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| Titel |
Surface radiation climatology derived from Meteosat First and Second Generation satellites |
| VerfasserIn |
Rebekka Posselt, Richard Müller, Jörg Trentmann, Reto Stöckli |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250039946
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| Zusammenfassung |
| A 25 year long continuous and consistently validated surface incoming shortwave (SIS)
radiation climate data record (CDR) from METEOSAT satellites is MeteoSwiss’ contribution
to CM SAF (Satellite Application Facility for Climate Monitoring). CM SAF is is a joint
activity of several national Meteorological Services within EUMETSAT’s satellite data
processing (SAF – Satellite Application Facilities). CM SAF generates, archives and
distributes widely recognized high-quality satellite-derived products and services
relevant for climate monitoring in operational mode with a special emphasis on the
retrieval of climate variables such as cloud parameters, radiation budget and water
vapor.
The SIS CDR by MeteoSwiss and DWD is generated using an extended Heliosat
algorithm which exploits the attenuation of radiation by clouds from the METEOSAT visible
channel, and using the MAGIC (Mesoscale Atmospheric Global Irradiance Code) radiative
transfer model that accounts for water vapor, ozone and aerosol absorption on clear sky
radiation fluxes. The dataset is compared to reference surface radiation datasets from ISCCP,
GEWEX and ERA interim. Ground based measurements of the BSRN (Baseline surface
radiation network) and ASRB (Alpine surface radiation budget) network are used as
validation sources to estimate the uncertainty of the SIS CDR and of the reference
datasets.
In order to satisfy the dataset accuracy required for climate variability and change studies,
discontinuities due to changes in satellite instrumentation must be avoided. Therefore, a
selfcalibration technique within the Heliosat algorithm is applied. It uses the 95% percentile
of the measured radiance distribution obtained in a selected (nearly) always cloudy region in
the southern Atlantic. The overlap period between two satellites/instruments (Meteosat7 and
Meteosat8 in 2005) is used to examine and validate the performance of the selfcalibration.
First validation results show a good agreement for both satellite generations (within +/-
4Wm-2). Larger differences are mainly apparent in highly vegetated regions (Tropics,
summer Europe) which are due to different spectral characteristics of the satellite
instruments.
Special attention is also drawn to the radiative influence of snow. Snowy and cloudy areas
have to be separated and the reflective properties of snow have to be considered. Multispectral
approaches are not applicable in the generation of a long surface radiation time series as
former satellite generations had less spectral channels and the whole time series should be
generated with the same algorithm. Thus, a time-series approach is applied which
employs only a visible channel and is based on the very low temporal variability
of snow compared to clouds. In case of snow the Heliosat algorithm is slightly
altered in order to account for reflected solar radiation. It is however found that this
algorithm is not able to decide whether a bright pixel is due to snow (on a clear day) or
clouds which results in an underestimation of the surface solar radiation in those
regions.
The SIS CDR is available at satellite resolution (for the Meteosat First Generation
Satellites (Meteosat 2-7, 1982-2005) about 2.5 km horizontally (at the subsatellite point)
every 30 minutes; for the Meteosat Second Generation Satellites (Meteosat 8-9,
2005-present) about 3 km (at the subsatellite point) every 15 min) . It will be made available
to the research community through the CM SAF website (www.eumetsat.int) in Fall 2010. |
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