Overview of the Stratosphere's Composition, Structure, and Dynamics
Refer to Figure 6.01, top panel, red curve.
1a. How does air density vary with altitude? What is air density at the top of the stratosphere?
Refer to Figure 6.01, top panel, black curve.
1b. How does temperature vary with altitude up to 60 km?
1c. What it the tropopause?
2a. What is potential temperature?
2b. What is the usual temperature structure of the troposphere and what does such a situation result in? What is convection?
2c. What is the usual temperature structure of the stratosphere? Does convection ever occur in the stratosphere?
2d. What is static stability? How does it relate to the stratosphere?
2e. How do changes in potential temperature with height determine the static stability of air?
Refer to Figure 6.01, bottom panel.
2f. What can you conclude about the static stability of the stratosphere?
2g. What are isentropic surfaces? How does stratospheric air move in relation to such surfaces?
2h. Why is potential temperature used as a vertical coordinate?
3a. What is the heterosphere and why does it exist?
3b. What exists below the heterosphere?
Refer to Figure 6.02.
4a. How does the position of the tropopause vary with latitude?
4b. What are the four regions into which the stratosphere is divided? What characterizes each region?
The Brewer-Dobson Circulation
1a. What is the Brewer-Dobson Circulation? What does Figure 6.03 reveal about this circulation pattern?
1b. What happens to water vapor as it is lifted by the Brewer-Dobson Circulation into the tropical stratosphere?
1c. Why is ozone concentrated in the polar regions even though its photochemical source region is in the tropical lower and middle stratosphere?
2a How fast (or slow) is lifting by the Brewer-Dobson Circulation in the tropics?
2b. How is the air lifted out of the tropical troposphere into the lower stratosphere characterized?
2c. What happens to most of the air entering the lower stratosphere?
2d. Why is ozone created in the tropical stratosphere?
2e. Where is the ozone layer?
2f. Why do CFCs have such a long lifetime?
3a. Describe the Brewer-Dobson Circulation poleward of 30N and 30S.
3b. What is another reason that ozone accumulates in the lower stratosphere of the extratropics?
4a. What is a Rossby wave?
4b. What is a standing planetary wave? How do such waves move?
4c. What happens when these standing planetary waves reach the stratosphere?
4d. What happens to the polar night jet described in Section 2.4.2c when these waves deposit their easterly momentum?
4e.What phenomenon arises as a result of wave breaking in the stratosphere? How is such a situation imbalanced?
4f. What happens to the air in the polar stratosphere immediately after a stratospheric sudden warming episode?
4g. What sort of vertical and meridional motions result? What is this circulation cell called?
4h. In what way does the existence of the Brewer-Dobson Circulation modify the radiative equilibrium temperature of the winter polar stratosphere?
5a. Why is there less wintertime wave activity in the southern hemisphere than in the northern hemisphere?
5b. What happens inside the Antarctic polar vortex?
6a. What is the QBO?
6b. List the three QBO-related features.
6c. What is the thermal wind relationship?
6d. Explain the relationship between positive and negative horizontal temperature gradients and vertical wind shear (i.e., the way that the zonal wind changes with height).
6e. What kind of temperature anomaly is associated with a transition zone between a westerly and easterly wind? How about between an easterly and westerly wind?
6f. Explain the top and bottom panels of Figure 6.09.
6g. How does the QBO modify the Brewer-Dobson Circulation in the tropics?
6h. What sort of ozone anomaly results in the tropics and in the subtropics during a descending westerly phase of the QBO?
6i. What sort of ozone anomaly results in the tropics and in the subtropics during a descending easterly phase of the QBO?
Refer to Figure 6.12.
7a. What is the age of air for the stratosphere?
7b. Why is the age of air in the troposphere relatively uniform?
7c. What happens to most of the air that enters the stratosphere through the tropical tropopause via Brewer-Dobson lifting motion?
