loss at the poles.

• As a result the poles are cool and the equator is warm.• The question is how does the earth restore the balance since

we know that the poles are not at absolute zero and the equator does not melt lead.

– To restore balance, the atmosphere transports warm air poleward and cool air equatorward.


General Circulation of the Atmosphere (Single-Cell Model)

– The british meteorologist George Hadley devised a gedanken to try to understand how the earth comes into balance.

– Single-Cell model assumes that:• Earth’s surface is uniformly covered with water (so that

differential heating between the land and water does not come into play).

• The sun is always directly over the equator (so that the winds will not shift seasonally).

• The earth does not rotate (so that the only force we need to deal with is the pressure gradient force).


General Circulation of the Atmosphere (Single-Cell Model) – For the single-cell model described above, the air

near the equator heats and as it does it becomes less dense.

– The air near the equator then begins to rise and the surface pressure drops.

– Conversely, the air near the poles cools and becomes more dense.

– The air near the poles then begins to sink and the surface pressure rises.


General Circulation of the Atmosphere (Single-

Cell Model) – At this point, a pressure gradient force develops

between the poles and the equator and the cold air flows from the high pressure at the poles to the low pressure at the equator.

– The opposite occurs in the upper portion of the troposphere. The rising air at the equator strikes the tropopause and must flow both north and south towards the poles which creates high pressure aloft.

– At the poles, the sinking air creates low pressure aloft and another pressure gradient develops in the upper part of the troposphere between the poles and equator.


General Circulation of the Atmosphere

(Single-Cell Model)


General Circulation of the Atmosphere (Single-Cell Model)

– Obviously, this circulation does not really occur on the earth.

• Earth is not uniformly covered with water.• The sun does not always stay above the equator.• Coriolis forces will turn the winds to the right.

– However, Hadley’s gedanken did provide grounds for further study.


General Circulation of the Atmosphere (Three-Cell Model)

– In the three-cell model, the first step is to remove the restriction of a non-rotating earth.

– Although this model is much more complex than the single-cell model, there are some similarities.

– As with the single cell model, the air at the equator is heated by the sun, becomes less dense, and rises to the tropopause creating a low pressure at the surface.

– In addition, above the equator, the rising air creates high pressure aloft.

– The poleward flow of air at upper-levels is then deflected by the Coriolis force toward the right in the NH and to the left in the SH.



– As the air aloft moves northward, it’s volume is slowly reduced.

• The volume is reduced because of the spherical surface.• As you move closer to the poles the latitude lines get closer

together to reflect the reduced radius. – The slowly reducing volume creates a region of

convergence near 30°. – The air becomes so compacted at 30° that it must sink.– The sinking air strikes the surface and spreads out in all

directions.– This creates a belt of high pressure called subtropical

highs.



– The air from the high pressure belt that is directed south curves to the right and eventually flows into the equatorial surface low (trade winds).

– The air from the high pressure belt that is directed northwards is again deflected to the right (westerlies).

– The subsiding air produces generally clear skies and warm surface temperatures. Here are where the major deserts of the world are found.

– This region is know as the horse latitudes.



– The mild air from the high pressure belt that is called the westerlies moves poleward and encounters cold air moving down from the poles.

– The cold air from the poles meets the air from the subtropical high in the horse latitudes.

– This convergent zone causes the air to rise once again, and is called the polar front.




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Sea-level Pressure and Surface Winds (January)


Sea-level Pressure and Surface Winds (July)


Air Masses– An air mass is a very large body of air whose

properties of temperature and moisture are fairly similar/uniform.

– Typically an air mass will cover several thousand kilometers.

– For a large mass of air to develop uniform characteristics,


the source region should be generally flat and of uniform composition with light winds.

– The best regions are the snow and ice covered arctic plains, the sub-tropical and tropical oceans and desert regions.

Air Masses (Classifications)

– Air masses are usually classified according to temperature and humidity, both of which usually remain fairly uniform in an horizontal direction.

– Grouped into four general categories according to their source region.

