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
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...
| Main Authors: | , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2018-03-01
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| Series: | Atmospheric Chemistry and Physics |
| Online Access: | https://www.atmos-chem-phys.net/18/3619/2018/acp-18-3619-2018.pdf |
| Summary: | 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 14 km of altitude
with strong updrafts up to 40 m s<sup>−1</sup>. 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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| ISSN: | 1680-7316 1680-7324 |