An Evaluation of Depth Resolution Requirements for Optical Profiling in Coastal Waters

Abstract

Wave perturbations induce uncertainties in subsurface quantities determined from the extrapolation of optical measurements taken at different depths. An analysis of these uncertainties was made using data collected in the northern Adriatic Sea coastal waters over a wide range of environmental conditions with a profiling system having a 6-Hz acquisition rate, ?0.1 m s?1 deployment speed, radiance sensors with 20° full angle field of view, and irradiance collectors of ?1-cm diameter. The uncertainties were quantified as a function of the depth resolution of radiance and irradiance profiles through the percent differences between the subsurface values computed from full and reduced resolution profiles (the latter synthetically created by removing data from the former). The applied method made the analysis independent from instrument calibration; from perturbations induced by instrument self-shading, deployment structure, and bottom effects; and from environmental variability caused by seawater and illumination changes during casts. The results displayed a significant increase in uncertainties with decreasing depth resolution. For instance, in the 443?665-nm spectral range with a depth resolution of 12.5 cm, the largest uncertainties were observed for the subsurface downward irradiance, Ed(0?, ?), and the near-surface diffuse attenuation coefficient, Kd(?), with spectral average uncertainties of 5.5% and 11.7%, respectively. With the same depth resolution, the smallest uncertainties were observed for the subsurface upwelling radiance, Lu(0?, ?), and upward irradiance, Eu(0?, ?), showing spectral average values of 1.0% and 0.6%, respectively. The uncertainties in the irradiance reflectance, R(?); the Q factor, Qn(?); and the normalized water-leaving radiance, LWN(?), gave values in keeping with those of the quantities used for their computation. The uncertainties were also analyzed as a function of sea state Ss and diffuse attenuation coefficient Kd at 490 nm. These values were used to estimate the depth resolution requirements restricting below given thresholds the wave-induced uncertainties in the computed subsurface optical quantities. To satisfy a 2% maximum uncertainty in the 443?665-nm spectral range, for the specific instrumental and environmental conditions characterizing the data used in the analysis, results suggested minimum depth resolutions of 11, 40, 3, and 2 cm, for Lu(0?, ?), Eu(0?, ?), Ed(0?, ?), and Kd(?), respectively.