Two Global Data Bases of Photosynthetically Active Radiation

The sunlight which is used by plants, Photosysnthetically Active Radiation (PAR), is the part of the solar spectrum between 0.4 and 0.7 microns, or blue to red. Two global data bases of PAR and sola radiation flux at the surface have been generated by the Science Directorate of Langley Research Centre of NASA. One data base was developed by using data from the Clouds and Earth Radiant Energy System (CERES) instruments aboard the Terra and Aqua spacecraft over the period March 2000 to the present. The other data base was developed by the Surface Radiation Budget Project in collaboration with the Global Energy and Water Experiment (GEWEX) and covers a 23-year period. The CERES data base has data in two forms. The first form is for each footprint of the CERES scanning radiometer and covers the swath of Earth viewed by the instrument as the spacecraft goes around the Earth. The second form has data on a1 degree latitude by 1 degree longitude grid and covers the Earth. PAR is given for every three hours, daily means and monthly means. For many plant types the diffuse PAR is important, so the data base includes total and diffuse PAR. PAR and solar radiation at the surface are retrieved from CERES measurements by use of a radiative transfer code. Cloud properties are critical for the computation and are derived from MODIS (MODerate resolution Imaging Spectrometer), also on Terra and Aqua. Aerosol optical depth is also an important parameter and is retrieved from MODIS except when clouds fill more than half of the CERES footprint, in which case aerosol properties from the Model for Atmospheric Transport andChemistry (MATCH) are used. For clear sky other parameters included are solar elevation angle, surface albedo, precipital water, elevation and total column ozone. For cloudy sky, cloud optical depth, height and phase are considered. The CERES PAR data base was validated by comparison with measurements from NOAA's SURFRAD network, which has seven sites across the coterminous U. S. Total PAR is measured by a LI-COR quantum sensor, which has an estimated uncertainty of 10%. For all-sky conditions 80% of the CERES PAR measurements are within 10% of the LI-COR results and for clear sky conditions 90% of CERES PAR are within 10% of LI-COR. The Rotating Shadowband Spectrometer (RSS) is more accurate than the LI-COR, and CERES PAR agrees slightly better with RSS than does LI-COR. The RSS also provides the ratio of direct to diffuse PAR. For a sample of 270 cases, the mean ratio from RSS is 2.6 and from CERES is 1.9. If only clear-sky cases are considered, the comparison is better. The cloudy cases differ largely because CERES makes only one measurement over a 20 km region during an overpass, whereas the RSS measures only over a small area but integrates over a 15 minute period. The Surface Radiation Budget Project used data from the International Satellite and Cloud Climatology (ISCCP) Project to provide information about clouds, from which the PAR and solar radiation flux at the surface are computed. . This data set provides estimates of the all-sky PAR parameters spanning from July 1983 through December 2007 at the same 1 degree x 1 degree resolution as CERES. The quantities are available at the 3-hourly time resolution and averages corresponding to daily, monthly averaged 3-hourly and monthly averaged. The SRB fluxes have also been validated relative to the SURFRAD measurements and found to have daily averaged mean differences < 1% and RMS differences of 15% for the period 1998 through 2007. These two data bases provide the PAR and solar radiation flux at the top of the canopy which are needed by models of biosphere-atmosphere interactions. Maps of these data products show the geographical distribution of PAR and solar radiation fluxes for each month. These maps are generated for clear, i.e. cloud free, conditions as well as for conditions with observed clouds. The difference between these two maps shows the effects of clouds to reduce the PAR and solar radiation flux. This information enables investigations of the effects of clouds in these interactions. The variation of PAR and solar radiation flux with time of day is also available. PAR and radiation fluxes at the top of the atmosphere and at the surface are available from the CERES data base at http:\\ceres.larc.nasa.gov/products.php?product=SYN-AVG-ZAVG These radiation fluxes are available for the surface from the Surface Radiation Budget data base at http:\\eosweb.larc.nasa.gov/project/srb/srb¬_table