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Airborne lidar probes undersea plankton

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Published on Mar 30, 2012

http://spie.org/x86675.xml

Lidar measurements of nonlinear sub-surface waves in a fjord agree with theoretical predictions based on in-situ profiles, opening the way for large-scale studies of internal-wave mixing processes.

Waves that propagate on density boundaries within the ocean are less familiar than their counterparts on the surface, but they are an important component of ocean dynamics. Nonlinear waves, whose amplitudes affect their propagation, are particularly important in the conversion of tidal energy into turbulent energy, which produces vertical mixing. Mixing raises nutrients from the rich, deep water to the surface layer where photosynthesis can occur. Because tidal motion takes place on larger spatial and temporal scales than turbulence, it is difficult to get a complete picture of this process.

Internal-wave measurements are typically made with in-water sensors or microwave radars above the surface. The former can provide very detailed information about the wave and the properties of the water column, but they are limited in spatial coverage. The latter can cover large areas quickly, but they detect internal waves indirectly through their impact on surface roughness. As a result, radars can only spot very strong waves, and they cannot give any depth-dependent information.

Airborne lidar (light detection and ranging) provides depth-resolved information, like in-water sensors, but it also offers the wide-area coverage of airborne radar. For waves to be apparent in a lidar signal, they must propagate through some tracer that scatters light, such as a layer of phytoplankton. Fortunately, various mechanisms often produce such a layer near the pycnocline, the interface between two zones of different density.1 These layers have been detected by airborne lidar.2, 3 We previously observed internal waves perturbing such layers,3, 4 but none of these observations had coincident in-water measurements for comparison. In our recent work, we identified numerous internal waves in lidar data from Washington's West Sound on Orcas Island and we compared the lidar observations with in situ measurements of water stratification and optical properties.5

We installed the National Oceanic and Atmospheric Administration's Fish Lidar6 in the back of a four-seat Cessna-177 aircraft with the rear seats removed (see Figure 1) and flew at an altitude of 300m and a speed of 40ms−1 (see video below7). We collected the scattered light from the water column with two telescopes whose fields of view were matched to the laser beam divergence. One accepted light with the polarization of the transmitted beam, and the other received light with the orthogonal polarization. We visually inspected the orthogonal channel for internal waves.

Read the complete article on the SPIE Newsroom http://spie.org/x86675.xml

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