Susceptibilities and Resilience in Aerosol-Cloud Interaction

The contribution of aerosols continues to pose an important uncertainty in anthropogenic climate forcing (IPCC Assessment Report 5). However, over the last decade and with increasingly detailed process representation in climate models, forcing estimates tend to decrease (Figure 1). Such a trend would back the notion of clouds as resilient or buffered systems, which has emerged from small-scale cloud modeling (Stevens & Feingold, 2009): Like chemical buffer solutions that keep a constant pH-value at the addition of an acid or base, the complex interplay of cloud processes and their dependence on the cloud regime might compensate aerosol perturbations on local to global scales.

In this project, we study cloud adjustments to aerosol perturbations and aim to quantify the degree of resilience observed for different cloud regimes and resolutions. Our approach is based on quantifying the link strengths in aerosol-cloud-radiation-precipitation interaction (Figure 2) by susceptibilities dln(effect)/dln(cause), i.e. relative sensitivities or effective exponents, respectively.

Connecting the process to the global scale, we compiled the regional climate model COSMO-ART-M7. To capture major processes from aerosol emission to radiation and precipitation, the model is coupled to a modal aerosol scheme and includes aerosol activation and heterogeneous freezing as well as two-moment cold and warm cloud microphysics.