Clouds and aerosols are inseparably coupled, linked via complex pathways of interaction whose outcome manifests in the macroscopic properties of precipitation and radiation fields. On the one hand, aerosol particles are required as cloud condensation nuclei from which cloud droplets do form. On the other hand, primary ice formation in the heterogeneous freezing range from 0 to approximately -40°C requires ice nucleating particles (INP) to be present in the aerosol reservoir. The ways in which aerosols and cloud particles interact are however controlled by the dynamics and thermodynamics of the atmospheric environment. Thermodynamics are considered to dominate the cloud microphysical properties because they control the amount of water vapour that is available for being transferred to either the liquid or the ice phase. This dominance makes it difficult to isolate aerosol-related effects in observations of cloud properties. As a consequence, virtually all currently available numerical weather prediction models do not take any interaction between aerosol and cloud properties into account. Nevertheless, it is well known that aerosol properties are subject to strong spatio-temporal variability which implies that under certain situations aerosols are likely to have considerable effects on the microphysical properties of clouds. Another consequence is that the lack of aerosol-cloud interaction or the assumption of constant aerosol conditions will introduce uncertainties to numerical simulations.
The direct numerical simulation of aerosol-cloud interaction processes is challenging because the spatiotemporal scales that are needed to be covered reach from the turbulent microscale up to the scale of precipitation processes. It is thus subject to observations to disentangle the signal of aerosol-cloud interaction processes from the dynamical and thermodynamically dominated observable meteorological features. Nonetheless, also the observation of aerosol-effects on cloud properties is difficult to achieve, considering that atmospheric conditions and aerosol load are frequently correlated, as it is for instance the case for southwesterly large-scale flows over central Europe which are usually accompanied by Saharan dust outbreaks. In addition, the northern hemisphere, for which currently most cloud and aerosol datasets exist, is expected to be permanently affected by considerable aerosol loads, be it from natural sources such as deserts and forests, or from anthropogenic sources such as industry or traffic. Actual contrasting of aerosol-free to aerosol-burden cloud environments under otherwise constant meteorological conditions may thus be hardly achievable at a single site or perhaps overall in the northern hemisphere.