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| Titel |
Aerosol processing in stratiform clouds in ECHAM6-HAM |
| VerfasserIn |
David Neubauer, Ulrike Lohmann, Corinna Hoose |
| Konferenz |
EGU General Assembly 2013
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 15 (2013) |
| Datensatznummer |
250074727
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| Zusammenfassung |
| Aerosol processing in stratiform clouds by uptake into cloud particles, collision-coalescence,
chemical processing inside the cloud particles and release back into the atmosphere has
important effects on aerosol concentration, size distribution, chemical composition and
mixing state.
Aerosol particles can act as cloud condensation nuclei. Cloud droplets can take up further
aerosol particles by collisions. Atmospheric gases may also be transferred into the cloud
droplets and undergo chemical reactions, e.g. the production of atmospheric sulphate.
Aerosol particles are also processed in ice crystals. They may be taken up by homogeneous
freezing of cloud droplets below -38Ë C or by heterogeneous freezing above -38Ë C. This
includes immersion freezing of already immersed aerosol particles in the droplets and contact
freezing of particles colliding with a droplet. Many clouds do not form precipitation
and also much of the precipitation evaporates before it reaches the ground. The
water soluble part of the aerosol particles concentrates in the hydrometeors and
together with the insoluble part forms a single, mixed, larger particle, which is
released.
We have implemented aerosol processing into the current version of the general
circulation model ECHAM6 (Stevens et al., 2013) coupled to the aerosol module HAM (Stier
et al., 2005). ECHAM6-HAM solves prognostic equations for the cloud droplet
number and ice crystal number concentrations. In the standard version of HAM,
seven modes are used to describe the total aerosol. The modes are divided into
soluble/mixed and insoluble modes and the number concentrations and masses of different
chemical components (sulphate, black carbon, organic carbon, sea salt and mineral
dust) are prognostic variables. We extended this by an explicit representation of
aerosol particles in cloud droplets and ice crystals in stratiform clouds similar to
Hoose et al. (2008a,b). Aerosol particles in cloud droplets are represented by 5
tracers for the chemical components as well as 5 tracers for aerosol particles in ice
crystals. This allows simulations of aerosol processing in warm, mixed-phase (e.g.
through the Bergeron-Findeisen process) and ice clouds. The fixed scavenging ratios
used for wet deposition in clouds in standard HAM are replaced by an explicit
treatment of collision of cloud droplets/ice crystals with interstitial aerosol particles.
Nucleation scavenging of aerosol particles by acting as cloud condensation nuclei or ice
nuclei, freezing and evaporation of cloud droplets and melting and sublimation of ice
crystals are treated explicitly. In extension to previous studies, aerosol particles from
evaporating precipitation are released to modes which correspond to their size.
Cloud processing of aerosol particles changes their size distribution and hence
influences cloud droplet and ice crystal number concentrations as well as precipitation
rate, which in turn affects aerosol concentrations. Results will be presented at the
conference.
Hoose et al., JGR, 2008a, doi: 10.1029/2007JD009251
Hoose et al., ACP, 2008b, doi: 10.5194/acp-8-6939-2008
Stevens et al., 2013, submitted
Stier et al., ACP, 2005, doi: 10.5194/acp-5-1125-2005 |
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