7d. What happens to the air that makes it to the top of the mesosphere? (Refer also back to Section 3.8.2.)
7e. How long does it take air parcels to be transported from the ground to the lower mesosphere?
7f. What characterizes younger air? What characterizes older air?
7g. How long does it take to cycle most of the air in the troposphere (as opposed to an individual parcel) through the upper stratosphere and lower mesosphere?
Atmospheric Waves and Transport of Tracers
1. Figure 6.13 shows the evolution of a large (synoptic) scale weather system in the middle (extratropical) latitudes of the northern hemisphere over an 8-day period. It shows this through the effect the system has on the total ozone field through mixing processes. What can we conclude from the eight panels about the effect of such a weather system on the north-south (meridional) distribution of ozone?
2a. What is a wave-1 planetary wave? Refer to the top panel of Figure 6.14.
2b. What is a wave-2 planetary wave? Refer to the bottom panel of Figure 6.14.
2c. What are synoptic scale waves? (Large-scale weather disturbances, as we saw in Figure 6.13, are referred to as synoptic scale waves.)
2d. What are the results of stronger wintertime wave activity in the northern hemisphere for ozone distribution in the Arctic lower stratosphere?
2e. What are the two main causes of wave dissipation in the stratosphere? Give a brief description of these two causes.
2f. What is the well-mixed region where wave breaking occurs called? Where does it occur?
Refer to Figure 6.18.
3a. What happens in the 2-day period shown to ozone concentrations over the Arctic and to the position of the polar vortex?
3b. What is the net effect of the mixing caused by wave activity?
Stratosphere-Troposphere Exchange
1. What is Stratosphere-Troposphere Exchange (STE)?
2. In what two ways do blocking anticyclones (highs) lower the total ozone over a region?
3. How do blocking highs affect STE?
4. What are the air characteristics of air inside a cutoff low pressure?
4. How do cutoff lows affect STE?
ANSWERS
Overview of the Stratosphere's Composition, Structure, and Dynamics
1a. Air density falls off very quickly with altitude. Air density at the top of the stratosphere is nearly zero.
1b. The temperature of the atmosphere at first decreases with altitude in the troposphere (surface and 11 km); it first holds steady and then slowly increases in the stratosphere (11 km to 47 km); finally, it drops in the mesosphere up to 60km.
1c. The tropopause is the region where the temperature structure of the atmosphere changes. It marks the boundary between the troposphere, wherein temperatures fall with altitude, and the stratosphere, wherein temperatures rise with altitude.
2a. Potential temperature is defined as the temperature an air parcel would have if it were compressed adiabatically (i.e. without any heat being added or taken away, such as would happen if water vapor condensed out of the parcel) from its existing pressure to a reference pressure of 1000 millibars.
2b. The temperature structure of the troposphere is typically one of colder air above warmer air. Air heated at the surface will rise into the colder air. Convection refers to the overturning and mixing of warmer and colder air layers.
2c. The temperature structure of the stratosphere is one of warmer air over colder air. No, convection never occurs in the stratosphere because of this temperature structure.
2d. Static stability is a measure of the buoyancy of the air and it is based on the potential temperature of the air. Because warm air rises and cold air sinks, any parcel of air in the stratosphere that is forced upwards would be colder than its surroundings (since temperatures rise in the stratosphere with altitude). The air parcel would quickly sink back down, though it would overshoot slightly because of its momentum. The parcel would then be warmer than its surroundings, and it would rise, once again bypassing its equilibrium level. The parcel would oscillate about this level at a certain period. Static stability is a measurement of how quickly the air parcel would oscillate, which is another way of saying it is a measure of the buoyancy of the air. In the stratosphere, because of the temperature structure of warmer air overlaying colder air, the air is said to be stably stratified.
2e. If potential temperature rises with height, the air is said to be stably stratified. If potential temperature falls with height, the air is said to be unstably stratified. If potential temperature does not change with height, the air is said to be neutrally stratified.
