Solar absorption in the atmosphere—Estimates from collocated surface and space-born observations

Solar radiation is the primary source of energy for the Earth's climate system. While the incoming and outgoing solar fluxes at the top-of-atmosphere can be quantified with high accuracy, large uncertainties still exist in the partitioning of solar absorption between surface and atmosphere. To compute best estimates of absorbed solar radiation at the surface and within the atmosphere representative for Europe during 2000-2010, we have combined ground-based observations of surface downwelling solar radiation (GEBA, BSRN) with collocated satellite-retrieved surface albedo (MODIS) and top-of-atmosphere net irradiance (CERES EBAF, 1° resolution ). Over Europe land, this combination yields annual mean surface and atmospheric absorption of 117 ±6 Wm¯² (42 ±2 % of TOA incident irradiance) and 65 ±3 Wm¯² (23 ±1 %). The fractional atmospheric absorption of 23% represents a robust estimate largely unaffected by variations in latitude and season, thus, making it a potentially useful quantity for first order validation of regional climate models. Preliminary results representative for the global scale support the finding of a spatially and seasonally fairly robust fractional atmospheric absorption of around 22-24%.

These estimates are based on quality assessed surface data. First of all, we examined the temporal homogeneity of the monthly GEBA time series beyond 2000 and find the vast majority to be suitable for our purposes. The spatial representativeness of the GEBA and BSRN sites for their collocated 1° CERES EBAF grid cells we assess by using a satellite-derived surface solar radiation product (CM SAF) at 0.03° spatial resolution. We find random representation errors of on average 3 Wm¯² or 2% (normalized by point values), hence, of similar order as the uncertainty of pyranometer measurements.

Care is taken to identify and quantify uncertainties, which arise mostly from the datasets themselves, in particular surface albedo and ground-based solar radiation. Other sources of uncertainty, like the spatial coverage by surface sites, the multiplicative combination of spatially averaged surface solar radiation and surface albedo, and the spatial representativeness of the point observations, are either negligibly small or can be corrected for.

Hakuba, M.Z., Folini, D., Schaepman-Strub, G., and Wild, M. 2014: Solar Absorption over Europe from collocated surface and satellite observations, J. Geophys. Res., accepted, doi:10.1002/2013JD021421.