Airborne lidar probes undersea waves

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.

Figure 1. Wide-angle photo of the lidar in the back of the aircraft. Laser power supply, cooling unit, and receiver electronics are on the right, and the two detector modules (white) can be seen on the optical package on the left. See also the video below⁷ about its observations. (Photo courtesy of Joseph Shaw, Montana State University.)

Several features of the observed internal waves pointed to nonlinear propagation. First, the amplitude of a linear wave would be much smaller than the thicknesses of the water layers, but the waves we measured were much larger. In one example, the background layer depth was about 4m, and the maximum depth of the internal wave was 7.8m (see Figure 2). Second, the waves had non-symmetric shapes: sharper on the bottom and more rounded on the top. Third, these waves were powerful enough to create currents that modulated the surface roughness, rendering them visible from the aircraft.

Figure 2. Relative lidar return (color bar at the top) as a function of distance along the flight track and depth reveals a nonlinear wave. Each lidar profile has been normalized by its maximum value, and the depth of that maximum is plotted as a black line.

In situ sensors near the location described in Figure 2 showed a density profile that could be approximated by two layers.⁵ They also revealed a strong optical-scattering layer just below the pycnocline, confirming that the lidar was measuring the region of interest. We used the layer densities estimated from the in situ measurements, along with the layer depth and wave amplitude from the lidar data, in the Korteweg–de Vries equation⁸ to predict a propagation speed of 5cms⁻¹ for a nonlinear wave. A linear approximation gave a speed of 16cms⁻¹. We made a second pass over the area along the same direction 31 minutes later and found that the wave had moved 87m, so the wave speed was 4.7cms⁻¹. This result is more evidence for the nonlinear nature of the wave.

Through comparison with in-water measurements, we have shown that the characteristics of nonlinear internal waves can be inferred from airborne lidar data. The next step is to use airborne lidar for large-scale (100km) studies of ocean mixing produced by internal waves.