An approach to the retrieval of thin cloud optical depth from a Cimel sun-photometer

Abstract

In this work we proposed a technique to retrieve optical depth for optically thin clouds (clouds that enable the transmission of solar radiation through them). 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. It is based on the computation of apparent cloud optical depths and 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, 38.6ºN, 7.9ºW, 293 m asl) in order to obtain a climatology of optical depths for optically thin clouds over middle-latitude regions. A comparison with MODIS retrievals is presented. Main features regarding annual variability from 2007 to 2010 are also reported.