High-frequency observation of floating algae from AHI on Himawari-8

Himawari-8 (H8) is a geostationary meteorological satellite launched by JAXA (Japan Aerospace Exploration Agency) and is now operated by JMA (Japan Meteorological Agency). It takes measurements at a temporal resolution of 10 min for full disk view. Although designed as a meteorological satellite, the Advanced Himawari Imager (AHI) onboard H8 has three visible (460, 510 and 640 nm), one near infrared (860 nm) and two shortwave infrared bands (1610 and 2257 nm) to observe the Earth system. In this study, the Floating Algae Index (FAI) developed for ocean color satellites (Hu, 2009) is adapted to process AHI data for the first time and applied for waters of Lake Taihu, China. For a total of 18 near-cloud-free images, a correlation coefficient (r) of 0.92 was obtained between the algae area derived from AHI FAI and that from the Moderate Resolution Imaging Spectroradiometer (MODIS) FAI, and the mean percentage difference is similar to 5% in algae coverage. More concurrent images (n = 80) were collected for a comparison between AHI and Geostationary Ocean Color Imager (GOCI), as GOCI is also a geostationary satellite, resulting in a correlation coefficient of 0.91 and percentage deviation of similar to 8% in observed algae coverage. These results indicate that H8/AHI can obtain reliable observations of floating algae at ultrahigh temporal resolutions (10 min). Especially, such ultrahigh-frequency measurements show that part of Lake Taihu (e.g., Meiliang Bay) experienced more frequent events of floating algae (mostly > 60%) than that observed by GOCI (generally < 40%). These results indicate that ultrahigh-frequency measurements are important not only for efficient environmental monitoring but also for the scientific understanding of algae dynamics.