Spaceborne Lidar in the Study of Marine Systems

Abstract

Satellite passive ocean color instruments have provided an unbroken ∼20-year record of global ocean plankton properties, but this measurement approach has inherent limitations in terms of spatial-temporal sampling and ability to resolve vertical structure within the water column. These limitations can be addressed by coupling ocean color data with measurements from a spaceborne lidar. Airborne lidars have been used for decades to study ocean subsurface properties, but recent breakthroughs have now demonstrated that plankton properties can be measured with a satellite lidar. The satellite lidar era in oceanography has arrived. Here, we present a review of the lidar technique, its applications in marine systems, a perspective on what can be accomplished in the near future with an ocean- and atmosphere-optimized satellite lidar, and a vision for a multiplatform virtual constellation of observational assets that would enable a three-dimensional reconstruction of global ocean ecosystems.

Keywords: aerosols; atmospheric corrections; clouds; lidar; ocean plankton; remote sensing.

Figure 1.

Illustration of the lidar time-of-flight ranging technique. (a)The laser transmits a short (e.g., 15 ns) pulse of laser light which is directed downward. (b) As the laser pulse travels toward Earth, photons are scattered from air molecules and cloud/aerosol particles in the atmosphere. (c) Shortly thereafter, the pulse penetrates the ocean where photons are also scattered by water molecules and suspended particles. Some of the scattered photons in the atmosphere and ocean are intercepted by the telescope, and the magnitude of this signal is recorded as a function of time by detectors located in the receiver. (d) Using the speed of light, time is converted to distance, creating a vertically-resolved profile of received backscatter.

Figure 2.

The HSRL technique (Box 1) relies on the spectral separation between 180˚ backscatter from seawater and suspended particles (e.g., phytoplankton). The spectrum of particulate backscatter is nearly identical to that of the transmitted single-frequency laser pulse. Molecular backscatter, on the other hand, is shifted (~7.5 GHz at 532 nm) and broadened by Brillouin scattering processes (Hickman et al. 1991).

Figure 3.

Along-track ‘curtain’ plots acquired with the NASA airborne HSRL-1 instrument on the May 2016 during a NAAMES deployment. (top panel) Vertically resolved aerosol backscatter in the atmosphere along the flight track in the North Atlantic. Vertical scale is in kilometers. (bottom panel) Vertically resolved diffuse attenuation coefficients in the ocean along the flight segment delineated by dashed pink arrows. Vertical scale is in meters. From 35° N to ~40° N, the transect sampled oligotrophic conditions with significant subsurface features north of 38° N. A strong near-surface bloom was encountered between ~41° N and ~43° N, followed by more mesotrophic waters with significant subsurface biomass between ~10 and 20 m depth.

Figure 4.

Results from the SABOR field campaign, which encompassed 24 flights with the HSRL-1 and 23 ocean sampling stations on the RV Endeavor. (a) MODIS K_d at 488 nm values (Lee et al. 2005) for July 18, 2014 (background color) and K_d retrieved with the HSRL-1 along a flight track on the same day (white outlined data; modified from Hair et al., 2016). HSRL-1 K_d values were calculated at 10 m depth and converted to 488 nm by accounting for the difference in pure seawater absorption. (b) K_d matchup data from HSRL-1 and MODIS for all flights during the SABOR campaign (modified from Hair et al., 2016). (c) Comparison of b_bp profiles from HSRL-1 (532 nm, red line) and in situ measurements (529 nm, black line) from a Wet Labs ECO BB3 instrument (modified from Schulien et al., 2017). (d) Matchup comparison of HSRL-1 and in situ b_bp data from the 16 offshore SABOR stations where overboard optical casts had near-coincident HSRL measurements (modified from Schulien et al., 2017). Colors indicate the optical depth of each sample.

Figure 5.

Sampling of the global ocean with CALIOP. (a) CALIOP ground tracks achieved within a single 16‐day repeat cycle. Red lines = 55° to 65° North latitude section used to compare CALIOP and MODIS data coverage in panel d. (b) CALIOP-based climatological annual average phytoplankton biomass (C_phyto) for the 2006 to 2012 period reported by Behrenfeld et al. (2013). (c) Location of all field b_bp data in the NASA SeaBASS data archive. These data required 13 years to collect, yet still leave most of the ocean unsampled in space and time. By comparison, CALIOP can provide an unbiased global sampling of b_bp every 16 days that can be used for global ocean science investigations and to refine algorithms for passive ocean color retrievals. (d) Comparison of CALIOP and MODIS pixel coverage per month for the 55° to 65° North latitude section identified in panel a (from Behrenfeld et al. 2016). Filled and unfilled symbols = Total number of 1° latitude × 1° longitude ice‐free ocean pixels per month with valid CALIOP and MODIS b_bp data, respectively.

Figure 6.

Simplified block diagram of primary components in the advanced spaceborne ocean-atmosphere optimized lidar discussed in Sections 6 and 7.

Figure 7.

Artistic rendering of a virtual ocean observing constellation including complementary HSRL, ocean color, and polarimeter instruments supplemented by in situ Bio-Geo-Argo floats that extend the depth-resolving capability of the lidar.