Short-wave radiative forcing at the surface for cloudy systems at a midlatitude site

Abstract

Six-year data (2006–2011) of short-wave (SW) radiation, aerosol optical depth and cloud fraction were used in the evaluation of cloud radiative forcing (1-min data) at the surface in the SW spectral range at Granada station (South-Western Europe). Three different systems were analysed in this study: cloud radiative forcing (CRFSW), cloud–aerosol radiative forcing (CARFSW) and aerosol radiative forcing under cloudy conditions (ARFSW). Average values of these variables presented a clear dependence on solar zenith angle (SZA), for example, at SZA=30°, the results were CRFSW=−78 W m−2, CARFSW=−100 W m−2 and ARFSW=−22 W m−2, and the values decreased to CRFSW=−50 W m−2, CARFSW=−69 W m−2 and ARFSW=−19 W m−2 at SZA=60°. These three variables exhibited a similar pattern: they increased in the absolute magnitude up to moderate SZAs and strongly decreased towards zero for high SZAs. The hemispherical fractional sky cover (SCV) and the fractional sky cover inside the octant where the Sun is placed (SCV-Sun) also played a key role in the determination of cloud forcing. As expected, the strongest cloud effect appeared when clouds covered the Sun. However, when SCV-Sun was low or moderate and total SCV was over 0.5, there was a high likelihood of enhancement occurrence (i.e., positive cloud forcing values). Finally, the evolution of CRFSW values during a case study (ranging from −600 to +200 W m−2) with a wide variety of cloud conditions could be explained by the temporal evolution of SCV and SCV-Sun.