MODIS inherent optical property (IOP) modelling

A model has been developed at PML to determine the Inherent Optical Properties (IOPs) of the oceans from space. Historically, ocean colour data from satellites such as the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) the Medium Resolution Imaging Spectrometer (MERIS) and the Moderate Resolution Imaging Spectroradiometer (MODIS) have been used to determine chlorophyll concentration based on empirical, band-ratio algorithms. However, such algorithms are fraught with problems: they do not always apply in optically complex waters such as around our coasts; chlorophyll is limited in its scope for validating ecosystem model output and; chlorophyll is limited in determining phytoplankton functional types. By determining the absorption and backscatter (IOPs) analytically from the satellite radiance measurements, we can begin to close the gap between remote sensing and ecosystem modelling efforts. Each of these components affects the in-water light field in a different manner, so this new information is helping us to implement improved optical schemes within ecosystem models. Figure 1 shows a selection of absorption and backscatter model outputs. The area within the red circle in figure 1d shows a highly absorbing algal bloom (as verified by in-situ data and appearing green in absorption image of figure 1a); the areas within the blue ellipses contain suspected coccolithophores which are known to scatter light at all wavelengths (and so appear green in figure 1c).

Total Absorption
RGB Image

Figure 1: MODIS data of 15 July 2006 12:50 UTC over the north east Atlantic Ocean, showing the model implementation for fields of: a) total absorption at 443 nm; b) absorption due to phytoplankton at 443 nm; c) backscatter at 551 nm and d) the simulated true colour image.

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