Solar radiation measurements compared to simulations at the BSRN Izaña station. Mineral dust radiative forcing and efficiency study

Abstract

This paper presents a comparative study of shortwave downward radiation (SDR) measurements and simulations, obtained with the radiative transfer model LibRadtran, at the Baseline Surface Radiation Network (BSRN) site of Izaña Atmospheric Observatory (IZA, Spain). The analysis is based on cloud-free days between March 2009 and August 2012 (386 days), including aerosol-free and Saharan mostly pure mineral dust conditions and comparing the day-to-day, annual, and interannual variability. The observed agreement between simulations and measurements is excellent: the variance of daily measurements overall agrees within 99% with the variance of daily simulations, and the mean bias (simulations–measurements) is –0.30 ˙ 0.24 MJm–2 (–1.1 ˙ 0.9%) for global, –0.16 ˙ 0.34 MJm–2 (–0.4 ˙ 0.9%) for direct, and +0.02 ˙ 0.25 MJm–2 (+0.9 ˙ 9.2%) for diffuse SDR. Furthermore, the diurnally averaged aerosol radiative forcing (∆DF) and radiative forcing efficiency (∆DFeff) due to Saharan mostly pure mineral dust events has been computed at Izaña Observatory. The mean ∆DF values are –7 ˙ 1, –96 ˙ 5, and 44 ˙ 2 Wm–2 for global, direct, and diffuse BSRN SDR, respectively (mean aerosol optical depth, AOD, at 500 nm of 0.18 ˙ 0.01), whereas the mean ∆DFeff values are –59 ˙ 6, –495 ˙ 11, and 230 ˙ 8 Wm–2 per unit of AOD at 500 nm for global, direct, and diffuse BSRN SDR, respectively. These values highlight the importance of scattering processes for mineral dust aerosols: the ratio between ∆DF and the corresponding SDR without aerosols is ~ 2.5% for diffuse SDR versus 0.2% for direct SDR. This illustrates the significant potential of mineral dust particles to cool the Earth-atmosphere system.