Radiometric Sensitivity and Signal Detectability of Ocean Color Satellite Sensor Under High Solar Zenith Angles

Abstract

New generation ocean color imagers on geostationary orbits are designed to provide a much higher temporal resolution along with enhanced spatial and spectral resolutions that will open up obvious opportunities for improving the sampling frequency and resolving diurnal variability of phytoplankton and other biogeochemical properties in dynamic coastal waters. Despite the capabilities of such new generation sensors to detect the diurnal cycles of various ocean phenomena, there is a lack of knowledge on their radiometric sensitivity and signal detectability for observing the ocean color at morning or evening hours. This paper aims to explore the capability of geostationary satellite ocean color sensor for detecting ocean biogeochemical properties [chlorophyll (CHL); total suspended matter (TSM); colored dissolved organic matter (CDOM)] under high solar zenith angles (SZAs). The analysis is based upon simulations from the vector radiative transfer model for the coupled ocean–atmosphere system (PCOART-SA), which considers the earth curvature effects. The unitless differential signal-to-noise ratio ( $\Delta $ SNR) is used as a discriminant parameter to indicate the radiometric sensitivity to variation of different biogeochemical properties. The results showed that the SZAs have a significant impact on the signal detectability for CHL variation. For typical shelf water (CHL $= 1\,\,\mu \text{g}$ /L, TSM = 1 mg/L, CDOM = 0.15 m−1), with the typical observation zenith angle (OZA) = 30°, changes on the order of $\Delta $ CHL $= 0.024\,\,\mu \text{g}$ /L (2.4% to background CHL) were detectable when SZA = 30°; when SZA > 75°, the detectable minimal $\Delta $ CHL increased to $0.77~\mu \text{g}$ /L (77%), indicating the difficulty of detecting CHL under high SZA. For CDOM, the detectability of changes ( $\Delta $ CDOM) was also found to be closely related to the SZAs, i.e., changes on the order of ten times depending on the SZA conditions. However, even under extremely high SZA conditions (SZA = 80°, OZA = 30°), $\Delta $ CDOM = 0.007 m−1 which is about 4.7% of the background CDOM was still detectable at 412 nm. On the other hand, under high SZA conditions (SZA = 80°, OZA = 30°), $\Delta $ TSM = 0.211 mg/L (2.1% to the background TSM) was also detectable. Overall, our results indicate that under high SZAs conditions, the geostationary satellite ocean color sensor may experience difficulty in detecting a slight change in CHL variation in productive waters, but it still can detect small changes in TSM and CDOM contents despite a reduced sensitivity at the steeper SZAs.