Synergistic detection of chlorophyll-a concentration vertical profile by spaceborne lidar ICESat-2 and passive optical observations

The tremendous improvement for satellite remote sensing has provided mankind with the feasibility of observing large-scale ocean phenomena from space. Passive remote sensing of ocean color has delivered continuous records of ocean surface information, although there are inherent limitations to this natural light-dependent measurement. The development of spaceborne lidars breaks through the restrictions of passive techniques that are unavailable for obtaining the vertical characteristics of water column, and provides a new approach to the detection of ocean subsurface. In this study, the active spaceborne lidar ICESat-2 and passive optical satellite are fused, to establish an inversion method of lidar equation and semi-analytical model. The diffuse attenuation coefficient ( k d ) was estimated in various areas of the global oceans, and vertical profiles of chlorophyll-a concentration ( Chla ) were further obtained. The results showed that Mean Absolute Errors (MAE) of Chla derived by the proposed method with BGC-Argo data were in the range of 0.003 -0.14 mg/m 3 , and the Mean Absolute Percentage Error (MAPE) was between 12.5 % and 26.9 %; Comparison with ocean color products indicated that R 2 of Chla was in the range of 0.84 -0.98, and MAE of k d was in the range of 0.004 -0.021 m-1 . The multiple validation approaches have demonstrated the effectiveness of water column profile detection with ICESat-2 lidar and passive satellite, which contributes to the enhanced understanding of ocean subsurface phytoplankton and their spatial or temporal variations.