ATOC 5750: Homework IIroma6744/HW/5750_ass2.pdfRobert Marshall ATOC 5750: Homework II Chapter 3...

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Robert Marshall ATOC 5750: Homework II Chapter 3 Questions 1. For subtropical deserts, summer precipitation is clearly on the equatorial side and winter precipitation is on the poleward side. 2. “Cold deserts include the central Asian deserts, the Great Basin Desert of North America, and the Patagonian Desert of South America” (Warner, 69). Cold deserts are called cold because they have much colder winter temperatures than hot deserts do. 3. Foggy deserts include coastal portions of the Atacama, Peruvian, and Namibian Deserts, western coastal parts of the Sahara, and Baja California. The Peru Current in the South Pacific is near the Atacama and Peruvian Deserts. The California Current is near Baja California. The Benguela Current in the South Atlantic is near the Namibian Desert, and the Canary Current is near the west coast of the Sahara. 4. The South American cold desert experiences less severe cold-weather outbreaks than its North American counterpart because the zonal width of South America at higher latitudes is much less than that of North America, so it is more difficult to escape the moderating effect the ocean has on temperatures. Further, the polar vortex in the Southern Hemisphere is considerably stronger than the polar vortex in the Northern Hemisphere, meaning intrusions of cold air bottled up in polar regions into midlatitudes occur less readily in the Southern Hemisphere. 5. Central parts of Asia and, to a lesser extent, North America experience aridity that is partly attributable to continentality. In central parts of the United States, however, moisture is also available from the Gulf of Mexico, requiring a shorter trajectory than from the mean zonal west- to-east flow bringing moisture from the Pacific onshore. 6. “Among the deserts of the world, the following are wholly or partly caused by different orographic effects: the Eurasian Deserts from Russia to Mongolia, the Monte Desert, the Patagonian Desert, all the deserts of North America, and the semi-arid Great Plains of North America” (Warner 34). 7. In the summer hemisphere, subtropical anticyclones migrate poleward, and in the winter hemisphere, subtropical anticyclones migrate equatorward. These seasonal shifts of subtropical highs dictate precipitation patterns in subtropical deserts, bringing considerably more precipitation to the equatorward portions of subtropical deserts in the summer months, and considerably more precipitation to the poleward sides in winter months. For example, consider figure 3.13 on page 84. In the Northern Hemisphere summer subtropical highs shift northward, while Southern Hemisphere subtropical highs also shift northward (since it is winter in the SH and equatorward is north in the SH). These shifts in turn displace the ITCZ northward and bring summer rains to the African Sahel and southern Sahara. As the figure shows, we indeed see significantly more precipitation in the summer in these areas. In the NH winter, subtropical anticyclones move southward, bringing the midlatitude storm track to lower latitudes as well. These storms will on occasion penetrate far enough south to bring precipitation to the northern

Transcript of ATOC 5750: Homework IIroma6744/HW/5750_ass2.pdfRobert Marshall ATOC 5750: Homework II Chapter 3...

Page 1: ATOC 5750: Homework IIroma6744/HW/5750_ass2.pdfRobert Marshall ATOC 5750: Homework II Chapter 3 Questions 1. For subtropical deserts, summer precipitation is clearly on the equatorial

Robert Marshall

ATOC 5750: Homework II

Chapter 3 Questions

1. For subtropical deserts, summer precipitation is clearly on the equatorial side and winter

precipitation is on the poleward side.

2. “Cold deserts include the central Asian deserts, the Great Basin Desert of North America, and

the Patagonian Desert of South America” (Warner, 69). Cold deserts are called cold because

they have much colder winter temperatures than hot deserts do.

3. Foggy deserts include coastal portions of the Atacama, Peruvian, and Namibian Deserts, western

coastal parts of the Sahara, and Baja California. The Peru Current in the South Pacific is near the

Atacama and Peruvian Deserts. The California Current is near Baja California. The Benguela

Current in the South Atlantic is near the Namibian Desert, and the Canary Current is near the

west coast of the Sahara.

4. The South American cold desert experiences less severe cold-weather outbreaks than its North

American counterpart because the zonal width of South America at higher latitudes is much less

than that of North America, so it is more difficult to escape the moderating effect the ocean has

on temperatures. Further, the polar vortex in the Southern Hemisphere is considerably stronger

than the polar vortex in the Northern Hemisphere, meaning intrusions of cold air bottled up in

polar regions into midlatitudes occur less readily in the Southern Hemisphere.

