Oral Session 1 | Monday, October 3, 12:20–12:40 | Abstract 604

Ocean color drones: not necessarily a suitable alternative for optical remote sensing under cloudy skies

Using low-flying ocean color drones seems to be an obvious alternative when satellite remote sensing is prevented by clouds. These drones can measure the ocean reflectance from underneath the cloud layer and in addition have the benefit of avoiding the need for atmospheric corrections. However, simple measurements and spectral radiative transfer calculations show that the drones will have their received reflected light signals significantly influenced by cloud-light being specularly reflected on the water surface. We compare water leaving signals from different water types under clear skies and under cloudy skies, where optical properties have been selected to represent water types ranging from the oligotrophic Atlantic Ocean, via the turbid English Channel, to a highly scattering glacial influenced Norwegian fjord, and we find that for blue-green light coming from a typical clear ocean, the fraction of the water-leaving light that is reaching the drones may be reduced from about 85% under clear skies to about 35% under cloudy skies, or in other words, the “blinding” fraction reflected from the surface increases from 15% to 65%. For red light it is even worse, with a reduction from 55% to only 2%. It is better, though, for more turbid waters, reaching a smallest cloud-effect in glacial influenced water, where the average water-leaving fraction is reduced from 95% to 60%. Altogether, the results show that if drones are used to measure ocean color under cloudy skies, they should be used with care and preferably only over highly scattering waters. 

Børge Hamre, University of Bergen, 0000-0002-0135-0542

Arne Kristoffersen, University of Bergen, Norway

David McKee, University of Strathclyde, Scotland

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