Long-term ground-based irradiance observations: An under-sampled survey of applications in climate research

Along with satellite observations and gridded model calculations of the Earth's radiation budget, spatially-sub-sampled components of the surface budget are quantified by numerous ground-based irradiance measurements. Some of these measurement programs have been continuous for several decades and analysis of the associated irradiance records are beginning to emerge within the context of long-term climate change issues. The extent of variability in these records on many time scales, including the demonstrated lack of significant variability, can be utilized in climate diagnostics. While surface radiation is often considered as a primary component of radiatively forced climate change, direct observations of surface radiation also document not only the magnitude of local radiative input to the surface energy budget, but also are indicative of realized changes in the atmosphere. Questions as to the validity and representativeness of surface observations will be addressed in this presentation using quantitative comparisons. The usefulness of these long-term ground-based measurement programs are examined using comparisons to satellite time series as well as radiative transfer calculations, both specific to local conditions and in the broader applications of large scale atmospheric circulation models. It is seen that on an interannual timescale that individual ground sites are representative of not only the local and extended regional spatial variability, but also can be highly correlated with large noncontiguous regions of the globe, suggestive of radiation teleconnection patterns. The data used in this analysis derive primarily from the 30-year solar irradiance record at each of 5 NOAA sites widely distributed around the globe. This a meager sample but unique in combined duration, high-temporal resolution, and uniform mode of observation. All 5 sites were centrally calibrated and maintained over then entire period of record and provide information on the time scales of minutes to decades. The data from this limited network provide distinct and unmatched investigative opportunities, including multi-scale irradiance variations and the impact of clouds these spatial and temporal scales. Cloud effects in the data have been identified through their higher-frequency signature and their resulting influence to total atmospheric transmission. Adequate averaging intervals to remove local influences of the 3-dimensional effects of clouds are seen to extend greater than one day. The reliability, precision, and accuracy of these ground based data contribute to their value and current applications. Nevertheless, efforts to improvement and extend the observations are ongoing with a national and international programmatic emphases.