Mechanism of attenuation coefficient variability of underwater upward radiance

Upward radiance is the signal source of ocean colour remote sensing. Its attenuation characteristics along with water depth not only determine the water-leaving radiance but are also the key parameters that reflect water-quality conditions. Assuming that the water is optically homogeneous in the vertical direction, this study calculates the attenuation coefficient of upward radiance (ACUR) under conditions with different zenith angles and absorption and scattering coefficients using the radiative transfer model coupled with multiple scattering. Then, the attenuation mechanism of upward radiance is discussed. The results obtained indicate the following. (1) The ACUR under the condition of a large zenith angle gradually decreased and subsequently increased with the increasing scattering coefficient when the absorption was less. The variation trend of the ACUR was gradually weakened with increasing absorption coefficients. (2) The ACUR at a small zenith angle increased approximately linearly with the increasing scattering coefficient; subsequently, this trend gradually disappeared. At the same time, the change rate of ACUR first increased and then decreased with increasing scattering when the absorption coefficient increased. (3) The absorption coefficient has the greatest contribution to the ACUR. When the scattering/absorption ratio is between 0.5 and 37, the corresponding contribution ratio is in the range of 90.3–98.5%. (4) When the ratio is less than 1.5, the contribution of the zenith angle to the ACUR is greater than that of the scattering coefficient, and the corresponding contribution ratios ranged from 2.2% to 7.3% and from 3.7% to 17.9%. With the increasing ratio, the contribution of the scattering coefficient to the ACUR was more than that of the zenith angle, and then this trend became stable. The results were validated by field observation data from Lake Taihu (China).