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Clouds play an important role in the Earth climate system. The amount of radiation reflected by the Earth-atmosphere system into outer space depends not only on the cloud cover and the total amount of condensed water in the Earth atmosphere but also the size of droplets and their thermodynamic state is also of importance.
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The information about microphysical properties and spatial distributions of terrestrial clouds on a global scale can be obtained only with satellite remote sensing systems. Different spectrometers and radiometers deployed on space-based platforms, measure the angular and spectral distribution of intensity and polarization of reflected solar light. Generally, the measured values depend both on geometrical and microphysical characteristics of clouds. Thus, the inherent properties of clouds can be retrieved (at least in principle) by the solution of the inverse problem. The accuracy of the retrieved values depends on the accuracy of measurements and the accuracy of the forward radiative transfer model.
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In particular, it is often assumed that clouds can be represented by homogeneous and infinitely extended in the horizontal direction plane-parallel slabs. The range of applicability of such an assumption for real clouds is very limited as is shown by observations of light from the sky on a cloudy day. For example, the retrieved cloud optical thickness is apparently dependent on the viewing geometry. This, of course, would not be the case for an idealized plane-parallel cloud layer. However, both the state-of-art radiative transfer theory and computer technology are not capable to incorporate 3-D effects into operational satellite retrieval schemes. As a result, cloud parameters retrieved should be considered as a rather coarse approximation to reality.
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However, even such limited tools produce valuable information on terrestrial clouds properties. For example, it was confirmed by satellite measurements that droplets in clouds over oceans are usually larger than those over land. This feature, for instance, is of importance for the simulation of the Earth’s climate.
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A new Semi-Analytical CloUd Retrieval Algorithm (SACURA) for the determination of cloud characteristics from space including the cloud liquid water path, the cloud altitude, the spherical albedo, the size of particles in clouds, the cloud thermodynamic state and also a number of other parameters important for climate studies, remote sensing and light propagation in terrestrial atmosphere.
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The cloud top height is derived from measurements in the oxygen A-absorption band. The cloud thermodynamic state is obtained using different spectral signatures of liquid water as compared to ice in the spectral range 1550-1670nm. Measurements at 440nm and 1555nm are used to get the cloud optical thickness and the size of particles in clouds.
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The algorithm is restricted to the case of optically thick clouds (the optical thickness ). It is planned to be supplemented in the future by the exact radiative transfer calculations at < 5.
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