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output
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Direct/diffuse ratio — ^
uA
eme to
1000 7—————Sensitivity : alfa turbidity parameter
b : 5
(b) total insolation | [ 28
- - - -Direct/diffus
e 24
T
£900 4 20
2 alfa=0.0, 0.5, 1.0, 2.0
= —_— r 169
= =
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3 200147720717 Nm cat Leda alfa=0.0 =
be
- 8
op wie sw vw wm nme my maw wim es mes alfa=0.5
ade ed E um m'a m'a di nq bet m e w m M S alfa=1.0
Migebee er m ms. mane ms 4 m se alfa=2.0r 4
700 ; T T T T T T 0
330 335 340 345 350, 355 360 365 370
Fig 1. (b)
Sensitivity : beta turbidity parameter
1000
©
©
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a
Total insolation wm)
|
po ©
un Hu i
co os
oO —
o 8
Direct/diffuse
co
e
e
A.
total insolation l. 10
= = direct/diffuse
700 ACT HE là
Julian day
Fig 1. (c)
There. was a decrease in total insolation with increase in
precipitable water (w), but direct/diffuse ratio remained almost
unaltered with increase in w. The decrease in insolation was
about 20% for w values between 0.0-0.5cm. But the rate of
decrease in insolation was less for w values beyond 0.5cm.
While there were less effects on total insolation to changes in c
turbidity parameter, the direct-diffuse ratio was substantially
reduced with increase in o. The rate of decrease is more for a
between 0.0-0.5. Similarly, the increase in B from 0.0 to 0.1
resulted in 80% decrease in direct-diffuse ratio. The rate of
decrease in ratio was relatively less for B beyond 0.1. In
contrary to c, there was 2-596 reduction in total insolation with
increase in p.
3.2 Validation of daily total surface insolation
For validation, the retrieved daily and pentad mean (five day
average) daily total surface insolation for the period 1st to 30th
December 1998 were compared with the insolation derived by
one of the conventional methods which is called bright sunshine
hours (BSS) method.
IAPRS & SIS, Vol.34, Part 7, "Resource and Environmental Monitoring", Hyderabad, India,2002
E Mean absolute bias=1.8968 MJm
E
= 201R°=055
oO
9 454 . v e
2
E e
9 10-
g v, ve
$ 51 * 6 € *
a
0 I i T 1
0 5 10 15 20 25
BSS derived (MJm ?)
Fig2. Validation of retrieved daily total insolation for the
period 1% to 30™ December 1998 over six stations in India
The 1:1 line in the figure 2 showed that the mean absolute bias
(MAB) between retrieved and BSS derived daily total was
1.8968 MJm? with R2-0.55. But the MAB has been reduced to
1.762 MJm? (fig 3) between pentad mean daily total retrieved
and BSS derived insolation with high R of 0.61. The pentad
mean daily insolation image product over India is shown in
figure 3.
vc 20 =
= Mean absolute bias=1.762 MJm
< 45 {R*=0.61
3 t
d + «t? .
s 10 41 $ e n d ee
e e
9 9
9
0 0 ‘ : :
0 5 10 15 20
BSS derived (MJm?)
Fig 3. Validation of five-day average daily total surface
insolation during December 1998 over six stations in
India
The cloud movement and persistence , the total atmospheric
precipitable water are two highly variable parameters spatially
and temporally throughout the day which influence the
scattering and absorption processes of insolation to a great
extent. The present study has three limitations, which seemed
to have resulted into observed deviations between retrieved and
BSS derived insolation. Only five image acquisitions between
5:00 to 17:00hrs. IST instead of hourly/half-hourly slots are
used to derive daily total. This might not have truly represented
the clear and cloudy conditions. The NOAA AVHRR TOVS