2f. Potential temperature always rises with height in the stratosphere, so the stratosphere is always stably stratified.
2g. If we choose a particular potential temperature, all the air with this potential temperature will form a surface called an isentropic surface. Stratospheric air tends to move along constant isentropic surfaces for many days because vertical motions are so small in the stratosphere.
2h. Potential temperature is used as a vertical coordinate because motions in the stratosphere tend to occur on constant isentropic surfaces, so we can substitute a vertical geometric coordinate with potential temperature.
3a. The heterosphere is a region of atmosphere above 120 km where the different gases that make up the air begin to separate (stratify) according to their molecular weight. Lighter gases accelerate more than heavier gases. The heterosphere exists because at 120 km, the air is so thin that individual molecules are able to accelerate to very high speeds before bumping into another molecule.
3b. Below the heterosphere, the atmosphere has a well-mixed composition because of winds, convection, and circulation patterns. Stratification by weight does not occur. This layer from the surface to 120 km is referred to as the homosphere.
4a. In the tropics, the tropopause is located at an altitude of about 16 km or 50,000 feet. In the polar regions, the tropopause is as low as 8 km or 30,000 feet.
4b. Tropics: Stretching from about 20S to 20N latitude. Ozone is photochemically produced in this region by the high energy UV radiation from the Sun that reaches this region. Ozone is transported out of this region by a broad Equator-to-pole circulation pattern.
Middle latitudes: Also called the "surf zone" because this region is characterized by a turbulent looking mixture of different air masses, each of which contains different amounts of ozone, thus it resembles the surf zone on a beach, which is characterized by turbulent mixing of ocean water and sand. Tropical air and polar air are mixed together in weather systems in the stratospheric surf zone.
Polar vortex: An isolated region of very cold air that exists in the winter time when a strong stratospheric jet stream (called the polar night jet) develops along the boundary of sunlight and polar winter darkness. The polar night jet isolates the air over the polar region from the rest of the stratosphere. It exists only in the winter hemisphere above the polar (Arctic or Antarctic) region.
Lowermost stratosphere: Adjacent to the tropopause where we find a mixture of stratospheric and tropospheric air (identifiable by their different chemical compositions).
The Brewer-Dobson Circulation
1a. The Brewer-Dobson Circulation is an Equator-to-pole circulation pattern that features slow lifting motion in the tropics and sinking motion in the polar latitudes.
1b. Water vapor is "freeze dried" as it moves vertically through the cold equatorial tropopause. Condensation and precipitation occurs as temperatures drop below -80C, leaving the lower stratosphere relatively depleted in water vapor.
1c. Lifting by the Brewer-Dobson Circulation carries ozone out of its tropical lower and middle stratosphere photochemical source region and into the lower polar stratosphere, where it accumulates due to sinking motion.
2a. The lifting action by the Brewer-Dobson Circulation in the tropics is on the order of only 20 to 30 meters per day.
2b. Air lifted out of the tropical troposphere into the lower stratosphere is very dry, with low ozone, and high CFC levels.
2c. Most of the air entering the lower stratosphere never makes it to the upper stratosphere, as air density drops by about 90% between 16 km and 32 km altitude. Thus, nearly 90% of the air mass entering the stratosphere at 16 km moves toward the middle latitudes rather than being carried upward to 32 km.
2d. Ozone is created in the tropical stratosphere because it is here that the Sun is positioned high overhead during the day all year long (i.e. sunlight is most intense), with enough of the necessary UV light to photodissociate O2 molecules and form ozone.
2e. Ozone is slowly created in the lower and middle tropical stratosphere, but the slow lifting action allows ozone to build up to a maximum density around 27 kilometers. It is this that is referred to as the ozone layer.
