Pôle Alpin Risques Naturels (PARN) Alpes–Climat–Risques Avec le soutien de la Région Rhône-Alpes (2007-2014)
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Réf. Hüsler & al 2014 - A

Référence bibliographique
HÜSLER F., JONAS T., RIFFLER M., MUSIAL J.P., WUNDERLE S., A satellite-based snow cover climatology (1985-2011) for the European Alps derived from AVHRR data. The Cryosphere, 8, 83-90, 2014 doi: 10.5194/tc-8-73-2014

Abstract : Seasonal snow cover is of great environmental and socio-economic importance for the European Alps. Therefore a high priority has been assigned to quantifying its temporal and spatial variability. Complementary to land-based monitoring networks, optical satellite observations can be used to derive spatially comprehensive information on snow cover extent. For understanding long-term changes in alpine snow cover extent, the data acquired by the Advanced Very High Resolution Radiometer (AVHRR) sensors mounted onboard the National Oceanic and Atmospheric Association (NOAA) and Meteorological Operational satellite (MetOp) platforms offer a unique source of information. In this paper, we present the first space-borne 1 km snow extent climatology for the Alpine region derived from AVHRR data over the period 1985–2011. The objective of this study is twofold: first, to generate a new set of cloudfree satellite snow products using a specific cloud gap-filling technique and second, to examine the spatiotemporal distribution of snow cover in the European Alps over the last 27 yr from the satellite perspective. For this purpose, snow parameters such as snow onset day, snow cover duration (SCD), melt-out date and the snow cover area percentage (SCA) were employed to analyze spatiotemporal variability of snow cover over the course of three decades. On the regional scale, significant trends were found toward a shorter SCD at lower elevations in the south-east and south-west. However, our results do not show any significant trends in the monthly mean SCA over the last 27 yr. This is in agreement with other research findings and may indicate a deceleration of the decreasing snow trend in the Alpine region. Furthermore, such data may provide spatially and temporally homogeneous snow information for comprehensive use in related research fields (i.e., hydrologic and economic applications) or can serve as a reference for climate models.

Mots-clés
 

Organismes / Contact

Auteurs / Authors :

  • HÜSLER F., Institute of Geography and Oeschger Center for Climate Change Research, University of Bern, Bern, Switzerland
  • JONAS T., WSL Institute for Snow and Avalanche Research SLF, Davos Dorf, Switzerland
  • RIFFLER M., Institute of Geography and Oeschger Center for Climate Change Research, University of Bern, Bern, Switzerland
  • MUSIAL J.P., Institute of Geography and Oeschger Center for Climate Change Research, University of Bern, Bern, Switzerland
  • WUNDERLE S., Institute of Geography and Oeschger Center for Climate Change Research, University of Bern, Bern, Switzerland

(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
  Snow cover    

Pays / Zone
Massif / Secteur
Site(s) d'étude
Exposition
Altitude
Période(s) d'observation
  European Alps       1985-2011

(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

 Spatial distribution of snow cover (1991-2011)

Generally, altitude is the main influencing factor due to the temperature gradient decreasing with height. [...] The SCD gradually decreases along the altitudinal steps and reaches a minimum of <25 days in the lowest regions in the south-west as well as in the south towards the Po Valley in Northern Italy. Similarly, regional differences become apparent. For example, the SCD in the south-western parts is much lower (i.e., 40 days on average) than in the north-eastern part (i.e., 75 days on average), even though lying at about the same altitude range (i.e., between 600 and 700ma.s.l.).

Annual Variation of snow cover (1985-2011)

The seasonal snow cover generally starts to increase in November, reaches a maximum between January and February and starts to melt out in mid- March. By the end of May, almost all snow has disappeared except for the very high areas lying above 2500ma.s.l.
Focussing on the entire Alpine region, the snow cover peaks in January and February with a monthly median of 68 %, that corresponds to an area of approximately 155 000 km2.
Besides the general annual behavior, the subregions exhibit some interesting features: the north-western part of the Alps shows constantly higher values of SCA due to higher elevations while the south-western part (comparable in terms of altitudinal distribution) shows lower values in winter and relatively high values starting in April. The SCA decrease in the south-west and north-west, hosting the highest elevations, however, seems to be slightly slower than in the eastern regions by virtue of the melting process being decelerated as elevation increases.

Inter-annual variation of snow cover (1991-2011)

It can assumed that the SCD in the years 1998, 2002 and 2003 was below average, with 2007 being the poorest winter on record. In contrast, comprehensive above-average snow cover conditions are found in 1992, 1993, 2009 and 2010.
It is striking that some anomaly patterns seem to occur systematically: firstly, a uniform pattern with above- (below-) average snow conditions, which prevails over the entire Alps (e.g., 1992, 1998, 2002 and 2007). Secondly, a pronounced north–south pattern can be observed in 1991, 1994 and 2005 when the Alpine ridge roughly divides the anomalies in a positive and a negative region. A third pattern follows an altitudinal gradient, for example, in 2003, 2006, 2008 and 2009. Lower levels clearly show below- (above-)average conditions while the higher altitudinal levels approach average conditions. Fourth, an east–west pattern, characterized by a meridional anomaly gradient, is presumed in 1996, 2000, 2004 and 2010. [...]
Generally it can be observed, that the altitudinal variation is more pronounced in SCMD while the variability in the snow onset is lower between different altitudes.
Regarding the inter-regional differences, some interesting features become visible. Besides the lower altitudes, the inter-annual spatial variations are more apparent in the southern regions of the Alps, where the SCD and SCMD parameters especially show a higher spread in the higher regions.

