| Summary: | The diurnal variation of the diffuse attenuation coefficient for downwelling irradiance at 490 nm <inline-formula><math display="inline"><semantics><mrow><mo stretchy="false">(</mo><msub><mi>K</mi><mi mathvariant="normal">d</mi></msub><mrow><mo>(</mo><mn>490</mn><mo>)</mo></mrow><mo stretchy="false">)</mo></mrow></semantics></math></inline-formula> has complex characteristics in the coastal regions. However, owing to the scarcity of in situ data, our knowledge on the diurnal variation is inadequate. In this study, an optical-buoy dataset was used to investigate the diurnal variation of <inline-formula><math display="inline"><semantics><mrow><msub><mi>K</mi><mi mathvariant="normal">d</mi></msub><mrow><mo>(</mo><mn>490</mn><mo>)</mo></mrow></mrow></semantics></math></inline-formula> in the coastal East China Sea, and to evaluate the <inline-formula><math display="inline"><semantics><mrow><msub><mi>K</mi><mi mathvariant="normal">d</mi></msub><mrow><mo>(</mo><mn>490</mn><mo>)</mo></mrow></mrow></semantics></math></inline-formula> L2 products of geostationary ocean color imager (GOCI), as well as the performance of six empirical algorithms for <inline-formula><math display="inline"><semantics><mrow><msub><mi>K</mi><mi mathvariant="normal">d</mi></msub><mrow><mo>(</mo><mn>490</mn><mo>)</mo></mrow></mrow></semantics></math></inline-formula> estimation in the Case-2 water. The results of validation show that there was high uncertainty in GOCI L2 <inline-formula><math display="inline"><semantics><mrow><msub><mi>K</mi><mi mathvariant="normal">d</mi></msub><mrow><mo>(</mo><mn>490</mn><mo>)</mo></mrow></mrow></semantics></math></inline-formula>, with mean absolute percentage errors (MAPEs) of 69.57% and 68.86% and root mean square errors (RMSEs) of 0.70 and 0.71 m<sup>−1</sup> compared to buoy-measured <inline-formula><math display="inline"><semantics><mrow><msub><mi>K</mi><mrow><mrow><mi mathvariant="normal">d</mi><mn>12</mn></mrow></mrow></msub><mrow><mo>(</mo><mn>490</mn><mo>)</mo></mrow></mrow></semantics></math></inline-formula> and <inline-formula><math display="inline"><semantics><mrow><msub><mi>K</mi><mrow><mrow><mi mathvariant="normal">d</mi><mn>13</mn></mrow></mrow></msub><mrow><mo>(</mo><mn>490</mn><mo>)</mo></mrow></mrow></semantics></math></inline-formula>, respectively. Meanwhile, with the coefficient of determination (R<sup>2</sup>) of 0.71, as well as the lowest MAPE of 27.31% and RMSE of 0.29 m<sup>−1</sup>, the new dual ratio algorithm (NDRA) performed the best in estimating <inline-formula><math display="inline"><semantics><mrow><msub><mi>K</mi><mi mathvariant="normal">d</mi></msub><mrow><mo>(</mo><mn>490</mn><mo>)</mo></mrow></mrow></semantics></math></inline-formula> in the target area, among the six algorithms. Further, four main types of <inline-formula><math display="inline"><semantics><mrow><msub><mi>K</mi><mi mathvariant="normal">d</mi></msub><mrow><mo>(</mo><mrow><mn>490</mn></mrow><mo>)</mo></mrow></mrow></semantics></math></inline-formula> diurnal variation were found from buoy data, showing different variabilities compared to the area closer to the shore. One typical diurnal variation pattern showed that <inline-formula><math display="inline"><semantics><mrow><msub><mi>K</mi><mi mathvariant="normal">d</mi></msub><mrow><mo>(</mo><mn>490</mn><mo>)</mo></mrow></mrow></semantics></math></inline-formula> decreased at flood tide and increased at ebb tide, which was confirmed by GOCI images through the use of NDRA. Hydrometeorological factors influencing the diurnal variations of <inline-formula><math display="inline"><semantics><mrow><msub><mi>K</mi><mi mathvariant="normal">d</mi></msub><mrow><mo>(</mo><mn>490</mn><mo>)</mo></mrow></mrow></semantics></math></inline-formula> were also studied. In addition to verifying the predominant impact of tide, we found that the dominant effect of tide and wind on the water column is intensifying sediment resuspension, and the change of sediment transport produced by them are secondary to it.
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