New ways of mixed-phase cloud and cirrus research by means of field studies with combined fluorescence, dual-FOV polarization lidar and cloud radar: CCNC-CDNC and INPC-ICNC closure studies

Vertical profiling with ground-based lidars and radars offers excellent opportunities to monitor life cycles of mixed-phase clouds (MPCs) and cirrus fields in a coherent way over hours to days. In this presentation, we will discuss the new potential of modern lidar techniques in combination with cloud radar methods (a) to explore the evolution of MPCs, separately in terms of liquid- and ice-phase properties and (b) to study in detail the contribution of heterogenous ice nucleation to cirrus formation processes. The discussion is based on measurements performed in the framework of the MOSAiC expedition (life cycles of long-lasting Arctic MPCs), and intensive field studies at Dushanbe, Tajikistan (water and mixed-phase cloud evolution in aged dust and  dust-haze mixtures in central Asia), at Leipzig, Germany  (cirrus evolution in aged Canadian wildfire smoke), and in Cyprus and Punta Arenas, Chile (MPC evolution in the polluted Eastern Mediterranean vs the MPC evolution  over the pristine Southern Ocean). Of central importance are closure studies in which retrieved cloud condensation nucleus concentrations (CCNC) and cloud droplet number concentrations (CDNC) as well as ice-nucleating particle concentrations (INPC) and ice crystal number concentrations (ICNC) are compared.

In the case of mixed-phase clouds, the CCNC and INPC information is derived from extinction and backscatter lidar observations in combination with the POLIPHON (Polarization Lidar Photometer Networking) method. Relevant INP types in the free troposphere are wildfire smoke particles and mineral dust. By means of the fluorescence and the polarization lidar techniques a clear identification of fluorescing smoke and non-fluorescing, but polarizing dust particles is possible. The recently introduced dual-field-of-view (dual-FOV) polarization lidar method allows continuous monitoring of CDNC at about 75 to 100 m above the base of liquid-dominated cloud layers, occurring for example at the top of stratiform MPC systems. ICNC information is provided in the ice virga region using the synergy of cloud radar reflectivity and lidar extinction measurements. As will be shown, we observed long lasting Arctic mixed-phase cloud decks with permanently occurring liquid-dominated cloud top layers with CDNC typically ranging from 50-300 cm^-3 and the continuous production of ice crystals with ICNC of typically 0.1-1 per liter.

In the case of cirrus field studies, an important step forward was the integration of fluorescence lidar measurements into the combined lidar-radar observations.  Now, we were able to clearly identify and quantify fluorescing wildfire smoke particles serving as INPs in the upper troposphere. Strong smoke plumes in the tropopause region fruwently occurred in 2023 and 2025. The smoke INPC can now be determined within the cirrus top layers in which ice nucleation takes place.  We are able to answer the question whether a smoke INP reservoir in ice clouds can be depleted quickly during a cirrus life cycle or not and how important the smoke impact on heterogenous ice nucleation for cirrus formation is. The synergy of cloud radar and lidar observations again delivers ICNC information. The INPC-ICNC closure studies provided clear indications for a significant impact of smoke on cirrus formation over Leipzig, Germany.