The impact of heterogeneous ice nuclei on cirrus cloud convection

For cold cirrus clouds, it is always assumed that heterogeneous ice nuclei (IN) can only affect the dominant ice formation mechanism (i.e. homogeneous freezing of aqueous solution droplets) at low vertical velocities, i.e. for synoptically driven situations. However, these results were obtained exclusively by using box model simulations. Thus, possible impacts and feedbacks of dynamics on different scales are not included. We use the an-elastic, non-hydrostatic model EULAG including a recently developed ice microphysics to reinvestigate the competition of homogeneous freezing and heterogeneous nucleation within the same environment. For this purpose we use idealized simulations of potentially unstable layers in the upper troposphere, lifted by a synoptic updraft. For comparison, simulations with pure homogeneous nucleation and with competing homogeneous and heterogeneous nucleation within the same environment are carried out. Additionally, the number concentration of heterogeneous IN was varied in the range 5-50 per liter, representing typical values as observed in the tropopause region. Our simulations show that already a small amount of heterogeneous IN (e.g. 10 per liter) can significantly change the cirrus cloud properties as well as the local dynamical structure inside the cloud. Finally, the radiative impact of these cirrus clouds is investigated; here, we also find strong changes due to heterogeneous IN.