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| Titel |
The global middle-atmosphere aerosol model MAECHAM5-SAM2: comparison with satellite and in-situ observations |
| VerfasserIn |
R. Hommel, C. Timmreck, H. F. Graf |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1991-959X
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| Digitales Dokument |
URL |
| Erschienen |
In: Geoscientific Model Development ; 4, no. 3 ; Nr. 4, no. 3 (2011-09-16), S.809-834 |
| Datensatznummer |
250001789
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| Publikation (Nr.) |
 copernicus.org/gmd-4-809-2011.pdf |
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| Zusammenfassung |
In this paper we investigate results from a three-dimensional
middle-atmosphere aerosol-climate model which has been developed to study the
evolution of stratospheric aerosols. Here we focus on the stratospheric
background period and evaluate several key quantities of the global
distribution of stratospheric aerosols and their precursors with observations
and other model studies. It is shown that the model fairly well reproduces in
situ observations of the aerosol size and number concentrations in the upper
troposphere and lower stratosphere (UT/LS). Compared to measurements from the
limb-sounding SAGE II satellite instrument, modelled integrated aerosol
quantities are more biased the lower the moment of the aerosol population is.
Both findings are consistent with earlier work analysing the quality of SAGE
II retrieved e.g. aerosol surface area densities in the volcanically
unperturbed stratosphere (SPARC/ASAP, 2006;
Thomason et al., 2008; Wurl et al., 2010).
The model suggests that new particles are formed over large areas of the LS,
albeit nucleation rates in the upper troposphere are at least one order of
magnitude larger than those in the stratosphere. Hence, we suggest that both,
tropospheric sulphate aerosols and particles formed in situ in the LS are
maintaining the stability of the stratospheric aerosol layer in the absence
of direct stratospheric emissions from volcanoes. Particle size distributions
are clearly bimodal, except in the upper branches of the stratospheric
aerosol layer where aerosols evaporate. Modelled concentrations of
condensation nuclei (CN) are smaller than measured in regions of the aerosol
layer where aerosol mixing ratios are largest. This points to an
overestimated particle growth by coagulation.
Transport regimes of tropical stratospheric aerosol have been identified from
modelled aerosol mixing ratios and correspond to those deduced from satellite
extinction measurements. We found that convective updraft in the Asian
Monsoon region significantly contributes to both stratospheric aerosol
load and size. The timing of formation and descend of layers of fine mode
particles in the winter and spring polar stratosphere (CN layer) are well
reproduced by the model. Where temperatures in the stratosphere increase with
altitude, nucleation is unlikely to occur. Nevertheless, in these regions we
find a significant concentration of fine mode aerosols. The place of origin
of these particles is in the polar stratosphere. They are mixed into the
mid-latitudes by planetary waves. There enhanced condensation rates of
sulphuric acid vapour counteract evaporation and extend aerosol lifetime in
the upper branches of the stratospheric aerosol layer.
Measured aerosol precursors concentrations, SO2 and sulphuric acid
vapour, are fairly well reproduced by the model throughout the stratosphere. |
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