Photosynthetically Available Radiation (PAR)
Table of Contents
1 - Product Summary
This algorithm estimates daily average photosynthetically available radiation (PAR) at the ocean surface in Einstein m-2 d-1. PAR is defined as the quantum energy flux from the Sun in the 400-700nm range. For ocean color applications, PAR is a common input used in modeling marine primary productivity. Implementation of this algorithm is contingent on the availability of observed top-of-atmosphere radiances in the visible spectral regime that do not saturate over clouds. The algorithm is applicable to MODIS, MERIS, SeaWiFS, and VIIRS, but it can be operated on all ocean color sensors. The PAR product is included as part of the standard Level-2 OC product suite and the Level-3 PAR product suite.
Algorithm Point of Contact: Robert Frouin, Scripps Institute of Oceanography
2 - Algorithm Description
Observed Top-of-Atmosphere (TOA) radiances in in the 400-700nm range that do not saturate over clouds.
Daily average PAR in Einstein m-2 d-1
A detailed description of the algorithm as implemented for SeaWIFS is provided in the whitepaper Frouin, Franz, and Wang, "Algorithm to estimate PAR from SeaWiFS data Version 1.2 - Documentation", which is also summarized in the SeaWIFS postlaunch technical report volume 22, Frouin, R., B.A. Franz, and P.J. Werdell (2002). The algorithm implementation has since been generalized to allow operation on any sensor with a sufficient set of spectral bands.
As noted, the PAR algorithm requires a set of sensor bands in the visible regime for which the observed TOA radiances do not typically saturate over clouds. This means that at least a few sensor bands in the visible domain can measure changes in radiance from very dark ocean to very bright clouds. Many ocean color sensor bands are designed to saturate (stop changing) at radiances well below that limit, but they may include a subset of bands with higher radiance limits (e.g., MODIS bands designed for land applications). Here we define the sensor bands used for each applicable sensor.
The PAR product is not computed if TOA radiance is negative or solar zenith angle exceeds 90-deg, or if the Level-2 flags indicate LAND, HIGLINT or SEAICE. A failure condition is indicated in Level-2 by setting the PAR value for that pixel to the _FILLVALUE and setting the Level-2 flags to indicate PRODFAIL.
3 - Implementation
|Product Short Name||par|
|Level-2 Product Suite||OC|
|Level-3 Product Suite||PAR|
4 - Assessment
A limited set of Level-2 satellite-to-in-situ match-up validation results are available for each sensor from the validation tool of the SeaWiFS Bio-Optical Archive and Storage System (SeaBASS). Links to those match-ups are provided below.
As detailed in Frouin, Franz, and Werdell (2002), however, the validity of the PAR product improves with temporal compositing. The PAR product is primarily intended for use as a Level-3 product, where data have been composited over weekly to monthly time-scales.
5 - References
Frouin, R., McPherson, J., Ueyoshi, K., & Franz, B. A. (2012). A time series of photosynthetically available radiation at the ocean surface from SeaWiFS and MODIS data. Remote Sensing of the Marine Environment II. http://dx.doi.org/10.1117/12.981264
Frouin, R., Franz, B. A., & Werdell, P. J. (2002). The SeaWiFS PAR product. ,In: S.B. Hooker and E.R. Firestone, Algorithm Updates for the Fourth SeaWiFS Data Reprocessing, NASA Tech. Memo. 2003-206892, Volume 22, NASA Goddard Space Flight Center, Greenbelt, Maryland, 46-50.The SeaWiFS PAR product
Frouin, R. & Pinker, R. T. (1995). Estimating Photosynthetically Active Radiation (PAR) at the earth's surface from satellite observations. Remote Sensing of Environment, Volume 51, Issue 1, January 1995, Pages 98-107, ISSN 0034-4257. http://dx.doi.org/10.1016/0034-4257(94)00068-X
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