Long-term changes in the radiative effects of aerosols and clouds in a mid-latitude region (1985-2010)

As clouds and aerosols are the main sources of uncertainty in the determination of the energy balance of the Earth, there is a growing interest in the evaluation of their radiative effects. Hence, in this work, long-term data of shortwave radiation from 13 locations over Spain (South-Western Europe) are used to investigate, for the first time, the radiative effects of clouds and aerosols in the period 1985-2010. In particular, monthly radiation data from ground-based observations and radiative transfer simulations fed with reanalysis data of ozone, water vapour and surface albedo, are used to evaluate the cloud and aerosol radiative effect (CARE). Annual values of the CARE become less negative from Northern to Southern stations. For instance, the annual CARE values for Bilbao (North), Valladolid (Centre), and Murcia (South) are -82, -46, and -42Wm^-2, respectively. CARE averages exhibit a clear seasonal pattern with the strongest contribution during spring and summer months. Particularly in these seasons, there is a very high correlation between CARE values and sunshine duration, number of cloud-free days, and temperature. Additionally, a significant decrease of the radiative effects of the clouds and aerosols is observed over Spain in the last 26 years. Overall, the linear trend of the mean annual CARE series over Spain is statistically significant with positive sign, 3.1Wm^-2 per decade. The significant trend values at individual stations range between 2.9 and 5.2Wm^-2 per decade. Seasonal trends in summer and spring are larger than in autumn and winter. Finally, the radiative effects of water vapour and ozone were also evaluated showing an annual mean over Spain of about -10Wm^-2 and -1Wm^-2, respectively. However, no significant trends were observed for these two variables between 1985 and 2010.