Radiative Processes in the Climate System - Climate Dynamics - page 75

absorption is not highly dependent on wavelength and occurs in addition to
line absorption.
Carbon dioxide is also a key greenhouse gas, responsible for about 25%
of the global atmospheric absorption of terrestrial radiation. CO 2 absorption
between 13.5 and 16.5 mm is particularly relevant, since these wavelengths
are near the peak in the terrestrial emission spectrum (see Figs.4.3b and 4.5) .
The pronounced divot near 9.6 mm in the spectrum in Figure 4.5 i s due to O 3
absorption. Also apparent in the earth's emission spectrum are absorption lines
of methane and nitrous oxide.
The atmosphere is relatively transparent to radiation at certain wavelengths,
in regions of the spectrum called atmospheric windows . A prominent longwave
window is found between about 8 mm and 13 mm, bordered by H 2 O and CO 2
absorption bands and interrupted by the strong O 3 absorption line at 9.6 mm.
Most of the terrestrial radiation that emerges from the top of the atmosphere
has wavelengths in this range. Instruments mounted on satellites take advan-
tage of these windows to observe the earth system. The longwave IR window
channel, for example, is at 10.7 mm. Energy emitted from the surface at this
wavelength passes through the atmosphere with minimal absorption by mol-
ecules in the atmosphere, so the observed emission temperature represents the
actual surface temperature.
Emission spectra can be used to infer vertical distributions of atmospheric
constituents when the temperature profile is known, or to infer the temperature
profile when the vertical distribution of a constituent is known. For example,
Figure 4.6 shows a terrestrial spectrum observed over the tropical Pacific by an
infrared detector on the Nimbus 4 satellite. Here, both wavelength and wave-
number are indicated on the x - axis. (Wavenumber, k , is 2p/l.) The dashed lines
are Planck curves for various temperatures. By matching a segment of the ob-
served spectrum with a Planck curve, one can assign an emission temperature
to that part of the spectrum. For example, the energy emitted in the wavelength
range 11-13 mm is being emitted at about 295 K. The fact that this is the at-
mospheric longwave window, and the temperature is high, suggests that this
emission originates from the surface.
Examine the longwave spectrum in the vicinity of the 15 mm CO 2 absorp-
tion band. This section of the spectrum fits the Planck curve for a temperature
of about 215 K. According to Figure 2.8, a temperature of 215 K is character-
istic of an elevation of about 10 km, so the CO 2 emissions detected by the sat-
ellite originate in the upper tropopause. Emission by H 2 O occurs at about 270
K, in the lower troposphere, since the sections of the spectrum at which H 2 O
is radiatively active (e.g., near 20 mm and 7.5 mm) approximate the blackbody
curve for that temperature. This is consistent with Figure 2.30 , which shows
that water vapor in the atmosphere is primarily located in the lower tropo-
sphere. In contrast, CO 2 is well mixed in the atmosphere.
Another way to represent the influence of various constituents on the flow
of longwave and shortwave radiation through the atmosphere is by construct-
ing absorption spectra . In contrast to the emission spectra shown in Figures
4.3- 4.6, an absorption spectrum displays the fraction of the radiation at a
given wavelength that is absorbed as a function of wavelength—that is, the
absorptivity—as that radiation passes through the atmosphere. Figure 4.7a