Comparison of Satellite, Model, and In Situ Values of Photosynthetically Available Radiation (PAR)

Photosynthetically available radiation (PAR) incident at the sea surface penetrates into the water column and drives oceanic primary production. Ecosystem models to estimate phytoplankton biomass and primary production require an estimate of sea surface PAR, which is available from satellite ocean color imagery and atmospheric model predictions. Because the PAR values could come from either source, it is important to understand the variability and accuracies of each. We performed spatial and temporal analyses covering multiple years and seasons, and clear/cloudy conditions. We compare values derived from the imagery to those from the models and to in situ measurements in the Gulf of Mexico to validate the imagery and models and to assess PAR variability based on source. Averaged over space or time, the relative errors in PAR between the six sources (two satellite, three model, and in situ) are generally less than 5%-7%, but they can vary up to 11%. However, the errors and biases on a daily or pixel-by-pixel basis are larger, and the averages can mask seasonal trends.