2f. The UV light required to break down CFCs is very intense. Such UV light is only encountered in the upper stratosphere. However, few CFC molecules actually make it to such high altitudes because air density is so thin: most CFC molecules are just recycled back into the troposphere outside the tropics due to sinking motion associated with the Brewer-Dobson Circulation. CFC lifetimes are very long because of the slow circulation speed and the very low air density at the altitudes where there is the necessary highly energetic UV light required to break them down.
3a. Poleward of 30N and 30S, the circulation becomes downward as well as poleward. This tends to increase ozone concentrations in the lower stratosphere of the middle and high (i.e., extratropical) latitudes.
3b. Ozone also accumulates in the lower stratosphere of the extratropics because the lifetime of an ozone molecule increases. This is because there are fewer free O atoms, in turn because there is little UV photolysis of oxygen molecules. So ozone is not easily destroyed in the lower stratosphere.
4a. A Rossby wave is a large-scale wave system in the atmosphere that arises due to a combination of a north-south (meridional) temperature gradient and the rotation of the Earth, which gives rise to the Coriolis deflection. The Rossby wave has a horizontal scale on the order of thousands of kilometers and avertical scale on the order of several kilometers.
4b. A standing planetary wave is a type of Rossby wave with a very long wavelength (on the order of 10,000 kilometers) generated by large-scale topographical features (like the Rocky Mountains and Himalaya-Tibet complex. Such waves are either stationary or move slowly westward (i.e. their motion is described as easterly). They also move slowly upward into the stratosphere.
4c. When these standing planetary waves reach the stratosphere, they deposit their momentum. Since their motion is easterly, the momentum they deposit is easterly.
4d. The deposition of easterly momentum into the westerly polar night jet slows up the polar night jet in a process known as wave breaking.
4e. The phenomenon of the stratospheric sudden warming occurs as warmer air from lower latitudes intrudes into the polar region. It arises due to wave breaking. Temperatures warm rapidly, producing a thermodynamically imbalanced situation.
4f. Wintertime radiational cooling quickly occurs in order to reestablish thermodynamic equilibrium.
4g. The wintertime radiational cooling in the polar region is accompanied by sinking motion (since colder air is denser and sinks). This sinking motion establishes an Equator-to-pole meridional motion in the winter hemisphere, as sinking air in the polar region is balanced by a poleward flow of air from the tropical stratosphere. This is required by mass continuity. The tropical air is itself replaced by rising air from the tropical troposphere, again due to mass continuity requirements. The resulting circulation pattern is the Brewer-Dobson Circulation.
4h. The Brewer-Dobson circulation keeps the winter time polar region much warmer than it would otherwise be. It keeps it on the order of 40K or 72F warmer than it would other wise be (-73C/-99F instead of -113C/-171F).
5a. There is less wintertime wave activity in the southern hemisphere because of the lack of large-scale topographical features (like the Rockies and Himalaya-Tibet complex). Indeed, the southern hemisphere is almost entirely water poleward of 55S to the Antarctic continent. This also means that the land-sea temperature contrasts are smaller in the southern hemisphere.
5b. Conditions inside the Antarctic polar vortex become very cold owing to the near total isolation from tropical and middle latitude influences.
6a. The QBO is an oscillation of the east-west wind in the tropical stratosphere that occurs every 22 to 34 months. The QBO arises from internal dynamics of tropical waves instead of the seasonal cycle. When the winds are easterly, it is referred to as an easterly wind regime, when winds are westerly, it is referred to as a westerly wind regime.
6b. The three QBO-related features are the equatorial zonal wind QBO, the temperature QBO, and the Brewer-Dobson Circulation QBO.
6c. The thermal wind relationship is a physical relationship between the zonal wind and the vertical temperature gradient.
6d. A horizontal temperature gradient that increases from pole to equator is said to be positive, while a horizontal temperature gradient that decreases from pole to Equator is said to be negative. The greater the gradient, the greater the vertical shear of the zonal (geostrophic) wind. A positive horizontal temperature gradient is associated with increasing westerly winds with height (referred to as positive wind shear), while a negative horizontal temperature gradient is associated with decreasing westerly or increasing easterly winds with height (referred to as negative wind shear).
