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

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 (CRF_SW), cloud–aerosol radiative forcing (CARF_SW) and aerosol radiative forcing under cloudy conditions (ARF_SW). Average values of these variables presented a clear dependence on solar zenith angle (SZA), for example, at SZA=30°, the results were CRF_SW=−78 W m⁻², CARF_SW=−100 W m⁻² and ARF_SW=−22 W m⁻², and the values decreased to CRF_SW=−50 W m⁻², CARF_SW=−69 W m⁻² and ARF_SW=−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 (S_CV) and the fractional sky cover inside the octant where the Sun is placed (S_CV-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 S_CV-Sun was low or moderate and total S_CV was over 0.5, there was a high likelihood of enhancement occurrence (i.e., positive cloud forcing values). Finally, the evolution of CRF_SW 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 S_CV and S_CV-Sun.