Geoscience Reference
In-Depth Information
−=
−
+−
(
αα
σ
)
S
TT
CS
CL
0
FH H
/
4
4
.
(4.38)
_
i
4
C
CL
CS
CS
CL
Because a
CS
< a
CL
, the first term on the right-hand side of Eq. 4.38 is nega-
tive, indicating that shortwave cloud forcing is always negative, a cooling ef-
fect. The second term on the right-hand side is the longwave cloud forcing. In
the absence of clouds, most of the thermal emission from the climate system
comes from atmospheric water vapor and, as seen in
Fig. 2.30,
nearly all the
water vapor is located in the lower troposphere—below about 800 hPa, or
about 2 km. Therefore,
T
CS
>
T
CL
, and the longwave cloud forcing is positive,
a heating effect. The sign of
F
C
depends on which of these two terms is larger.
Estimates based on satellite observations indicate that, in the global average,
the magnitude of the longwave forcing is 31.1 W/m
2
, and the magnitude of the
shortwave forcing is 48.4 W/m
2
. From these measurements, then, the presence
of clouds cools the climate system
(
C
=− As discussed in
chapter
11,
this does not inform us about the role of clouds as climate changes.
Locally, clouds can have either a heating or a cooling effect. Positive values
of
F
C
are observed over the western Pacific warm pool, for example, where
deep convective clouds form. Negative values of
F
C
are observed in association
with the marine stratus clouds that form at low levels over the cold-tongue re-
gions of the Pacific and Atlantic. Cloud forcing in these regions helps maintain
the cold surface waters of the eastern ocean basins.
F
17.3
W/m
2
).
4.8 REFERENCES
Fröhlich, C., 2006: Solar irradiance variability since 1978. Revisions of the PMOD
composite during solar cycle 21. Space Science Reviews,
125
, 53-65.
Goody, R. A., and G. D. Robinson, 1951: “Radiation in the troposphere and lower
stratosphere.”
Quarterly Journal of the Royal Meteorological Society
77: 153.
Hanel, R. A., et al., 1972: “The Nimbus 4 infrared spectroscopy experiment.”
Journal of
Geophysical Research
77: 2629-2641.
Kiehl, J. T., and K. T. Trenberth, 1997: “Earth's global mean energy budget.”
Bulletin of
the American Meteorological Society
78: 197-208.
4.9 EXERCISES
4.1. The temperature of the blackbody curve that most closely matches an
observed spectrum is called the
brightness temperature
of the emitting
surface. Using Wein's displacement law, estimate the brightness temperature
for the sun and for the earth from an examination of their spectra in
Figures 4.4 a
nd
4.5
.
4.2. Sketch the emission spectrum from the sun at the top of the atmosphere,
using the same
y
-axis as in
Figure 4.4
but extending the
x
-axis to 40 mm.
Now, try to draw the terrestrial spectrum observed at the surface (see
Fig.
4.5
) to scale on your graph.
4.3. Calculate the solar constant for Mars.