6e. The transition zone between a westerly and easterly wind is associated with a warm temperature anomaly. The transition zone between an easterly and westerly wind is associated with a cold temperature anomaly.
6f. The top panel of Figure 6.09 shows a descending westerly QBO phase in which a warm temperature area is associated with positive wind shear. The bottom panel of Figure 6.09 shows a descending easterly QBO phase in which a cold temperature area is associated with negative wind shear.
6g. The QBO descending easterly phase maintains colder temperatures between the overlying easterlies and underlying westerlies, resulting in less infrared cooling to space than normal, which in turn results in greater total heating in the tropics (since solar UV heating is about constant). This heating will speed up the Brewer-Dobson Circulation. Conversely, the QBO descending westerly phase maintains warmer temperatures between the overlying westerlies and underlying easterlies, resulting in more infrared cooling to space than normal, which in turn results in lesser total heating in the tropics (again, since solar UV heating is about constant). This cooling will slow up the Brewer-Dobson Circulation.
These upward and downward motions associated with the QBO in the tropics are balanced by motions in the opposite sense in the subtropics, resulting in a QBO-induced meridional circulation. Such a circulation modifies and exists on top of the Brewer-Dobson Circulation in the region between 15S and 15N.
6h. The descending westerly phase of the QBO is associated with a vertical circulation pattern that produces downward motion in the tropics and upward motions in the subtropics, which acts against the Brewer-Dobson Circulation, slowing and weakening it. Because of this weakened circulation, and because the vertical gradient of ozone mixing ratio is positive in the lower stratosphere, ozone production can proceed for a longer time. This results in a positive ozone anomaly in the tropics and a negative ozone anomaly in the subtropics.
6i. The opposite situation occurs from that in 7a: the descending easterly phase of the QBO is associated with a vertical circulation pattern that produces upward motion in the tropics and downward motion in the subtropics, which acts to enhance the Brewer-Dobson Circulation. Because of this strengthened circulation, and because the vertical gradient of ozone mixing ratio is positive in the lower stratosphere, ozone production proceeds for less time. This results in a reversal of the column anomalies: a negative anomaly occurs in the tropics and a positive anomaly occurs in the subtropics.
7a. The age of air for the stratosphere refers to the average amount of time it takes for air parcels to be transported from the ground to a specific latitude and altitude region in the stratosphere.
7b. The age of air in the troposphere is relatively uniform because air parcels here are quickly mixed in convection and weather systems.
7c. Nearly 90% of the air entering the stratosphere through the tropical tropopause via Brewer-Dobson lifting motion never makes it to the top of the tropical stratosphere. This air instead finds its way into the extratropical lower and middle stratosphere, where it enters the descending branch of the Brewer-Dobson Circulation.
7d. Air that makes it to the top of the mesosphere undergoes vigorous overturning. In the mesosphere, during the solstices, a single pole-to-pole circulation exists with rising motion at the summer pole and sinking air at the winter pole.
7e. It takes air parcels 4.5 to 5 years to be transported from the ground to the lower mesosphere.
7f. Younger air, such as is found in the tropical lower stratosphere, contains high CFC amounts and low concentrations of chlorine radicals. Older air, on the other hand, contains low CFC amounts and high concentrations of chlorine radicals.
7g. Only a small fraction of all tropospheric air ever makes it to the upper stratosphere/lower mesosphere. The time needed to cycle most tropospheric air that high is many decades.
Atmospheric Waves and Transport of Tracers
1. We can conclude from the eight panels of Figure 6.13 that such a northern hemispheric, extratropical weather system pulls high-ozone polar air southward and westward, while pulling low-ozone tropical air northward and eastward. In this way, the system mixes air with different ozone (and other trace gas constituents) from different latitudes.
