UAV-Borne Quasi-3-D Observation of Physical–Biological Coupling in a Dipole Eddy by Integrating Radar Altimetry and Ocean LiDAR

Abstract

3-D remote sensing of the upper ocean remains a major challenge in ocean observation. Conventional dynamic remote sensing can effectively detect sea surface height (SSH), but it lacks the capability to synchronously sense subsurface structures, which limits observations of 3-D physical and biogeochemical variability in the upper ocean. To address this limitation, we develop an uncrewed aerial vehicle (UAV)-borne quasi-3-D remote sensing system that integrates a dual-frequency Ka/Ku radar altimeter and a dual-wavelength blue–green ocean light detection and ranging (LiDAR) system. A flight campaign of over 5800 km was carried out over a mesoscale dipole eddy in the northern South China Sea (SCS), enabling synchronous high-resolution measurements of sea level anomaly (SLA) and subsurface chlorophyll-a (Chl-a) profiles. The SLA retrieved from UAV-borne radar altimetry agreed with the 0.25° Data Unification and Altimeter Combination System (DUACS) and Surface Water and Ocean Topography (SWOT) 2-km products, with root-mean-square errors of 0.44 and 0.51 cm and mean biases of +0.006 and −0.001 cm, respectively. The Chl-a profiles retrieved from UAV-borne LiDAR agreed with synchronous in situ fluorescence observations, with an RMSE of <inline-formula> <tex-math notation="LaTeX">$0.071~\mu $ </tex-math></inline-formula>g<inline-formula> <tex-math notation="LaTeX">$\cdot $ </tex-math></inline-formula>L−1, an MAE of <inline-formula> <tex-math notation="LaTeX">$0.067~\mu $ </tex-math></inline-formula>g<inline-formula> <tex-math notation="LaTeX">$\cdot $ </tex-math></inline-formula>L−1, and a mean bias of <inline-formula> <tex-math notation="LaTeX">$+ 0.067~\mu $ </tex-math></inline-formula>g<inline-formula> <tex-math notation="LaTeX">$\cdot $ </tex-math></inline-formula>L−1. These synchronous observations further revealed the coupled physical and bio-optical structure of the dipole eddy, showing that an approximately 10 cm decrease in SLA coincided with an approximately 12 m shoaling of the subsurface chlorophyll maximum (SCM) and an approximately <inline-formula> <tex-math notation="LaTeX">$0.15~\mu $ </tex-math></inline-formula>g<inline-formula> <tex-math notation="LaTeX">$\cdot $ </tex-math></inline-formula>L−1 increase in subsurface Chl-a. These results support the feasibility of synchronous quasi-3-D sensing of upper ocean dynamical and bio-optical structure and inform the development of future spaceborne 3-D ocean remote sensing missions.