GOME-MERIS cloud products inter-comparison on global scale
Stefano Casadio(1), Diego Loyola(2) and Claus Zehner(3)
(1) SERCO, via Galileo Galilei, 00044, Frascati (RM), Italy
(2) German Aerospace Center (DLR), D-82234 Wessling, Oberpfaffenhofen, Germany
(3) ESA, via Galileo Galilei, 00044, Frascati (RM), Italy
This paper presents the inter-comparison of the operational GOME and MERIS cloud products: cloud fraction, cloud-top height/pressure and cloud-top albedo. The two instruments are flying onboard different platforms (ERS-2 and ENVISAT respectively). They are characterized by different space resolution, are based upon different measurement principles, and a significant time delay (25 minutes) exists between their measurements. Nevertheless, it is demonstrated here that cloud parameters estimated from GOME and MERIS can be inter-compared to assess their quality with excellent precision.
Accurate determination of cloud information out of GOME/ERS-2 is needed for the (near-real-time) retrieval of ozone and other trace gases and it is crucial for more demanding applications like aerosol retrieval and the retrieval of trace gas concentrations in the troposphere. The cloud fraction is retrieved in GOME using the polarization measurement devices with a footprint size of 16x40 km2, while the cloud-top height and cloud-top albedo are derived using the spectral measurements in and around the oxygen A-band at 760 nm with a pixels size of 320x40 km2.
Although the primary mission of MERIS is the measurement of sea colour in the oceans and in coastal areas with high space resolution, MERIS is also capable of retrieving cloud top pressure, water vapour total column, and aerosol load over land. The MERIS cloud optical thickness is estimated from measurements of channel 10 at 753.75 nm.. The MERIS cloud top pressure/height is estimated from reflected solar radiation within the oxygen A-band absorption centred at 760 nm. It is based on the assumption that the mean photon path length of the reflected solar radiation is related to the amount of absorption measured in the O2 A-band. In a cloudy atmosphere the mean photon path length is primary determined by the air mass above the cloud, the cloud top pressure. Hence the cloud top pressure can be estimated through the ratio of measured radiance ‘in’ and ‘out’ absorption window channels.
The space resolution of MERIS (1x1 km2) is much higher than for GOME (320x40 km2), therefore an averaging procedure has been developed in order to resample the MERIS data onto the GOME ground pixel. The MERIS pre-processing algorithm consists of:
1. Selection of MERIS pixels contained in the GOME pixel.
2. Selection of valid MERIS products contained in the selected measurement set.
3. Computation of statistical quantities of selected products (i.e. mean, std, % valid products).
To process the 30Gb of GOME and 1.2Tb MERIS data needed for this study the ESRIN's Grid on-Demand Infrastructure and Services handling was used. The Grid on-Demand, which is run from ESRIN, offers access to, and support for, science-oriented Earth Observation GRID services and applications, including access to a number of global geophysical Envisat and ERS-2 products and ESA's toolkits like BEAT and BEAM.
Comparison results are available from January to June 2003 where both GOME and MERIS data were available on a global scale. It turns out that GOME cloud fractions are in excellent agreement with corresponding MERIS results (correlation coefficient > 0.98). Cloud-top height and cloud-top albedo results will also be critically discussed.
Keywords: ESA European
Space Agency - Agence spatiale europeenne,
observation de la terre, earth observation,
satellite remote sensing,
teledetection, geophysique, altimetrie, radar,
chimique atmospherique, geophysics, altimetry, radar,