Abstract
Short-term (sub-diurnal) biological and biogeochemical processes cannot be fully captured by the current suite of polar-orbiting satellite ocean color sensors, as their temporal resolution is limited to potentially one clear image per day. Geostationary sensors, such as the Geostationary Ocean Color Imager (GOCI) from the Republic of Korea, allow the study of these short-term processes because their orbit permit the collection of multiple images throughout each day for any area within the sensor’s field of regard. Assessing the capability to detect sub-diurnal changes in in-water properties caused by physical and biogeochemical processes characteristic of open ocean and coastal ocean ecosystems, however, requires an understanding of the uncertainties introduced by the instrument and/or geophysical retrieval algorithms. This work presents a study of the uncertainties during the daytime period for an ocean region with characteristically low-productivity with the assumption that only small and undetectable changes occur in the in-water properties due to biogeochemical processes during the daytime period. The complete GOCI mission data were processed using NASA’s SeaDAS/l2gen package. The assumption of homogeneity of the study region was tested using three-day sequences and diurnal statistics. This assumption was found to hold based on the minimal diurnal and day-to-day variability in GOCI data products. Relative differences with respect to the midday value were calculated for each hourly observation of the day in order to investigate what time of the day the variability is greater. Also, the influence of the solar zenith angle in the retrieval of remote sensing reflectances and derived products was examined. Finally, we determined that the uncertainties in water-leaving “remote-sensing” reflectance (Rrs) for the 412, 443, 490, 555, 660 and 680 nm bands on GOCI are 8.05 × 10−4, 5.49 × 10−4, 4.48 × 10−4, 2.51 × 10−4, 8.83 × 10−5, and 1.36 × 10−4 sr−1, respectively, and 1.09 × 10−2 mg m−3 for the chlorophyll-a concentration (Chl-a), 2.09 × 10−3 m−1 for the absorption coefficient of chromophoric dissolved organic matter at 412 nm (ag (412)), and 3.7 mg m−3 for particulate organic carbon (POC). These Rrs values can be considered the threshold values for detectable changes of the in-water properties due to biological, physical or biogeochemical processes from GOCI.
Highlights
Ocean waters are highly dynamic due to environmental factors such as heating of the surface ocean layer, fluctuation in wind intensity, surface currents, tidal cycles, changes in vertical mixing layers and variation of sunlight radiation
An analysis of the complete Geostationary Ocean Color Imager (GOCI) time series in the GCWS region was conducted to provide an understanding of the long term variability in remote sensing reflectances (Rrs) and biogeochemical products to provide a context for interpreting the short term variability
With the possible exception of the phytoplankton fluorescence signal in Rrs(680), the remaining GOCI Rrs and biogeochemical products studies demonstrated no measurable sub-diurnal to day-to-day variability of in-water properties in the GCWS
Summary
Ocean waters are highly dynamic due to environmental factors such as heating of the surface ocean layer, fluctuation in wind intensity, surface currents, tidal cycles, changes in vertical mixing layers and variation of sunlight radiation. These dynamics produce changes in marine ecosystem processes, such as ocean primary production, carbon stocks, export production and phytoplankton community composition and their effects can be measured at different time scales, from decades to years, all the way to days or hours Longer term variations, such as seasonal, interannual and decadal patterns in phytoplankton stocks, optical properties and primary production, have been extensively studied using low earth orbit (LEO) assets (e.g., [1,2,3,4]) such as the Sea-Viewing Wide Field-of-View Sensor (SeaWiFS) [5], Moderate Resolution Imaging Spectroradiometer (MODIS) [6] and the Visible Infrared Imaging Radiometer Suite (VIIRS) [7]). GEO missions such as these three have the capability to provide quasi-global coverage at low and mid-latitudes [9]
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