IMOS - Satellite Remote Sensing - MODIS - 01 day - Net Primary Productivity (GSM model and Eppley-VGPM algorithm)

The Aqua satellite platform carries a MODIS sensor that observes sunlight reflected from within the ocean surface layer at multiple wavelengths. These multi-spectral measurements are used to infer the concentration of chlorophyll-a (Chl-a), most typically due to phytoplankton, present in the water. Modelling is then used to compute an estimate of the Net Primary Productivity (NPP).The model used is based on the standard vertically generalised production model (VGPM). The VGPM is a "chlorophyll-based" model that estimates net primary production from chlorophyll using a temperature-dependent description of chlorophyll-specific photosynthetic efficiency. For the VGPM, net primary production is a function of chlorophyll, available light, and the photosynthetic efficiency. The only difference between the Standard VGPM and the Eppley-VGPM is the temperature-dependent description of photosynthetic efficiencies, with the Eppley approach using an exponential function to account for variation in photosynthetic efficiencies due to photoacclimation. The similarity between these models (VGPM vs E_VGPM) is described more extensively in a paper by Elena-Carr et al (2006.). VGPM is a Depth Integrated Model(DIM) described by Behrenfeld and Falkowski (1997), with modification due to Eppley (1972) as implemented by Antoine and Morel (1996).There are multiple retrieval algorithms for estimating Chl-a. These data (eppley_npp_chl_gsm) use the Garver-Siegel-Maritorena (GSM) method implemented in the SeaDAS processing software l2gen and described in “Chapter 11, and references therein, of IOCCG Report 5, 2006, (http://ioccg.org/wp-content/uploads/2015/10/ioccg-report-05.pdf). This product is recommended for use in the VGPM model. The product Eppley_NPP_chl_gsm is experimental, in the sense that it is challenging to validate due to the scarcity of insitu measurments. It should only be used with caution informed by understanding of the algorithms.Production of contemporary MODIS data ceased at 2024/12/26. Users requiring similar Ocean Colour Radiometry data beyond 2024 should consider the VIIRS instruments on the SNPP and NOAA20+ series of satellites, also provided by IMOS.

Aqua卫星平台搭载中分辨率成像光谱辐射计（Moderate Resolution Imaging Spectroradiometer, MODIS）传感器，可在多个波长下观测海洋表层内反射的太阳光。这些多光谱测量数据被用于反演水体中叶绿素a（chlorophyll-a, Chl-a）的浓度，而该叶绿素a主要由浮游植物产生。随后通过建模计算净初级生产力（Net Primary Productivity, NPP）的估算值。
本次采用的模型基于标准垂直广义生产模型（vertically generalised production model, VGPM）。VGPM是一种“基于叶绿素”的模型，通过针对叶绿素特异性光合效率的温度依赖性描述，从叶绿素浓度估算净初级生产力。对于VGPM而言，净初级生产力是叶绿素浓度、有效光照以及光合效率的函数。
标准VGPM与Eppley-VGPM的唯一区别在于光合效率的温度依赖性描述方式：Eppley方法采用指数函数，以解释光适应作用导致的光合效率变化。
Elena-Carr等人（2006年）的一篇论文对这两种模型（VGPM与Eppley-VGPM）的相似性进行了更为详尽的阐述。
VGPM是由Behrenfeld与Falkowski（1997年）提出的深度积分模型（Depth Integrated Model, DIM），后经Eppley（1972年）修正，并由Antoine与Morel（1996年）实现应用。
目前存在多种用于估算Chl-a浓度的反演算法。
本数据集（eppley_npp_chl_gsm）采用Garver-Siegel-Maritorena（GSM）算法，该算法已集成于SeaDAS处理软件的l2gen模块中，相关说明详见《国际海洋颜色协调组（International Ocean-Colour Coordinating Group, IOCCG）报告第5号》（2006年）第11章及其中引用文献（http://ioccg.org/wp-content/uploads/2015/10/ioccg-report-05.pdf）。
该数据集被推荐用于VGPM模型的计算。
Eppley_NPP_chl_gsm数据集属于试验性产品，由于原位测量（in situ measurements）数据匮乏，其验证工作存在较大难度。
使用该数据集时，应在充分理解相关算法的前提下谨慎操作。
当代MODIS数据的生产工作已于2024年12月26日终止。对于2024年之后仍需同类海洋颜色辐射测量数据的用户，可考虑使用由IMOS提供的、搭载于SNPP及NOAA20+系列卫星上的可见光红外成像辐射仪套件（Visible Infrared Imaging Radiometer Suite, VIIRS）传感器。