Quantification of Cloud Susceptibilities to Ice Nuclei

The impact of aerosols on cloud properties is currently not well established. This is largely attributed to the interdependencies of aerosols and cloud microphysical processes, among which primary ice formation contributes to considerable uncertainties. Although it is known that in a large range of thermodynamic conditions aerosol particles are required to initiate ice formation, identifying and characterizing the effect of specific ice nuclei is among current scientific efforts.

Here we attempt to quantify the change of cloud macrophysical and radiative properties with varying aerosol background concentrations. We adapt the concept of susceptibilities for mixed-phase and ice clouds which has up to now been applied primarily to liquid clouds, defining the susceptibility S as the derivation of a macrophysical quantity Q with respect to aerosol concentrations N. A focus of our study is the comparison of different model approaches. The classical method is the direct comparison of two independent model runs, where the whole range of microphysical and macrophysical feedbacks contributes to different cloud properties in a perturbed simulation. Our alternative method relies on a single simulation which incorporates two microphysical schemes in parallel with different aerosol concentrations. Since in the latter case only microphysical feedbacks contribute to the properties of perturbed clouds, we expect to be able to draw conclusions on the importance of macrophysical feedbacks on perturbed clouds in different cloud regimes.

First results are shown, comparing a persistent Arctic mixed-phase stratocumulus cloud layer and other idealized cloud regimes with respect to their sensitivity to different ice nuclei species. Results are based on three-dimensional large eddy simulations with a two-moment representation of cloud microphysical interactions.