基于VIIRS卫星遥感影像的南海近岸海域透明度时空变化特征

Spatio-temporal variations of Secchi disk depth in coastal waters of the South China Sea from VIIRS observations

水体透明度是评价水下光场分布的重要指标,反映了水下生物、物理和化学动力变化过程。利用不同光学类型的现场数据,分析南海近岸海域透明度对遥感反射光谱的影响,构建基于VIIRS波段设置的透明度遥感评价模型,经过验证,应用于2013—2016年VIIRS数据,揭示了南海近岸海域透明度的时空变化特征,同时讨论了VIIRS和MODIS的海洋环境监测能力。结果表明,当透明度<5.3 m时,在555 nm处出现明显反射峰,而当透明度>5.3 m时,拐点出现在488 nm处。VIIRS的R_(rs)(488)/R_(rs)(555)与透明度显著相关(R~2=0.77),适于南海近岸海域透明度的遥感评价。将该模型应用于VIIRS数据,发现南海近岸海域透明度表现为离岸较近海域透明度低于较远海域,西部低于东部。VIIRS具有较高的空间分辨率和性能,环境监测能力优于MODIS。

Water transparency is an important ecological indicator for evaluating the underwater light field,which determines the biological,physical and chemical processes in waters.Based on the field measurements from case I and case II waters,the effect of Secchi disk depth on remote sensing reflectance of water was performed to develop a Secchi disk depth estimation algorithm for coastal waters of the South China Sea.The optical model,validated by using local data in October 2015,was used with monthly VIIRS data from 2013 to 2016 to assess the temporal and spatial variability of the Secchi disk depth in coastal waters of the South China Sea.Furthermore,a comparison of VIIRS and MODIS for monitoring marine environment was performed in this study.Results demonstrated that the Secchi disk depth highly controlled the remote sensing reflectance of water at bands of 488 and 555 nm.A reflected peak was found at 555 nm when the Secchi disk depth higher than 5.3 m,while on the condition of the Secchi disk depth lower than 5.3 m,the peak moved to the waveband of 488 nm.The band ratio of VIIRS,R_(rs )(488)/R_(rs )(555),was highly correlated to the Secchi disk depth in coastal waters of the South China Sea,which was then applied to the calibrated VIIRS images and found that the Secchi disk depth was higher in the offshore than nearshore and also in the east than west.Compared to MODIS data,the VIIRS instrument has higher spatial resolution and calibration activities,which will provide more ocean color products in future.