5. Central parts of Asia and, to a lesser extent, North America experience aridity that is partly

attributable to continentality. In central parts of the United States, however, moisture is also

available from the Gulf of Mexico, requiring a shorter trajectory than from the mean zonal west-

to-east flow bringing moisture from the Pacific onshore.

6. “Among the deserts of the world, the following are wholly or partly caused by different

orographic effects: the Eurasian Deserts from Russia to Mongolia, the Monte Desert, the

Patagonian Desert, all the deserts of North America, and the semi-arid Great Plains of North

America” (Warner 34).

7. In the summer hemisphere, subtropical anticyclones migrate poleward, and in the winter

hemisphere, subtropical anticyclones migrate equatorward. These seasonal shifts of subtropical

highs dictate precipitation patterns in subtropical deserts, bringing considerably more

precipitation to the equatorward portions of subtropical deserts in the summer months, and

considerably more precipitation to the poleward sides in winter months. For example, consider

figure 3.13 on page 84. In the Northern Hemisphere summer subtropical highs shift northward,

while Southern Hemisphere subtropical highs also shift northward (since it is winter in the SH

and equatorward is north in the SH). These shifts in turn displace the ITCZ northward and bring

summer rains to the African Sahel and southern Sahara. As the figure shows, we indeed see

significantly more precipitation in the summer in these areas. In the NH winter, subtropical

anticyclones move southward, bringing the midlatitude storm track to lower latitudes as well.

These storms will on occasion penetrate far enough south to bring precipitation to the northern

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reaches of the Sahara. We see in figure 3.13 that indeed northern parts of the Sahara receive

more precipitation during winter months than during summer months.

8. Substrates found in deserts include sand (sometimes in the form of dunes), clay, stones and

pebbles, salt flats, as well as solid rock surfaces.

Chapter 3 Problems:

1. Arid maritime areas are adjacent to the west coasts of deserts because cold upwelling current

run along the west coasts of the continents. This means the surface water is cold in these

regions, creating high static stability, suppressing convection and precipitation. Further, these

arid maritime areas, like the coastal deserts to their east, are in the subtropics where the large-

scale, planetary circulation generates subsidence, further enhancing stability and further

suppressing precipitation.

2. Southern Sahara in July-September; Mexico and SW US in July-September; South Central Asia

from June-September; Northern Australia from December-February.

Chapter 4 Questions:

1. Net radiation is the net flux of radiation energy (energy per time per area) received by the

surface. Radiation inputs to the surface include shortwave solar radiation as well as

downwelling longwave radiation from the atmosphere and clouds. Radiation losses from the

surface include upwelling longwave radiation and reflected shortwave radiation. Comparing the

radiation fluxes absorbed by the surface (SW↓ + LW↓) to those lost from the surface (SW↑ +

LW↑) tells us whether net radiation is positive or negative—whether the surface receives more

radiation than it loses or puts out more radiation than it receives. Net radiation tends to be

negative during night and at high latitudes and positive during day and at lower latitudes.

2. In an oasis, we have ample surface moisture beneath dry, desert air. This sets up a strong

moisture gradient near the surface in turn leading to rapid evaporation of water from the

surface. This evaporation requires latent heat, which it takes from the surface, keeping the

surface relatively cool. Since the surface loses energy to facilitate evaporation, we have a strong

upward latent heat flux associated with the rapid evaporation. The cool surface underlying hot

desert air results in a downward sensible heat flux. During daytime, the net radiation flux will

be positive into the surface. Assuming the ground heat flux is small, since both net radiation

and the sensible heat flux are putting energy into the surface, the upward latent heat flux must

essentially be equal and opposite to the combined effect of net radiation and sensible heating,

implying that the latent heat flux can be greater in magnitude than net radiation in oases.

Downwind of an oasis, on the other hand, the desert surface is quite dry. The dryness of the

surface begets a small latent heat flux. During the day, the surface will heat more readily than

the overlying air, resulting in an upward sensible heat flux rather than the downward sensible

heat flux we saw in the oasis. Net radiation is again positive into the surface during the day. In

this case, assuming a small ground heat flux, the primary balance will be between net radiation

and the sensible heat flux.

