Exploring the potential of Aeolus lidar mission for ocean color applications

The Atmospheric Laser Doppler Instrument (ALADIN) onboard the Aeolus wind mission was the first High Spectral Resolution Lidar operating in the Ultra Violet (UV) region deployed in space. This study explores and documents the feasibility of deriving ocean optical properties using data from ALADIN. A three steps (i.e. data screening, analytical estimation of the total in-water signal contribution, Look Up Table-based estimation of the in-water attenuation) retrieval algorithm was developped combining data analysis and signal simulations from a radiative transfer model. The algorithm has been implemented using the signal acquired by the Mie channel, and tested for 1-year of Aeolus observations. This approach allowed estimating the first Aeolus derived Ocean Color (OC) products in terms of the total in-water signal contribution and the in-water attenuation term in a spectral region (355 nm) not covered, during Aeolous lifetime, by operational OC products. The validation process involved comparing these products with both Biogeochemical-Argo (BGC-Argo) field measurements and satellite OC dataset distributed by ESA Ocean Color Climate Change Initiative across a set of 7 selected oceanic regions representing diverse open-ocean scenarios. These validation exercises attested the general accordance between OC reference measurements and the proposed Aeolus OC parameters. Thus, this study was able to provide statistical evidence of the sensitivity of the retrieved Aeolus in-water lidar attenuation term to the CDOM variability on a temporal/seasonal and spatial/regional basis. A preliminary estimation of the uncertainty associated to the retrieved quality controlled Klid, was performed assuming the radiometric noise as unique source of uncertainty. As a result, a median/average value of absolute relative percent difference of about 50/80% was obtained. Limits of the developed technique, possible improvements, potential adaptation to planned/future space lidar missions are discussed.