Observations of surface solar radiation variability from space and ground

Trends in surface solar radiation have been reported at numerous locations and the findings have attracted attention due to possible climatic implications of the observed decreases. Global trends are difficult to determine from ground observations due to the limited number of observing sites and issues related to instrument maintenance and calibration. Most available estimates of large or global scale distributions of radiative fluxes come from synthesis of ground observations and from model estimates. Several efforts are underway to use satellites for estimating radiative fluxes both at the surface and at the top of the atmosphere. In addition to the methodology used to infer such fluxes, the accuracy of the derived values depends on the quality of the satellite data, instrument calibration, spatial and temporal resolution of the satellite observations that enter the computations, and the spatial and temporal resolution to which these estimates are amalgamated. When observed trends in satellite estimates and selected ground observations of high quality have been compared, the direction of the trends was found to be similar. In this presentation, the scope of such comparison will be extended to additional location and an independent estimate of trends will be made for all-sky and clear-sky conditions. An inference scheme developed at the University of Maryland as driven with about twenty years of satellite observations from the NOAA/NASA Pathfinder ISCCP project will be used. High quality ground observations like those resulting from the BSRN activities will be used to evaluate to what extend the satellite and ground observations are consistent in detecting variability under both atmospheric conditions. It is hoped that the use of the diverse observing systems will help to identify possible limitations in each one.