Geoscience Reference
In-Depth Information
Stricker,
2000
). Allen et al. (
1998
) recommend 0.25 and 0.5 for global applications.
The coeficient (
a + b
) can be interpreted as the transmissivity of the cloudless sky,
whereas
a
is the transmissivity of the clouds. Other values for the constants can be
found in Iqbal (
1983
). The strength of expression (
2.17
) lies in the fact that it contains
the two domin
ant
variables that determine daily mean global radiation: the yearly
cycle (through
K
↓
24
) and cloudiness (through
n
/
d
). However, it assumes a ixed
transmissivity of the cloudless atmosphere.
Question 2.7:
Refer to the graphs of relative sunshine duration for Valencia and Oslo
in
Figure 2.3
. The following additional data are given:
Daylength (hour)
K
0
(W m
-2
)
January
June
January
June
Valencia
9.5
14.7
120
356
Oslo
6.6
17.8
29.6
358
a) Explain why Oslo (located much more Northerly) has a higher daily mean radiation
at the top of the atmosphere in June than Valencia (igure not needed).
b) Estimate the daily average number of sunshine hours for Valencia and Oslo, for Jan-
uary and June. Explain why the relative contrast in daily sunshine hours between
January and June is larger for Oslo than for Valencia.
c) Estimate the daily mean global radiation in Valencia and Oslo for January and June.
A parameter that is often used to characterize the transmissivity of the cloudless atmo-
sphere is the Linke turbidity factor
T
L
. This factor uses the transmissivity of a clean,
dry atmosphere as a reference (as this reference atmosphere is dry, it is a hypothetical
atmosphere). In such a reference atmosphere attenuation takes place only due to Ray-
leigh scattering and absorption by gases other than water vapour (mainly CO
2
, O
2
and
ozone). The transmissivity of the real atmosphere is expressed in the number of clean,
dry atmospheres needed to attain the same transmissivity:
T
=
( )
m
L
=
−
δ
mT
(2.18)
τ τ
θ
r
e
b,cdarL
,
b
bvcda
where
δ
b,cda
is the broadband optical thickness of the clean, dry atmosphere and
τ
bv,cda
is the vertical broadband beam transmissivity of the clean, dry atmosphere. These
values depend on the exact deinition of the reference atmosphere (see Ineichen
and Perez,
2002
), but a typical value for
τ
bv,cda
is 0.9. As the Linke turbidity fac-
tor is a broadband quantity, it will depend on the spectral composition of the radia-
tion arriving at the surface. As the latter depends on the relative optical mass (some
wavelengths become depleted if the path length becomes longer), so does the Linke
turbidity. Usually, values are reported that are valid for a relative optical mass of 2,
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