Réf. Durand & al. 2009a

Référence bibliographique complète

DURAND, Y. LATERNSER, M., GIRAUD, G., ETCHEVERS, P., LESAFFRE, B., MÉRINDOL, L. 2009. Reanalysis of 44 yr of climate in the French Alps (1958–2002): methodology, model validation, climatology, and trends for air temperature and precipitation. Journal of Applied Meteorology and Climatology, 48, 429-449, DOI: 10.1175/2008JAMC1808.1.

Abstract: Since the early 1990s, Météo-France has used an automatic system combining three numerical models to simulate meteorological parameters, snow cover stratification, and avalanche risk at various altitudes, aspects, and slopes for a number of mountainous regions in France. Given the lack of sufficient directly observed long-term snow data, this ‘‘SAFRAN’’–Crocus–‘‘MEPRA’’ (SCM) model chain, usually applied to operational avalanche forecasting, has been used to carry out and validate retrospective snow and weather climate analyses for the 1958–2002 period. The SAFRAN 2-m air temperature and precipitation climatology shows that the climate of the French Alps is temperate and is mainly determined by atmospheric westerly flow conditions. Vertical profiles of temperature and precipitation averaged over the whole period for altitudes up to 3000 m MSL show a relatively linear variation with altitude for different mountain areas with no constraint of that kind imposed by the analysis scheme itself. Over the observation period 1958–2002, the overall trend corresponds to an increase in the annual near-surface air temperature of about 1°C. However, variations are large at different altitudes and for different seasons and regions. This significantly positive trend is most obvious in the 1500–2000-m MSL altitude range, especially in the northwest regions, and exhibits a significant relationship with the North Atlantic Oscillation index over long periods. Precipitation data are diverse, making it hard to identify clear trends within the high year-to-year variability.

Mots-clés
Climatology, French Alps, Observations, SAFRAN model, Temperature, Précipitation, Trends

Organismes / Contact
GAME/CNRM-CEN (CNRS/Météo-France) - Corresponding author: Yves Durand, Météo-France, CNRM-CEN, 1441 rue de la Piscine, 38400 Saint-Martin d’Hères, France. E-mail: yves.durand@meteo.fr

(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

Temperature (near-surface air  temperature) and precipitation (24-h rainfall)

     

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

23 massifs:

43 selected validation sites:

(Les Gets, Les Contamines, La Clusaz Haute, Vallorcine, Chamonix, Megeve, Les Houches, Beaufort, Bourg St. Maurice, Val d’lsère, Pralognan, Peisey-Nancroix, St. Martin de B., Bonneval, Bessans, Valloire, St Sorlin, Chambery, St Pierre de C., St Pierre d’E, Revel, La Ferrière, SMH, Allemond, Autrans, Villard de L., Besse, Vaujany, Bourg d’Ois, St Christ. En O., La Grave, St Etienne en D., Lus La Cr., Briançon, Le Monetier, Orcières, St. Veran, Arvieux, Ceillac, Embrun, Barcelonnette, Beuil, Luceram)

Chablais
Aravis
Bauges
Chartreuse
Vercors
Mont-Blanc
Beaufortin
Haute-Tarentaise
Haute-Maurienne
Vanoise
Maurienne
Belledonne
Grandes-Rousses
Thabor
Oisans
Pelvoux
Champsaur
Dévoluy
Queyras
Parpaillon
Ubaye
Alpes-Azuréennes
Mercantour

600–2700 m
900–2700 m
600–2100 m
600–2100 m
600–2400 m
1200–3600 m
900–3000 m
900–3600 m
1200–3600 m
900–3600 m
600–3000 m
600–3000 m
900–3300 m
1500–3000 m
900–1200 m
1200–3600 m
1200–3300 m
600–3000 m
1200–3000 m
900–3300 m
1200–3000 m
600–2700 m
1200–3000 m
  1958–2002

(1) - Modifications des paramètres atmosphériques
Reconstitutions
 
Observations

Analyzed temperature and precipitation trends:
Annual mean air temperature [1958–2002] temporal trend for three representative observed series locations: [...] Nice for the southern Alps, Annecy for the northern Alps, and Villard-de-Lans for the central Alps. All locations exhibit a clear temperature increase over the past 40 yr of about 1.58°C for Annecy and Nice and 1.18°C for Villard-de-Lans (with higher variability). These results, required for the same period as the modeled results, are relevant for the last 45 yr, but cannot be extrapolated to longer periods such as the whole century. They exclude particularly the important 1940s and 1950s decades, as can be seen when comparing the Annecy series.

