Abstract

The distribution of irradiance in the upper ocean was examined from sensors mounted on an Autonomous Underwater Vehicle (AUV). Apparent and inherent optical properties along with physical variability ranging from scales O(10 cm) to O(1 km) were collected off the coast of Oregon during the summer of 2004. Horizontal wavenumber spectra of downwelling irradiance showed that irradiance varied as a function of wavenumber and depth. The analysis indicates that irradiance variability between 1 and 20 m spatial scales was attributed to the focusing effects of surface wave geometry. The dominant wavelength of focusing at depths of 2 – 6 m was about 2 m for ~6 m s-1 wind speeds.

© 2005 Optical Society of America

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References

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Annu. Rev. Fluid Mech. (1)

Melville, W. K., �??The role of surface-wave breaking in air-sea interaction,�??�?? Annu. Rev. Fluid Mech. 28, 279-321 (1996).
[CrossRef]

Appl. Opt. (1)

J. Geophys. Res. (2)

Stramski, D., and J. Tegowski, �??Effects of intermittent entrainment of air bubbles by breaking wind waves on ocean reflectance and underwater light field,�??�?? J. Geophys. Res. 106, 31,345-31,360 (2001).
[CrossRef]

Terrill, E. J., W. K. Melville, and D. Stramski, �??Bubble entrainment by breaking waves and their influence on optical scattering in the upper ocean,�??�?? J. Geophys. Res. 106, 16815-16823 (2001).
[CrossRef]

J. Opt. Soc. Am. (2)

Oceanologia (2)

Dera, J., S. S. Sagan, and D. Stramski, �??Focusing of sunlight by sea surface waves: new results from the Black Sea,�??�?? Oceanologia 34, 13-25 (1993).

Dera, J., and J. Olszewski, �??Experimental study of short-period irradiance fluctuations under an undulating sea surface,�??�?? Oceanologia 10, 27 (1978).

Opt. Express (1)

Other (1)

Dera, J, Marine Physics, (Elsevier, Amsterdam, 1992, p.516).

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Figures (6)

Fig. 1.
Fig. 1.

An example of AUV level mission. The AUV level flights were conducted at 5 different depths during daytime hours while winds were blowing from north to south with a magnitude of about 6.5 m s-1 (wind stress ~ 0.054 N m-2). Left Panels: wind speed and direction (top), short wave radiation (middle), and depths of AUV level flights as a function of time (bottom). Right Panel: Spatial position of AUV flights.

Fig. 2.
Fig. 2.

From top to bottom: Spatial series of absorption and attenuation from AC-9+ , downwelling irradiance from OCI-200 for 2 m depth (red) and 4 m depth (blue), and normalized downwelling irradiance. Wavenumber spectra of IOP and irradiance for different depths are given in Figs. 3 and 4.

Fig. 3.
Fig. 3.

Horizontal Wavenumber spectra of attenuation c488 (left) for different depths. Wavenumber spectra of turbidity (right) from the microstructure package. Turbidity which resolves spatial variability to about 10 cm, shows the red shape of IOP wavenumber spectra. Note: pumping rate of water limited the spatial resolution of AC9 signals to about 2 m (or 0.5 cpm). Most of the IOP variability is at scales greater than 100 m.

Fig. 4.
Fig. 4.

Horizontal wavenumber spectra of downwelling irradiance Ed at 489nm wavelength. Wave focusing of light appears to be responsible for the spatial energy at large wavenumbers (for example see Zaneveld et al., [9,10]).

Fig. 5.
Fig. 5.

Left Panel: Horizontal wavenumber spectra of pitch (red) and roll (blue) at a depth of 2 m. Right Panel: Coherence between Ed at 2 m (Fig. 4) and vehicle’s pitch and roll at 2 m; Ed-pitch coherence (red) and Ed-roll coherence (blue). Coherences between Ed and the vehicle’s pitch and roll in the 0.05 - 1 cpm wavenumber band are quite small and do not contribute the to irradiance measurement.

Fig. 6.
Fig. 6.

Normalized area preserving spectra of irradiance for 5 different depths shown in Fig. 4. Spectral estimates for wavenumbers greater than 0.01 cpm are plotted.

Equations (2)

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z f L = 2 δ + 1 ( 8 δ ) ,
z c L = 0.5 tan π 2 + a sin { n 1 sin ( a tan ( 2 πδ ) ) } a tan ( 2πδ ) ,

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