Characterization of volcanic aerosol properties over liquid water clouds from combined Infrared and Polarized observations

After the Eyjafjallajökull eruption in spring 2010, the emitted plumes were advected over the Atlantic Ocean, thousands of kilometers away from the sources and sometime in areas covered with low level clouds. The retrieval of these volcanic plume properties which are of uncertain composition and overlying a surface of high and variable reflectance is a challenge for standard radiometric observations based on visible wavelengths. The A-Train provides an unprecedented observational environment, which includes polarized observations from both active and passive instruments onboard CALIPSO and PARASOL. Polarized observations offer the possibility to retrieve the aerosol optical depth above liquid water clouds with a limited error induced by the cloud reflectance variability. Infrared (IR) observations have long been used to characterize volcanic emitted material and it is one of the advantages of the CALIPSO mission to combine IR and lidar instruments to provide co-located observations directly exploitable to improve the characterization of aerosol and clouds. We will describe the underlying principle of the different methodologies based on Polarized and Infrared A-Train observations as well as their applications on a few case studies where we characterize the properties of various kinds of aerosols above liquid water clouds with an emphasis on volcanic ash optical and microphysical properties retrieval.