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| Titel |
The lapse-rate feedback leads to polar temperature amplification. |
| VerfasserIn |
Rune Grand Graversen, Peter Langen, Thorsten Mauritsen |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250098835
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| Publikation (Nr.) |
 EGU/EGU2014-14551.pdf |
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| Zusammenfassung |
| The atmospheric temperature will change in response to a radiative forcing of the climate
system, but the temperature change may not be constant with height in the atmosphere. The
dependence of the temperature change on hight gives rise to the lapse-rate feedback. In a
warmer climate, the saturated mixing ratio of water vapour increases more at lower than
at upper levels in the troposphere. Therefore due to enhanced latent heat release,
the atmosphere tends to warm more in the upper than in the lower troposphere
in regions where strong convection is present, such as at tropical latitudes. This
results in enhanced radiation back to space, and in a more efficient cooling of the
Earth system. This is contributing to a negative lapse-rate feedback. The opposite
situation prevails at the high latitudes where stable stratification conditions in the
lower troposphere result in a larger warming of the surface-near atmosphere than
of the upper troposphere. This is contributing to a positive lapse-rate feedback.
Hence the lapse-rate feedback is assumed to be negative at low, and positive at high
latitudes.
Here we explore the lapse-rate feedback and its effect on the climate system using a
slab-ocean climate model, the Community Climate System Model version 4. By locking
the temperature change throughout the troposphere to that at the surface in calls
to the radiation code, the lapse-rate feedback is suppressed on-line in the model.
Doubling-of-CO2 experiments where the lapse-rate feedback is suppressed are
compared with experiments where it is retained. In a similar way the surface-albedo
feedback is suppressed by keeping the surface albedo fixed in the entire model
system.
On the basis of model versions where either one or both of the feedbacks are suppressed,
we are able to separate the effect of the surface-albedo and lapse-rate feedback. For instance
we can estimate the contribution to the polar temperature amplification due to each of the
feedbacks. The results show that the lapse-rate feedback contributes significantly to the
amplification of the surface temperature in the polar areas. The lapse-rate feedback accounts
for around 15 %, and the surface-albedo feedback 40 % of the amplification in the Arctic. In
the Antarctic, 20 % of the amplification can be attributed to the lapse-rate feedback and
60 % to the surface-albedo feedback. Further it is found that the surface-albedo
and lapse-rate feedbacks interact considerably at high latitudes to the extent that
they cannot be considered independent feedback mechanisms at the global scale. |
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