Geoscience Reference
In-Depth Information
Wavelength (
μ
m)
5
10
15
20
25
20
288 K
Figure 5.10
The net
exchange of longwave
radiation between a surface
at 288 K and a cloudless
atmosphere at 263 K.
(Redrawn from Monteith and
Unsworth, 1990, published
with permission.)
263 K
10
0
CO
2
H
2
O
O
3
H
2
O
CO
2
H
2
O
atmosphere at 263 K. The net longwave flux is upward (outward) partly because
the surface temperature is higher than the effective average temperature of the
atmosphere above, and partly because the effective emissivity of the overlying
atmosphere is less than that of the surface.
In practice, there is some linkage between the temperature of the surface and air
temperature, and a great deal of the downward longwave radiation originates in
the lower atmosphere comparatively close to the surface. For this reason, both the
upward and downward longwave radiation fluxes are linked to near-surface (2 m)
air temperature and in clear sky conditions there is therefore often a reasonably
strong approximately linear relationship between air temperature and both upward
and downward longwave radiation.
When clouds are present, more of the outgoing longwave radiation is absorbed
and returned to the surface and this is the basis of the simple empirical formula
much used in hydrological applications for estimating the daily average net
longwave radiation,
L
n
d
, which has the form:
d
4
L
=−
f
e¢s
T
(5.22)
n
air
where
T
air
is the daily average air temperature. Because water vapor makes an
important contribution to the absorption of outgoing longwave radiation and
emission of downward radiation, the effective emissivity,
e
′
, in Equation (5.22)
depends on the humidity content of the air and is estimated by:
e¢
=−
0.34
0.14
e
(5.23)
d
where
e
d
is the daily average vapor pressure in kPa. The factor
f
is an empirical
cloud factor
which is calculated from (
S
d
/
S
d
clear
), i.e., the ratio of the estimated surface
solar radiation given by Equation (5.16) or (5.17) in ambient conditions to the
