Chemical Composition of the Atmosphere
1. Explain why water is an important component in the study of Earth's atmosphere.
2. What are atmospheric trace species? Name several.
- 3a. List several greenhouse gases.
3b. Explain how greenhouse gases affect Earth's radiation budget.
3c. What is the current trend in greenhouse gas concentration in the atmosphere? Give an example of a way scientists determine ancient concentrations of gases such as methane.
4. What additional atmospheric components (beyond major and trace species) are important in characterizing Earth's atmosphere? Why are they important?
- 5a. When do scientists believe that ozone first become a constituent of Earth's atmosphere? What event is this associated with? Refer to Figure 2.02.
5b. How is the development of higher life forms linked to the development of an oxygen-containing atmosphere? Refer again to Figure 2.02.
Vertical Structure of the Atmosphere
- 1a. What property of the atmosphere is used to define layers? Why?
1b. What is the rate of change of temperature with height called (see Section 3.3.1)?
Using Figure 2.04, explain what happens to the temperature as you ascend through the following layers of the atmosphere.
1c-i. troposphere
1c-ii. stratosphere
1c-iii. mesosphere
1c-iv. thermosphere
What is the name and approximate height at the boundary between the
1d-i. troposphere and stratosphere (see Section 3.4)
1d-ii. stratosphere and mesosphere (see Section 3.5)
1d-iii. mesosphere and thermosphere (see Section 3.6)
2. What is responsible for weather systems in the troposphere?
- 3a. Why is vertical movement of air small in the stratosphere?
3b. What causes the temperature to increase in the stratosphere?
- 4a. What is the distinction between number density (n) and mass density (d)?
4b. How is atmospheric pressure related to density?
4c. How does density change as you ascend through the atmosphere?
Horizontal Structure of the Atmosphere
1. Why do you expect the annual temperature changes at the equator to be small compared to high latitudes? Figures 2.08a-b show such changes.
Refer to Figures 2.09 and 2.10 to answer the following questions.
2a. What is the general latitudinal trend in temperatures as you move poleward from the Equator in the troposphere? In the stratosphere?
2b. Are tropospheric changes more severe in the summer or winter hemisphere? Explain.
2c. What evidence is there of a weak wave structure in the tropospheric mean temperature field in the winter hemisphere?
2d. Do you find the same temperature variation with latitude in the stratosphere? Explain.
2e. How does the stratospheric northern hemisphere winter temperature structure compare to that of the southern hemisphere winter?
2f. What are regions of coldest temperatures in the stratosphere termed?
3a. What is the zonal wind?
3b. What latitude region experiences least change in zonal winds and why?
3c. What latitude range experiences the greatest changes in zonal winds and why?
4a. Why is there more variability in the tropospheric zonal wind fields in the winter hemisphere?
4b. How do the zonal wind fields in the troposphere compare to those in the stratosphere?
4c. Explain why the south polar vortex is more isolated than the north polar vortex.
Refer to Figure 2.14 to address the following questions.
5a. Are higher ozone levels associated with higher or lower geopotential height? Explain.
5b. What happens to the total column ozone over a geographic location as a region of low pressure moves over the region?
5c. Are these local ozone fluctuations considered long term or short term?
6a. What is the cause of stratospheric sudden warmings?
6b. How do stratospheric sudden warmings affect ozone levels?
6c. Why are warmings more prevalent in the northern hemisphere?
7a. Explain what the quasi-biennial oscillation is.
7b. What causes the regular QBO of the tropical stratosphere?
7c. Why is the QBO of interest to scientists studying ozone?
8a. What is the El Niño Southern Oscillation (ENSO)?
8b. Is there evidence that ENSO has an effect on stratospheric ozone? Explain.
Atmospheric Dynamics
1a. What is the Coriolis force?
1b. How are air parcels affected by the Coriolis force in the northern hemisphere? Southern hemisphere?
1c. Is the Coriolis force an equally important factor in describing atmospheric motion at all latitudes? Explain.
2a. What is the thermal wind?
2b. Based on thermal wind balance computations, what happens to the zonal wind in the northern hemisphere as you ascend in the atmosphere? Where would the change in zonal wind be the greatest?
