High-frequency Observation Of Floating Algae Bloom 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),AHI(Advanced Himawari Imagers)is the sensor onboard Himawari-8,and the greatest advantage of AHI is its temporal resolution of full disk up to 10 minutes,along with broad spatial coverage.Although designed as a meteorological satellite,the Himawari-8/AHI 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,these characteristics make it with great potential to realize ocean color remote sensing.In this study,the Level-0 data of Himawari-8/AHI was calculated and analyzed,the 6S model(Second Simulation of the Satellite Signal in the Solar Spectrum)was subsequently modified to do atmospheric correction,then the Rr,(Rayleigh-corrected reflectance)is derived.The Rrc,were applied to produce the FAI(Floating Algae Index)(Hu,2009).The FAI developed for ocean color satellites is adapted to process Himawari-8/AHI data for the first time and applied for waters of Lake Taihu,China.The threshold of Himawari-8/AHI FAI in Taihu Lake was further determined and found to be-0.008.By evaluation of the spatial and temporal patterns of FAI,a correlation coefficient(r)of 0.92 was obtained between the algae area derived from Himawari-8/AHI FAI and that from the Moderate Resolution Imaging Spectroradiometer(MODIS)FAI,and the mean percentage difference is～5%in algae coverage for a total of 18 near-cloud-free images.More concurrent images(n=80)were collected for a comparison between Himawari-8/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～8%in observed algae coverage.Furthermore,Himawari-8/AHI FAI was also applied on algae bloom cases happened in the Yellow Sea and the East China sea to demonstrate the applicability of Himawari-8/AHI FAI in other waters.These results indicate that Himawari-8/AHI can obtain reliable observations of floating algae at ultrahigh temporal resolutions(10 minutes).For example,it was found 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%)in October,201 7.This study proves that Himawari-8/AHI with ultra-high frequency can detect several natural phenomena missed by GOCI,and Himawari-8/AHI can provide effective observation when GOCI fails due to cloud cover;hence,Himawari-8/AHI offers greater possibilities and better opportunities to observe the dynamics and contingencies of the Earth system.Ultra-high frequency observations are significant not only for ecological and environmental monitoring but also for better scientific understanding of algal dynamics.In addition to the five times higher temporal resolution than GOCI,the Himawari-8/AHI has a broader spatial coverage.Himawari-8/AHI data products(such as Himawari-8/AHI FAI proposed in this study)can play an important role in the scientific research and observation of more inland waters and coastal ecosystems as well as other marine waters.