Kaicun Wang¹, Robert E. Dickinson²

¹ State Key Laboratory of Earth Surface Processes and Resource Ecology, College of Global Change and Earth System Science, Beijing Normal University, Beijing, China;

² Department of Geological Sciences, The University of Texas at Austin, Austin, Texas, USA.

 

Abstract: Atmospheric downward longwave radiation at the surface (L_d) varies with increasing CO₂ and other greenhouse gases. This study quantifies the uncertainties of current estimates of global L_d at monthly to decadal timescales and its global climatology and trends during the past decades by a synthesis of the existing observations, reanalyses, and satellite products. We find that current L_d observations have a standard deviation error of ~3.5Wm^-2 on a monthly scale. Observations of L_d by different pyrgeometers may differ substantially for lack of a standard reference. The calibration of a pyrgeometer significantly affects its quantification of annual variability. Compared with observations collected at 169 global land sites from 1992 to 2010, the L_d derived from state-of-the-art satellite cloud observations and reanalysis temperature and humidity profiles at a grid scale of ~1° has a bias of ±9 W m^-2 and a standard deviation of 7 W m^-2, with a nearly zero overall bias. The standard deviations are reduced to 4 W m^-2 over tropical oceans when compared to L_d observations collected by 24 buoy sites from 2002 to 2011. The -4 W m^-2 bias of satellite L_d retrievals over tropical oceans is likely because of the overestimation of L_d observations resulting from solar heating of the pyrgeometer. Our best estimate of global means L_d from 2003 to 2010 are 342±3 W m^-2 (global), 307±3 W m^-2 (land), and 356±3 W m^-2 (ocean). Estimates of L_d trends are seriously compromised by the changes in satellite sensors giving changes of water vapor profiles.