Global analysis of cirrus origins using satellite observations and Lagrangian trajectories

In addition to their formation mechanisms, the origin of cirrus clouds (liquid-origin or in situ) can significantly influence their microphysical and radiative properties. Liquid-origin cirrus, which form through the freezing of water droplets from the mixed-phase region upon reaching cirrus temperatures (below -38°C), are typically characterized by high ice crystal concentrations and are associated with a strong cooling effect. In contrast, in situ-origin cirrus consist of ice crystals that form directly within the cirrus regime via homogeneous or heterogeneous freezing. Large cloud systems often comprise a mixture of both types. However, the global occurrence, distribution, and environmental conditions associated with these cirrus types remain poorly understood.

This study investigates cirrus clouds by integrating satellite observations with reanalysis data. Observations from lidar-radar satellite instruments (DARDAR-Nice) provide detailed retrievals of cirrus microphysical properties, including ice water content and ice crystal number concentration. To trace the origins of cirrus clouds, we employ Lagrangian air mass trajectories based on ERA5 reanalyses, using the FLEXPART Lagrangian model. This analysis is conducted for every satellite retrieval pixel, with a horizontal resolution of 1.7 km and a vertical resolution of 60 m. Environmental conditions at and preceding cirrus formation, as well as those in the mixed-phase regime for liquid-origin cirrus, are examined along the trajectories. Aerosol data from CAMS reanalyses are also included to assess their influence.

A combined cloud-aerosol dataset, derived from satellite observations and reanalysis tools, is compiled for one year of global data. The global occurrence of liquid-origin cirrus is analyzed in relation to ice crystal formation drivers and the resulting microphysical and radiative properties of these clouds, as retrieved by the DARDAR-Nice product. The relative occurence of insitu- and liquid-origin ice in cirrus is also assessed. The role of aerosols in cirrus formation processes will briefly be explored.