Distribution spatiale de la couverture neigeuse (1991-2011)

En règle générale, l’altitude est le principale facteur qui influence la distribution de la couverture neige, étant donné que le gradient de température décroit avec l’altitude. […] La durée de la couverture neigeuse (SCD) a petit à petit diminué, en suivant le gradient altitudinal, pour atteindre un minimum inférieur à 25 jours dans les régions du sud ouest les plus basses ainsi que dans le sud, aux alentours de la vallée de Po en Italie du nord. De manière similaire, les différences régionales deviennent plus apparentes. Ainsi, la SCD dans la partie sud ouest est bien plus faible (40 jours de moyenne) que dans la partie nord-est (75 jours de moyenne), bien que se situant aux mêmes tranches d’altitude (entre 600 et 700m asl).


Variations annuelles de la couverture neigeuse (1985-2011)

La couverture neigeuse saisonnière commence généralement à augmenter aux alentours de Novembre, atteint son maximum entre janvier et février et comme à fondre vers mi-mars. Vers la fin mai, toute la neige a généralement disparue, à l’exception des zones d’altitude située au-dessus de 2500m.
Lorsque l’on se concentre sur l’intégralité de la région alpine, on peut observer une couverture neigeuse très importante en Janvier et Février, avec un recouvrement de 68% qui correspond à une surface d’environ 155 000m².
Mis à part le comportement général annuel, certaines régions présentent des propriétés intéressantes : la partie nord-ouest des Alpes montre constamment les valeurs de couverture neigeuse les plus importante alors que la partie sud-ouest montre les valeurs les plus basses en hiver mais des valeurs relativement importante à partir d’avril. La zone occupée par la neige diminue dans les partie sud-ouest et nord-ouest, qui hébergent les altitudes les plus importantes. Cependant, ces tendances semblent malgré tout moins importante que dans les régions de l’est, les processus de fonte étant modéré par les altitudes élevées.


Variations de couverture neigeuse interannuelles (1991-2011)

Nous pouvons supposer que la durée de la couverture neigeuse, pour les années 1998, 2002 et 2003 était moins importante que la moyenne, avec  2007 l’année la plus pauvre en termes neigeux. Au contraire, des conditions neigeuses supérieures à la moyenne ont pu être observées en 1992, 1993, 2009 et 2010.
Il est frappant d’observer la récurrence de certains schémas d’anomalie. Tout d’abord, un schéma uniforme avec des conditions neigeuses supérieures (inférieures) à la moyenne, qui prévaut sur la totalité des Alpes (1992, 1998, 2002, 2007). Ensuite, un schéma marqué d’orientation nord-sud, qui peut être observé en 1991, 1994 et 2005 lorsque la chaîne alpine divise les anomalies entre une région positive et une région négative. Un troisième schéma suit, quant à lui, le gradient altitudinal, par exemple en 2003, 2006, 2008 et 2009. Les plus bas niveaux mettent en évidence des conditions inférieures (supérieures) à la moyenne alors que les plus hautes altitudes connaissent des conditions moyennes. Un dernier, et quatrième, schéma, qui suit une orientation est-ouest, se caractérise par un gradient d’anomalie méridional, est supposé s’être produit en 1996, 2000, 2004, et 2010.
De manière générale, les variations altitudinales sont plus prononcées dans l’étude du premier jour de fonte de la couverture neigeuse (snow cover melting day – SCMD), alors que l’apparition de la première neige est moins soumise aux variations d’altitude.
En ce qui concerne les différences interrégionales, certaines caractéristiques intéressantes apparaissent. En dehors des altitudes les plus basses, les variations spatiales interannuelles sont plus apparentes dans les régions du sud des Alpes, ou la durée de la couverture neigeuse ainsi que le SCMD montrent une diffusion plus importante dans les zones les plus hautes.

Modélisations
 
Hypothèses
 

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

Two different time periods are considered in the study at hand. First, 1985 to 2011 for the monthly mean snow cover time series and second, 1991–2011 for spatial applications relying on daily data coverage.

In this study, we use a combination of spatial and temporal gap-filling techniques to mitigate the influence of clouds.

Dans cette étude, nous avons pris en compte deux échelles temporelles. Tout d’abord 1985-2011 pour la moyenne des séries décrivant la couverture neigeuse mensuelle. Ensuite, 1991-2011 pour les applications spatiales en rapport avec la couverture quotidienne des données.

Dans cette étude, nous avons utilisé une combinaison de techniques destinées à compléter les erreurs issues de la présence d’une couverture neigeuse, à la fois à l’échelle spatiale et à l’échelle temporelle.


(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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