Monitoring the Bio-optical State of the Baltic Sea Ecosystem with Remote Sensing and Autonomous In Situ Techniques - The Baltic Sea Basin - page 418

Fig. 20.5 RGB composite images from 19 August 2002 over Himmerfjärden, both in 300 m,
full resolution (FR) and 1.2 km reduced resolution (RR). This comparison shows that the 300 m
resolution of MERIS is more appropriate to view coastal bays than the common 1 km resolution of
ocean colour sensors. Note the images have not been corrected for environmental effects (Kratzer
and Vinterhav 2010 )
CDOM absorption at 440 nm. Besides the optical in-water components, one can
also derive so-called inherent optical properties describing the propagation of light
in the water, such as absorption and scattering, as well as the diffuse attenuation
coefficient, K d, characterizing the rate of light attenuation. Level 3 products are
space and/or time binned data sets of level 2 products which are used to generate
seasonal climatologies and to analyse long-term global trends.
20.2.1.4 Limitations and Challenges
The low sun elevation in the high-latitude regions of the Baltic basin limits the avail-
ability of satellite data from approximately early March to late October. Further
limitations are caused by intermittent cloud cover. The extent of cloud cover in the
Baltic Sea area is about 40-50% in summer and about 60-70% in winter (Karlsson
1996 ) . Before the retrieval of the water quality parameters cloud masking techniques
are applied to the satellite data so that only cloud-free areas are used in the process-
ing. Due to high CDOM concentrations in the Baltic Sea, the absorption is very high,
especially in the blue part of the spectrum. Thus, Baltic Sea water is relatively dark
compared to other seas leading to especially low water signals. Therefore, a high
signal-to-noise ratio is a crucial sensor requirement for Baltic Sea remote sensing.