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| Titel |
Sub-micrometer Aerosol Particles in the UT/LMS Region as Measured by CARIBIC and Modeled Using the MIT-CAM3 Global Climate Model |
| VerfasserIn |
Annica Ekman, Markus Hermann, Peter Groß, Jost Heintzenberg, Dong-Chul Kim, Chien Wang |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250051760
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| Zusammenfassung |
| In recent years, more and more aerosol microphysical processes have explicitly been
incorporated in global atmospheric models. These developments make it possible to
investigate in more detail the influence of different aerosol processes on clouds and climate.
In particular, the influence of boundary layer particle formation on the number of available
cloud condensation nuclei and related radiative forcing has been studied intensively in
the last years. Deep convective clouds are considered to be the most important
controllers of new particle formation in the upper troposphere/lowermost stratosphere
(UT/LMS) region. However, the exact formation path and spatial and temporal
extent of the formation events are not well known. Hence, model simulations of
these processes are associated with large uncertainties. In the present study, we
compare global distributions of sub-micrometer aerosol particle concentrations
obtained from the CARIBIC project (Civil Aircraft for Regular Investigation of the
Atmosphere Based on an Instrument Container) with global model simulations using
the MIT-CAM3. The MIT-CAM3 is a state-of-the art global climate model with a
two-moment interactive aerosol physics and chemistry module involving seven
different aerosol compounds and mixtures. The main aim of the comparison is
to learn more about what governs the formation as well as spatial and temporal
distribution of the UT/LMS sub-micrometer aerosol. Furthermore, we investigate the
performance of a state-of-the-art global climate model in terms of its representation of
aerosol processes (and the interaction of aerosols with clouds) in the UT/LMS,
which may indicate where more general model improvement and development is
needed.
We find that the simulated global average nucleation mode particle number concentration
(here particles with diameter between 4 and 12 nm, N4-12) agrees fairly well with
observations but the spatial correlation is poor. For the Aitken mode particles (particles with
diameter larger than 12 nm, N12), the model overestimates the median concentration but the
geographical distribution is in rather good agreement with observations (correlation
coefficients between 0.68 and 0.71). In general, the largest overestimate by the model appears
to be located downwind the source regions and over South America. A clear seasonal
variation of N12 is displayed in both the observations and in the model simulations, with
overall higher concentrations in the northern hemisphere during the northern hemisphere
summer. The seasonal variation in the N4-12 concentration is less conspicuous, but there
is a tendency to higher summertime concentrations over Southeast Asia and the
US.
High concentrations (>5Ã104 particles/cm3STP) of N4-12 particles are in the
CARIBIC data found in a broad altitude interval between 310 and 370 K potential
temperature. This is in agreement with previous studies and indicates that new particles are
generated in deep convective outflows close to the tropopause and then transported
into the UT/LMS region. The model simulations also display a maximum in the
N4-12 number concentration around 340K, but the maximum value is substantially
lower. The vertical extent of the maximum is also much narrower compared to the
observations. This is most likely an indication of too little mixing and transport in the
model around the tropopause. Comparing the observed and modeled vertical aerosol
distributions, the pattern is similar for N12 as for N4-12. A maximum in particle number
concentration is found both in the model and in the observations around 340-350K, but the
observations display high N12 values over a larger altitude range compared to the
model. Again the modeled vertical gradient of the N12 concentration is steeper
than in the observations, indicating again too weak mixing around the tropopause. |
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