4 An Introduction to FY - 3 / MERSI , Ocean Colour Algorithm , Product and Application

Ocean colour is the water-leaving radiance in the visible and near-infrared just above the ocean surface owing to selective absorption and scattering by phytoplankton and its pigments such as chlorophyll, as well as dissolved organic matter and suspended particulate matter in the subsurface ocean waters. Ocean colour carries useful information concerning biogeochemical properties of the water body. Ocean colour remote sensing can be applied in investigating the optical properties of upper ocean layers, biological productivity, global carbon and biogeochemical cycles in the oceans. When an ocean-colour sensor measures the radiance backscattered by the ocean-atmosphere system, the signal it received is largely dominated by the atmosphere. Ocean colour retrieval from satellite measured top-of-the-atmosphere (TOA) radiances over the oceans requires removal of the atmospheric and ocean surface interfering effects, a process termed atmospheric correction which is the key step in ocean colour data processing. Water-leaving radiance is the basic product in ocean colour remote sensing, and can be used to derive the water inherent optical property (IOP) and constituent's concentration, photosynthetically active radiation (PAR), red-tide index. Other oceanic applications such as primary productivity and global carbon can be investigated further. Besides water-leaving radiance, concentration of phytoplankton pigments such as chlorophyll a is another standard ocean colour product. Diffuse attenuation coefficient, fluorescence line height, photosynthetically available radiation, concentrations of particulate organic and inorganic carbon, chromophoric dissolved organic matter and suspended particulate matter can also be provided. With the advantage of spatial coverage and frequent overpass, satellite instruments are widely used in ocean research at local and regional scales. Since the launch of the Coastal Zone Colour Scanner (CZCS) in 1978, ocean colour remote sensing has been focused on studying the spatial and temporal evolution of phytoplankton in open oceanic areas. The main purpose is to improve our understanding of the carbon cycle and the role of the ocean in climate change. After CZCS, instruments with improved spatial and spectral