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Volume 18, issue 5
Atmos. Chem. Phys., 18, 3619-3639, 2018
https://doi.org/10.5194/acp-18-3619-2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Special issue: Results from the ice nucleation research unit (INUIT) (ACP/AMT...

Atmos. Chem. Phys., 18, 3619-3639, 2018
https://doi.org/10.5194/acp-18-3619-2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 13 Mar 2018

Research article | 13 Mar 2018

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

Karoline Diehl1 and Verena Grützun2 Karoline Diehl and Verena Grützun
  • 1Institute of Atmospheric Physics, University of Mainz, Germany
  • 2Institute of Meteorology, Hamburg, Germany

Abstract. In deep convective clouds, heavy rain is often formed involving the ice phase. Simulations were performed using the 3-D cloud resolving model COSMO-SPECS with detailed spectral microphysics including parameterizations of homogeneous and three heterogeneous freezing modes. The initial conditions were selected to result in a deep convective cloud reaching 14km of altitude with strong updrafts up to 40ms−1. At such altitudes with corresponding temperatures below −40°C the major fraction of liquid drops freezes homogeneously. The goal of the present model simulations was to investigate how additional heterogeneous freezing will affect ice formation and precipitation although its contribution to total ice formation may be rather low. In such a situation small perturbations that do not show significant 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 in comparison to solely homogeneous freezing, (2) contact and deposition freezing in comparison to immersion freezing, and (3) small fractions of biological ice nucleating particles (INPs) in comparison to higher fractions of mineral dust INP. The results indicate that the modification of precipitation proceeds via the formation of larger ice particles, which may be supported by direct freezing of larger drops, the growth of pristine ice particles by riming, and by nucleation of larger drops by collisions with pristine ice particles. In comparison to the reference case with homogeneous freezing only, such small perturbations due to additional heterogeneous freezing rather affect the total precipitation amount. It is more likely that the temporal development and the local distribution of precipitation are affected by such perturbations. This results in a gradual increase in precipitation at early cloud stages instead of a strong increase at later cloud stages coupled with approximately 50% more precipitation in the cloud center. The modifications depend on the active freezing modes, the fractions of active INP, and the composition of the internal mixtures in the drops.

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In deep convective clouds reaching altitudes of 14 km, heavy rain is often formed involving the ice phase. Ice nucleating particles (INPs) are responsible for heterogeneous freezing at middle and lower altitudes. Cloud model simulations indicate that INPs may effect a gradual increase in precipitation at early cloud stages instead of a strong increase at later cloud stages. Simultaneously, the local distribution of precipitation is changed, with more precipitation in the cloud center.
In deep convective clouds reaching altitudes of 14 km, heavy rain is often formed involving the...
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