Aerosol impacts on cirrus cloud formation and properties using in-situ and lidar measurements during CIRRUS-HL campaign

Cirrus clouds cover about 30% of the Earth’s surface and play a crucial role in the Earth’s radiation balance. They are composed of ice crystals with various sizes and nonspherical shapes. Ice crystals can form through either homogeneous freezing or heterogeneous freezing depending on the ambient temperature, humidity, updraft, and the availability of INPs, and hence possess different properties. Their radiative effects strongly depend on the formation processes and cloud microphysical, thermal, and optical properties. Furthermore, global aviation affects the Earth’s radiation balance by increasing cloudiness due to contrail formation and exerting an indirect effect on the microphysical properties of naturally-formed cirrus clouds. Aviation is responsible for 3-4% of anthropogenic effective radiative forcing and more than half of them stems from contrails and contrail-induced cirrus. Experimental and numerical studies have been carried out in the past years to understand contrails and contrail-induced cirrus as well as their climate effects. Unfortunately, however, the parameterization of ice crystal properties in global climate model and the estimate of radiation forcings are still inadequate. Compared with the intensive studies on cirrus clouds in the tropics and midlatitude regions, cirrus cloud measurements and model studies at high latitudes are sparse, although cirrus clouds at high latitudes attract more attention in recent years because the Arctic undergoes faster warming than other regions of the globe. The airborne measurements from the ML-CIRRUS mission revealed that cirrus clouds with enhanced PLDR appear in areas of high aviation emissions. Nevertheless, observational evidence of indirect effects of aviation exhaust on the changes of cirrus properties is still missing. Thanks to the foundational work of ML-CIRRUS, the CIRRUS-HL mission in June-July, 2021, with upgraded instrumentation was designed to characterize cirrus cloud at both high- and midlatitudes and to investigate aviation impact, radiation, and aerosol-cloud interactions. It collected more details in the simultaneous profiling of cirrus cloud and aerosol properties. In this study, we focus on the comparison of particle linear depolarization ratios (PLDR) of cirrus clouds with the airborne lidar WALES from two specific flights under similar cloud formation processes during CIRRUS-HL. Their microphysical properties (i.e. ice crystal size and number concentration) are also determined and compared based on the analysis of simultaneous in-situ measurements. The analysis is also extended to all the flights for statistical results. Furthermore, the characterization of aerosol load, especially aviation soot, will be identified in the regions of ice crystal formation and evolution and their correlations with cirrus cloud properties are finally able to be further determined.