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Di Biagio, C; di Sarra, A; Eriksen, P; Ascanius, SE; Muscari, G; Holben, B (2012). Effect of surface albedo, water vapour, and atmospheric aerosols on the cloud-free shortwave radiative budget in the Arctic. CLIMATE DYNAMICS, 39(4-Mar), 953-969.

This study is based on ground-based measurements of downward surface shortwave irradiance (SW), columnar water vapour (wv), and aerosol optical depth (tau) obtained at Thule Air Base (Greenland) in 2007-2010, together with MODIS observations of the surface shortwave albedo (A). Radiative transfer model calculations are used in combination with measurements to separate the radiative effect of A (Delta SWA), wv (Delta SWwv), and aerosols (Delta SW tau) in modulating SW in cloud-free conditions. The shortwave radiation at the surface is mainly affected by water vapour absorption, which produces a reduction of SW as low as -100 Wm(-2) (-18%). The seasonal change of A produces an increase of SW by up to +25 Wm(-2) (+4.5%). The annual mean radiative effect is estimated to be -(21-22) Wm(-2) for wv, and +(2-3) Wm(-2) for A. An increase by +0.065 cm in the annual mean wv, to which corresponds an absolute increase in Delta SWwv by 0.93 Wm(-2) (4.3%), has been observed to occur between 2007 and 2010. In the same period, the annual mean A has decreased by -0.027, with a corresponding decrease in Delta SWA by 0.41 Wm(-2) (-14.9%). Atmospheric aerosols produce a reduction of SW as low as -32 Wm(-2) (-6.7%). The instantaneous aerosol radiative forcing (RF tau) reaches values of -28 Wm(-2) and shows a strong dependency on surface albedo. The derived radiative forcing efficiency (FE tau) for solar zenith angles between 55A degrees and 70A degrees is estimated to be (-120.6 +/- A 4.3) for 0.1 < A < 0.2, and (-41.2 +/- A 1.6) Wm(-2) for 0.5 < A < 0.6.



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