| SCIAMACHY, on board Envisat,
has been in operation now
for almost three years. This
UV/visible/NIR spectrometer measures the solar irradiance, the earthshine
radiance scattered at nadir and from the limb, and the attenuation of solar
radiation by the atmosphere during sunrise and sunset, from 240 to
2380 nm and at moderate spectral resolution. Vertical columns and
profiles of a variety of atmospheric constituents are inferred from the
SCIAMACHY radiometric measurements by dedicated retrieval algorithms. With the
support of ESA and several international partners, a methodical SCIAMACHY
validation programme has been developed jointly by Germany, the Netherlands and
Belgium (the three instrument providing countries) to face complex requirements
in terms of measured species, altitude range, spatial and temporal scales,
geophysical states and intended scientific applications. This summary paper
describes the approach adopted to address those requirements.
Since
provisional releases of limited data sets in summer 2002, operational SCIAMACHY processors established at DLR on behalf of ESA
were upgraded
regularly and some data products – level-1b spectra, level-2 O3,
NO2, BrO and clouds data – have improved significantly.
Validation results summarised in this paper
and also reported in this special issue
conclude that for limited periods
and geographical domains they can already be used for atmospheric research.
Nevertheless,
current
processor
versions still experience known limitations that hamper
scientific usability in other periods and domains.
Free from
the constraints of
operational processing, seven scientific institutes (BIRA-IASB, IFE/IUP-Bremen,
IUP-Heidelberg, KNMI, MPI, SAO and SRON) have developed their own retrieval
algorithms and generated SCIAMACHY data products, together addressing nearly
all targeted constituents. Most of the UV-visible data products
– O3, NO2, SO2, H2O total columns; BrO, OClO slant columns; O3, NO2, BrO profiles –
already have acceptable, if not excellent, quality.
Provisional
near-infrared column products
– CO, CH4, N2O and CO2 –
have already
demonstrated their potential for a variety of applications.
Cloud and aerosol parameters are retrieved, suffering from calibration with the exception of cloud cover.
In any case,
scientific users are advised to read carefully validation reports before using
the data. It is required and anticipated that SCIAMACHY validation will continue
throughout instrument lifetime and beyond
and will accompany regular processor upgrades. |