This paper provides a review of the development of profiling oceanographic lidars. These can provide quantitative profiles of the optical properties of the water column to depths of 20 to 30 m in productive coastal waters and to depths of 100 m for a blue lidar in the open ocean. The properties that can be measured include beam attenuation, diffuse attenuation, absorption, volume scattering at the scattering angle of 180 deg, and total backscattering. Lidar can be used to infer the relative vertical distributions of fish, plankton, bubbles, and other scattering particles. Using scattering as a tracer, lidar can provide information on the dynamics of the upper ocean, including mixed-layer depth, internal waves, and turbulence. Information in the polarization of the lidar return has been critical to the success of many of these investigations. Future progress in the field is likely through a better understanding of the variability of the lidar ratio and the application of high-spectral-resolution lidar to the ocean. Somewhat farther into the future, capabilities are likely to include lidar profiling of temperature in the ocean and an oceanographic lidar in space.

We have developed approximate expressions for the aperture-averaging factor of optical scintillation in the turbulent atmosphere. For large apertures and weak path-integrated turbulence with a small inner scale, the variance of signal fluctuations is proportional to the −7/3 power of the ratio of the aperture diameter to the Fresnel zone size. If the inner scale is large, the variance is proportional to the −7/3 power of the ratio of the aperture diameter to the inner scale. In strong path-integrated turbulence, two scales develop. That portion of the variance associated with the smaller scale is proportional to the −2 power of the ratio of the aperture diameter to the phase coherence length. That portion of the variance associated with the larger scale is proportional to the −7/3 power of the ratio of the aperture diameter to the scattering disk. These simple approximations are within a factor of 2 of the measurements.

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Churnside, J. H., and Donaghay, P. L. 2009. Thin scattering layers observed by airborne lidar. – ICES Journal of Marine Science, 66: 778–789. More than 2000 km of thin (<3 m) optical scattering layers were identified in 80 000 km of airborne lidar data collected from a variety of oceanic and coastal waters. The spatial characteristics of thin layers varied dramatically from (i) those that were self-contained features consistently <3–4 m thick over their 1–12 km extent to (ii) those that were clearly parts of much longer layers that had gaps and/or regions where the layer became more intense and much thicker than the 3-m criterion. The characteristics of the lidar signal suggest that plankton was the most likely source of scattering. Examples from upwelling regions, areas with large fresh-water influx, and warm-core eddies are presented. The results are quite consistent with the characteristics observed in studies of thin plankton layers in fjords and near-coastal waters. These layers exhibit great spatial variability that is difficult to observe using traditional methods, and examples of layer perturbations by both linear and non-linear internal waves are presented. The results suggest that airborne lidar can be a powerful tool not only for detecting and mapping the spatial extent of thin scattering layers and linking their occurrence to larger scale physical processes, but also for tracking their evolution over time and guiding the ship-based sampling needed to understand their composition, dynamics, and impacts. Such a capability will be crucial in future studies designed to test the hypothesis that thin plankton layers have the spatial extent and intensity to play a key role in controlling the recruitment of fish larvae, biogeochemical cycling, trophic transfer processes, plankton biodiversity, and harmful algal bloom dynamics.

A scanning-laser glint meter designed for field measurements of sea-surface slope statistics is described. A narrow laser beam is scanned in a line, and specular reflections (glints) are counted in bins according to their slope angle. From normalized glint histograms, moments to the fourth order are calculated, and slope probability density functions are approximated with a Gram–Charlier expansion. Field measurements with this instrument show good agreement with previous results when the stability (essentially air–sea temperature difference) is near neutral (zero). Under conditions of negative stability (warm ocean), both the mean-square slope and the probability density function kurtosis increase.