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Cloud Parameters

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5.3.1 Cloud Parameters

For SCIAMACHY information on cloud parameters are of twofold importance. Firstly, cloud parameters such as cloud fraction, top pressure (or height), and optical thickness (albedo) are required to correct the cloud impact on the trace gas concentrations. Secondly, due to its wide spectral range and despite its relatively low spatial resolution, SCIAMACHY data enables determination of important parameters for climate research like thermodynamic phase or geometrical thickness.

Cloud Fraction (CF): In order to correct for the effect of clouds, a fast and reliable cloud fraction algorithm is required for SCIAMACHY. Nearly all cloud fraction algorithms for SCIAMACHY use the PMD measurements, as their higher temporal readout frequency translates into a higher spatial resolution as compared to data from channel 1 to 8. The basic principle of the algorithms is that cloud albedo is much higher than the Earth’s surface albedo (see fig. 5-8). Therefore, a pixel that is contaminated by clouds will have a higher detector signal than one that is cloud free. Cloud fractions can therefore be determined through comparison of PMD intensities. Several derivates of cloud fraction algorithms using PMD data are currently applied to SCIAMACHY data, such as the Optical Cloud Recognition Algorithm (OCRA, Loyola et al. 1998) used in the operational processing, the SCIAMACHY PMD Cloud Algorithm (SPCA, Yan 2005), the Heidelberg Iterative Cloud Retrieval Utilities (HICRU, Grzegorski et al. 2004) or the SCIAMACHY PMD Identification of Clouds and Ice/snow (SPICI, Krijger et al. 2005). (fig. 5-8)

fig. 5-8:

Clouds over Europe on July 9th, 2005. Cloud coverage as seen in a RGB composite (right) from MODIS on-board TERRA and cloud fraction (left) determined with OCRA using SCIAMACHY PMD data (images: IUP-IFE, University of Bremen and Dundee Satellite. Receiving Station)

Cloud Top Height (CTH): CTH can be estimated from measurements in the solar backscatter spectral ranges by using the changes in the penetration depth of solar photons due to strong changes in the absorption of trace gases with known vertical distributions such as O2 (see fig. 5-9). The idea for the CTH retrieval from O2 A-band absorption was originally proposed by Yamomoto and Wark (1961).


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