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

TLDR

Theory and seaborne measurements are presented for the near‑infrared (700‑900 nm) water‑leaving reflectance in turbid waters, and theory predicts that the NIR spectrum shape is largely determined by pure water absorption and is thus almost invariant. The study outlines applications of the NIR similarity spectrum to atmospheric correction of ocean color data and to quality control of seaborne, airborne, and spaceborne reflectance measurements in turbid waters. A similarity NIR reflectance spectrum is defined by normalizing at 780 nm, calculated from seaborne measurements, compared with laboratory‑derived spectra, and its shape is analyzed theoretically and via radiative transfer simulations. Simulations confirm the similarity spectrum is valid for waters ranging from moderately to extremely turbid, and measurement uncertainties and the critical air‑sea interface correction for low reflectances are quantified.

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 10−4 or total suspended matter concentration of order 0.3 g m−3) to extremely turbid (e.g., reflectance at 780 nm of order 10−1 or total suspended matter of order 200 g m−3). 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.