2c. Where is the jet stream located?
3a. How is potential temperature defined?
3b. Why are potential temperature surfaces useful for studying atmospheric chemical constituents?
4a. What is potential vorticity?
4b. What can potential vorticity be used for?
5a. List several types of waves in the atmosphere and give brief descriptions of them.
5b. What impacts do wavemean flow interactions have on stratospheric motions?
ANSWERS
Chemical Composition of the Atmosphere
1. Water vapor is important in atmospheric science for two reasons. First, water in its liquid form is as essential to all life as air. Second, water in the lowest level of the atmosphere in various forms (vapor, liquid, solid) creates many of the observed weather features like clouds, rain snow, ice, and fog. As a consequence of its properties and its abundance in the troposphere, water vapor concentration is one of the most important considerations in understanding meteorology and climate.
2. Trace gases are atmospheric constituents that exist in small amounts, typically at the level of parts per million by volume (ppmv) or parts per billion by volume (ppbv). Trace species include water vapor, carbon dioxide, ozone, methane, various oxides of nitrogen, neon, and helium, as well as manmade chlorofluorocarbons (CFCs).
3a. Greenhouse gases include carbon dioxide, water vapor, ozone, and methane, as well as the CFCs.
3b. Greenhouse gases affect Earth's radiation budget by the way in which they absorb incoming shortwave energy from the Sun (solar radiation) and reradiate it upward and downward as longwave radiation in the form of infrared or thermal energy, which we can feel as sensible heat.
3c. Greenhouse gases are increasing in our atmosphere. Figure 2.1 shows a plot of methane measurements made in Cape Meares, Oregon between 1979 and 1984 which shows roughly a 3% increase in just 5 years. Similar measurements from all over the globe show a similar trend. Measurements of the chemical composition of bubbles in ice cores dug up in the arctic and Antarctic show that the methane remained constant at about 0.7 ppm for thousands of years and the increase has only occurred relatively recently on geological time scales. Similar trends are observed in carbon dioxide, which we know arises from burning (cooking, heating, clearing land etc.), and the chloroflurocarbons, which do not occur naturally.
4. There are atmospheric suspended particulates referred to as aerosols that exist in the form of solids or liquids. These are involved in a variety of atmospheric phenomena including clouds, fog, dust, some stratospheric aerosols (including the Junge layer which is composed of sulfuric acid droplets arising from volcanic emissions into the stratosphere), and polar stratospheric clouds (PSC's) composed of water and hydrates of nitric acid. Aerosols may have significant atmospheric consequences due to their influence on the radiation balance of the Earth and via heterogeneous chemical reactions which have been shown to be responsible in part for the polar ozone hole phenomena which have received much attention in recent years.
Ions are compounds that are electrically imbalanced due to the gain or loss of electrons. They are present at all altitudes in the atmosphere. In the troposphere ions are produced by lightning, cosmic rays, and the decay of radioactive elements in Earth's crust. In the stratosphere and mesosphere charged particles (protons) from the Sun can penetrate and cause some ionization.
5a. Ozone first became a constituent of Earth's atmosphere about 2.5 billion years ago with the appearance of free oxygen in the atmosphere, an even intimately associated with the appearance of algal and cyanobacterial life forms, which produced oxygen as a byproduct.
5b. The development of higher life forms is linked to the development of an oxygen-containing atmosphere, since such an atmosphere was necessary to permit formation of a protective ozone shield in the stratosphere. By blocking out biologically harmful UV radiation, ozone permitted the development of higher life forms, including land animals.
Vertical Structure of the Atmosphere
1a. The property of temperature change with height is used to define different layers in the atmosphere. The different ways the temperature changes with height in the atmosphere are associated with the chemistry, composition, and density of the atmosphere at different levels.
1b. The rate of change of temperature with height in the atmosphere is called the lapse rate.
