Measurements of high-frequency light fluctuations induced by sea surface waves with an Underwater Porcupine Radiometer System

Abstract

Under clear skies the underwater light field within the near-surface ocean shows large fluctuations caused by focusing of sunlight by surface waves. The downwelling light at near-surface depths can fluctuate greatly on times scales as short as milliseconds and distances less than 1 cm. Specially designed radiometers and measurement strategies are required to adequately characterize these fluctuations. We developed an Underwater Porcupine Radiometer System which has a capability to measure light fluctuations with a sampling frequency of 1 kHz. This instrument is equipped with 23 radiometric sensors for measuring time series of the downward plane irradiance, Ed(t), at several light wavelengths and the downwelling radiance L(t) at λ = 532 nm for different zenith angles within two orthogonal azimuthal planes. We describe the critical components of the Porcupine instrument and measurement protocols, and present example results from measurements made at near-surface depths in the ocean. We show that the irradiance collector of a few millimeters in diameter or smaller is required to provide adequate measurement of light flashes produced by wave focusing. The measurements with larger irradiance collectors can result in deceptive reduction of the measured intensity of fluctuations. The brightest flashes of irradiance or radiance can exceed the time-averaged irradiance or radiance by an order of magnitude and the duration of flashes is typically on the order of milliseconds to tens of milliseconds. The intensity of light fluctuations decreases rapidly with depth and is higher at longer light wavelengths compared with shorter wavelengths within the visible spectral range. Copyright 2011 by the American Geophysical Union.