Diurnal dynamics and coastal challenges in chlorophyll-a monitoring using Himawari-8 satellite data

Chlorophyll-a concentration estimated from satellite measurements is useful to monitor phytoplankton biomass. However, traditional methods of using polar orbit satellites only provide low frequency observation, inadequate to detect hourly changes that often trigger ecological events. With high temporal frequency data, geostationary sensors are expected to solve this problem. This research investigates whether high frequency data from Himawari-8, when applied rigorous quality control, can be reliable for oceanographic applications. A large dataset of Himawari-8 pixels was collocated with Sentinel-3 during four different three-day periods, corresponding to solstices and equinox in 2019. After applying a homogeneous filter (CV <0.2), full-disk analysis has exhibited high correlations between two datasets with r ≈ 0 . 82 in June, September, and December. However, the correlation decreases in March with r=0.685 due to misestimation of chlorophyll-a concentration at high viewing zenith angle. In low viewing zenith angle areas, Himawari-8,overestimates chlorophyll-a concentration compared to Sentinel-3, while in areas with higher viewing zenith angle of above 35°, chlorophyll-a concentration is underestimated by up to 12%–19% compared to Sentinel-3. These misestimations are mostly located in the viewing zenith angle range of 45°to 55°. For temporal analysis, one-hour timeseries analysis shows a very low day-to-day variation in offshore oligotrophic gyres (<0.05 mg/ m 3 ). In the coastal area, the noon variation can peak up to 0.2 mg/ m 3 . Although these patterns are similar to the known diurnal variation, when phytoplankton absorbs available sunlight and nutrients to grow, there is no direct evidence to prove that observed chlorophyll-a variations are caused by the growth of phytoplankton. • Large-scale intercomparison of ocean color data from geostationary satellites. • High consistency found between Himawari-8 and Sentinel-3 chlorophyll-a concentration. • Applied quality filtering improved agreement in chlorophyll-a data between sensors. • Himawari-8 chlorophyll-a is underestimated at high viewing angles near coasts. • Further study is needed to link observed daily variation and phytoplankton growth.