1c.i. Troposphere - temperatures decrease with altitude
1c.ii. Stratosphere - temperatures increase with altitude
1c.iii. Mesosphere - temperatures decrease with altitude
1c.iv. Thermosphere - temperatures increase with altitude
1d.i. Tropopause - 12 km
1d.ii. Stratopause - 50 km
1d.iii. Mesopause - 85 km
2. In the troposphere, in general, warmer air at the surface lies beneath colder air higher up. Such a temperature structure is unstable, since warm air tends to rise. When large regions of warm air rise and mix into colder regions, we have the phenomenon of convective overturning. Much like the bubbles in a pot of boiling water, convection consists of rising warm air and sinking cold air. This is ultimately responsible for all observed weather systems we see in the troposphere.
3a. In the stratosphere, the lowest temperatures are found at the bottom and the highest at the top. Because warm air rises, vertical motions are naturally suppressed in the stratosphere, leading to vertical stratification of the air masses it contains.
3b. Although only a minor constituent even in the stratosphere, ozone strongly absorbs solar ultraviolet radiation and reemits it as thermal longwave radiation in all directions (much like Earth's surface does with visible light). This thermal longwave radiation warms the stratosphere, causing temperatures to increase. Indeed, it is this "ozone layer" that is responsible for the very existence of the "stratosphere."
4a. The number density (n) is given by n = Nv/V where Nv is the number of molecules in a given volume of space and V is the volume of that space. The mass density (d) is given by d = mv/V where mv is the mass in a given volume and V is the volume of that space. The distinction involves number of molecules versus mass in a given volume. We can translate from (d) to (n) through the relationship d = mn, where m is the mass of each molecule.
4b. The relationship between atmospheric pressure (P) and density (d) in the atmosphere is given by a form of the ideal gas law: P = dRT.
4c. At lower altitudes where pressure is higher, molecules are closer together and the density is greater. Higher up, where atmospheric pressure is lower, molecules are spread farther apart and therefore the density is lower.
Horizontal Structure in the Atmosphere
1. Annual changes in temperature at the Equator are small (as shown in Figure 2.08a) because in the tropics the amount of solar radiation received does not vary much over the course of the year. As a result, the annual (or seasonal) changes in temperature are fairly small, as demonstrated by the relatively smooth, layered appearance of the isotherms in Figure 2.08a.
At high latitudes (as shown in Figure 2.08b), the tilt of Earth's axis results in significant variations in solar insolation. As a result, temperature changes over the course of a year are quite dramatic. Figure 2.08b shows temperature changes of up to 25K occurring in the lower troposphere and upper stratosphere.
2a. In general, tropospheric temperatures decrease uniformly as you move poleward from the equator while stratospheric temperatures decrease monotomically from the summer pole to the winter pole.
2b. From Figures 2.09 and 2.10 you can see that in the troposphere temperatures decrease as you move poleward from the tropics in both hemispheres. The changes are more severe in the winter hemisphere with a noticeable drop in temperature poleward of about 30° latitude. At 30° latitude, isotherms are clustered tightly together indicating that temperature changes through this region are rather dramatic.
2c. A weak wave structure in the tropospheric mean temperature field in the winter hemisphere is evident primarily in the northern hemisphere winter (Figure 2.09). It is evident by the undulating isotherms poleward of about 30N in Figure 2.09. The structure is due to two important features in the northern hemisphere: large-scale topographic features (like the Rockies and the Himalaya-Tibet complex) and greater land-sea temperature contrasts.
2d. In the stratosphere, temperatures decrease monotomically as you move from the summer pole toward the winter pole. The region of sharpest temperature changes appears between 30° and 60° latitude in the winter hemisphere, placing it somewhat poleward of the region identified in the troposphere.
2e. The structure of temperatures is more zonal during the southern hemisphere winter (July) than is the case in the northern hemisphere winter (January). This is also a consequence of the two important northern hemisphere features mentioned above in 2c; specifically, large-scale topographic features and greater land-sea temperature contrasts. In both cases, the coldest temperatures are found over the pole of the winter hemisphere (where it is dark for 6 months).
2f. The coldest stratospheric temperatures are found over the pole of the winter hemisphere. The region of cold temperatures is known as the polar vortex, a term which describes the appearance of this air mass in the zonal wind fields.
3a. The zonal wind is the component of the wind field blowing parallel to zones of latitude, i.e., from west to east or vice versa.
