Geoscience Reference
In-Depth Information
normally dominates the net radiation flux and in the absence of cloud, fog, or
heavy aerosol contamination, the loss of solar radiation through the clear air
boundary layer is comparatively small. The profile of longwave radiation through
the boundary layer can be influenced by variations in the concentration of radia-
tively active atmospheric constituents and the heat content of the air. During the
night, longwave radiation dominates and the nighttime potential temperature
profile may induce vertical changes in the net radiation flux. Regardless of its
cause, energy conservation means vertical divergence of the net radiation flux will
be associated with local heating or cooling of the air, the amount being directly
related to the rate of change of net radiation with height and given by:
∂
q
∂
R
r
c
v
=
n
(5.29)
p
∂
t
∂
z
Important points in this chapter
Separation of wavebands
: most solar radiation is in the wavelength range
0.15-4
●
m and most longwave radiation in the Earth system is in the wave-
length range 3-100
μ
μ
m, this allows separate consideration of the two streams.
Blackbody radiation laws
: describe radiation for an
idealized
emitting and
absorbing
blackbody
surface, and include
Planck's Law
describing the
spectrum;
We i n's L aw
, giving the wavelength of peak emission; and
Stefan-
Boltzmann Law
, giving the total flux of radiation.
●
Gray surfaces
: are imperfect blackbodies for which radiation exchange at
wavelength
l
is calculated from blackbody radiation using multiplicative
factors, i.e.,
emissivity
(
e
l
) for emitted radiant energy;
absorptivity
(
a
l
) for
absorbed radiant energy,
reflectivity
(
r
l
), for emitted radiant energy; and
transmissivity
(
t
l
) for transmitted radiant energy.
Kirchov's principle
requires
that
e
l
=
a
l
.
●
Integrated parameters for natural surfaces
: because there is little overlap
between the waveband for solar and longwave radiation, radiation exchange
for natural surfaces is often described by wavelength integrated surface
properties, i.e.,
albedo
(
a
), the integrated reflectivity for solar radiation, and
emissivity
(
e
), the integrated emissivity for longwave radiation.
●
Typical values of parameters
: daily average values albedo are typically
∼
8%
●
for water,
∼
80% for fresh and
∼
40% for dirty snow,
∼
23% for bare soil and
agricultural crops, and
∼
12% for forests; emissivity is typically
∼
95
±
5%.
Top of atmosphere solar radiation
: maximum value changes by about
±
1.6%
●
with day of year around the '
solar constant
'
S
o
= 1367 W m
−2
.
Maximum solar radiation at the ground
: if there were no atmosphere the
daily total solar radiation reaching the ground could be calculated using
an (albeit complex) formula from the latitude of the site (
f
),
solar declination
●
