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Tropospheric Aerosol Parameters

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For SCIAMACHY two algorithms are currently implemented to derive cloud top height from O2 A-band measurements. These are the Fast Retrieval Scheme for Clouds from the Oxygen A-band (FRESCO, Koelemeijer et al. 2001, Fournier et al. 2006) and the Semi-Analytical Cloud Retrieval Algorithm (SACURA, Rozanov and Kokhanovsky 2004). It is worth mentioning that in addition to the O2 A-band, there is also the option to derive CTH information from the O2-O2 absorption (Acarreta et al. 2004a) or the Ring effect (Joiner et al. 1995, de Beek et al. 2001). (fig. 5-9)

fig. 5-9:

The top-of-atmosphere reflectance in the O2 A-band as a function of cloud top height. (Graphics: IUP-IFE, University of Bremen)

Cloud Geometrical Thickness (CGT): O2 A-band absorption can also be used to obtain an estimate of the CGT (Asano et al. 1995). The CGT values represent an estimate of the light absorption inside a cloud and are therefore suited reducing uncertainties in the cloud top altitude measurements. This method was applied to SCIAMACHY data by Kokhanovsky and Rozanov (2005).

Cloud Optical Thickness (COT) and Effective Radius (Reff,cld): Measurements of the Earth reflectance spectrum in the VIS or NIR range outside strong gaseous absorption bands permit derivation of the COT and Effective Radius (Nakajima and King 1990, Platnick et al. 2003). Kokhanovsky et al. (2005, 2006) applied the method to SCIAMACHY data. In the case COT and Reff,cld are known for a liquid water cloud, the liquid water path can be estimated.

Cloud Phase Index(CPI): SWIR reflectance measurements are also suitable for obtaining the cloud phase index CPI (Knap et al. 2002, Acarreta et al. 2004b, Kokhanovsky et al. 2005, 2006), as the dependence of the single scattering albedo on the particle size is different for liquid water and ice.

In the GOME/SCIAMACHY data processing environment mainly two cloud retrieval algorithms are exploited. One is FRESCO, which uses measurements inside and outside the O2 A-band (758-778 nm). FRESCO is developed for cloud correction of trace gas retrievals, like O3 and NO2. It simultaneously retrieves an effective Cloud Fraction (CFeff) and an effective Cloud Top Height (CTHeff) assuming that the cloud can be represented as a bright Lambertian surface with a fixed albedo value of 0.8. The second cloud algorithm employed in the operational processing is the combination of OCRA with SACURA. OCRA delivers cloud fraction as input for SACURA to determine CTH and COT. In addition to that, an improved version of SACURA delivers cloud geometrical thickness, effective radius and the cloud thermodynamical state (Kokhanovsky et al. 2005, 2006).

5.3.2 Tropospheric Aerosol Parameters

Aerosols are characterised by high spatial or temporal variability and a mixture of chemically and physically different particles, dependent on their origin (P√∂schl 2005). Detection of aerosol by spaceborne instruments utilise their effect on the reflected solar radiation observed at the top of the atmosphere. Contributions from Earth’s surface reflections (see fig. 5-7) and atmospheric gases (see fig. 5-3) need to be separated using available information on the surface properties and the effects of gases. Thus the signatures of aerosols can be derived and can be used to retrieve aerosol properties.


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