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| Titel |
Influence of a dynamic background albedo scheme on mid-Holocene North African rainfall |
| VerfasserIn |
Freja Vamborg, Victor Brovkin, Martin Claussen |
| 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 |
250054764
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| Zusammenfassung |
| Precipitation in North Africa during the mid-Holocene was significantly increased compared
to today, presumably due to a change in orbital forcing. It has been hypothesised that a
vegetation-precipitation feedback acted to increase the response of precipitation to this
forcing. The key parameter for the strength and sign of this feedback is the surface albedo.
Thus, to correctly asses the importance of this feedback, the change in surface albedo must be
modelled in an appropriate way. Albedo schemes in general circulation models tend to
simulate the dynamic behaviour of the canopy, but not of the surface below it, which may
lead to a misrepresentation of the albedo change. Here we present a new albedo scheme, that
takes the dynamic behaviour of the surface below the canopy into account. Using the MPI
Earth System Model (MPI-ESM) we investigate the effect of using this dynamic scheme
versus the static version that is usually used. The dynamic scheme represents the seasonal
cycle of albedo and the correspondence between annual mean albedo and vegetation
cover in a more consistent way than the static scheme. It thus gives a better estimate
of albedo change between the two time periods. The present-day dry bias in the
Sahel of the standard MPI-ESM is reduced and the sensitivity of precipitation to
mid-Holocene external forcing is increased by around one third. The dynamic albedo
scheme leads to increased vegetation variability in the remaining desert region,
indicating a higher frequency of green spells, thus reaching a better agreement with the
vegetation distribution as derived from pollen records. We also show that without the
dynamic albedo scheme, the vegetation-precipitation feedback in the MPI-ESM is very
weak. |
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