Radiance transmittance measured at the ocean surface

Area of Science:

• Ocean optics
• Remote sensing
• Aquatic science

Background:

• Radiance transmittance (Tr) is a key optical parameter defined as the ratio of water-leaving radiance (Lw(0+)) to sub-surface upwelling radiance (Lu(0-)).
• A theoretical constant value of approximately 0.54 for Tr has been widely adopted in ocean optics and remote sensing, but it has not been empirically validated in situ.
• Accurate Tr values are essential for interpreting ocean color data and understanding marine radiative transfer.

Purpose of the Study:

• To present the first in situ measurements of zenith radiance transmittance (Tr) across a broad spectrum (350-700 nm) in diverse oceanic environments.
• To validate the widely used theoretical Tr value (~0.54) against field measurements.
• To assess the performance of a novel setup for simultaneous measurement of upwelling and water-leaving radiance.

Main Methods:

• Developed and deployed a robust field setup for simultaneous in situ measurement of sub-surface upwelling radiance (Lu(0-)) and water-leaving radiance (Lw(0+)).
• Collected radiance measurements across oceanic waters, covering the spectral range from 350 to 700 nm.
• Calculated radiance transmittance (Tr) from the measured radiance data and compared it with the theoretical value of 0.54.

Main Results:

• Measured Tr values in oceanic waters were found to be generally consistent with the theoretical value of 0.54, with a mean relative difference below 10%.
• The best agreement between measured and theoretical Tr was observed in the 400-600 nm spectral range, with an average difference less than 5%.
• Larger discrepancies (average difference < 15%) were noted for wavelengths longer than 600 nm, attributed to low radiance values and environmental conditions.

Conclusions:

• The study provides the first direct field validation of the theoretical radiance transmittance (Tr) value in oceanic waters.
• The results increase confidence in the application of the ~0.54 Tr value for ocean optics and remote sensing, particularly within the 400-600 nm spectrum.
• The successful simultaneous measurement of Lu(0-) and Lw(0+) validates the experimental setup for future optical studies in aquatic environments.