• Polar (P)• Tropical (T)• Continental (c)• Maritime (m)



– P (polar) indicates that the source region is in the polar latitudes and this air mass is typically cold.

– T (tropical) indicates the the source region is the tropic latitudes and this air is typically warm.

– The letter that precedes the latitudinal location of the source region indicates the moisture content.

– m (maritime) indicates that the air mass originated over an ocean/maritime region which will be typically moist or humid.

– c (continental) indicates that the air mass originated over a land/continental region and it will be dry.



– A cT air mass in North America will be warm and dry and originate from the deserts of Mexico and the Southwest.

– A mT air mass may originate over the Gulf of Mexico and be warm and moist.


Air Masses (cP and cA)

– Originates over the ice- and snow-covered regions of northern Canada and Alaska.

– This air is cold and dry.– Air is modified east of the Appalachians and occasionally

along the Rocky Mountains as this cold air is forced over the mountains. This warms the air by compression and the temperatures rises dramatically.

– cP air masses are responsible for the generation of lake-effect snows in the Great Lakes region.

– In the summer, cP air masses have more moisture because they have melted and evaporated some of the snow. This colder and more moist air brings relief of the summer heat to the central and eastern US.


Air Masses (cP and cA)


Air Masses (mP)

– Occasionally cold moist air from the Aleutian Islands movers over the Pacific coast.

– This air is cool, moist, and conditionally unstable.– Temperatures near the surface are near 50F, but a small

distance aloft the air is once again below freezing.– When this air is forced aloft much of the water vapor

condenses out into rain.


– Once it leaves the coast and into the mountain ranges it loses most of it's moisture and it turns into cool dry air.

– Only occasionally do mP air masses make it to the east coast.

Air Masses (mP)


Air Masses (mT)

– These air masses have their origin in warm tropical waters.– For the US, the primary source region for the Midwest and East

Coast is the Gulf of Mexico and for southern California is the Southern Pacific Ocean.

– The presence of the Gulf of Mexico provides for interesting weather in the Midwest and along the East Coast.

– In winter, cold polar air tends to dominate weather, but occasionally mT air is drawn northward out of the Gulf of Mexico.

– When these air masses collide, the warm very moist air from the Gulf quickly loses its moisture and it falls out as heavy snows. These snows generally occur from the Midwest to the Northeast.

– In the spring the temperature contrast is less, but the winds are still strong and strong thunderstorms and tornadoes occur.


Air Masses (mT)


Air Masses (cT)

– Source regions are the desert southwest and the Mexican Plateau.

– Very hot and dry air.– Since the air is typically subsiding in this region

due to topographic effects, the air is generally very stable and therefore clouds cannot form.


– If this air mass moves into the Great Plains and stagnates over that region for a period of time, a heat wave and/or drought can occur.

Fronts– A front is the transition zone between two air

masses of different densities. The region where two differing air masses come in contact.

– Density differences are most often caused by temperature differences.

– Since air masses are three-dimensional objects the


boundaries between these air masses are surfaces (frontal surfaces or frontal zones).

– Since air masses also move then so do the frontal surfaces as well.

Fronts (Stationary)

– A stationary front is a frontal boundary that is not moving.

– On weather maps, stationary fronts are marked by alternating semicircles in red (point toward cold air) and triangles in blue (point toward warm air) facing in opposite directions.


– Common in winter along the eastern side of the Rocky Mountains. Unable to cross the barrier, the cold air shows little or no westward movement.

Fronts (Cold)

– A cold front represents a zone where cold, dry, stable polar air is replacing warm, moist, conditionally unstable subtropical air.

– The front is drawn as a solid blue line with the triangles along the front pointing to the direction of the frontal movement.

– Criteria used to locate a front on a surface map:• Sharp temperature contrasts over a short distance.• Changes in the air’s moisture content.• Changes in wind direction.• Pressure and pressure changes.• Clouds and precipitation patterns.


