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| Titel |
Towards improving the formation of drizzle in marine stratiform clouds |
| VerfasserIn |
V. Sant, A. Seifert, R. Posselt, U. Lohmann |
| Konferenz |
EGU General Assembly 2012
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250063245
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| Zusammenfassung |
| Due to their proximity to the surface and their vast expanse over global oceans,
marine stratus and stratocumulus induce a net cooling towards the Earth’s radiative
budget. These low clouds are very susceptible to changes in meteorological and
environmental conditions such that the amount of formed precipitation, although small,
may be altered significantly. The formation of drizzle is highly dependent on the
onset of the collision-coalescence process, which is related to the concentration of
cloud condensation nuclei (CCN) and/or turbulence, but has also been recognised
to feed back onto both microphysics and dynamics of the cloud. The three-way
interaction of cloud microphysics, dynamics and precipitation formation in marine
stratiform clouds is complex and has a significant impact on the clouds radiative
properties.
To achieve a more physical representation of the droplet spectrum in low clouds an
additional drizzle drop class with radii between 25-100 μm is introduced to the traditionally
existing classes of cloud liquid water and rain. The idea is to improve the microphysical, but
possibly also dynamical or thermodynamical, mechanisms responsible for the precipitation
onset. A new parameterization to describe the collision-coalescence processes between three
drop classes has been developed based on the stochastic collection equation and
solved for truncated moments. For polluted environments specifically, i.e. high CCN
concentrations, where precipitation formation may be retarded, the additional drizzle drop
class improves the evolution of the drop spectrum and possible influences of giant
CCN such as large sea salt aerosols towards enhancing the collision-coalescence
process.
Results comparing the new parameterization to a resolved spectral description of the
microphysics within a 1D kinematic cloud model revealed to be very promising for
different CCN concentrations and vertical updraft regimes. Furthermore, with the goal
of improving marine stratiform clouds on a global scale, the effects of the new
parameterization with the prognostic treatment of precipitation is investigated in the
general circulation model ECHAM5-HAM will also be presented. In addition, the
radiative effect of the drizzle drop class can be taken into account for the latter
case. |
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