2a. A planetary wave is a Rossby wave with a horizontal dimension (wavelength) in excess of 10,000 kilometers. A wave-1 planetary wave refers to a planetary wave with a single ridge and a single trough straddling the entire planet at that latitude. In the case of Figure 6.14, top panel, the wave-1 planetary wave is centered at about 60N and has a wavelength of about 20,000 km.
2b. A wave-2 planetary wave refers to a planetary wave with two ridges and two troughs (high-low, high-low) straddling the planet at that latitude. In the case of Figure 6.14, bottom panel, the wave-2 planetary wave is centered at about 60N and has a wavelength of about 10,000 km.
2c. Synoptic scale waves are Rossby waves with a wavelength of 1000 to 4000 km.
2d. Stronger winter time wave activity in the northern hemisphere produces a stronger Brewer-Dobson Circulation, which in turn results in a weaker polar vortex, warmer polar stratospheric temperatures (due to the stronger Brewer-Dobson Circulation), and an earlier breakup of the polar vortex. The stronger Brewer-Dobson Circulation results in more ozone accumulated in the Arctic lower stratosphere than in the Antarctic lower stratosphere.
2e. Thermal dissipation refers to the process by which radiative cooling in warmer regions occurs faster than the radiative cooling in colder regions, resulting in a more uniform temperature field. Lessening the large-scale temperature difference associated with Rossby wave formation results in Rossby wave dissipation.
Wave breaking refers to a rapid north-south mixing process of air parcels that occurs when a vertically propagating wave reaches the stratosphere, increasing in strength as it rises into less dense air until the wave breaks. Breaking is characterized by rapid and irreversible meridional mixing in which various constituents (such as ozone) become thoroughly mixed so that source region characteristics (i.e., tropical air or polar air) are lost. Wave breaking occurs most often in the winter middle latitudes.
2f. The well-mixed region of the middle latitude stratosphere where wave breaking occurs is called the surf zone, since the characteristics of the air parcels are thoroughly mixed much like the surf zone on a beach where ocean waves break and mix up the water and sand. The stratospheric surf zone occurs on the equatorward side of the polar night jet in the winter hemisphere.
3a. Figure 6.18 shows how large changes in ozone concentration occurs over a 2-day period over the Arctic due to the influence of large-scale wave activity. This wave activity distorts the polar vortex and allows repeated intrusions of low-ozone middle latitude air to occur. Several tongues of high-ozone air that represents filaments of the polar vortex moves over eastern Scandinavia, central Asia, and the Central Pacific as the vortex itself is pulled and stretched in response to atmospheric wave activity.
3b. The net effect of the irreversible mixing caused by wave activity is to weaken the latitudinal (meridional) gradient of ozone between the polar, middle latitude, and tropical regions that is created in the mean by the Brewer-Dobson Circulation.
Stratosphere-Troposphere Exchange
1. STE refers to the way in which material in air parcels actually cross between the troposphere and stratosphere through the tropopause.
2. Blocking highs lower total ozone over a region in two ways. First, the anticyclonic flow around the high brings lower latitude air poleward. Such air has lower amounts of ozone than polar air. Second, the warmer temperatures associated with a blocking high causes isentropic surfaces to bend upward, which bends the tropopause upward, extending the overall depth of the troposphere. Ozone amounts in the troposphere are lower than those in the stratosphere, so overall ozone amounts in the blocking high are lower.
3. By pushing tropospheric air poleward and bending the tropopause upward, blocking highs increase the time it takes STE processes to occur.
4. Air inside a cutoff low contains characteristics of its polar source region, which includes cold temperatures, high potential vorticity, and high ozone amounts.
5. Cutoff lows are capable of large scale convection. Within convective updrafts, tropospheric air is transported across the tropopause into the stratosphere, speeding up STE processes. Sometimes this large scale convection can erode the tropopause and create a well mixed region of tropospheric and stratospheric air, while the tropopause reestablishes itself higher up.
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