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| Titel |
New particle-dependent parameterizations of heterogeneous freezing processes: sensitivity studies of convective clouds with an air parcel model |
| VerfasserIn |
K. Diehl, S. K. Mitra |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1680-7316
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| Digitales Dokument |
URL |
| Erschienen |
In: Atmospheric Chemistry and Physics ; 15, no. 22 ; Nr. 15, no. 22 (2015-11-18), S.12741-12763 |
| Datensatznummer |
250120166
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| Publikation (Nr.) |
 copernicus.org/acp-15-12741-2015.pdf |
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| Zusammenfassung |
| Based on the outcome of laboratory results, new particle-dependent
parameterizations of heterogeneous freezing were derived and used to improve
and extend a two-dimensional spectral microphysics scheme. They include (1)
a particle-type-dependent parameterization of immersion freezing using the
numbers of active sites per mass, (2) a particle-type and size-resolved
parameterization of contact freezing, and (3) a particle-type-dependent
description of deposition freezing. The modified microphysical scheme was
embedded in an adiabatic air parcel model with entrainment. Sensitivity
studies were performed to simulate convective situations and to investigate
the impact of ice nuclei concentrations and types on ice formation. As a
central diagnostic parameter, the ice water fraction (IWF) was selected, which is
the relation of the ice water content to the total amount of water in
the condensed form. The following parameters were varied: initial aerosol
particle number size distributions, types of ice nucleating particles, final
temperature, and the fractions of potential ice nucleating particles. Single
and coupled freezing processes were investigated. The results show that
immersion freezing seems to be the most efficient process. Contact freezing
is constrained by the collision kernel between supercooled drops and
potential ice nucleating particles. The importance of deposition freezing
lies in secondary ice formation; i.e., small ice particles produced by
deposition nucleation trigger the freezing of supercooled drops by
collisions. Thus, a broader ice particle spectrum is generated than that by
immersion and contact freezing. During coupled immersion–contact and
contact–deposition freezing no competition was observed, and both processes
contribute to cloud ice formation but do not impede each other. As already
suggested in the literature, mineral dust particles seem to be the most
important ice nucleating particles. Biological particles are probably not
involved in significant ice formation. The sensitive parameters affecting
cloud properties are temperature, aerosol particle composition and
concentration, and particle size distribution. |
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