Geoscience Reference
In-Depth Information
3.2
Airborne Observation of Vertical Profiles
of Solar Irradiance and Data Processing
The concern of the spectral observations of solar irradiances was to calculate
radiative flux divergences and it conditions both the observational scheme and
the methodology of data processing. It is necessary to distinguish two different
cases: observations under overcast and clear sky conditions. The observations
either of upwelling or of downwelling irradiance were accomplished using
one
instrument
through the upper and lower opal glasses in turn.
Theobservationsofthesolarirradiancesintheovercastskywereaccom-
plishedoutofthecloud(abovethecloudtopandbelowthecloudbottom)and
within the cloud layer at every 100m. As the implementation of the experiment
under the overcast conditions needed both a horizontal homogeneity of the
cloud and its stability in time, the observations were accomplished as fast as
possible with measuring of only one pair of the irradiances (upwelling and
downwelling) at every altitude level. Besides, only one circle of observations
was needed as usual. We need to stress that cases of homogeneous and stable
cloudiness are rare so the quantity of observations for the overcast sky are less
than in the clear sky.
The main component of the uncertainty during irradiance observations
under overcast conditions is the random error due to the heterogeneity of
illumination (Table 3.1). It leads to distortions of the vertical profiles of the
spectrum, as Fig. 3.2 demonstrates. The filtration of these distortions was
possible using the smooth procedures, but the standard algorithms (Anderson
1971; Otnes and Enochson 1978) turned out to be ineffective in this case. Thus,
it was necessary to elaborate the special one (Vasilyev A et al. 1994).
The smooth procedure of distortions of the spectral downwelling and up-
welling irradiances provides the replacement of the irradiance value at every
altitude level with the weighted mean value over this level and two neighbor
(upper and below) levels:
1
1
1
F
↓
(
z
i
)
−1
β
j
f
↓
(
z
i
+
j
),
F
↑
(
z
i
)
−1
β
j
f
↑
(
z
i
+
j
),
=
=
−1
β
j
=
1,
(3.2)
=
=
=
j
j
j
β
j
are the weights of smoothing (common for all wavelengths, altitudes
and types of the irradiances);
f
↓
(
z
i
),
f
↑
(
z
i
) are the observational results of the
downwelling and upwelling irradiances at level
z
i
;
F
↓
(
z
i
),
F
↑
(
z
i
)arethevalues
of the irradiances calculated during the secondary data processing. Weights
where
β
j
in (3.2) have been obtained from the demands of the physical laws.
As the radiative flux divergence has to be positive, the net radiant flux does
not increase with the optical thickness increasing (from the top to the bottom
ofthecloud)accordingtoSect.1.1.Thatistosay,thefollowingconditionhas
tobefulfilledfortheresultsof(3.2):
F
↓
(
z
i
)−
F
↑
(
z
i
)
F
↓
(
z
i
−1
)−
F
↑
(
z
i
−1
)
≥
(3.3)
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