 |
| Titel |
Global variability of cloud condensation nuclei concentrations |
| VerfasserIn |
Risto Makkonen, Olaf Krüger |
| Konferenz |
EGU General Assembly 2017
|
| Medientyp |
Artikel
|
| Sprache |
en
|
| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 19 (2017) |
| Datensatznummer |
250150464
|
| Publikation (Nr.) |
 EGU/EGU2017-14932.pdf |
|
|
|
|
|
| Zusammenfassung |
| Atmospheric aerosols can influence cloud optical and dynamical processes by acting as cloud
condensation nuclei (CCN). Globally, these indirect aerosol effects are significant to the
radiative budget as well as a source of high uncertainty in anthropogenic radiative forcing.
While historically many global climate models have fixed CCN concentrations to a certain
level, most state-of-the-art models calculate aerosol-cloud interactions with sophisticated
methodologies based on interactively simulated aerosol size distributions. However,
due to scarcity of atmospheric observations simulated global CCN concentrations
remain poorly constrained. Here we assess global CCN variability with a climate
model, and attribute potential trends during 2000-2010 to changes in emissions and
meteorological fields. Here we have used ECHAM5.5-HAM2 with model M7 microphysical
aerosol model. The model has been upgraded with a secondary organic aerosol
(SOA) scheme including ELVOCs. Dust and sea salt emissions are calculated online,
based on wind speed and hydrology. Each experiment is 11 years, analysed after a
6-month spin-up period. The MODIS CCN product (Terra platform) is used to
evaluate model performance throughout 2000-2010. While optical remote observation
of CCN column includes several deficiencies, the products serves as a proxy for
changes during the simulation period. In our analysis we utilize the observed and
simulated vertical column integrated CCN concentration, and limit our analysis
only over marine regions. Simulated annual CCN column densities reach 2⋅108
cm−2 near strong source regions in central Africa, Arabian Sea, Bay of Bengal and
China sea. The spatial concentration gradient in CCN(0.2%) is steep, and column
densities drop to <50% a few hundred kilometers away from the coasts. While the
spatial distribution of CCN at 0.2% supersaturation is closer to that of MODIS
proxy, as opposed to 1.0% supersaturation, the overall column integrated CCN
are too low. Still, we can compare the relative response of CCN to emission and
meteorological variability. Most evident pattern of high temporal correlation is found over
North Atlantic ocean, extending throughout Europe and up to Gulf of Mexico.
All of these regions show a generally decreasing trend throughout the decade in
control simulations and MODIS CCN, and the simulations including the emission
trends clearly improve the simulations with climatological emissions. In regions
where the observed intra-annual cycle correlates well with sea-spray emissions,
the long-term annual correlation usually remains poor. This could indicate that
the model is unable to capture the natural variability in marine aerosol emissions. |
|
|
|
|
|
|