3b. The zonal wind shows little variability near the equator. In the tropics, the heating provided by solar radiation is fairly uniform both temporally and geographically. As a result, the temperature field is fairly smooth and the zonal winds fairly weak.
3c. The latitude range around 60N and 60S experiences the greatest changes in zonal winds. It is here that seasonal changes in differential heating between the poles and tropics are maximized. At 60° latitude, differential heating in the winter hemisphere is much greater than in the summer hemisphere. This results in sharper temperature gradients and hence stronger winter zonal winds. Zonal winds weaken in the summer hemisphere around 60° latitude as differential heating lessens and temperature gradients are weaker. The seasonal variability in zonal winds is greatest here.
4a. In the winter hemisphere, the zonal wind fields show more variability because of enhanced wave activity, stronger thermal gradients, and the north/south motion of weather fronts.
4b. In the stratosphere (10 mb level), the zonal wind fields are much smoother in both summer hemispheres than the corresponding tropospheric (300 mb) fields, and both winter hemispheres show a much stronger change in zonal wind speeds moving poleward of the Equator than the corresponding tropospheric fields. This is shown in Figures 2.12 and 2.13.
4c. As you move poleward in the southern hemisphere winter, the gradient in the zonal wind field is very strong, much stronger than is the case in the northern hemisphere. The strength of this wind field results the southern polar vortex being even more strongly confined and isolated than is the case with the northern polar vortex.
5a. An ozone "high" is associated with a geopotential "low." A high pressure system will lead to a reduction of column ozone.
5b. A low pressure system will increase the total column ozone over a geographic location.
5c. Like the pressure systems that generate them, these local ozone fluctuations are short term.
6a. These stratospheric warmings result from the rapid movement of the polar vortex from a roughly polar symmetric circulation to a circulation that is offset from the pole.
6b. Both temperatures and ozone levels undergo extremely rapid increases as the polar vortex moves off of the pole during the breakdown of the circulation.
6c. Warmings are more prevalent in the northern hemisphere than the southern hemisphere because there is much less topography in the southern hemisphere, which results in less north-south wave activity associated with large-scale weather systems. That is, there is less meridional mixing in the form of an undulating jet stream in the southern hemisphere, and the southern polar vortex is less prone to major displacements off the South Pole.
7a. The direction of the winds in the tropical stratosphere are observed to reverse from easterly to westerly and back to easterly again approximately every 26 to 28 months. This reversal of winds in the tropics is known as the quasi-biennial oscillation or QBO.
7b. Convective activity and other forcings in the tropics generate a variety of atmospheric waves, some of which propagate vertically from the troposphere into the stratosphere (similar to a wave propagating onto a beach). As the waves dissipate in the stratosphere (similar to wave breaking on the shoreline), they deposit easterly or westerly momentum, causing an acceleration of the winds in the case of easterly momentum deposition, and causing a deceleration in the case of westerly momentum deposition. The alternating wind accelerations and decelerations are related to whether there are easterly winds or westerly winds in the lower stratosphere.
7c. The phase of the QBO appears to be related to the magnitude of ozone concentrations found within the Antarctic polar vortex.
8a. Large variations in the ocean temperature of the equatorial Pacific Ocean have become known as the El Niño-Southern Oscillation (ENSO) phenomena. Typically, the waters of the Eastern Pacific near South America are quite cool as a result of upwelling. During a warm ENSO or "El Niño" year, the warmer waters of the western Pacific migrate eastward and substantially warm the waters of the eastern Pacific.. This warm water phenomenon was originally referred to as the "El Niño" by Peruvian fisherman who observed its irregular occurrence around Christmas time, hence the name "the Child." The southern oscillation refers to the seesaw of pressure between the Pacific and the Indian Ocean described by Walker and Bliss [1932]. This pressure seesaw is directly related to the sea surface temperatures of the eastern and central Pacific, bringing about the name ENSO. The counterpart to the warm ENSO is the cold ENSO or "La Niña," when the waters of the eastern Pacific become even colder than normal.
