INTERNATIONAL JOURNAL OF CLIMATOLOGY Int. J. Climatol. 30: 2088–2104 (2010) Published online 25 March 2010 in Wiley Online Library (wileyonlinelibrary.com) DOI: 10.1002/joc.2107 Comparison of satellite-derived TOA shortwave clear-sky fluxes to estimates from GCM simulations constrained by satellite observations of land surface characteristics Valentine G. Anantharaj, a Udaysankar S. Nair, b * Peter Lawrence, c Thomas N. Chase, c Sundar Christopher b and Thomas Jones b a Geosystems Research Institute, Mississippi State University, Box 9652, Mississippi State, MS 39762, USA b Earth System Scientific Center, National Space Science and Technology Center (NSSTC), 320 Sparkman Drive, Huntsville, AL 35805, USA c Cooperative Institute for Research in Environmental Studies, University of Colorado, Boulder, CO 80309, USA ABSTRACT: Clear-sky, upwelling shortwave flux at the top of the atmosphere  S ↑ TOA  , simulated using the atmospheric and land model components of the Community Climate System Model 3 (CCSM3), is compared to corresponding observational estimates from the Clouds and Earth’s Radiant Energy System (CERES) sensor. Improvements resulting from the use of land surface albedo derived from Moderate Resolution Imaging Spectroradiometer (MODIS) to constrain the simulations are also examined. Compared to CERES observations, CCSM3 overestimates global, annual averaged S ↑ TOA over both land and oceans. However, regionally, CCSM3 overestimates S ↑ TOA over some land and ocean areas while underestimating it over other sites. CCSM3 underestimates S ↑ TOA over the Saharan and Arabian Deserts and substantial differences exist between CERES observations and CCSM3 over agricultural areas. Over selected sites, after using ground- based observations to remove systematic biases that exist in CCSM computation of S ↑ TOA , it is found that use of MODIS albedo improves the simulation of S ↑ TOA . Inability of coarse resolution CCSM3 simulation to resolve spatial heterogeneity of snowfall over high altitude sites such as the Tibetan Plateau causes overestimation of S ↑ TOA in these areas. Discrepancies also exist in the simulation of S ↑ TOA over ocean areas as CCSM3 does not account for the effect of wind speed on ocean surface albedo. This study shows that the radiative energy budget at the TOA is improved through the use of MODIS albedo in Global Climate Models. Copyright  2010 Royal Meteorological Society KEY WORDS radiation energy budget; shortwave fluxes; global circulation model; CERES; MODIS land surface albedo; ocean albedo; vegetation albedo Received 1 December 2008; Revised 30 December 2009; Accepted 12 January 2010 1. Introduction Research over the past three decades shows that anthro- pogenic activities have the potential to significantly impact global and regional climates. One such anthro- pogenic activity that has received considerable atten- tion in recent years is the increased emission of green- house gases, specifically carbon dioxide (CO 2 ). Increas- ing concentration of greenhouse gases results in a reduction of net outgoing longwave radiation from the earth–atmosphere system leading to an increase in sur- face temperatures. Observations show increasing surface temperature trends in recent decades (IPCC, 2007), which has been partly attributed to increasing atmospheric CO 2 concentrations. Identification of increasing CO 2 as the primary cause of warming trends in surface air tempera- ture is based primarily on Global Climate Model (GCM) * Correspondence to: Udaysankar S. Nair, National Space Science and Technology Center (NSSTC), 3046, Huntsville, AL 35805, USA. E-mail: nair@nsstc.uah.edu simulations. As GCMs are extensively utilised in assess- ments of climate change, realism of different processes simulated by GCMs is of significant interest. Analysis of the top of the atmosphere (TOA) radiative energy budget, characterised by an approximate balance between the net incoming solar radiation and outgoing terrestrial longwave radiation, is important in climate change studies. In recent decades, radiative energy bud- get at the TOA has been monitored using satellite-borne instruments such as the Earth Radiation Budget Exper- iment (ERBE) and Clouds and Earth’s Radiant Energy System (CERES). These satellite sensors provide datasets that allow evaluation of the performance of GCMs from a global energy budget perspective. Net incoming shortwave radiation at the TOA, the main energy input into the earth–atmosphere system, is modulated by the planetary albedo, which is the proportion of the incoming solar radiation reflected back to space by the land and ocean surface, clouds and atmospheric aerosols. Bender et al. (2006) compared planetary albedo from 20 GCMs used in the Fourth Assessment Report (AR4) of the Intergovernmental Panel Copyright  2010 Royal Meteorological Society