Observed and SAFRAN-analyzed annual mean temperature and precipitation trends for 21 sites in the French Alps since 1958:
[...] It is difficult to simulate precise geographical locations with the modeled results, which do not take into account small-scale orographic effects at these locations. In addition, some observation sites have also been greatly influenced by surrounding urbanization, as is probably the case for Megeve. However, the mean air temperature trends for the 21 sites are 0.025°C yr-1 for the observed data and 0.028°C yr-1 for the SAFRAN simulated data. The results are therefore of the same order of magnitude even if the analysis overestimates the values for many points such as those in the Vercors massif.

Similar remarks can be made for the precipitation trends, especially in the northern Alps where both SAFRAN analyses and observations show a small temporal increase, often overestimated by the model. Trends are rather weak in the southern Alps for this parameter. As very few observation series cover the full temporal period and as the observations are above all representative of the winter season, these values are difficult to interpret both in time and spatially. The mean precipitation trends are +1.6 mm.yr-1 for the observed data and +2.6 mm.yr-1 for the SAFRAN simulated data. Note the clear latitudinal difference with a positive trend both for the observations of the northern Alps and the SAFRAN results, and no real trend in the south. However, even if the positive trend values are consistent with those of [spatially averaged SAFRAN parameters], they are not statistically significant. Moisselin et al. (2002) point out the lack of consistency and of significance of most of the observed precipitation series over the southeast of France and these data are the main SAFRAN inputs. It is therefore impossible to draw valid conclusions on the scale of the observation site concerning these trends. However, considering cross validations only, which is the purpose here, we observe a consistent positive trend for precipitation in the northern Alps and no trend in the southern Alps, both for observed data and SAFRAN results.

SAFRAN analyzed temperature and precipitation trends at 1800 m MSL [1958–2005]

Temperature trends:
The mean [SAFRAN filtered daily temperatures] values over the entire Alps exhibit the classical shape of the last years characterized by a plateau until the 1970s, followed by a more pronounced increase of about +1°C. All the different regional areas show the same features modulated by the latitudinal variability and temporal smoothing. These characteristics [already been pointed out by Trenberth et al. (2007) for a larger spatial scale but with the same magnitude for the temperature trend] are mainly due to the increase of the daily minimum temperatures as quoted by Moisselin et al. (2002) and Beniston (2005). [...] The correlation between filtered NAO index and temperature over the working area is about 0.7, which corroborates the previous results [see references in the study] and indicates a mutual influence on this finer scale. The latitudinal variability is consistent with the discussion in Prömmel et al. (2007).

Detailed results show an overall rise of about +1.5°C for Chablais (the northernmost French massif) over the last 30 yr. The winter half-year increase was almost +2°C and the summer increase about +1.5°C with a constant very limited variation but higher variability during late summer. All foothill massifs (Chablais–Vercors) including Mont-Blanc, Beaufortin, and Belledonne show in general the same behavior. Chartreuse is the most extreme massif with a net winter rise of almost +2.5°C. The Mercantour massif is well representative of the central and southern massifs. The most striking difference to the northern massifs is a strong temperature decrease in early winter (–2°C) since the mid-1980s followed by only a slight increase in midwinter but an increasing trend in late winter (up to +3°C), which implies only a slight increase (+0.5°C) on the scale of the overall winter season. All central and southern massifs roughly follow this pattern with Haute Tarentaise-Vanoise-Maurienne being the least distinctive and Queyras-Parpaillon-Ubaye being the most pronounced.

Precipitation trends (rain and snow):
As in several other studies, no clear temporal trend or clear relationship with the NAO index can be found for any of the concerned areas that exhibit only a clear latitudinal variability between the northern and southern Alps. [...] Indications of a possible small positive trend in the north and of a very flat shape in the south are not statistically significant and allow no conclusions to be drawn. [...]

Detailed results (not shown here) show flat mean shapes in Chablais both for winter and summer seasons but with larger interannual and interseasonal variations. The Grande Rousses massif presents a significant increase during the summer period (~70 mm per decade) and is one of the only massifs to show a small positive trend, whereas Mercantour shows a small negative trend especially during the winter season. However, Chablais presents two extreme values for the last two winters (2001 and 2002) and Mercantour includes three very high values during recent winters (1997, 1998, 2001) while its snowfall rises to a high point at the end of the 1970s before dropping.