Fronts (Cold)


Fronts (Cold; Cross Section)

– At the cold front, the cold, dense air wedges under the warm air, forcing the warm air upward.

– Behind the front the air cools quickly (Freezing level dips as it crosses the front).

– Winds shift.


– Leading edge of the front is steep. Vertical rise to horizontal distance is 1:50

Fronts (Cold)


Fronts (Warm)

– A warm front is the opposite of a cold front.– In this case, warm air displacing cold air.– The front is drawn as a solid red line with

semicircles pointing in the direction of the movement of the warm front.

– The direction of movement is determined from the weather conditions.

– In this case the warm is less dense than the air it is displacing so it rides up over the top of the dome of cold air.

– This makes the ascent of the air much slower than compared to a cold front.


Fronts (Warm)


Polar Front Theory– From the concept discussed earlier about air

masses, it is easy to see why fronts move.– A previous slide showed two air masses bumping

into each other forming a stationary front.– It has also been discussed earlier that cold air

needs to move southward to warm up while the warm air needs to move northward to redistribute heat energy.

– At the same time there is no reason that the high pressures associated with each air mass need to be exactly the same.


Polar Front Theory– Indeed, the pressures are very likely to be

different.– The pressure in one air mass may be relatively

high compared to one air mass but low compared to another air mass (two different air masses both with high pressure).

– As we discussed in the section on winds, air will flow from higher pressure to lower pressure.

– Thus, if there is any difference in the pressures the air will begin to move.

– As it does move, the relationship between the air masses will begin to change.


Polar Front Theory– At some point, the cold air will begin to move south

and the warm will begin to move north as we would expect. This restores the energy balance at the poles and equator.

– This is the point where the shape of the stationary front begins to change.


– There are six steps in the idealized life cycle of a wave cyclone.

Polar Front Theory (Stationary Front)


– Stationary front represents a trough of lower pressure with higher pressure on both sides.

– Cold air to the north and warm air to the south flow parallel to the front, but in opposite directions.

– This flow sets up cyclonic wind shear (change in wind speed or direction over a certain distance).

– Visualize this concept by placing a pen between the palms of your hands and move your left hand toward your body, the pen turns cyclonically.

Polar Front Theory (Frontal Wave)

– A wavelike kink forms along the front. One theory suggests that small changes in the temperature field at the surface causes a small imbalance in the wind fields and that causes the wave to form.


– Cold front now begins to push southward and a warm front now begins to push northward.

– Region of lowest pressure (central pressure) is at the junction of the two fronts.

– Green shaded area shows location of precipitation in the frontal wave stage.

Polar Front Theory (Open Wave)

– A fully developed open wave forms in 12 to 24 hours.– Central pressure is now much lower, and several isobars

encircle the wave’s apex.


– Tighter packed isobars create a stronger cyclonic flow, as the winds move counterclockwise and inward towards the low pressure center.

– Precipitation forms ahead of the warm front and behind the cold front.

– The region between the warm and cold fronts is called the warm sector. Weather tends to be partly cloudy, but scattered showers and thunderstorms form if the air is conditionally unstable.

Polar Front Theory (Mature; Initial Occlusion)

– As the open wave continues moving east, the central pressure continues to decrease and the winds blow stronger as the open wave develops into a mature cyclone.


– The faster moving cold front constantly inches closer to the warm front, making the warm sector smaller and smaller.

– At this point, the storm is usually most intense, with clouds and precipitation covering a large area.

Polar Front Theory (Advanced Occlusion)

– The point of occlusion where the cold front, warm front, and occluded front all come together is called the triple point.

– At the triple point, the cold and warm fronts appear very similar to the open wave stage. Occasionally, under the right conditions, a new


surface cyclone can form in this location, this is called a secondary low.

– The central pressure of the main low gradually increases.

– The supply of energy provided by the rising warm, moist air does not get to the center of the storm because the warm sector is getting more and more removed.

Polar Front Theory (Cut-off Cyclone)

– The supply of energy provided by the rising warm, moist air does not get to the center of the storm because the warm sector is getting more and more removed.


