Retrieval of Cirrus properties by Sunphotometry: A new perspective on an old issue

Cirrus clouds are important modulators of the Earth radiation budget and continue tobe one of the most uncertain components in weather and climate modeling. Sunphotometers are widely accepted as one of the most accurate platforms for measuring clearsky aerosol optical depth (AOD). However, interpretation of their measurements isambiguousinthe presenceofcirrus.DerivationofavalidAODundercirrusconditionswasfocused previously on correction factors, rather than on derivation of cirrus cloud opticalthickness (COT). In the present work, we propose a new approach that uses the totalmeasured irradiance to derive cirrus COT and ice particle effective diameter (D