Geoscience Reference
In-Depth Information
distinction is made between direct and diffuse radiation (see Eq. (
2.4
)). Second, a
broadband transmissivity that does not take into account the direction of the beam is
frequently used, denoted by
τ
b
and deined as:
↓
K
K
0
τ
b
=
.
(2.16)
↓
This transmissivity is used when no distinction is made between direct and diffuse
radiation.
Question 2.4:
The ratio between the amount of diffuse radiation (
D
) and the global
radiation (
K
↓
) is an important indicator of the nature of the radiation that reaches Earth's
D
K
↓
(see also
Figure 2.6
):
surface. What is (approximately) the value of
a) On a sunny day without clouds
b) On an overcast day
Question 2.5:
See
Figure 2.6
. On May 22 and 23, at 12 UTC, the solar zenith angle is
2
d
d
about 32 degrees. The ratio
is about 0.974 for these dates and the solar constant
Sun
Sun
can be taken as 1365 W m
-2
.
a) Estimate
I
at 12 UTC for May 22 and May 23 from
Figure 2.6
.
b) Estimate the broadband beam transmissivity
τ
b
θ
at 12 UTC for both days.
c) Estimate the broadband transmissivity
τ
b
at 12 UTC for both days.
d) Is the difference in broadband beam transmissivity between both days due mainly to
differences in absorption, or differences in scattering?
Question 2.6:
In this section a range of transmissivities has been introduced. Collect
them and note down the following for each of them: symbol, meaning (what does a
given value mean in physical reality?) and deinition (mathematical relationship to var-
ious radiation lux densities).
For
τ
b
various empirical m
odel
s exist. An empirical model often used to estimate the
daily mean solar radiation (
K
↓
24
) from sunshine duration data is:
24
↓
K
K
ab
n
N
==+
τ
,
(2.17)
24
b
24
↓
d
0
where
n
is the hours of bright sunshine,
N
d
is the day length (in hours) and
a
and
b
are
empirical constants. The overbar denotes temporal averaging, in this case of a period
of 24 hours. Typical values for the Netherlands are
a
= 0.2 and
b
= 0.55 (DeBruin and
Search WWH ::
Custom Search