atoc 5750: homework iiroma6744/hw/5750_ass2.pdfrobert marshall atoc 5750: homework ii chapter 3...
TRANSCRIPT
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
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.
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:
, 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.