Polarized lidar and ocean particles: insights from a mesoscale coccolithophore bloom

Abstract

Oceanographic lidar can provide remote estimates of the vertical distribution of suspended particles in natural waters, potentially revolutionizing our ability to characterize marine ecosystems and properly represent them in models of upper ocean biogeochemistry. However, lidar signals exhibit complex dependencies on water column inherent optical properties (IOPs) and instrument characteristics, which complicate efforts to derive meaningful biogeochemical properties from lidar return signals. In this study, we used a ship-based system to measure the lidar attenuation coefficient ( α ) and linear depolarization ratio ( δ ) across a variety of optically and biogeochemically distinct water masses, including turbid coastal waters, clear oligotrophic waters, and calcite rich waters associated with a mesoscale coccolithophore bloom. Sea surface IOPs were measured continuously while underway to characterize the response of α and δ to changes in particle abundance and composition. The magnitude of α was consistent with the diffuse attenuation coefficient ( K d ), though the α versus K d relationship was nonlinear. δ was positively related to the scattering optical depth and the calcite fraction of backscattering. A statistical fit to these data suggests that the polarized scattering properties of calcified particles are distinct and contribute to measurable differences in the lidar depolarization ratio. A better understanding of the polarized scattering properties of coccolithophores and other marine particles will further our ability to interpret polarized oceanographic lidar measurements and may lead to new techniques for measuring the material properties of marine particles remotely.

The influence of particle concentration and bulk characteristics on polarized oceanographic lidar measurements

“…4b,c with fig. 2 in Collister et al 2020). However, the single‐particle model underestimated δ in the region where scattering was strongly coupled to calcite concentration, and overestimated δ in the region of strong scattering near the coast where b bp and b b ′ became decoupled (Fig.…”

“…( b ) Values of δ from the RMSE optimized solution plotted against measured values of δ (RMSE = 0.023, = 0.66, ϕ p = 0.02). ( c ) Model estimates of δ (red) from the RMSE optimized model solution plotted as a time series alongside lidar measurements of δ from Collister et al (2020).…”

“…The color map shows the corresponding value of RMSE comparing modeled and measured values of δ . ( b ) Modeled values of δ from the RMSE optimized model solution (RMSE = 0.019, = 0.58, ϕ acid = 0.0, ϕ ′ = 0.19) plotted against measured values of δ from Collister et al (2020). The solid line shows the 1:1 relationship, and the dashed line shows the results of a least‐squares linear regression (slope = 0.78 ± 0.031 [95% confidence interval]; intercept = 0.042 ± 0.006 [95% confidence interval]; r 2 = 0.75, df = 783; SSE = 0.58).…”

“…The model was parameterized using a dataset of in situ inherent optical properties that were collected concurrently with oceanographic lidar measurements of δ made during the CoccoMix research expedition in the North Atlantic ( see Collister et al 2020 for more details of the cruise). For the duration of the expedition, δ was measured at a distance along the beam of 6.5 m from the sea surface using a shipboard lidar system mounted at the bow of the ship and positioned at an angle of 35° from nadir.…”

Airborne Lidar Observations of a Spring Phytoplankton Bloom in the Western Arctic Ocean

“…The backscattering ratio has been independently used to infer properties of scattering particles in the ocean [52,53]. Depolarization from that same lidar has been used to identify a coccolithophore bloom [54]. Another attempt used the ratio δ/b bp instead of δ to discriminate different types of scattering particles [31].…”