305
Symposium on Remote Sensing for Resources Development and Environmental Management / Enschede / August 1986
Mapping of available solar radiation at ground
Ehrhard Raschke & Martin Rieland
Institute for geophysics and meteorology, University of Cologne, FR Germany
Abstract :
The high correlation between the backscatterance and transmittance of the cloudy atmosphere
for solar radiation allows rather accurate estimates of the total solar radiation reaching the
ground from operational daylight images taken from meteorological, geostationary satellites.
The reduced ISCCP data sets of the B3 - format (30-50 km) enables still accuracies of 4 to 8
percent for monthly averages. Results were obtained from Meteosat and GMS - data.
1. Introduction
Many scientific disciplines are interested in
accurate time series of avalaible solar radiation
over the entire globe. The amount of available
solar radiation is important to know for power
industry and for agriculture. For meteorological
aspects the solar radiation reaching the ground,
often named as global radiation, is an important
factor for the energy budget of the ground. It
causes small scale (spatial and temporal)
phenomena, such as convection, but it also
influences the Global Climate of the earth. Recent
studies made by Woods (1984) for instance show
remarkable heating rates in the upper ocean layers
due to the absorption of solar radiation. Although
solar heating below the mixed layer is weak, it can
be significant on longer time scales.
The determination of global radiation by evaluating
satellite data may be the only way to complete the
sparse worldwide distribution of ground based
measurements. Methods to map the downward solar
radiation at ground on the basis of operational
daylight images Of the earth from satellites make
use of the fact that the reflectance or
backscatterance of the atmosphere for solar
radiation is highly correlated with its
transmittance. Only relatively small corrections
for some effects are required.
A method, originally developed by Moser and Raschke
(1982), is applied to B3-data sets from the
geostationary satellites Meteosat (Gratzki,1985)
and GMS (Riel and,1985). These data samples are the
basis for worldwide cloud retrievals within the
ISCCP (International Satellite Cloud Climatology
Project, WCP-Report 42,1982).
2. The Model
While solar radiation is propagating through the
atmosphere its amount and spectral distribution is
changed due to interaction with atmospheric
components (e.g. clouds (absorption, scattering),
aerosols and dust (mainly scattering), water vapor
content (absorption) and ozone content
(absorption)). The influence of clouds, which is
the most important, is taken into account by
evaluation of satellite measurements. The remaining
factors, some of them are mentionend above, are
calculated by radiative transfer calculations
(Two-stream-approx.,Kerschgens 1978) for certain
mean standard atmosphere profiles (see Tab. 2.1).
The idea of the model is the inference of the
(2.1)
Mo = Mon * Moo + (1-M On) ^ Momln
Mo ! global radiation
Mq 0 : maximum global radiation,
case I
M Bmln : minimum global radiation,
case II
M 0n Ï normalized global
radiation, 0 < M Gn < 1
(2.2)
Mr - M.
* M,
+ (1-Mnn) * M f
(2.3)
Mon - g(6o,Mr„)
(2.4)
Moo = f(6o)
atmospheric transmittance for solar radiation (0.2
^m < X < 4.0 ^m) from the reflected solar radiation
at the top of the atmosphere, which can be measured
by satellites. The global radiation and the
reflected solar radiation at the top of the
atmosphere are parameterized as linear
relationships between two cases :
(I) cloudfree atmosphere
(II) atmosphere with optically very thick clouds
(6 > 40)
M R : reflected solar radiation
Mrmin: minimum reflected solar
radiation, case I
maximum reflected solar
radiation, case II
M Rn : normalized reflected solar
radiation, 0 < Mr„ < 1
The relationship between the normalized global
radiation (or atmospheric transmittance) M D n and
the normalized reflected radiation M Bn is
investigated by radiative transfer calculations.
M 0 n is developed as a function of 8 0 and the
normalized reflected radiation h Rn , ' for different
mean standard atmospheres (see Tab. 2.1).
For different mean standard atmosphere profiles the
maximum global radiation M 00 is developed as a
function of the sun zenith angle 0 O .
The figures 2.1 and 2.2 show some examples for Moo
and Mon.
