Chlorophyll-a Monthly Frequency for High Anomalies

An additional sub-indicator will be provided to evaluate the intra-annual changes in chlorophyll-a concentration anomalies in each Exclusive Economic Zone (EEZ) using the NOAA VIIRS chlorophyll-a ratio anomaly product produced daily for the globe at 2 km spatial resolution. The daily global VIIRS chlorophyll-a concentrations are produced from the NOAA Multi-Sensor Level 1 to Level 2 (MSL12) processing of the VIIRS sensor on the Suomi SNPP satellite. [Wang et al., 2017; Wang et al.,2014] This anomaly product is defined as the daily chlorophyll-a concentration subtracted from a rolling 61-day mean baseline with a 15 day lag (based on Stumpf et al., 2003), then normalized to the rolling 61 day mean to create the proportional difference anomaly. The processing steps are outlined below. 1. Classify and count pixels as moderate, high or extreme anomalies. For each day in the reporting year, pixels in the global EEZ area (as defined in World EEZ v11) that are classified as: moderate (in the 90th percentile), high (in the 95th percentile) and extreme (in the 99th percentiles). The number of days a given pixel is classified as moderate, high, or extreme within each month is then calculated. The number of days in each month where the pixel has valid data is also counted. 2. Calculating the monthly statistics Because these anomalies are based on daily observations, data gaps are expected due to cloud cover, sun glint, high sensor zenith angle, high sun zenith angle, and other possible algorithm flags. To avoid bias due to non-valid data retrievals, the frequencies are normalized using the number of days in the month with valid observations, as follows: Relative frequency of classified pixel chlorophyll-a anomalies= αc / ε Where α = the number of days in the month with a classified (moderate, high or extreme anomaly) anomaly Where c indicates the anomaly classification (moderate, high, or extreme anomaly) Where ε = the number of days in the month with valid data Finally, the monthly mean of the relative frequencies for each class is calculated for each EEZ, resulting in 3 monthly values, one value in each of the 3 classes for each country. For more information see the Global Manual of Ocean Statistics Available on the UNEP Document Repository (pages 19-24).本研究将提供一项额外子指标，用于评估每个专属经济区（Exclusive Economic Zone, EEZ）内叶绿素a（chlorophyll-a）浓度异常的年内变化，所用数据为美国国家海洋和大气管理局（National Oceanic and Atmospheric Administration, NOAA）生产的VIIRS叶绿素a比例异常产品，该产品以2千米空间分辨率每日生成全球范围数据。 全球每日VIIRS叶绿素a浓度数据由搭载于Suomi国家极轨伙伴卫星（Suomi National Polar-orbiting Partnership, Suomi SNPP）上的VIIRS传感器，通过NOAA多传感器1级至2级（Multi-Sensor Level 1 to Level 2, MSL12）处理流程生成[Wang等，2017；Wang等，2014]。该异常产品的定义为：以滞后15天的61日滑动平均为基准，将当日叶绿素a浓度与该基准值相减，再将差值归一化至该61日滑动平均基准，以得到比例差值异常值（依据Stumpf等，2003）。 具体处理步骤如下： 1. 像元异常分类与计数 在报告年度的每日，将全球专属经济区（依据"World EEZ v11"划定）内的像元划分为三类：中度异常（处于90百分位数）、重度异常（处于95百分位数）及极端异常（处于99百分位数）。随后计算单幅像元在各月内被归类为中度、重度或极端异常的天数，并同时统计该像元在对应月份内具备有效数据的天数。 2. 月度统计量计算 由于该异常值基于每日观测数据生成，因此会因云覆盖、太阳耀斑、传感器高天顶角、太阳高天顶角及其他算法标记等因素出现数据缺失。为避免无效数据反演带来的偏差，需以当月有效观测天数为基准对异常频率进行归一化，具体公式如下： 分类像元叶绿素a异常相对频率 = α_c / ε 其中，α为当月被划分为中度、重度或极端异常的天数；c代表异常分类等级（中度、重度或极端异常）；ε为当月拥有有效数据的天数。 最后，针对每个专属经济区计算各异常分类等级的相对频率月度均值，最终为每个国家得到3个月度数值，分别对应中度、重度、极端异常三类等级。 更多详情可参阅联合国环境规划署（United Nations Environment Programme, UNEP）文档库中的《全球海洋统计手册》（第19-24页）。