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 (CRFSW), 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 degrees, the results were CRFSW = -78 W m(-2), CARF(SW) = -100 W m(-2) and ARF(SW) = -22 W m(-2), and the values decreased to CRFSW = -50 W m(-2), CARFSW = -69 W m(-2) and ARF(SW) = -19 W m(-2) at SZA = 60 degrees. 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 (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.