– The storm system will continue to die out and gradually dissipate.

– The entire life cycle of a wave cyclone can last from a few days to over a week.

Polar Front Theory (Family of Cyclones)

– Figure shows a series of wave cyclones at various stages of development along the polar front in winter.

– This is a family of cyclones.


– To the north of the polar front, there are cold anticyclones; to the south over the Atlantic Ocean is the warm semipermanent Bermuda High.

– Front has developed a series of loops and at the apex of each loop is a cyclone.

Locations of Development


Three-Dimensional Structure– Up to this point, our discussions have focused on the

horizontal structure and lifecycle of extra-tropical cyclones.

– However, extra-tropical cyclones are three-dimensional.

– Why is it that some frontal waves develop into huge cyclonic storms, whereas others simply dissipate in a day or two?

– This is one of the most complex challenges in weather forecasting.

– Indeed, surface conditions influence the situation, but it’s the three dimensional structure that plays the crucial role in the development of these storms.


Three-Dimensional Structure– What controls the surface pressure? – Total mass of air above the surface

• If there is a net removal of air in the column, the surface pressure decreases.

• If there is a net gain of air in the column, the surface pressure increases.

– Temperature of air above the surface• Warming the column causes it to expand – less dense –

lowering the surface pressure.• Cooling column causes it to contract – more dense –

raising the surface pressure.


Three-Dimensional Structure (Advection)

– Before we can talk about the role that the vertical structure has on an extra-tropical cyclone, we need to define some terms.


– Advection is the transport of some quantity by the winds.

• Moisture advection is the transport of moisture by the winds.

• Temperature advection is the transport of heat by the winds.

– Look for those places where the isolines of the quantity are being pushed by the winds.

Three-Dimensional Structure (Convergence/Divergence)– The next important concept is convergence/divergence.– The best analogy that can be given is traffic on an interstate

highway when one or more highways intersect.– Consider the Interstate-64 and Interstate-270 intersection in

west county.– Where the ramps of the interstates merge, there is

convergence.– Where a ramp leaves the interstate, there is divergence.

– A second analogy is what happens to traffic as traffic either speeds up or slows down.

– When the traffic slows down the cars have a tendency to pile up (convergence) and when traffic speeds up the traffic thins out (divergence).


Three-Dimensional Structure (Convergence/Divergence)

– Simplistic convergence/divergence models.



– Simplistic convergence/divergence models.– Low pressure at both the upper-levels and surface

corresponds to convergence of air at these levels.


– This “piles up” the air in the column above the surface low which will increase the air pressure in the column and cause the surface low to weaken.

– A piling up of mass in the column means pressure of the column increases.


– Best case scenario for a developing surface low.– High pressure at upper-levels corresponds to

divergence of air at upper-levels.– Low pressure at the surface corresponds to

convergence of air at the surface.


– If upper-level DIV > low-level CON then surface low deepens and strengthens.

– In this case, mass is being “evacuated” from the column and the column will continue to lower in pressure.


– The question becomes where does this kind of movement occur in the atmosphere?

– Consider a ridge trough/pair.• The trough begins as cold air plunges south.• The ridge begins as warm air pushed north.

– As the air moves from the ridge to the trough the air slows down to go around the base of the trough.

– As it leaves the base of the trough, it speeds up.– This creates a region of convergence on the west

side of the trough and divergence on the east side of the trough.


Three-Dimensional Structure (Vorticity)

– When something spins, it has vorticity. The faster it spins, the greater the vorticity.

– In meteorology, vorticity is the measure of the spin of air parcels.

– Vorticity is three-dimensional in nature, for this discussion, our concern will be with the spin of horizontally flowing air about a vertical axis.

• Much like an ice skater spins about an imaginary vertical axis.

– When viewed from above, air that spins cyclonically (counterclockwise) has positive vorticity and air that spins anticyclonically (clockwise) has negative vorticity.


