The study of ocean colour with satellite sensors provides a wealth of
information on the composition of the upper water layer with respect to
chlorophyll concentration, the presence of phytoplankton, and the concentration
of particulate and dissolved matter. The measurements are based on the fact that
part of the sun light can enter the water, is reflected or absorbed by
substances in the water and then exits the water again to be observed by the
satellite instrument.
Here, the GOME and SCIAMACHY instruments were used to measure ocean colour, even
though these instruments were designed for atmospheric trace gas measurements
and therefore lack the spatial resolution typical for ocean colour instruments.
In this study the visible wavelength range (i.e. 400nm – 600nm) of GOME
calibrated level-1 spectra (spatial resolution of 40x320km2) from January 2002
to March 2003, and SCIAMACHY uncalibrated level-0 data (spatial resolution of
30x60km2) from August 2002 to August 2003 were used. Ocean colour was obtained
by the ratio of the sun-normalized radiance at top of atmosphere in the
wavelength ranges of 425 nm - 450nm (where chlorophyll concentration has strong
absorption) to 550 nm – 560 nm (where absorption is insensitive to chlorophyll
concentration (hinge point)).
Results are compared with those of the SEAWIFS instrument from NASA, which is
calibrated with chlorophyll concentrations. While ocean colour retrieved from
GOME data agrees qualitatively well with that of SEAWIFS, SCIAMACHY data still
suffer from calibration problems in the visible spectra used.