藻华是一种常见的自然灾害现象，当在沿岸爆发时，常常危害生态环境和人类社会的生产生活，造成巨大经济损失。如何对其进行有效的监测与预测，成为全球性的关注热点。卫星观测是监测大空间尺度藻华的唯一有效手段。然而，目前在轨运行的卫星往往时空分辨率低，不能满足及时有效监测藻华发生发展的需求。

Harmful algal bloom is one of the most disastrous natural hazards that frequently endanger the ecological environment and human life and activity, resulting in also significant economic losses. Therefore, it has been a hot topic worldwide in recent decades to study and monitor algal blooms. Comparing to the traditional shipborne or station-based monitoring methods, satellite observations provide a much more effective approach to monitor algal blooms. However, the spatial-temporal resolutions of the currently operational satellites are far from enough to observe the detailed development of algal blooms.

本实验室光学海洋学研究组及时注意到日本新近发射的气象卫星Himawari-8/AHI。Himawari-8卫星作为静止气象卫星，其优势是获取地球全圆盘图的频率为每10分钟一次。Himawari-8上搭载有先进的葵花成像仪AHI，它有8个可见光、1个近红外和2个短波红外通道具备对海洋水色进行高时效性的连续观测潜力。团队新近成功实现了其大气校正，计算获得漂浮藻类指数(FAI)，确定太湖水域探测阈值为-0.008，漂浮蓝藻探测结果与水色界常用的MODIS和GOCI卫星的结果高度一致；同时AHI超高频率的监测结果表明，其在太湖（例如梅梁湖）的一部分水域中监测到了比GOCI所获取结果更频繁的漂浮藻类暴发情况（60％ vs. 40％）。该方法不仅可以对漂浮藻类藻华现象进行定性识别与定量分析，更可以依据超高的观测频率对藻华的日变化进行一个近连续性的观测，以捕捉到其他卫星，如韩国水色静止卫星GOCI，在监测藻华过程中可能遗漏的变化细节（Chen, Shang*, Lee, et al., 2019）。

The OOL (Optical Oceanography Lab) tried to utilize the data from the newly launched meteorological satellite Himawari-8 of Japan to assess the water quality and monitor the algal bloom in inland lakes. Himawari-8 satellite is a geostationary meteorological satellite, which allows for a full Earth disk view in around every 10 minutes. The Himawari-8 is equipped with an Advanced Himawari Imager(AHI), which takes radiometric measurements at 8 visible bands, one near-infrared band, and two short-wave infrared bands. The band configuration of AHI is very comparable to that of ocean color satellites. Therefore, AHI has great potentials in providing high-efficiency and continuous observation of ocean color data. In the recent efforts by Lee and Shang's group, the atmospheric correction for AHI imagery was carried out, and the Floating Algae Index (FAI) was calculated for demonstrations in Lake Taihu. It is found that the detection threshold of FAI for Lake Taihu waters was -0.008. The robustness of the atmospheric correction was later validated again concurrent measurements of MODIS and GOCI, where consistent results of detected floating algae were observed from those three satellite products. In addition, the use of AHI data could significantly elevate the frequency of detected floating Cyanophyta in the northern part of Lake Taihu (e.g., Lake Meiliang) than that obtained from GOCI (60% vs. 40%) (Chen, Shang *, Lee, et al., 2019). Results obtained in this study suggest that the ultra-high temporal resolution AHI data can qualitatively identify the floating algae and provide a quasi-continuous observation of the diurnal variation of algal blooms. AHI data is also able to capture the detailed variability in algae blooms within a short time period that other satellites (e.g., GOCI) may be deemed incapable.

图（1）2017年11月23日太湖漂浮藻类面积的日变化，蓝点与蓝色曲线表示Himawari-8/AHI FAI的观测结果；绿色红色分别表示GOCI AFAI和MODIS FAI的观测结果。下图为2017年11月23日太湖漂浮藻类分布，左图为FAI/AFAI图像，右图为二值图

Fig. 1. (Top) Diurnal change of the area of floating algal patches in Lake Taihu on November 23, 2017. Blue dot and curve: area derived from AHI FAI; red bar: area derived from GOCI AFAI; green bar: area derived from MODIS. (Bottom) Distribution of floating algae in Lake Taihu on November 23, 2017, with the FAI/AFAI images (left) and the binary images (right). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

因此，AHI提供了更大的可能性和更好的机会来观察生态系统中的动态事件和偶然事件，这不仅对于生态环境监测富有重要意义，且对于科学上更好地理解藻类动态变化也极为重要。同时，Himawari-8/AHI的空间覆盖范围超出现有的静止水色卫星，预期Himawari-8/AHI将在更多内陆水域、沿海生态系统以及大洋开阔海域的环境监测与科学研究中发挥巨大作用。最关键的是，马尾藻、团扇藻等漂浮藻类威胁核电取水，本实验室提供的超高频率的Himawari-8卫星观测已经在2018年4月博鳌论坛期间监测海南昌江核电遭遇的大型藻类侵袭发挥作用。未来融合多传感器拓展该技术的应用，对于及时预警、使得核电厂得以及时采取措施应对，减轻损失，具有重大意义。

This study also highlight that high temporal resolution satellite imagery could provide a more effective means to observe both dynamic events and occasional events in the ecosystem (Chen, Shang *, Lee, et al., 2019), which is of great significance for ecological environment monitoring and enables better understanding of algae blooms dynamic changes. In addition, the spatial coverage of Himawari-8/AHI is much more extensive than the existing geostationary satellites. It is expected that Himawari-8/AHI data could be employed in more inland waters, coastal waters, and open oceans for ecosystem monitoring and scientific researches. For example, floating algae, such as Sargassum and Padina, are great theaters to the nuclear power plant as they may prevent the water intaking. During the Boao Forum in April 2018, Hainan Changjiang nuclear power plant encountered two serious floating algae invasions, and two units of the nuclear power plant were forced to shut down. Note that the continuous floating algae products from Himawari-8 data provided by our group had provided the supports for the decision to shut down the two units. We expect that monitoring floating algae using Himawari-8 data would be essential to provide the necessary information for early warning, which will enable nuclear power plants to take timely measures to reduce losses.

图（2） Himawari-8/AHI和GOCI分别监测到2017年10月份太湖水面被漂浮藻华覆盖的频率

Fig. 2. The frequency of water surface covered by floating algae in October 2017, detected from AHI and GOCI, respectively. It was calculated by counting the times a pixel was covered by floating algae in that month, and then divided this number by the total times of cloud-free observations at that pixel in that month. The red arrow points to Meiliang Bay, where AHI shows a higher frequency of floating algae than GOCI. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Xinrong Chen; Shaoling Shang; Zhongping Lee; Lin Qi; Jing Yan; Yonghong Li. High-frequency observation of floating algae from AHI on Himawari-8. Remote Sensing of Environment, 2019, 227: 151–161.