Water optical properties in Scandinavian waters and airborne optical sensing

Water optical properties are crucial for airborne ocean optical sensing. Laser depth sounding has been used for many decades and is now a well-developed commercial technique for bottom depth charting. Airborne lidar also has the capability to generate depth profiles of water scattering properties. Recently, satellite based lidars have also been shown to measure water depth. The NASA satellite ICESat-2 (Ice, Cloud and land Elevation Satellite-2) carries a photoncounting laser altimeter used to measure, for example, the elevation of ice sheets, glaciers and sea ice. The raw data can also be used to measure water depth in shallow waters. Passive EO remote sensing from aircraft and satellites can also be used for shallow water bathymetry and environmental monitoring as well as for estimating various optical parameters. In order to plan and execute airborne EO sensing over water, a prior knowledge of optical parameters of the water is important. There are many data sources for optical water parameters including Secchi disc depths, turbidity profiles, phytoplankton, coloured dissolved organic matter (CDOM), humus substances and others. Many of these parameters are related to water colour and transmission. The paper will give examples of such data for the Baltic Sea and how they relate to geographical and seasonal variations. Examples of laser depth sounding in the Baltic will be given. Depth capability for airborne lidars are discussed in relation to new technology developments. Finally, the need for a comprehensive forecast model of water clarity is discussed including its relation to seasonal, geographical and weather variations.