Annual distribution:
The previously seen marked temperature increase is clear both in winter and summer seasons. The winter season exhibits fewer cold events, begins a bit later in the north, and ends earlier in the south. The summer season becomes clearly warmer over a longer time. The transition period between winter and summer temperatures appears to be decreasing in all regions, which denotes shorter intermediate seasons, consistent with the present personal feelings of many inhabitants.

The precipitation does not exhibit any temporal structure and we see mainly the latitudinal gradient as well as the main features of the southern Alps: dry in summer and winter and storms in autumn. Particularly in winter, year-to-year variability can be very high and appears to have increased even more in recent years. While the last decade is generally marked by low precipitation, some outstanding maximum years clearly stand out. The early winters of 1997, 1998, and 2001 have beaten all records in the south, but below 2000 m MSL precipitation fell predominantly in the form of rain. The midwinters of 1995 and 1999 brought record precipitation in the north falling as snow down to 1000 m MSL and the far south received large snow amounts down to low levels in 1993 and 1995. The year 2001 was an outstanding year for late-winter record snowfalls throughout the French Alps except in the far south (Mercantour, Alpes- Azuréennes) and Haute-Maurienne in the east. Even if it is standard to split the Alps into a northern and southern part, the central massifs, in particular, can show major deviations. Particularly in early summer, these central massifs can differ considerably from both northern and southern massifs, showing a strong increase (up to 100% over the whole period for Grandes-Rousses and Pelvoux).

Vertical trends:
[...] The Spearman’s rank correlation coefficient computed for the daily near-surface SAFRAN analyzed temperatures over the entire area of the Alps over 47 year at different elevations (from 600 to 3600 m MSL) shows a clear positive increase with time especially at midelevations (1500–2000 m MSL) [0.020°C yr-1 at 600 and 900m ; 0.026°C yr-1 at 1200M ; 0.034°C yr-1 at 1500m ; 0.033°C yr-1 at 1800m ; 0.031°C yr-1 at 2100m ; 0.029°C yr-1 at 2400m ; 0.020°C yr-1 at 2700m ; 0.016°C yr-1 at 3000m ; 0.014°C yr-1 at 3300m ; and 0.009°C yr-1 at 3600m]. [...] All levels except the highest (3600 m MSL) present a significant positive near-surface temperature increase over the limited study period [corresponding significance evaluated through a Student’s t test with a 95% confidence interval]. [The authors] have also determined such fits at the different elevations despite the 47 available years, which implies results only representative of this period. The limited accuracy of the linear assumption is visible through the values of the square of the correlation coefficient between raw and fitted values where only midelevation values are of little significance. However, all vertical levels (except the highest) exhibit a positive linear trend corroborated by their 95% confidence interval with a clear emphasis at midelevation and a weaker signal higher. [see Discussion of these results, p. 445].

The mean values obtained at midelevations correspond to those given by Trenberth et al. (2007) but with a larger confidence interval, mainly due to our short time series. Rebetez and Reinhard (2007) find a slightly higher value (0.057°C yr-1) for 12 Swiss stations over the 1975–2004 period. Beniston and Jungo (2002) also determined an altitudinal variation of temperature anomalies with minimum values at low elevations. These results can also be compared to the observed trend values, which well illustrate the variability in this mountainous area.

The similar study for precipitation does not show any significant results for our area over the same considered time period.

Link between temperature and precipitation trends:
Looking at snow precipitation trends in the light of temperature trends reveals that in the north, falling temperatures are associated with slightly rising snowfalls (early winter) and rising temperatures cause diminishing snowfalls (midwinter–early summer). Constant late-summer temperatures show no impact on snow precipitation trends, as would be expected. However, the example of Mercantour in the far south shows that strongly dropping early winter temperatures do not necessarily result in increasing snowfalls, since total precipitation is also decreasing. At Grandes-Rousses, in the central part, we see that strongly rising late-winter temperatures have hardly any effect either on snow or rain precipitation, but a strong early summer temperature increase is accompanied by a very strong rainfall increase and a slight snowfall decline. Finally, near-constant late-summer temperatures are accompanied by a strong positive rainfall trend but have no effect on snowfall. [...] Some elements of [the present] study are in common with those of Beniston [2003], especially the uncertainties concerning precipitation and the NAO–temperature link.

Modélisations
 
Hypothèses
 

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

The present study analyzes long-term climate series over the entire French Alps. Using 44 yr of newly reanalyzed atmospheric model data from the 40-yr European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis (ERA-40) project (ECMWF 2004), the ‘‘SAFRAN’’–Crocus–‘‘MEPRA’’ (SCM) model chain has been run on an hourly basis for a period starting in winter 1958/59. Results include air temperature and precipitation trends, as well as average conditions (spatial variability) and long-term trends (temporal variability) for various snow-cover parameters. [...]

