Aerosol–Cloud Interactions in Mixed-Phase Clouds and Their Role for Climate (Invited Presentation)

Aerosol-cloud interactions constitute the highest uncertainties in estimates of the anthropogenic radiative forcing. Uncertainties due to the phase and longevity of mixed-phase clouds (MPCs) influence the radiative balance and the hydrological cycle. The co-existence of cloud droplets and ice crystals in MPCs is thermodynamically unstable due to the lower saturation vapor pressure over ice than over liquid water. Thus, MPCs are expected to quickly glaciate, as ice crystals grow at the expense of cloud droplets. Measurements in the Arctic and orographic terrain have, however, shown that MPCs can persist over longer periods of time. In the Alps their presence can be confirmed by km-scale model simulations pointing to the dominant role of updraft velocities as a source of cloud droplets and such for the persistence of MPCs. In addition, the ice crystal number concentration (ICNC) in MPCs regularly exceeds the number concentration of ice nucleating particles (INP) by several orders of magnitude. Therefore, additional sources of ice crystals, e.g. secondary ice production and ice outside the cloud, have to contribute significantly to the ICNCs in MPCs. We made progress in understanding MPCs experimentally, through large eddy simulations and climate modelling, as will be presented in this talk.