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Effect of Surface Roughness on Lidar Overlap Function

Effect of Surface Roughness on Lidar Overlap Function. James Churnside NOAA Earth System Research Laboratory. Lidar Overlap Function. Ratio of flux on detector to flux through entrance pupil from specified range.

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Effect of Surface Roughness on Lidar Overlap Function

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  1. Effect of Surface Roughness on Lidar Overlap Function James ChurnsideNOAA Earth System Research Laboratory

  2. Lidar Overlap Function • Ratio of flux on detector to flux through entrance pupil from specified range. • For single scattering from far field, O < 1 if illuminated area is not within receiver field of view. • In near field, there is an additional effect due to focus mismatch, so O < 1 if image of illumination is not all on detector.

  3. Single-Scatter Lidar Equation • Too many functions of z. • Ideally, we would like O(z) = 1 for all values of z. • We could deal with O(z) = a single function that we could measure during calibration. • Since neither of those will apply, we can only hope that d ln(O)/dz is much less than 2α.

  4. Monte-Carlo Geometry SIMULATION PARAMETERS ParameterValue Height 5 m Tilt angle 15° T-R separation 0 cm, 8 cm T-R offset 0°, 0.92° T diameter 2.5 cm T divergence 1° R diameter 2.5 cm, 5 cm R field of view 1°, 4°

  5. Pierson–MoskowitzSpectrum 100 waves Random phase Random azimuth

  6. Surface Elevation 10 m square on surface, 5 m s-1 wind speed, 30 cm wave height

  7. Illumination and Field of view Wind = 5 m s-1, FOV = 1°, Depth = 5 m

  8. Wide FOV Receiver • 4° FOV • Complete overlap at surface • Complete overlap at all depths with no wind (flat surface) • Sharp drop with depth even at 1 m s-1 wind where rms slope = 5° (Cox, Munk)

  9. Resulting Attenuation • 4° FOV • High near surface • Down to 0.05 m-1 by about 5 m depth • Should be able to make attenuation measurements with reasonable accuracy

  10. Moderate FOV Receiver • 1° FOV, focused at ∞, axis parallel to transmitter • Incomplete overlap at surface • Incomplete overlap at depth with no wind (flat surface, dashed line) • Very low overlap with wind

  11. Resulting Attenuation • 1° FOV • High near surface with wind • Down to 0.05 m-1 by about 10 m depth • Still should be able to make attenuation measurements with reasonable accuracy (actual attenuation higher for narrow beam)

  12. Angled and Focused Pointed toward surface illumination and focused at surface

  13. Co-Axial Geometry Focused at infinity Focused at surface

  14. Conclusions • Measuring beam attenuation cby using a narrow lidar geometry and assuming single scattering may prove difficult when operating through the rough sea surface from a ship. • Measuring diffuse attenuation KDwith a wider field of view that collects multiple scattering is probably more accurate.

  15. Questions • Since the overlap function is random, can we select “good” shots to estimate beam attenuation? • Can we use wide angle illumination with wide and narrow receivers to measure KD and c? • Can we use α vs. β plots to estimate absorption from a surface vessel?

  16. Lidar Absorption Lidar attenuation vs scattering for flight through open-ocean water (blue), coastal water, and Columbia River plume.

  17. Questions • Since the overlap function is random, can we select “good” shots to estimate beam attenuation? • Can we use wide angle illumination with wide and narrow angle receivers to measure 2KD and KD+c? • Can we use α vs. β plots to estimate absorption from a surface vessel?

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