Retrieval and variability analysis of optically thin cloud optical depths from a Cimel sun-photometer

In this work we propose a technique to retrieve optical depths for optically thin clouds. This method complements the current performance of AERONET for optically thick cloud measurements using data eliminated by the cloud-screening algorithm that are not useful to derive aerosol properties, therefore inexpensively increasing the capabilities of the sun-photometers. The proposed method is based on the computation of the apparent cloud optical depths and on a forward scattering correction method that exploits state-of-the-art ice cloud scattering models. This complementary procedure is applied to Cimel sun-photometer measurements performed at the Évora Geophysics Centre (Portugal, 38.6°N, 7.9°W, 293 m asl) included in the AERONET network in order to obtain a climatology of optical depths for optically thin clouds over middle-latitude continental regions of the southwestern Iberian Peninsula. Main features regarding annual and seasonal variability and the relative changes in the infrared radiative forcing from 2007 to 2010 are also reported and analyzed