The seasonal disposition of solar radiation at the surface of the Earth: Highlights of cloud and aerosol forcing on broadband insolation, direct/diffuse PAR, and UVA/UVB/Erythema

The Surface and Atmosphere Radiation Budget (SARB) working group, part of NASA's Clouds and the Earth's Radiant Energy System (CERES) project, has developed a consistent global record for broadband SW insolation, the direct and diffuse PAR, UVA, UVB, and the UV Index – plus their respective cloud and aerosol forcings that presently covers 2000-2004. This globally gridded 3-hourly product named SYN (for synoptic) uses a highly modified version of the Fu & Liou radiation transfer model and MODIS data from EOS supplemented heavily with geostationary satellites for cloud properties. Previous UV products could not account for the effect of diurnally varying clouds that we account for with the inclusion of the geostationary data.

The biology community needs a record of the ultraviolet (UV) and shorter wavelength Photosynthetically Active Radiation (PAR) at the Earth's surface that accounts for temporal changes in cloudiness, aerosols, and surface characteristics across the globe for a number of research areas. Mercado et al. (2009), for example, implicated the role of aerosols from the Pinatubo volcano for a fluctuation much more sensitive to both ozone and solar in the atmospheric CO2 concentration in the early 1990s: Scattering in the stratosphere caused an increase at the surface of the diffuse component of PAR, which can be used more effectively by plants than direct PAR, leading to a more rapid uptake of CO2 by plants. Another important use of UV is by medical researchers, who earlier have been interested specifically in the erythemal component, a convolution of the spectrum expressed by the singular “UV Index”. Recent, more subtle epidemiological approaches suggest possibly opposing effects of UVA and UVB dosages. UVB at the surface much more sensitive to both ozone and solar zenith angle than is UVA.

The SYN product is validated with matching calculations and observations (from the broadband CERES instrument) for reflected SW at TOA. Surface validation is mainly through broadband networks; focused validation for PAR and UV is done at a few sites also having quality broadband measurements. An abbreviated (monthly only, but interannual) record of the surface fields is available on-line at the CAVE home page (google “CERES CAVE”, under “Global Maps”), which also enables on-line radiative transfer calculations of the same fields.