Focusing of sunlight by sea-surface waves: new measurement results from the Black Sea

Jerzy Dera; Slawomir Sagan; Dariusz Stramski

doi:10.1117/12.138865

22 December 1992 Focusing of sunlight by sea-surface waves: new measurement results from the Black Sea

Jerzy Dera, Slawomir Sagan, Dariusz Stramski

Proceedings Volume 1749, Optics of the Air-Sea Interface: Theory and Measurement; (1992) https://doi.org/10.1117/12.138865
Event: San Diego '92, 1992, San Diego, CA, United States

Abstract

Under clear skies, strong fluctuations in the downwelling irradiance, E_d, prevail in shallow water as a result of the focusing and defocusing of sunlight by surface waves. Such temporal fluctuations were measured in the Black Sea, usually at a depth of 1 m, from a fixed platform located 600 m off the coastline. A method of thresholding analysis was applied to 109 time-series records of E_d (525 nm), each of which lasted 10 min. The frequency of occurrence of flashes of intense foci (intensity exceeding the time-averaged irradiance, E_d, by > 50%) decreased exponentially with increasing flash intensity. The frequency and intensity of flashes, hence the slope of the exponential relationship, all varied with wind-wave conditions and atmospheric lighting conditions. The best conditions for wave focusing were characterized by light winds of 2 to 5 m s^-1, solar elevation > 40 degree(s), and diffuseness of surface irradiance < 40%. Then, at a depth of 1 m, the flashes > 1.5 E_d occurred at rates as high as 6 Hz. The most intense flashes exceeded E_d 5-fold at rates of 10^-3 Hz. These results, which are consistent with our previous observations, substantially improve the database on still poorly documented wave focusing effects.

Citation Download Citation

Jerzy Dera, Slawomir Sagan, and Dariusz Stramski "Focusing of sunlight by sea-surface waves: new measurement results from the Black Sea", Proc. SPIE 1749, Optics of the Air-Sea Interface: Theory and Measurement, (22 December 1992); https://doi.org/10.1117/12.138865

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