Model used:
SAFRAN [Système d’Analyse Fournissant des Renseignements Atmosphériques à la Neige] (Durand et al. 1993) is a meteorological application that performs an objective analysis of weather data available from various observation networks (including radar and satellite data) over the considered elevations and aspects of the different massifs. SAFRAN combines the observed information with a preliminary estimation generally provided by numerical weather forecasting models. The analysis method combines an optimal interpolation every 6 h and a variational interpolation over 6-h windows, providing hourly data for the main relevant atmospheric parameters affecting snow surface changes (i.e., air temperature, wind speed, air humidity, cloudiness, snow and rain precipitation, longwave radiation, and direct and scattered solar radiation). [...]

Air temperature and precipitation (total or snow), all provided by SAFRAN [...], are modeled for all 23 massifs of the French Alps in 300-m-altitude steps over elevations ranging at the most from 300 to 3600m MSL. These different massifs have been defined for their climatological homogeneity, especially with regard to precipitation fields (Pahaut et al. 1991). They are those used for operational avalanche hazard estimation in France and their characteristics have been well known for many years by local forecasters. Their boundaries coincide well with the main topographic features. However, for each massif, only existing elevations are considered and no ‘‘fictitious’’ extrapolations are made to higher or lower elevations, which can make comparisons difficult between massifs for certain elevation ranges. The output has an hourly resolution from 1 August 1958 to 31 July 2002 and covers 44 winter periods. By convention, winters are referred to by the year of the main part of the winter (e.g., 1959 means winter 1958/59).

Data and methods:
The meteorological analyses are based on both conventional observations and numerical atmospheric weather model outputs. Conventional observations include various kinds of datasets extracted from the operational databases of Météo-France and ECMWF. They cover the French Alps and adjacent areas of neighboring Italy and Switzerland within a grid of 43.15°–47.0°N and 4.45°–8.0°E. Data are concatenated into several different file types according to their contents and source. [...] All available conventional observations have been used and are recorded in several files and databases. Air pressure, air temperature, wind (meridian and zonal components), humidity, snow depth, new snow, and various parameters for weather type and cloudiness are available at their own observation frequency (hourly or by steps of 3 or 6 h). Precipitation, snow depths, and minimum–maximum temperatures are available only on a daily basis. Radiosonde and pilot balloon data are also used. The number of stations providing available data varies greatly with the hour, day, and year considered and is thus given only as a general indication. Individual files are incomplete in roughly two-thirds of all cases, in particular snow, weather type, and cloudiness along with minimum–maximum temperature and new snow amount. All these missing data and short observation series are the main reason for using meteorological analysis software such as SAFRAN that uses information available on a daily basis, even if sparse, without the constraint of full and homogeneous observation series. [...]

[The authors] chose to use retrospective analyses from ERA-40 (ECMWF 2004) that provide a uniform coverage of the entire study period, [and] uses both satellite and conventional observations to provide a full set of validated meteorological analysis parameters from the surface to the 0.1-hPa level (’65 km MSL) dating back to 1958. [...] Six parameters (air pressure, geopotential, air temperature, meridional wind, zonal wind, humidity) over a maximum of 16 elevation levels (from the surface up to the 300-hPa level at ~8.5 km MSL) within a regular grid of 1.5° latitude–longitude were extracted over the entire period. [...]

The results presented are mainly yearly or seasonal averages or amounts at different elevations of two selected variables: near-surface air temperature and 24-h rainfall [and some finer results discussed in the text...]. Elevation ranges are often referred to as low (~1000 m), mid- (1000–2000 m), and high altitudes (~2000 m); however, these terms should not be taken too literally since they only represent a rough graduation.

Model validation:
Before running the two first models in coupled mode, each was carefully validated in different contexts [see p. 432]


(2) - Effets du changement climatique sur le milieu naturel
Reconstitutions
 
Observations
 
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 des paramètres de l'aléa à des paramètres climatiques
Informations complémentaires (données utilisées, méthode, scénarios, etc.)
 
 

(4) - Remarques générales
 

(5) - Syntèses et préconisations

Summary:
The validations presented here and based on the SAFRAN analysis process show the robustness of the models used and their ability to reproduce the main meteorological features of several mountainous observation sites even when data are deliberately omitted from the analyses. The analyzed results on the massif scale can be considered to be representative of the climatology of the French Alps study area at different elevations during the considered period.

