Did the 2011 Nabro eruption affect cirrus optical properties?

We have evaluated eight years of CALIPSO/CALIOP backscatter and depolarization data to address the question whether volcanic aerosols have an impact on the microphysical and optical properties of cirrus clouds. The CALIOP lidar instrument allows to discriminate cloud and aerosol particles based on their actively sensed backscatter and depolarization properties. We focus on the eruption of the Nabro volcano in June 2011, which added about 1 Tg of SO2 to the lower stratosphere (Theys et al. 2013) and was the strongest eruption since Pinatubo in this regard.

Previous observational studies have come to differing conclusions regarding the impact of volcanic aerosols on cirrus properties (Sassen 1992, Luo et al 2002). Modelling studies of the Pinatubo eruption and of stratospheric sulfate geoengineering are also ambiguous in this regard (Lohmann et al. 2003, Kuebbeler et al. 2012, Cirisan et al. 2013).

We find that year-to-year variability, which is likely caused by synoptic processes and climatic oscillations such as the North Atlantic Oscillation (NAO) and the El Niņo-Southern Oscillation (ENSO), clearly outweighs any Nabro signal in the cirrus cloud backscatter and cirrus occurrence frequency at all latitudes and over land and ocean. We conclude that the cirrus-cloud radiative forcing caused by volcanically-induced changes in cirrus optical properties is negligible in the case of the Nabro eruption.