Diurnal Variation of the Diffuse Attenuation Coefficient for Downwelling Irradiance at 490 nm in Coastal East China Sea

Yu Zhang,Wenxi Cao,Wendi Zheng,Kai Zeng,Lin Deng,Yang Li,Yuezhong Yang,Yinxue Zhang,Guifen Wang,Zhantang Xu,Wen Zhou

doi:10.3390/rs13091676

Abstract

The diurnal variation of the diffuse attenuation coefficient for downwelling irradiance at 490 nm (Kd(490)) 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 Kd(490) in the coastal East China Sea, and to evaluate the Kd(490) L2 products of geostationary ocean color imager (GOCI), as well as the performance of six empirical algorithms for Kd(490) estimation in the Case-2 water. The results of validation show that there was high uncertainty in GOCI L2 Kd(490), 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−1 compared to buoy-measured Kd12(490) and Kd13(490), respectively. Meanwhile, with the coefficient of determination (R2) of 0.71, as well as the lowest MAPE of 27.31% and RMSE of 0.29 m−1, the new dual ratio algorithm (NDRA) performed the best in estimating Kd(490) in the target area, among the six algorithms. Further, four main types of Kd(490) 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 Kd(490) 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 Kd(490) 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.

Highlights

IntroductionThe diffuse attenuation coefficient for downwelling irradiance (Kd ) defines the rate of decrease of downwelling irradiance with water depth, which can be used, among other things, to compute primary production and describe the clarity of seawater [1,2,3,4,5]
The reliability of the optical buoy data has always been an important concern in its applications, because its radiometric measurements are affected by numerous factors [32]
The difference between the geostationary ocean color imager (GOCI) L2 Rrs (λ) and buoy Rrs12 (λ) was found to be smallest, which indicates that Rrs12 best characterizes the remote sensing signal of the upper-layer water column

Summary

Introduction

The diffuse attenuation coefficient for downwelling irradiance (Kd ) defines the rate of decrease of downwelling irradiance with water depth, which can be used, among other things, to compute primary production and describe the clarity of seawater [1,2,3,4,5]. Following the primary studies in the 1970s [6], the value of Kd at 490 nm (Kd (490)) is traditionally measured in situ at marine platforms or provided as a comment product by remote sensing satellite instruments [7]. Kd (490) of these waters is usually used to study the process of sediment transportation and Remote Sens. In highly dynamic coastal waters, Kd (490) shows significant diurnal variations because of tidal forcing, atmospheric and current disturbances [11,12]

Methods

Results

Discussion

Conclusion