Seaborne measurements of near infrared water-leaving reflectance: The similarity spectrum for turbid waters

Abstract

Theory and seaborne measurements are presented for the near infrared (NIR: 700-900 nm) water-leaving reflectance in turbid waters. According to theory, the shape of the NIR spectrum is determined largely by pure water absorption and is thus almost invariant. A ''similarity'' NIR reflectance spectrum is defined by normalization at 780 nm. This spectrum is calculated from seaborne reflectance measurements and is compared with that derived from laboratory water absorption measurements. Factors influencing the shape of the similarity spectrum are analyzed theoretically and by radiative transfer simulations. These simulations show that the similarity spectrum is valid for waters ranging from moderately turbid (e.g., water-leaving reflectance at 780 nm of order 10 Ϫ4 or total suspended matter concentration of order 0.3 g m Ϫ3 ) to extremely turbid (e.g., reflectance at 780 nm of order 10 Ϫ1or total suspended matter of order 200 g m Ϫ3). Measurement uncertainties are analyzed, and the air-sea interface correction is shown to be critical for low reflectances. Applications of the NIR similarity spectrum to atmospheric correction of ocean color data and to the quality control of seaborne, airborne, and spaceborne reflectance measurements in turbid waters are outlined.Although ocean color remote sensing has focused primarily on visible wavelengths (400-700 nm) where photosynthetic pigments have detectable absorption features, there is a growing interest in water-leaving reflectances at near infrared (NIR) wavelengths, usually taken as the range 700-1,000 nm. There are essentially three reasons for this. First, although for atmospheric correction over clear waters the NIR water-leaving reflectance can usually be taken as zero (Gordon and Wang 1994), for turbid waters it is essential to model or estimate NIR water-leaving reflectance in order to 1 Corresponding author (K.Ruddick@mumm.ac.be). 2 Present address: Polytechnic of Namibia, P/Bag 13388, Windhoek, Namibia. AcknowledgmentsThis study was funded by the Belgian Science Policy Office's STEREO program in the framework of the BELCOLOUR project SR/00/03, by the European Union under the REVAMP project EVG1-CT-2001-00049, and by PRODEX contract 15190/01. The captains, crews, and support staff of the research vessels Belgica and Zeeleeuw are thanked for their enthusiastic help with the seaborne measurements. Jean-Paul Huot and the scientists of the RE-VAMP project and the MERIS Validation Team are especially thanked for the many discussions that have helped to improve and control the quality of the seaborne measurements. Wolfgang Cordes of GKSS is acknowledged for the tests on polarization and droplet sensitivity of spectroradiometers. Rudiger Heuermann of TriOS and Jean-Pierre De Blauwe and André Pollentier of MUMM-Oostende are thanked for help with system design; Barbara Van Mol for help with figure preparation; Bouchra Nechad for discussions on the use of the near infrared (NIR) range for total suspended matter (TSM) retrieval; and Arnold Dekker for discussion of op...

Bio-Optical Characterization and Ocean Colour Inversion in the Eastern Lagoon of New Caledonia, South Tropical Pacific

“…These measurements however, are known to have residual uncorrected sky and sun glint signals manifested by a positive (approximately white) spectral shift [14,61]. This positive bias in the above-water R rs can be corrected using simple white offset corrections for waters where null near-medium-infrared water reflectance can be assumed [61], or using more elaborated correction schemes for turbid waters [87]. Such residual correction was not performed by Ouillon et al [38], which may explain the negative biases of the algorithm when applying R rs obtained by in-water approaches as for the ELNC in the present work.…”

Potential of High Spatial and Temporal Ocean Color Satellite Data to Study the Dynamics of Suspended Particles in a Micro-Tidal River Plume

“…This coefficient may vary strongly with wind speed for clear sky conditions because of reflection of brighter parts of the sky in the case of higher waves [32], but is approximately independent of wind speed under overcast skies. This is accounted for by switching between clear sky and cloudy sky models for ρ, according to the ratio Ls/Ed at 750 nm (see Equations (23) and (24) in [33]). For each of the 27 stations, the measured Rrs spectra were filtered, then averaged.…”

Ocean reflectance spectra at the red, near‐infrared, and shortwave infrared from highly turbid waters: A study in the Bohai Sea, Yellow Sea, and East China Sea

“…In recent years, satellite-derived NIR nL w (l), as well as some examples of the SWIR nL w (l), have been successfully used (Shi and Wang 2012a;Wang et al 2012a). On the other hand, Ruddick et al (2006) showed that the shape of the NIR nL w (l) spectrum is determined largely by pure water absorptions. Thus, the ratio of nL w (l) at the two NIR bands (e.g., 748 and 869 nm for MODIS) is almost a constant (similarity spectrum at the NIR bands; Ruddick et al 2006).…”

“…Thus, oceancolor products from satellite ocean-color sensors such as MODIS on the satellite Aqua have been routinely derived using the two MODIS NIR bands (748 and 869 nm; Gordon and Wang 1994), with the assumption of a black ocean in the NIR for the open ocean. For turbid waters in coastal regions, however, the NIR black-ocean assumption is often invalid (Ruddick et al 2000;Siegel et al 2000;Wang and Shi 2005), leading to errors in the satellite-derived ocean-color products (Wang 2007;Wang et al 2009b). Some modifications have been implemented to account for the NIR ocean contributions for productive (but not very turbid) nearshore or coastal waters (Stumpf et al 2003;Bailey et al 2010).…”