Pôle Alpin Risques Naturels (PARN) Alpes–Climat–Risques Avec le soutien de la Région Rhône-Alpes (2007-2014)
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Réf. Wastl & al - 2013

Référence bibliographique
WASTL C., SCHUNK C., MARVIN L., COCCA G, CONEDERA M., VALESE E., MENZEL A., 2013. ReLarge-scale weather types, forest fire danger, and wilfire occurence in the Alps. Agric. Forest Meteorol. 168, 15-25.

Abstract : In the Alps forest fires have burnt around 14,500 ha per year in the past decade. In this paper we studied large-scale (synoptic) weather patterns and the corresponding occurrence of forest fires in this complex topography. The database for our analysis comprised three main parts: a daily classification of weather types in the period 1951–2010, daily calculated forest fire danger indices at five selected stations in the Alps (1951–2010) and ten years of observed forest fires (2001–2010). Firstly we analyzed the frequency of the 11 different weather types and show that the Alps are a region where cyclonic flows in general, and westerly cyclonic in particular, are the dominating large-scale weather pattern due to their location in the westerlies of the global circulation system. Comparing the weather types with three calculated sub-indices of the Canadian Forest Fire Danger Rating System (FFMC, DC, DMC) at five selected sites (representative of the different climate regions in the Alps) revealed a strong dependence of meteorological forest fire danger on flow direction and cyclonality. Cyclonic weather types were characterized by a high relative humidity and in consequence a low fire danger, while the calculated fire danger in anticyclonic weather situations was significantly higher. Furthermore, strong regional differences occurred in dependence on the flow direction. Northerly winds resulted in low fire danger north of the Alps, due to orographic enhanced precipitation, and high forest fire danger south of the Alps, because of dry katabatic foehn winds. In general, the stations in the Northern Alps showed significantly lower fire index values than the stations in the south and additionally a stronger seasonal variation with considerably higher index values in summer. Regional differences were highest for the FFMC, followed by the DMC and the DC, and could be attributed to the time lag of different forest soil layers. DMC and DC relate to a rather thick soil layer which reacts very slowly and since weather types in the Alps usually change every 7th day, drying of this deep layer is too slow to reveal significant differences between the regions. The Alpine forest fire database was analyzed on a national basis to identify correlations between observed fires and large-scale weather types. 95% of the observed fires in the EU-defined Alpine Space in the past decade occurred in the two southern countries Italy and France. This was likely due to both favorable climatic conditions and better database quality in these two countries. Unfortunately, the datasets of some regions north of the Alps (e.g. in Switzerland, Germany, Austria) were very patchy. A strong human influence on the Alpine fire regime resulted in a generally low correlation between weather types and observed forest fires. Surprisingly, many forest fires occurred in conjunction with cyclonic weather types. This could be explained by the start date of the fire which was mostly at the end of a drought period when the largescale synoptical conditions had already turned to cyclonic. Nevertheless, most and biggest fires occurred during high pressure systems and other anticyclonic situations when the fuels were completely dry.

Mots-clés
 Alps, Synoptic weather types, Forest fire danger, Fire occurrence, Canadian Forest Fire Danger Rating System, Foehn

Organismes / Contact

Authors / Auteurs :

WASTL C., Chair of Ecoclimatology, Technische Universität München, Hans-Carl-von-Carlowitz-Platz 2, D-85354 Freising, Germany

SCHUNK C., Chair of Ecoclimatology, Technische Universität München, Hans-Carl-von-Carlowitz-Platz 2, D-85354 Freising, Germany

MARVIN L., Chair of Ecoclimatology, Technische Universität München, Hans-Carl-von-Carlowitz-Platz 2, D-85354 Freising, Germany

COCCA G., Ente Regionale per i Servizi all’Agricoltura e alle Foreste, Via Taramelli 12, I-20124 Milano, Italy

CONEDERA M., WSL, Swiss Federal Institute for Forest, Snow and Landscape Research, Insubric Ecosystems Research Group, Via Belsoggiorno 22, CH-6500 Bellinzona, Switzerland

VALESE E., Dipatrimento Territorio e Sistemi Agro Forestali, Università degli Studi di Padova, Viale dell’Università 16, I-35020 Legnaro, Padova, Italy

MENZEL A., Chair of Ecoclimatology, Technische Universität München, Hans-Carl-von-Carlowitz-Platz 2, D-85354 Freising, Germany

rattaché au projet : European Union through the Alpine Space ALPFFIRS project (no. 15-2-3-IT).