The annual mean air temperature at 1800 m MSL varies from 3.4°C in the north (Chablais massif) to 5.1°C in the south (Mercantour massif). The variations are slightly higher in winter (from –1.4° to +0.4°C) than in summer (from +8.3° to +9.9°C).

Year-to-year variability of annual precipitation can be very high (commonly 100% for annual data and much more seasonally) and regional trends exist. At 1800 m MSL, the maximum annual precipitation amounts to nearly 2000 mm in the northwestern foothills (particularly Chartreuse and Aravis), and decreases to less than half that amount toward the southeast (831 mm for Queyras). A secondary maximum is located in the extremesoutheast associated with the occurrence of northward Mediterranean flows.

Low-atmosphere vertical gradients have also been computed and exhibit a very linear shape over the entire area. From north to south, the mean near-surface vertical temperature gradient varies from –5.0° to –5.5°C (1000 m)-1. The annual vertical rainfall gradients exhibit a larger latitudinal dependence with values from north to south of 294, 195, 172, and 178 mm (1000 m)-1.

In terms of an overall temporal trend for the 1958–2002 observation period, the annual air temperature rose by about 1°C, mainly during the 1980s and 1990s. However, variations of this trend are large for different altitudes, seasons, and regions. The trends are most pronounced between 1500 and 2000 m MSL and exhibit some relationships with the NAO variations especially for northern massifs. Temperatures have risen in spring and fallen in autumn, reducing the intermediate seasons. This temperature drop in autumn and early winter is also at the root of the most striking regional differences. A large year-to-year variability is another common characteristic, often deviating far from smoothed trend lines. Temperatures have remained relatively homogeneous at high elevations, without significant trends.

Precipitation variability is very high, making it hard to detect clear trends. No relationship with NAO has been detected or any clear tendency or temporal trend. Regional differences split the French Alps into a northern and southern part. Whereas variations in the north are greater in summer, the southern massifs show higher variability in winter.

Références citées :

Beniston, M., 2003: Climatic change in mountainous regions: A review of possible impacts. Climatic Change, 59, 5–31. [Fiche Biblio]

Beniston, 2005: Mountain climates and climatic changes: An overview of processes focusing on the European Alps. Pure Appl. Geophys., 162, 1587–1606. [Fiche Biblio]

Beniston, and P. Jungo, 2002: Shifts in the distribution of pressure, temperature and moisture and changes in the typical weather patterns in theAlpine region in response to the behaviour of the North Atlantic Oscillation. Theor. Appl. Climatol., 71, 29–42.

Durand, Y., E. Brun, L. Mérindol, G. Guyomarch, B. Lesaffre, and E. Martin, 1993: A meteorological estimation of relevant parameters for snow models. Ann. Glaciol., 18, 65–71.

ECMWF, 2004: ‘‘ERA-40: ECMWF 45-years reanalysis of the global atmosphere and surface conditions 1957-2002’’ by Uppala, S., and Coauthors. ECMWF Newsletter, No. 101, ECMWF, Reading, United Kingdom, 2–21.

Moisselin, J. M., M. Schneider, C. Canellas, and O. Mestre, 2002: Les changements climatiques en France au XXe siècle (Climate change in France during the 20th century). Meteorologie, 38, 45–56.

Pahaut, E., E. Brun, and G. Brunot, 1991: L’organisation de la prévision du risque d’avalanches en France. Proceedings of the Symposium ANENA -CISA-IKAR, Chamonix, 4-8 June 1991, G. Brugnot et al., Eds., ANENA, 50–56.

Prömmel, K., M. Widmann, and J.-M. Jones, 2007: Analysis of the (N)AO influence on Alpine temperature using a dense dataset and high-resolution simulations. Extended Abstracts, 29th Int. Conf. on Alpine Meteorology, Vol. 1, Chamberg, France, Météo-France, 233–236. [Available from Météo-France/CNRM, 42 av. Gaspard Coriolis, 31057 Toulouse, France.]

Rebetez, and M. Reinhard, 2007: Monthly air temperature trends in Switzerland 1901–2000 and 1975–2004. Theor. Appl. Climatol., 91, 27–34, doi:10.1007/s00704-007-0296-2.

Trenberth, K. E., and Coauthors, 2007: Observations: Surface and atmospheric climate change. Climate Change 2007: The Physical Science Basis, S. Solomon et al., Eds., Cambridge University Press, 235–336.