Analysis of Nighttime Radiances Measured by VIIRS Satellite Sensor (NASA/NOAA) Over Coastal Waters at Seasonal and Daily Time Scales. Application to the Observation of River Discharges During Flooding Events

The observation of coastal waters using satellite remote sensing is of primary importance to gain understanding on the complex processes occurring in those highly dynamical ecosystems. The analysis of ocean color data relies on the interaction between the incident sunlight daytime radiation that propagates within the ocean-atmosphere system and the suspended material (hydrosols and aerosols) present in each medium. The goal of this study is to examine how relevant nighttime radiances as measured by the panchromatic Day-Night Band (DNB) of the Visible Infrared Imaging Radiometer Suite (VIIRS) satellite sensor could be for the detection and monitoring of river plumes in turbid waters. The spatial and temporal variation of the nighttime radiance is studied over areas that are subjected to intense river flooding events, namely, the regions of the delta of Mekong (Vietnam) and the delta of Rhone (France), at seasonal and daily scales and for various magnitudes of the lunar illumination. The VIIRS observations show that the variations of the radiance are closely related to seasonality of the water turbidity; higher radiances are observed during the wet season as a result of the intense rainfalls. The seasonal cycle of the radiance is noticeable near the full moon phase for both areas, thus demonstrating the high sensitivity of VIIRS sensor radiometry for detecting ocean changes at night. It was also noted that the radiance seasonal cycle remains distinguishable for the Rhone area for a lunar illumination that could be far from the full moon condition, namely, for a fraction value F of the full moonlight of 40%. The impact of the adjacency effects induced by the diffuse glow of city lights on the nighttime radiance was examined at seasonal scale. Such an impact could be as high as 40% over water targets, thus suggesting that aquatic areas located sufficiently far from coastal cities should be preferentially used to properly observe water turbidity at night. The analysis of flooding events at a daily time scale revealed that intense river inputs into the ocean could be satisfactorily distinguished, including for low fractions of lunar illumination (F approximate to 20%). The analysis of the influence of ocean dynamics on the spatiotemporal variation of the radiance during river flooding events highlighted that the wind-driven current is the major contributor to the displacement of the suspended matter within the Rhone river plume that could explain the rapid hourly variations of the nighttime radiance. This study demonstrates the high benefit of measuring the nighttime upward radiation over turbid waters using ocean color remote sensing, which topic has not been deeply investigated so far, for the analysis and monitoring of coastal processes, such as the transport of suspended matter.