Effect of surface albedo, water vapour, and atmospheric aerosols on the cloud-free shortwave radiative budget in the Arctic

This study is based on ground-based measurements of downward surface shortwave irradiance (SW), columnar water vapour (wv), and aerosol optical depth (τ) 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 (ΔSWA), wv (ΔSWwv), and aerosols (ΔSWτ) 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 ΔSWwv 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 ΔSWA 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 (RFτ) reaches values of −28 Wm−2 and shows a strong dependency on surface albedo. The derived radiative forcing efficiency (FEτ) for solar zenith angles between 55° and 70° is estimated to be (−120.6 ± 4.3) for 0.1 < A < 0.2, and (−41.2 ± 1.6) Wm−2 for 0.5 < A < 0.6.