Quantifying the global distribution of ice-nucleating particles and their climate impacts from model simulations with MADE3 in EMAC

Kurzfassung

Ice-nucleating particles (INPs) can exert important influences on the formation and the microphysical properties of cirrus clouds. However, the knowledge about the atmospheric distribution of INPs is still limited and consequently the understanding of their climate impacts is highly uncertain. We perform model simulations with a global aerosol-climate model coupled to a two-moment cloud microphysical scheme and a parametrization for aerosol-induced ice formation in cirrus clouds and present a global climatology of INPs in the cirrus regime. This novel INP climatology comprises, in addition to the broadly considered mineral dust and soot INPs, also crystalline ammonium sulfate and glassy organic particles. The simulated INP number concentrations range from about 1 to 100 L-1 and agree well with in-situ observations and other global model studies. While glassy organic INP concentrations are mostly low in the cirrus regime, our model results show large ammonium sulfate INP concentrations. By coupling the different INP-types to the microphysical cirrus cloud scheme, their ice nucleation potential under cirrus conditions is analyzed, considering possible competition mechanisms between different INPs. To quantify the impact of the INPs on the radiative forcing of cirrus clouds we compare a simulation with all different INP-types and a simulation with only homogeneous freezing. This results in a radiative forcing of -28 and -55 mW m-2, assuming a lower and an increased ice-nucleating potential of INPs. While the simulated impact of glassy organic INPs is mostly small and not significant, ammonium sulfate INPs introduce a considerable radiative forcing, which is nearly as large as the combined effect of mineral dust and soot INPs. The INP-cirrus effect due to anthropogenic INPs, considering the difference between present-day and preindustrial conditions, amounts to -29 mW m-2.