Three-Dimensional Structure (Earth’s Vorticity)

– Since the earth spins, it has vorticity. In the NH, the earth’s vorticity is always positive because the earth spins counterclockwise about its vertical axis.

– The amount of earth vorticity on an object depends on latitude.

– On the equator an observer would not spin about their vertical axis.


– A little further north, the observer would spin very slowly.

– At the North Pole, the observer would spin at a max rate of one revolution per day.

Three-Dimensional Structure (Relative Vorticity)

– Moving air will generally have additional vorticity relative to the earth’s surface (relative vorticity).

– It is the sum of two effects: the curving of the air flow (curvature) and the changing of the wind speed over a horizontal distance (shear).

– Air moving through a trough tends to spin cyclonically, increasing its relative vorticity.


– Air moving through a ridge tends to spin anticyclonically, the relative vorticity increases but this time in a negative direction.

– Vorticity maxima will be found in the base of mid- upper-level troughs.

Three-Dimensional Structure (CON/DIV and Absolute Vorticity)

– The sum of the earth’s vorticity and relative vorticity is called the absolute vorticity.

– As a parcel of air moves with the upper-level flow an increase (decrease) in absolute vorticity is related to upper-level convergence (divergence).


• Position 1: Air parcel has only slight cyclonic spin because the relative vorticity is negative due to curvature.

• Position 2: Relative vorticity due to curvature is zero which allows the earth’s vorticity to spin the parcel faster.

• Position 3: The parcel spins even faster as the curvature is cyclonic, which will add to the earth’s vorticity.

• Position 4: Parcel spins slower again as the curvature is once again zero.

Three-Dimensional Structure (CON/DIV and SFC Vorticity)

– Divergence aloft and the vorticity of surface air are related.

– Suppose the column in the figure has weak cyclonic spin.– If an area of upper-level DIV moves directly over the

column, the surface pressure will lower.– As the surface pressure lowers, air converges on it (like a

skater pulling their arms in close to spin faster).


– This squeezing of the air column causes it to shrink horizontally and stretch vertically.

– This increases the vorticity of the column.

– DIV aloft increases the cyclonic vorticity of surface cyclones.

Three-Dimensional Structure (Summary)


Three-Dimensional Structure (Putting It All Together)

– At 500 mb, there is a longwave trough with height lines and isotherms parallel.

– Surface stationary front is present at the surface.– Colder air is on the northern half of the map, while

warmer air is located to the south.


– Suppose a shortwave moves into the longwave trough disturbing the flow.


– As the flow aloft is disturbed, it begins to lend support for the development of surface pressure systems.

– Converging air forms above position 1 and diverging air forms above position 2.

– The converging air aloft causes the surface pressure to rise in the vicinity of “H”. Surface winds blow out from the high pressure at the surface and the air aloft sinks to replace it.


– The diverging air aloft causes the surface pressure to fall in the vicinity of “L”. Surface winds blow in toward the region of lower pressure as the diverging air aloft removes air from the column.

Three-Dimensional Structure (Putting It All Together)– Cold air moves in behind the cold front and warm air slides

up along the warm front.– These regions of warm and cold advection occur all the way

up to 500 mb.– Cold advection makes the air more dense and lowers the

height of the column from the surface to 500 mb which lowers the pressure in the trough and the trough deepens.


– Warm advection makes the air less dense and raises the height of the column from the surface to 500 mb which raises the pressure in the ridge and the ridge strengthens.

– The overall effect of the temperature advections amplifies the upper-level wave.


– Eventually, warm air curls around the north side of the low, and the storm occludes.

– Some storms can continue to deepen, but most do not as they move out from under the region of upper-level divergence.


– Surface low is now cut off from the warm air at the surface by cold, dry air.

Three-Dimensional Structure (Storm of the Century)


Real Meteorology EAS-135 Wind: Global Systems. Real Meteorology EAS-135 Chapter 10 Overview...

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Transcript of Real Meteorology EAS-135 Wind: Global Systems. Real Meteorology EAS-135 Chapter 10 Overview...