Frouin, Robert: Scripps Institution of Oceanography, UCSD (Project Lead)
Project Funding:
2014 - 2016
NRA: 2013 NASA: Terra and Aqua: Algorithms--Existing Data Products
Funded by NASA
Abstract:
The MODIS Level-3 standard PAR product, available from the NASA Ocean Color web site (http://oceancolor.gsfc.nasa.gov/), has provided long-tern PAR information (14 years of data from Terra and 11 years from Aqua) at a 4 km resolution on daily, 8-day, and monthly time scales over the global ocean with estimated RMS accuracy of 18%, 12%, and 8%, respectively, for individual sensors. The product has been extensively used in the ocean biology/biogeochemistry community. Indeed, knowing the spatial and temporal distribution of PAR over the ocean 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. The PAR data from individual MODIS instruments, however, exhibit biases that need to be taken into account to generate a consistent time series across sensors. It is also desirable to improve performance against in situ measurements, in terms of both standard deviation and bias.
In view of this, the project objectives are (1) to improve the MODIS PAR algorithm and (2) to correct the resulting PAR time series generated from individual instruments for biases due to limitations in the modeling and diurnal variability of clouds. The improvements envisioned concern the spectral integration of atmospheric functions, the ocean albedo parameterization, the specification of aerosol optical properties, and they include taking into account, using climatology, diurnal changes in cloudiness. Biases between PAR estimates by individual sensors will be determined by comparing PAR estimates obtained from one sensor with those obtained by combining estimates from three sensors observing at different local times, i.e., MODIS-Terra (10:30 am), SeaWiFS (12:00 pm), and MODIS-Aqua (1:30 pm), when they are all operating, and the resulting adjustment factors, spatially and temporally varying, will be applied to the individual sensors. This will require applying the algorithm modifications not only to MODIS, but also to SeaWiFS. Accuracy for individual and combined sensors will be quantified on daily, weekly, and monthly time scales against existing in-situ measurements and in comparisons with other radiation products.
Implementation of the improved PAR algorithm and routine production of bias-corrected PAR data from MODIS-Terra and -Aqua will be accomplished, as for the current PAR product, by the NASA Ocean Biology Processing Group (OBPG), who will make the time series and related/updated documents available to the public from their web site. The OBPG processing system has been designed to allow multiple re-processing as algorithms mature/improve and includes routine check of accuracy and control of quality.

The project will provide a unique and invaluable PAR dataset, consistent across sensors, extending into the future, for a wide range of research applications, such as primary production and carbon export modeling, ecosystem dynamics and mixed-layer physics, photochemical transformations of dissolved organic matter, and control of stable soluble iron in marine waters. The contribution of the project to understanding the role of the oceans in carbon cycling and climate change, one of NASA objectives in Earth science research is therefore expected to be very significant.
More details may be found in the following project profile(s):
