Full text: Remote sensing for resources development and environmental management (Volume 1)

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 
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 
Mr - M. 
* M, 
+ (1-Mnn) * M f 
Mon - g(6o,Mr„) 
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.

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