Cirrus cloud median microphysical and optical properties in the IWC-temperature space from a comprehensive airborne size distribution database

The detailed information on the particle size distributions (PSDs) of ice clouds is essential for various topics of radiative transport in a cloudy atmosphere. The new composite of in-situ PSD measurements, including 9 airborne campaigns with in total 137 flight hours in cirrus cloud conditions, is currently the most comprehensive data set for studying PSD parameters. The PSDs cover for all campaigns particle diameters down to 3 microns and are not affected by the so-called shattering effect. The database covers the complete cirrus cloud temperature range (185 - 235 K) and IWC from 10⁻⁶ to 1 g/m³ and and is thus especially well-suited to investigate optically thinnest clouds hitherto not included in PSD data bases.

Here, the database is used for detailed analysis of PSDs of cirrus clouds for two size modes, as Bartolomé García et al. (2023) have shown that ice cloud PSDs are better represented by bimodal PSDs. To this end, two log-normal functions are derived for the measured PSDs, one for small and one for large ice particles. An iterative approach for fitting the bimodal log-normal functions to the measured PSD by minimizing a cost function have been applied to the data with overall good fitting results.

Next, microphysical and optical properties such as ice water content (IWC), mean mass diameter (D), effective radius (R_eff) and extinction are determined for the total cirrus particle size range and also for each of the two size modes. For each parameter, median values are then computed at predefined IWC - temperature intervals.

Characteristics of the total size range as well as of the small and large size modes in the IWC-temperature space in terms of microphysical and optical properties will be presented for mid-latitudes and the tropics. The potential imprint of the results on currently applied cloud modules and cloud parameterization in global climate models will be investigated.

 

References: Bartolomé García, I., Sourdeval, O., Spang, R., and Krämer, M.: Technical note: Bimodal parameterizations of in situ ice cloud particle size distributions, Atmos. Chem. Phys., 24, 1699–1716, https://doi.org/10.5194/acp-24-1699-2024, 2024.