Cirrus cloud origin classification for seven years of IAGOS flights including a new origin category

Cirrus clouds exhibit varying radiative impacts depending on their origin of formation. For example, in situ origin cirrus clouds tend to have a warming effect, whereas liquid-origin cirrus clouds exhibit a rather cooling effect, though with large uncertainty. Understanding the global distribution of cirrus clouds, particularly with respect to their origin types is therefore essential for accurately assessing their radiative impacts. Consequently, analyzing their vertical and seasonal distributions is of key importance.

In this study, we use the refined cirrus origin index from the large-scale, Lagrangian model for the microphysical properties of cirrus clouds  (CLaMS-Ice: Krämer et al., 2026; Gasparini et al., 2025), which extends the existing classification of in-situ and liquid origin cirrus by introducing a third origin type termed dual-origin cirrus. Dual-origin cirrus are initially of liquid origin,  in which later in-situ ice nucleation occurs. They therefore bear the signature of both pure cirrus types. So far, this type is assigned to the liquid-origin cirrus. CLaMS-Ice was applied to seven years of passenger aircraft flight data from the European research infrastructure IAGOS.  The relative variability of the three types of cirrus cloud is then investigated along the IAGOS flight routes using the new origin index of CLaMS-Ice.

The variability of the cirrus types is examined with respect to 30 hPa layers around and below the tropopause in the northern mid-latitudinal regions of North America, the North Atlantic, and Western Europe, including a seasonal analysis. The total frequency of cirrus clouds is found to be highest over the North Atlantic, with a high fractional density across the upper troposphere, particularly in layers closest to the tropopause. In situ origin cirrus clouds show the highest fractional occurrence near the tropopause across all seasons and represent the dominant category among all cirrus types, with their fraction gradually decreasing at higher pressure levels (lower altitudes). In addition to the two cirrus categories of in-situ and liquid origin, a substantial fraction of the data falls into the dual-origin category. The fraction of dual-origin cirrus clouds is observed to be higher than that of liquid-origin cirrus clouds.  Our analysis reveals a significant contribution from this dual-origin cirrus class, highlighting the importance of distinguishing it when assessing cirrus cloud variability and their associated radiative impacts.

 

Gasparini, B., Atlas, R., Voigt, A., Krämer, M., and Blossey, P. N.: Tropical cirrus evolution in a kilometer-scale model with improved ice microphysics, Atmos. Chem. Phys., 25, 9957–9979, https://doi.org/10.5194/acp-25-9957-2025, 2025.

Krämer, M, J.-U. Grooß, P. Spichtinger, I. Bartolomé Garçia, and C. Rolf:  Large-scale Lagrangian 3D cirrus modeling with ClaMS-Ice; in preparation for ACP.