Principal Investigator (PI): Robert Frouin, Scripps Institution of Oceanography

The solar energy available for photosynthesis, known as photosynthetically active radiation (PAR), controls the growth of phytoplankton and, therefore, regulates the composition and evolution of marine ecosystems. Knowing the spatial and temporal distribution of PAR over the oceans is critical to understanding biogeochemical cycles of carbon, nutrients, and oxygen, and to address important climate and global change issues such as the fate of anthropogenic atmospheric carbon dioxide.

In view of this, the project objective is to produce a 12-year time series of PAR at the ocean surface from Sea-viewing Wide Field-of-view Sensor (SeaWiFS), Moderate Resolution Imaging Spectroradiometer (MODIS)—Terra, and MODIS—Aqua data. The product will cover the global oceans, with a spatial resolution of about 9.3x9.3 km (equal area grid) and a temporal resolution of one day. The time series will start in September 1997, i.e., at the beginning of the SeaWiFS operational phase.

Daily PAR will be computed using a mature algorithm, tested and evaluated, which has been applied operationally by NASA’s Ocean Biology Distributed Active Archive Center (OB.DAAC) to generate the SeaWiFS PAR product. The algorithm will be adapted/modified to include data from multiple ocean-color sensors. Combining data from satellite sensors with different equatorial crossing times will account for the diurnal variability of clouds. The advantage of using ocean-color sensors to estimate PAR is that they provide chlorophyll concentration, another key parameter in ocean primary productivity modeling. Furthermore, the same data pre-processing is required, i.e., PAR can be produced with little extra effort. In this way, studies of ecosystem dynamics are facilitated.

Consistency across time will be achieved by comparing estimates obtained using data from one or two sensors with those from the three sensors when they are all operating, determining adjustment factors, and applying those factors to reduce biases during the periods with only one or two sensors. Accuracy will be quantified on daily, weekly, and monthly time scales against existing in-situ measurements, and it will be monitored during the last years of the time series using long-term PAR sensors installed at selected sites. Eventually, the algorithm will be refined, and the time series re-processed.

Large-scale data production will be accomplished by OB.DAAC, who will make the time series and related documents available to the public from their web site. The processing system, including routine check of accuracy and control of quality, will be designed to operate during the entire lifetime of SeaWiFS and MODIS, and to accommodate future sensors with ocean-color capabilities. The project will provide an invaluable PAR dataset, with possibility of extension in the future, for studying the evolution of marine ecosystems and the role of the oceans in carbon cycling and climate change.