Uncertainties in the Geostationary Ocean Color Imager (GOCI) Remote Sensing Reflectance for Assessing Diurnal Variability of Biogeochemical Processes

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.

Assessment of the Number of Valid Observations and Diurnal Changes in Chl-a for GOCI: Highlights for Geostationary Ocean Color Missions

“…Such results appear inconsistent with the conventional understanding of the physical and biogeochemical characteristics of the open ocean, where changes in water constituents and the associated optical properties are considered negligible over the course of a day. For example, water columns in an open ocean region were assumed to be diurnally invariant by Concha, Mannino, Franz and Kim [ 25 ], and they used this assumption to gauge the uncertainties in GOCI ocean color products.…”

“…Real geostationary measurements from GOCI have been used to demonstrate its advantages over coastal oceans by tracking the diurnal changes in the sediment plumes of riverine estuaries [ 6 , 22 ], documenting the short-term spatial and vertical redistributions of harmful algal blooms [ 5 , 7 , 23 ], and detecting the continuous evolution of internal waves [ 24 ], etc. In the open ocean, however, the optical characteristics of the water column have commonly been considered diurnally invariant; this assumption has even been applied to determine the accuracy of the GOCI ocean color products [ 25 ]. In contrast, previous in situ investigations have demonstrated that the concentrations of water constituents and their optical properties vary considerably over a short period (daily or hourly) in clear oceanic waters [ 26 , 27 , 28 ].…”

Sampling Uncertainties of Long-Term Remote-Sensing Suspended Sediments Monitoring over China’s Seas: Impacts of Cloud Coverage and Sediment Variations

“…GOCI is the first geostationary ocean color satellite sensor, onboard the South Korean Communication, Ocean and Meteorological Satellite (COMS). It has successfully collected data covering geographical regions between 110 • E and 150 • E, and 2 • N and 50 • N, with a 500-m spatial resolution, since its launch in June 2010 [16,32]. GOCI was designed to measure six visible light bands (at 412, 443, 490, 555, 660 and 680 nm) and two near infrared bands (at 745 and 865 nm) with hourly observations during the daytime.…”

“…In most conditions, valid samplings are often limited: routine field surveys are generally conducted monthly by local authorities, such as the environmental or water quality agency. The most widely used Terra/Aqua MODIS satellites, designed to perform two observations a day, are particularly prone to disruption from cloud coverage and other poor weather conditions, as well as sun glints [16][17][18].…”

“…Thus, the geostationary-orbit (GEO) ocean color sensors have become prevalent to meet the demands of the high-frequency sampling of water quality. The first GEO ocean color satellite, the Geostationary Ocean Color Imager (GOCI), was launched by South Korea in 2010, and takes hourly measurements during the daytime [16]. GEO satellites, including GOCI, provide high-frequency sampling to capture the dynamic of coastal/inland waters [28].…”