Cloud Microphysical Properties Retrieval in the Presence of Strong Aerosol Events

Clouds are one of the main regulating factors of the Earth’s climate through scattering and absorption of solar radiation as well as emission and absorption of thermal radiation. In consequence, the study of cloud properties is extremely important for understanding their role in climate change mechanisms. Specifically, aerosol particles, natural or anthropogenic, influence cloud-related processes by changing cloud microphysical properties and their radiative properties. Moreover, in certain cases precipitation processes may be affected as described by Rosenfeld (1999, 2000). The algorithm of Nakajima and Nakajima (1995) is used to retrieve cloud optical thickness and effective particle radius from visible, near infrared and thermal infrared satellite spectral measurements. The algorithm is applied to measurements from the MODerate Resolution Imaging Spectroradiometer (MODIS) onboard the Terra spacecraft. Since the algorithm treats separately water and ice clouds, it is necessary to obtain information on cloud particle phase beforehand from a multi-spectral data analysis. The selected case studies refer to strong aerosol events originating in different regions of the Earth. Microphysical cloud properties are retrieved considering different atmospheric aerosol situations: a typical background situation that characterizes the area prior to the event; a “heavy” aerosol load during the event; “mixed” aerosol / background conditions affecting the area after the passage of the main aerosol plume. Cloud properties obtained for these distinct atmospheric situations are analysed in terms of their possible changes due to the interactions of the clouds with the aerosol particle plumes. Comparisons of the results with those from independent algorithms are presented in the companion paper by Cattani et al. (2004).