8b. ENSO effects on ozone have been observed in the TOMS total ozone observations. These calculations used the Southern Oscillation index (SOI) to calculate the impact of the ENSO on stratospheric ozone. There is a weak anticorrelation of the SOI and total ozone in the tropics, with a positive correlation in the southern hemisphere midlatitudes.
Atmospheric Dynamics
1a. The Coriolis force is an apparent force that appears in a rotating (noninertial) frame of reference, such as Earth, which spins about an axis. The spinning creates a constant centrifugal acceleration on Earth's surface. Objects moving from one latitude to another latitude appear to curve. The Coriolis force is an "apparent" force in that it does no work but instead arises only in the context of a noninertial frame of reference.
The Coriolis force arises from the conservation of angular momentum principle. An object on the ground has a fixed amount of angular momentum, based on Earth's rotation speed at that latitude. The axis of Earth's rotation passes through the poles, so that at these points, Earth is not rotating (rotation velocity is zero). At the Equator, Earth is rotating at maximum, since all points on the surface are a maximum distance from the axis of rotation.
An object such as an airplane that starts out at a particular latitude will have the same angular momentum as Earth at that latitude. If the plane flies toward the Equator, it gets farther away from Earth's axis of rotation, meaning that the airplane's rotation around this axis is decreasing. The solid planet underneath rotates from west to east. The plane, which has a constant amount of angular momentum, will appear to drift westward as it moves toward the Equator. The plane decelerates in that it isn't "keeping up with" Earth. The opposite happens if the plane flies toward the pole: the airplane's rotation around Earth's axis increases and the plane will appear to drift eastward as it accelerates faster than the rotating planet. In the northern hemisphere the deflection in both cases is toward the right. In the southern hemisphere, the deflection in both cases is toward the left.
1b. Our airplane analogy in 1a applies equally well to air parcels. In the northern hemisphere, air parcels moving from one latitude to another are deflected toward the right, and in the southern hemisphere, air parcels moving from one latitude to another are deflected toward the left. Specifically, in the northern hemisphere an air parcel moving toward the Equator will feel a deceleration toward the right (west) while an air parcel moving toward the North Pole will feel an acceleration toward the right (east). In the southern hemisphere an air parcel moving toward the Equator will feel a deceleration toward the left (west) while an air parcel moving toward the South Pole will feel an acceleration toward the left (east).
1c. No. The Coriolis force is not important in tropical latitudes where it becomes very small and hence geostrophic balance is not a good approximation to motions in the real atmosphere.
2a. The thermal wind is a relationship that derives from the balance between the horizontal temperature gradient and the vertical gradient of the zonal wind. This means that the magnitude of the vertical gradient of the geostrophic (zonal) wind is larger and is positive, where the magnitude of the horizonal temperature gradient is larger.
2b. Because the temperature decreases from Equator to pole in the troposphere, the thermal wind relationship tells us that the zonal wind increases with height. This applies in both hemispheres, so it applies in the northern hemisphere. The change in zonal wind would be greatest where the horizonal temperature gradient is largest, which in the northern hemisphere means most negative.
2c. The jet stream is located in teh upper troposphere at those latitudes where the temperature gradient is largest.
3a. Potential temperature is the temperature an air parcel would have if expanded or compressed adiabatically (i.e., without any heat being added or taken away) from its existing pressure to a reference pressure of 1000 mb.
3b. Air parcels in which no heat is added or lost tend to move on surfaces of constant potential temperature, referred to as isentropic surfaces.
4a. Potential vorticity is a combination of relative vorticity and the gradient of potential temperature (expressed as a scalar quantity) that is conservedunder frictionless, adiabatic conditions.
4b. Potential vorticity can be used to measure the stability of air. It can also be used as a tracer of atmospheric motion, since it is approximately conserved following the motion of an air parcel. When plotted on an isentropic surface, potential vorticity can illustrate the process of planetary wave breaking.
5b. Such interactions are referred to as eddites, or departures from the basic state flow in the atmosphere. They transport energy and momentum from one latitude to another. Such interactions drive the QBO phenomenon and the SAO (semiannual oscillation) phenomenon. They also help drive the mean meridional circulation of the middle atmosphere.
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