The work aims at the retrieval of the cloud optical thickness and estimate the cloud radiative forcing from ground-based spectral and broadband irradiance observations taken at the Atmospheric Physics Observatory of the University of Évora (southwestern Europe). The cloud optical thickness is derived from the ground-based spectral irradiance measurements, for 7 years of observations available, and its spatial representativeness is analyzed through comparisons with independent remote sensing retrievals from the MODIS cloud product, onboard the Terra and Aqua satellites. The surface cloud radiative forcing is estimated from ground-based broadband solar irradiance measurements for the same period and its variability is examined. The surface cloud forcing efficiency (cloud radiative forcing per unit of cloud optical thickness) is also presented. The cloud radiative forcing attained is in addition compared, for selected cases, with modeling results obtained from numerical simulations with the MesoNH mesoscale atmospheric model, showing a fairly good agreement.

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