Derived Depths in Opaque Waters Using ICESat‐2 Photon‐Counting Lidar.

Since turbid waters make it extremely difficult for light to penetrate the water column due to attenuation, performing satellite‐derived bathymetry using spectral images and/or photon‐counting lidars is a very challenging problem in turbid waters. The photon‐counting lidar borne on Ice, Cloud, and Land Elevation Satellite‐2 (ICESat‐2) can obtain high‐resolution and accurate height estimations of sea surfaces, that do not depend on the water clarity. Using the ocean surface waves calculated from ICESat‐2 and classical wave theory that models the relationship between the water depth and wavelength, we propose a method to estimate the depths in opaque waters. The performance is validated in different water qualities with a mean absolute percent error of 9%∼22%. The basic theory of estimating water depths in this study is entirely different from that of conventional laser bathymetry, and this method has the potential to fill the gap of space‐based bathymetric data sets in turbid waters. Plain Language Summary: In clear waters, optical instruments such as lidar and spectral cameras are the most used satellite‐based sensors to obtain water depths. These instruments can measure water depths by receiving the reflected light signal from the seafloor. However, as turbid waters make it extremely difficult for light to penetrate the water column, the seafloor is almost undetectable in turbid waters. Alternatively, the lidar on the Ice, Cloud, and Land Elevation Satellite‐2 (ICESat‐2) satellite provides unprecedented dense two‐dimensional geolocated points on the water surface, from which ocean wave characteristics can be precisely calculated. When ocean waves approach coastal areas, their wavelengths decrease as water depths decrease. The phenomenon of wavelength changes can be used to derive water depths in coastal areas, even operating in opaque waters. In this study, we investigate the technical process of how ICESat‐2 obtains high‐resolution ocean waves that propagate and evolve from the oceans to the coasts and the basic theory of estimating water depths by observing wavelength changes. With global‐scale ICESat‐2 data, this new method can be used to obtain the near‐shore water depth regardless of the water quality and can also be expanded to understand the wave characteristics in many specific coastal areas or even the globe. Key Points: A method to obtain near‐shore water depths even in opaque waters is proposed using Ice, Cloud, and Land Elevation Satellite‐2 (ICESat‐2) lidar dataICESat‐2 can observe high‐resolution ocean waves that propagate and evolve from the oceans to the coastsThe linear wave theory is modeled to estimate water depths by observing wavelength changes from ICESat‐2 [ABSTRACT FROM AUTHOR]