Outer Earth processes and systems - Physical Processes in Earth and Environmental Sciences - page 237

6 Outer Earth processes
and systems

6.1 Atmosphere

6.1.1 Radiation balance and heat transfer

formation of clouds and the precipitation/evaporation of
surface waters ( latent heat transfer ). Thus although the
troposphere away from the tropics is in radiation deficit
(Fig. 6.2), the overall positive net radiation from the
Earth's surface due to the greenhouse effect (Section 4.19)
means that the planet is in balance due to this transfer of
heat from low to high latitudes by oceanographic and tro-
pospheric circulation. To illustrate this, imagine that
Earth, like its Moon, has no atmosphere. Then at any one
time there would be a perfect energy balance between
incoming shortwave insolation to the side of Earth facing
the Sun and outgoing longwave reradiation from the
whole Earth. The mean surface temperature would then
be about 254 K, or

It is important initially to consider the net balance of
incoming solar radiation energy and outgoing reradiated
energy from Earth over a long time period. We need to
consider energy transformations also, like those between
conductive and convective heat energy, potential and
kinetic energy. In Fig. 6.1, 100 units of energy represent
the magnitude of the incoming shortwave solar radiation
flux (sometimes termed insolation) at the outer atmos-
phere; because of the Earth's planetary albedo, approxi-
mately 32 percent of this is reflected back into space. Of
this total reflection, about 23 percent is from clouds behav-
ing as perfect blackbodies and about 9 percent is directly
from Earth's surface. This reflected radiation plays no part
in the climate system. The remaining incoming radiation is
either absorbed by the Earth's surface as direct and diffuse
radiation (~49%) or absorbed by the atmosphere and
clouds (~20%). Note the smaller atmospheric absorption
compared with the large surface absorption. As seen previ-
ously (Section 4.19), the latter is converted into heat
energy and, by Wein's law, is reradiated into the atmos-
phere as longwave radiation, where most of it is absorbed
and then reemitted (at the same wavelengths). More long-
wave radiation in net terms is lost to space in this process
from the troposphere (~60%), chiefly from cool cloud tops,
than is absorbed (~17%). Together with the 19 percent of
shortwave radiation absorbed, this means that there is a
total absorption deficit of more than 35 percent.
Should the matter rest there, the Earth's troposphere
and surface would cool drastically, to below 0

C. This compares to the actual
mean surface temperature of around 14

19

C. The surplus

temperature of 31 C is due to the greenhouse effect,

whereby Earth's atmospheric gases absorb, reradiate,
and reabsorb significant portions of the outgoing infrared
radiation from the surface and make this energy available
for the lateral and vertical transport of heat energy by the
troposphere. It is only in dry desert areas that the Earth's
climate is dominated by radiative exchanges alone, with
high daily and low nightly temperatures.
The result of tropospheric heat transfer processes is a
mean thermal structure shown in Fig. 6.3. The warmest
temperatures, about 27 C, are at lowest latitudes,

decreasing toward the poles and vertically up to the top of
the tropopause at between 10 and 15 km elevation. The
troposphere is thickest at low latitudes, thinning toward
the poles. The greatest vertical and lateral gradients of T
occur toward the top of the troposphere, a prominent
boundary within the overall temperature structure of the
whole atmosphere. Note that in general, particularly away
from the tropics, the isotherms controlling air density
diverge from the major latitudinally (zonally) averaged,

C. So,
where does the “extra” energy come from? The deficit is
provided by the transfer of heat energy from the Earth's
surface by conduction and convective turbulent exchange
(called sensible heat transfer ) and as a by-product of the