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3. In arid areas, there tends to be more upwelling longwave radiation emitted from the surface

relative to non-arid areas since surface temperatures tend to be higher. There also is generally

more incoming shortwave radiation in arid regions since there are generally fewer clouds.

Reflected shortwave radiation is typically greater in arid regions because there tends to be less

vegetation and thus a higher albedo. Downwelling longwave radiation is probably less in arid

regions since there tends to be less water vapor in the overlying atmosphere as well as fewer

clouds. Diffuse solar radiation is generally smaller in arid regions since there are fewer clouds in

general. In arid areas, the magnitude of the latent heat flux will generally be smaller than in

non-arid areas because there tends to be less surface moisture in arid areas. Sensible heat flux

in arid areas is likely greater due to strong surface heating. Net radiation is generally greater in

arid regions because most arid regions are located at lower, subtropical latitudes with long days,

and there are few clouds to block incoming solar radiation. Ground heat flux can be higher or

lower in arid areas when compared to non-arid regions. Ground heat flux varies greatly with

substrate type since it is highly sensitive to the thermal diffusivity (which depends on thermal

conductivity, specific heat capacity, and density).

Chapter 4 Problems:

1.

For the sun, T=6000 K, and so:

480 nm corresponds to something between blue and green visible light, as expected.

For the Earth, T=300 K, yielding:

9.6 µm corresponds to infrared light, as expected.

2. The solar constant is 1368 Wm-2 when incident on a perpendicular surface. The whole surface of

the earth will not be perpendicular to the incoming solar radiation at once, and in fact only half

the earth will be receiving any sunlight at any given instance. The perpendicular area facing the

solar radiation is πre2 where re is the radius of the Earth. Then, the total radiant power incident

on the top of Earth’s atmosphere is given by (1368 Wm-2)* πre2. Now, we must divide this

power by the total surface area of the Earth (4πre2) to obtain the global average heat flux at the

top of the atmosphere:

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, as required.

3. The assumption of LE being very small relative to the other terms is not generally reasonable;

this assumption is only reasonable for arid regions. This would certainly not be a reasonable

assumption over oceans or over other moist and/or vegetated surfaces (or an oasis as discussed

earlier). The assumption of the annual-average G being near zero however is generally

reasonable. A non-zero annual-average G would imply global warming or cooling depending on

the sign, and any warming that is occurring at present is not so strong as to imply more than a

small annual-average ground heat flux.

4. The desert is so much warmer than the polar regions because the lag time for heating the land is

very substantial. Subtropical deserts receive ample solar radiation at all times of year, so the

land stays warm throughout the year. In polar regions during winter, days are short (or non-

existent at the right times and places), and strong radiational cooling of the land occurs. The

land cools through a deep layer since there is so little incoming radiation for such a large

fraction of the year. In the summer, when polar regions receive a lot of solar radiation, much of

the energy will indeed be balanced by the ground heat flux heating the land. But since the land

in polar regions starts off much, much colder than in the subtropics, it simply does not have the

time to absorb enough energy to reach the temperatures seen in subtropical deserts before

autumn returns, solar radiation decreases dramatically, and the land begins to cool off once

again.

5. The intensity on a horizontal surface would be S times the cosine of the zenith angle. If the sun

is directly overhead (zenith angle of zero), then the cosine of the zenith angle is one and the

intensity is simply S. For a zenith angle of 60 degrees, the cosine is ½, and so the intensity is ½S.

On the southern slopes of the sand dune, we essentially angle the horizon downward 20

degrees, increasing the angle of the sun above the new horizon to 50 degrees and decreasing

the zenith angle to 40 degrees. The intensity on the south slope is then S*cos(40°) or about

0.766*S. On the north slope, we in essence angle the horizon up 20 degrees, decreasing the

angle of the sun above the horizon to 10 degrees and increasing the zenith angle to 80 degrees.

The intensity on the north slope is then S*cos(80°) or about 0.174*S.

6. Summer temperatures for mid-latitude cold deserts are comparable to those of subtropical

deserts because mid-latitudes and subtropics receive about the same amount of solar radiation

in summer, as seen in figure 4.8. In winter, on the other hand, mid-latitude cold deserts receive

far less solar radiation than subtropical hot deserts. From figure 4.8 we see that in mid-winter, a

location at 20 degrees latitude receives well over twice the solar radiation a location at 45

degrees latitude receives.