Observation and simulation of the diffuse attenuation coefficient of downwelling irradiance in the Polar Ocean

The three-stream radiation transfer model is used to investigate the fluctuation in the underwater diffuse attenuation coefficient of downwelling irradiance in the polar ocean with a high solar zenith angle and different direct radiation proportions. First, the applicability of the three-stream radiation model in the polar region is validated by using 18 in situ observation data from September to October 2009 in the Beaufort Sea. Statistics show that in the absence of sea ice, the average relative errors between the simulation and observation values for 490 nm downwelling irradiance (E-d(490)) and its diffuse attenuation coefficient (K-d(490)) are 7.04% and 9.88%, respectively. At the stations surrounded by sea ice, the radiation is relatively small due to ice blocking, and the average relative errors simulated by the model reach 15.89% and 15.55%, respectively. Second, simulations with different chlorophyll concentrations and different proportions of direct radiation reveal that a high solar zenith angle has a greater impact on K-d(490) in the surface water. K-d(490) is less affected by the light field (affected by the solar zenith angle and the proportion of direct radiation) at depths greater than 30 m, and meets the linear relationship with the inherent optical parameters (the sum of the absorption coefficient and backscattering coefficient). The surface K-d(490) is still consistent with that at a depth of more than 50 meters under a high solar zenith angle, implying that the surface K-d(490) can also be considered as an inherent optical parameter at a high solar zenith angle (greater than 60 degrees). The relative error of obtaining surface K-d(490) by using the linear relationship at the 50 m layer is found to be less than 8% in the seawater with chlorophyll concentration greater than 0.05 mg m(-3). The effect of the solar zenith angle and proportion of direct radiation can be ignored when measuring the diffuse attenuation coefficient in the polar region. Finally, the model can correct the ice induced fluctuation in downward irradiance, allowing for optical research of seawater beneath the ice in the polar ocean.