Aviation-induced changes in cirrus cloud properties over Europe: Investigation with CALIPSO lidar measurements

Abstract

Cirrus clouds are ubiquitous high-level clouds consisting of ice crystals with a variety of sizes and shapes. They play a crucial role in the Earth’s radiation budget. Overall, they are supposed to have a warming effect, but their radiative effect is strongly determined by their microphysical properties, e.g. shape and size of the ice particles as well as the ice particle number concentration. Linear contrails and contrail cirrus induced by water vapor and soot emissions from air traffic in the upper atmosphere are expected to contribute a large part of the climate impact of avation. Furthermore, global aviation affects the Earth’s radiation balance by inducing an indirect effect on the microphysical properties of naturally-formed cirrus clouds. During the first lockdown due to COVID-19 in April 2020 air traffic over Europe was reduced by about 90%. We analysed CALIPSO lidar data measured over Europe during this strong air traffic reduction and compared it to CALIPSO measurements performed in the same region and months for 6 years before COVID-19. We found a significant reduction in the particle linear depolarization ratio (PLDR) of cirrus clouds in April 2020 compared to the previous years 2014-2019. Those findings fit well to former studies from aircraft measuremnts reporting, for the first time, an indication of an aviation impact on naturally formed cirrus clouds. Motivated by these results, long-term analysis were performed over Europe to investigate seasonality and trends of cirrus PLDR, which is found to be significantly correlated with the increase of civil aviation in the last 10 years before COVID-19. In our presentation we will show our results of the changes in optical properties of cirrus clouds related to the reduced air traffic over Europe during the COVID-19 lockdown in March/April 2020, and we will present our long-term analysis of the cirrus’ optical properties over Europe. Finally, we will link these findings to optical and microphysical properties from airborne measurements in the same region.