Geoscience Reference
In-Depth Information
Table 1.4 Estimates of mean global heat budget at the earth surface in W m
-2
Land
Oceans
Global
Reference
R
n
L
e
EH
L
e
EH
L
e
EH
n
n
Budyko (1974)
65
33
32
109
98
11
96
80
16
Baumgartner and
Reichel (1975)
66
37
29
108
92
16
96
76
20
Korzun
et al
. (1978)
65
36
29
121
109
12
105
89
16
Ohmura (2005)
62
36
26
125
110
15
104
85
19
up to 3.73 m y
−
1
have been inferred for the Gulf Stream in the western Atlantic (Bunker
and Worthington, 1976) and up to 4 or even5my
−
1
for the Gulf of Aqaba (Assaf and
Kessler, 1976). Much research has been directed in recent years to study the evolution of
today's climate in response to increasing greenhouse gases in the atmosphere. Although
the issue is far from resolved, there are some indications of an accelerating hydrologic
cycle in several regions (see, for example, Brutsaert and Parlange, 1998; Karl and Knight,
1998; Lins and Slack, 1999).
The strong linkage between the water cycle and climate is further illustrated by the
estimates of the mean global surface energy budget in Table 1.4. Over large areas and over
sufficiently long periods, when effects of unsteadiness, melt and thaw, photosynthesis
and burning, and lateral advection can be neglected, this surface energy balance can be
written as
R
n
=
L
e
E
+
H
(1.2)
where
R
n
is the specific flux of net incoming radiation,
L
e
is the latent heat of vapor-
ization,
E
is the rate of evaporation, and
H
is the specific flux of sensible heat into the
atmosphere. The major portion of the incoming radiation is absorbed near the surface
of the Earth, and it is transformed into internal energy. The subsequent partition of this
internal energy into long-wave back radiation, upward thermal conduction and convec-
tion of sensible heat
H
, and latent heat
L
e
E
, is one of the main processes driving the
atmosphere. Table 1.4 indicates that the net energy is mainly disposed of as evaporation.
Over the oceans, the latent heat flux
L
e
E
is on average larger than 90 percent of the net
radiation. But even over the land surfaces of the Earth,
L
e
E
is on average still larger than
half of
R
n
.
Because the global patterns of heating force the circulation of the planetary atmo-
sphere, the implications of this large latent heat flux are clear. As a result of the relatively
large latent heat of vaporization
L
e
, evaporation of water involves the transfer and redis-
tributiuon of large amounts of energy under nearly isothermal conditions. Because, even
at saturation, air can contain only relatively small amounts of water vapor, which can
easily be condensed at higher levels, the air can readily be dried out; this release of energy
through condensation and subsequent precipitation is the largest single heat source for the
