Geoscience Reference
In-Depth Information
August 1999, the elimination of direct beam
radiation caused diffuse radiation to drop from
680W m -2 at 10.30 a.m. to only 14W m -2 at 11.00
a.m.at Bracknell in southern England.
Between 1961 and the 1990, solar radiation
receipts globally decreased by some 4 percent, a
phenomenon termed 'global dimming'; amounts
then recovered again in the 1990s ('brightening').
The reason for these trends appears to have been
increased aerosol absorption (by black carbon)
and backscatter (by sulfates, nitrate and dust)
during the first period and a decrease in aerosol
loading subsequently. Sulfate aerosols have a
direct radiative forcing globally of - 0.4W m -2 ,
fossil fuel black carbon + 0.2W m -2 , and mineral
dust - 0.1W m -2 , out of a total aerosol direct
effect of - 0.5W m -2 . There is also an indirect
effect on clouds whereby aerosols increase the
2
Effect of the atmosphere
Solar radiation is virtually all in the short
wavelength range, less than 4
m (see Figure 3.1 ).
About 18 percent of the incoming energy is
absorbed directly by ozone and water vapor.
Ozone absorption is concentrated in three solar
spectral bands (0.20-0.31, 0.31-0.35 and
0.45-0.85μm), while water vapor absorbs to a
lesser degree in several bands between 0.9 and
2.1μm (see Figure 3.1 ). Solar wavelengths shorter
than 0.285
μ
m scarcely penetrate below 20km
altitude, whereas those >0.295
μ
m reach the
surface. Thus the 3mm (equivalent) column of
stratospheric ozone attenuates ultraviolet radia-
tion almost entirely, except for a partial window
around 0.20
μ
m, where radiation reaches the lower
stratosphere. About 30 percent of incoming solar
radiation is immediately reflected back into space
from the atmosphere, clouds and the earth's
surface, leaving approximately 70 percent to heat
the earth and its atmosphere. The surface absorbs
almost half of the incoming energy available at the
top of the atmosphere and re-radiates it outwards
as long (infrared) waves of greater than 3μm (see
Figure 2.1 ). Much of this re-radiated longwave
energy is then absorbed by the water vapor,
carbon dioxide and ozone in the atmosphere, the
rest escaping through atmospheric windows back
into outer space, principally between 8 and 13
μ
S olar radiation at the top of the atmosphere
A bsorbed by earth's surface
A bsorbed by air
A bsorbed by clouds
R eflected by earth's surface
R eflected by air
Reflected by clouds
500
400
300
m
(see Figure 3.1 ). This retention of energy by the
atmosphere is vital to most lifeforms, because
otherwise the average temperature of the earth's
surface would fall by some 40°C!
The atmospheric scattering, noted above, gives
rise to diffuse (or sky) radiation and this is
sometimes measured separately from the direct
beam radiation. On average, under cloud-free
conditions the ratio of diffuse to total (or global)
solar radiation is about 0.15-0.20 at the surface.
For average cloudiness, the ratio is about 0.5 at
the surface, decreasing to around 0.1 at 4km, as
a result of the decrease in cloud droplets and
aerosols with altitude. During a total solar eclipse
experienced over much of western Europe in
μ
200
100
0
90°N
60°N
30°N
Latitude
30°S
60°S
90°S
Figure 3.5 The average annual latitudinal disposition
of solar radiation in W m -2 . Of 100 percent radiation
entering the top of the atmosphere, about 20 percent
is reflected back to space by clouds, 3 percent by
air (plus dust and water vapor), and 8 percent by the
earth's surface. Three percent is absorbed by clouds,
18 percent by the air and 48 percent by the earth.
Source: After Sellers (1965). Courtesy of University of Chicago
Press.
 
 
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