CALIPSO observations of cirrus cloud properties: investigation of latitude differences and possible aviation impacts

Cirrus clouds, forming in the cold upper troposphere, are composed of ice crystals with various sizes and nonspherical shapes. They are observed at all latitudes covering over 30% of the Earth’s surface. Studies reveal that they have a significant impact on the radiation balance of our planet and, consequently, on the climate evolution. The radiative effect of cirrus clouds is strongly determined by the cloud microphysical, thermal, and optical properties. Furthermore, global aviation affects the Earth’s radiation balance by inducing contrails and exerting an indirect effect on the microphysical properties of naturally-formed cirrus clouds. In the last decades, the Arctic surface has been warming faster than other regions of the globe, which is known as Arctic Amplification. The thin and small-coverage cirrus clouds over the Arctic are presumed to largely contribute to it. Unfortunately, however, the optical and microphysical properties of cirrus clouds over the Arctic and the exact role they play in the elevated warming of the Arctic are far from understanding. Compared with the intensive studies of cirrus clouds in the tropics and midlatitude regions, cirrus cloud measurements and model studies at high latitudes are sparse. In this study, we present the comparisons of the particle linear depolarization ratio (PLDR) and occurrence rates of cirrus clouds at midlatitudes (35–60 ^oN; 30 ^oW–30 ^oE) and high latitudes (60–80 ^oN; 30 ^oW–30 ^oE) based on the analysis of lidar measurements of CALIPSO in the years 2018–2021. The results show that cirrus clouds at high latitudes appear at lower altitudes than the midlatitude cirrus clouds. The PLDR and occurrence rates of cirrus clouds at high latitudes are smaller than the midlatitude cirrus clouds. Furthermore, air traffic over Europe was significantly reduced in 2020 (starting from March) and only moderately reduced in 2021 due to the COVID-19 pandemic. Under this condition we are able to study the difference in the aviation impacts on the cirrus cloud properties at high latitudes and midlatitudes.