Photosynthetically active radiation from Clouds and the Earth's Radiant Energy System (CERES) products

Abstract

We describe a method that retrieves surface photosynthetically active radiation (PAR) and its direct and diffuse components from the Surface and Atmospheric Radiation Budget (SARB) product of Clouds and the Earth's Radiant Energy System (CERES). The shortwave spectrum in the SARB Edition 2 is calculated in 15 bands, 4 of which are used to develop the PAR, in conjunction with the look‐up tables described in this paper. We apply these look‐up tables to existing CERES Terra Edition 2 products. The new retrieved surface PAR is validated with LI‐COR PAR measurements at seven Surface Radiation Budget Network (SURFRAD) sites using data from March 2000 to June 2005. The relative bias of retrieved all‐sky PAR at the SURFRAD sites is 4.6% (positive sign indicating retrieval exceeds measurement), and 54% of the all‐sky samples are within the ±10% uncertainty of the LI‐COR PAR measurements. The satellite field‐of‐view (FOV) is more representative of the ground instrument FOV under clear conditions, so 89% of clear‐sky retrievals are within the uncertainty of the LI‐COR PAR measurements at SURFRAD sites with positive biases at most sites. The retrieved PAR is also validated at the Atmospheric Radiation Measurement (ARM) Southern Great Plains Central Facility (CF) site using data from October 2003 to June 2004 for those FOVs having both LI‐COR and Rotating Shadowband Spectroradiometer (RSS) ground measurements; for this small domain, all‐sky relative biases are again positive (1.9%) for LI‐COR but negative (−4.2%) for RSS. The direct‐to‐diffuse ratio derived from CERES is smaller than that from RSS for both clear and cloudy conditions. CERES also retrieves the broadband shortwave insolation, and the relative biases for the broadband retrievals are much less than those for PAR at the above sites. It appears that some of the ground‐based measurements of PAR do not have the fidelity of those for broadband shortwave insolation.