Thin scattering layers observed by airborne lidar

Abstract

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.

Optical Backscattering Measured by Airborne Lidar and Underwater Glider

“…The results for the cross-polarized return, which include the effects of both d and b bp , (Figure 6) show similar patterns to the unpolarized return lidar signal. The contrast between the scattering layers and background levels is greater, however, consistent with previous observations [30,31]. The scattering from pure sea water of 650 nm light at 124 • is less than that of 532 nm light at 180 • by a factor of 0.29, so the contrast from the scatterometer will be higher than that from the unpolarized lidar return.…”

“…This technique has been particularly effective in measuring enhanced scattering layers in the ocean [29,30] and dynamical processes such as internal waves [31,32]. A direct comparison of airborne lidar estimates of b bp with ship based measurements produced a root-mean square difference of 9.4 × 10 −4 m −1 over a range of values from 5 × 10 −4 m −1 to 9 × 10 −3 m −1 [33].…”

Layered organization in the coastal ocean: An introduction to planktonic thin layers and the LOCO project

“…For example, within this issue, shipbased methods are detailed in Rines et al and Sullivan et al; acoustic methods are detailed in Benoit-Bird et al and Holliday et al; methods using moored autonomous profilers are described in Sullivan et al; and methods using AUVs and gliders are described in Benoit-Bird et al, Moline et al, Ryan et al and Wang and Goodman. In addition to the methods and instruments described in this issue, recently developed remote sensing techniques such as Light Detection and Ranging (LIDAR) are providing new information about the spatial extent and depth range of thin layers over large swaths of the upper ocean (Churnside and Donaghay, 2009), while mathematical theory and modeling has also played a major role in understanding thin layer dynamics (Franks, 1995;Osborn, 1998;Leising, 2001;Stacey et al, 2007;Durham, et al, 2009). …”

“…Zaneveld and Pegau, 1998;Petrenko et al, 1998;Sullivan et al, 2005, this issue;Churnside and Donaghay, 2009). As phytoplankton thin layers typically contain a significant percentage of the total water column chlorophyll, a large percentage of the absorption (a) and scattering of light can occur within the thin layers (Sullivan et al, 2005, this issue).…”

Physical and optical properties of phytoplankton-rich layers in a coastal fjord: a step toward prediction and strategic sampling of plankton patchiness

“…Although accomplished in real time, this approach to identifying PRLs relies on fortuitous sampling over extensive time and space scales or in places of known patch formation (Dekshenieks et al 2001, Sullivan et al 2010a). An alternate approach to PRL identification has been to use airborne lidar for remote detection (Churnside & Donaghay 2009). Airborne surveys vastly increases the spatial scales over which areas can be sampled and the large-scale, long-term processes to which they can be related.…”