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. In deep convective clouds, heavy rain is often formed involving the icephase. Simulations were performed using the 3-D cloud resolving modelCOSMO-SPECS with detailed spectral microphysics including parameterizations ofhomogeneous and three heterogeneous freezing modes. The initial conditionswere selected to result in a deep convective cloud reaching 14 km of altitudewith strong updrafts up to 40 m s −1 . At such altitudes with correspondingtemperatures below − 40  ∘ C the major fraction of liquid dropsfreezes homogeneously. The goal of the present model simulations was toinvestigate how additional heterogeneous freezing will affect ice formationand precipitation although its contribution to total ice formation may berather low. In such a situation small perturbations that do not showsignificant effects at first sight may trigger cloud microphysical responses.Effects of the following small perturbations were studied: (1) additional ice formation via immersion, contact, and deposition modes incomparison to solely homogeneous freezing, (2) contact and depositionfreezing in comparison to immersion freezing, and (3) small fractions ofbiological ice nucleating particles (INPs) in comparison to higher fractionsof mineral dust INP. The results indicate that the modification ofprecipitation proceeds via the formation of larger ice particles, which may besupported by direct freezing of larger drops, the growth of pristine iceparticles by riming, and by nucleation of larger drops by collisions withpristine ice particles. In comparison to the reference case with homogeneousfreezing only, such small perturbations due to additional heterogeneousfreezing rather affect the total precipitation amount. It is more likely that thetemporal development and the local distribution of precipitation are affectedby such perturbations. This results in a gradual increase in precipitationat early cloud stages instead of a strong increase at later cloudstages coupled with approximately 50 % more precipitation in the cloudcenter. The modifications depend on the active freezing modes, the fractionsof active INP, and the composition of the internal mixtures in the drops.

atmospheric chemistry and physics

Model simulations with COSMO-SPECS: impact of heterogeneous freezing modes and ice nucleating particle types on ice formation and precipitation in a deep convective cloud