Pôle Alpin Risques Naturels (PARN) Alpes–Climat–Risques Avec le soutien de la Région Rhône-Alpes (2007-2014)
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Réf. Giorgi et al. 2016

Référence bibliographique
GIORGI F., TORMA C., COPPOLA E., BAN N., SCHÂR., SOMOT S., (2016) Enhanced summer convective rainfall at Alpine high elevations inresponse to climate warming, Nature Geoscience DOI : 10.1038/NGEO2761

Abstract : Global climate projections consistently indicate a future decrease in summer precipitation over the European Alps. However, topography can substantially modulate precipitation change signals. For example, the shadowing eect by topographic barriers can modify winter precipitation change patterns and orographic convection might also play an important role. Here we analyse summer precipitation over the Alpine region in an ensemble of twenty-first-century projections with high-resolution (12 km) regional climate models driven by recent global climate model simulations. A broad-scale summer precipitation reduction is projected by both model ensembles. However, the regional models simulate an increase in precipitation over the high Alpine elevations that is not present in the global simulations. This is associated with increased convective rainfall due to enhanced potential instability by high-elevation surface heating and moistening. The robustness of this signal, which is found also for precipitation extremes, is supported by the consistency across models and future time slices, the identification of an underlying mechanism (enhanced convection), results from a convection-resolving simulation, the statistical significance of the signal and the consistency with some observed trends. Our results challenge the picture of a ubiquitous decrease of summer precipitation over the Alps found in coarse-scale projections.

Mots-clés
 

Organismes / Contact

Authors / Auteurs :

  • GIORGI F., Earth System Physics Section, The Abdus Salam International Centre for Theoretical Physics, I-34151 Trieste, Italy.
  • TORMA C., Earth System Physics Section, The Abdus Salam International Centre for Theoretical Physics, I-34151 Trieste, Italy.
  • COPPOLA E., Earth System Physics Section, The Abdus Salam International Centre for Theoretical Physics, I-34151 Trieste, Italy.
  • BAN N., Institute for Atmospheric and Climate Science, ETH Zurich, 8092 Zurich, Switzerland.
  • SCHÂR., Institute for Atmospheric and Climate Science, ETH Zurich, 8092 Zurich, Switzerland.
  • SOMOT S., Météo-France/CNRS, CNRM, Centre National de Recherches Météorologiques, F-31100 Toulouse, France.

(1) - Paramètre(s) atmosphérique(s) modifié(s)
(2) - Elément(s) du milieu impacté(s)
(3) - Type(s) d'aléa impacté(s)
(3) - Sous-type(s) d'aléa
precipitation

convective rainfall

   

Pays / Zone
Massif / Secteur
Site(s) d'étude
Exposition
Altitude
Période(s) d'observation
Alpes Alps        

(1) - Modifications des paramètres atmosphériques
Reconstitutions
 
Observations

 

Modélisations
 
Hypothèses
 

Informations complémentaires (données utilisées, méthode, scénarios, etc.)



(2) - Effets du changement climatique sur le milieu naturel
Reconstitutions
 
Observations

The hypothesis that topography might locally modify precipitation change patterns is particularly relevant for the European Alps, whose complex physiography is well known to strongly affect local climate characteristics.

While most global climate models (GCMs) project a ubiquitous decrease in summer precipitation over the Alps in response to global warming, this response can be substantially affected by topography. Such a modulation would in fact point to the added value of using high-resolutionmodels in regional climate projections.
A number of studies demonstrated the added value of RCMs in reproducing different characteristics of topographically forced precipitation.

Contrary to the GCMs, the RCMs produce an area of increased summer precipitation along theAlpine chain embedded in the largescale regional drying.
The summer precipitation change DS is positive along the Alpine chain and negative in the surrounding low-elevation regions. This pattern is relatively invariant across the three time slices and it is generally consistent across models. despite some inter-model variability of the spatial structure of the DS, all RCMs exhibit this topographical modulation regardless of the sign and intensity of the large-scale precipitation change (mostly driven by the GCMs).

It can be seen that the projected increase in precipitation over the Alpine chain is essentially attributable to an increase in convective rainfall, which locally overcomes the large-scale regional drying.

Our findings have important implications for the assessment of impacts over the Alpine region, as they challenge the validity of the ubiquitous summer drying projections suggested bymany GCMs, as well as medium-resolution (25-50 km) RCM experiments2,15. They point to the added value provided by high-resolution models (here the RCMs) in a climate change context, specifically in the simulation of convection and associated topographic feedbacks.

 

L’hypothèse selon laquelle la topograhie peut affecter localement les modèles d’évolution de précipitations est particulièrement pertinente dans les Alpes Européennes, où la physiographie complexe est connue pour affecter, de manière importante, les caractéristiques climatiques locales

Alors que la plupart des modèles climatiques globaux (GCMs), mettent en avant une diminution globale des précipitations estivales dans les Alpes, en réponse au réchauffement climatique, on observe que cette réponse peut être affectée de manière importante par la topographie. De telles modifications mettent en avant l’intérêt de l’utilisation de modèles à haute résolution dans les projections climatiques régionales.

A l’inverse des modèles climatiques globaux, les modèles climatiques régionaux prévoient une augmentation des précipitations estivales, le long de la chaîne alpine, à l’intérieur des zones décrites comme en cours d’assèchement, à une échelle plus large. Le signal de changement d’échelle dans la modification des précipitations estivales (The summer precipitation change DS) est positif le long de la chaîne alpine et négatif dans les régions alentours de plus basse altitude. Ce schéma ne varie globalement pas au cours des 3 périodes étudiées et se retrouve dans les différents modèles. Malgré quelques variations entre les modèles qui décrivent la structure spatiale des DS (Downscaling signals), tous les modèles climatiques régionaux mettent en valeur l’influence topographique, et ce sans tenir compte des signes et de l’intensité de l’évolution des précipitations à une plus large échelle.

Nous avons pu observer que la projection de l’augmentation des précipitations le long de la chaîne alpine est essentiellement conséquence de l’augmentation des pluies convectives, qui modifient, à l’échelle locale, les modèles d’assèchement que l’on retrouve à une échelle plus large.

Nos découvertes ont une importance particulière dans le cadre de l’analyse des impacts dans la région alpine, étant donné qu’ils contredisent la validité des conclusions de nombreux modèles tant globaux que de résolution moyenne, qui prévoient un assèchement progressif de la zone. Nos recherches mettent donc en avant l’utilisation des modèles à haute résolution (dans notre cas les modèles climatiques régionaux) dans un contexte de changement climatique, particulièrement dans la simulation des phénomènes de convection et des retours topographiques associés.

Modélisations
 
Hypothèses
 

Sensibilité du milieu à des paramètres climatiques
Informations complémentaires (données utilisées, méthode, scénarios, etc.)
 

 


(3) - Effets du changement climatique sur l'aléa
Reconstitutions
 
Observations
 
Modélisations
 
Hypothèses
 

Paramètre de l'aléa
Sensibilité du paramètres de l'aléa à des paramètres climatiques
Informations complémentaires (données utilisées, méthode, scénarios, etc.)
 
 
 

(4) - Remarques générales
 

(5) - Syntèses et préconisations
 

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