(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
  Forêts alpines feux de forêts  

Pays / Zone
Massif / Secteur
Site(s) d'étude
Exposition
Altitude
Période(s) d'observation
Alps Alps       1951-2010

(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

Weather types and forest fire danger :

FFMC :

At all five stations and for all weather types the index values were considerably higher during summer than winter (77.6 compared to 66.1 on average). This fact is due to the strong temperature dependency of the FFMC and most other forest fire danger indices. High temperatures and strong radiation accelerate the drying of the upper litter layer (of which the FFMC is representative) and hence the meteorological forest fire danger rises much faster after a precipitation event in summer than in winter. Furthermore, winter in Alpine valleys and plains are usually characterized by cold air and high relative humidity, which also reduces the calculated forest fire danger.
The highest index values occurred for the anticyclonic weather types EA, SA, NA and prevailing high pressure systems (H) which are usually associated with dry conditions and a lot of sunshine.

DMC :

The differences between summer and winter season were greater than for the FFMC which can be attributed to the deeper soil layer that is considered in the DMC.
Comparing different weather types, the highest DMC values were connected with anticyclonic weather types (EA, WA, NA, H), while the cyclonic situations EC, WC and NC produced the lowest DMC values. However, the differences between the single weather types within the stations were not as large as for the fast-reacting FFMC.

DC :

Generally, a strong correlation between precipitation amount and the moisture content in a deep soil layer could be found.
The differences between cyclonic and anticyclonic weather types were negligible. Weather conditions in Central Europe are simply too variable and change too fast to influence the moisture conditions in a deep forest layer.

Forest fire and weather types :

Most forest fires have been observed in conjunction with anticyclonic conditions (EA, NA, H). This is not surprising since anticyclonic weather types are usually associated with dry conditions and a high meteorological forest fire danger (Fig. 3). However, on the other hand also in the cyclonic types NC and L a lot of forest fires occurred.
Cyclonic weather types are usually associated with humid conditions and low forest fire danger (see Fig. 3), but in summer cyclonic weather types are not always accompanied by large-scale precipitation. Since the convective activity in such weather conditions is usually very high, local precipitation can be exceptionally intense, while adjacent areas remain dry. With strong convection the frequency of lightning increases which can also act as ignition source in the Alpine region. Furthermore, the wind speed in cyclonic weather types is higher, also increasing the fire danger and the fire rate of spread.

 

Différents types de climat et danger relatif aux feux de forêt :

FFMC :

Sur les 5 stations d’études, et pour les différents types de climat, la valeur de l’indice [relatif au déclenchement des feux de forêts] est bien plus haute en été qu’il ne l’est en hiver (77.6 comparé à 66.1 en moyenne). Cela est dû à la forte dépendance du FFMC à la température. Les hautes températures and les fortes radiations accélèrent le séchage des couches supérieures du sol, ce qui entraîne une augmentation plus importante du danger relatif au déclenchement des feux de forêts, après un épisode de précipitations en été, qu’après un épisode de précipitations en hiver. De plus, l’hiver dans les vallées alpines, ainsi que dans les plaines, se caractérise généralement par un air froid et une humidité relative importante, réduisant ainsi le danger relatif au déclenchement de feux calculé.
On retrouve les valeurs de l’indice les plus importantes pour les climats anticycloniques de types EA, SA, NA et pour les systèmes à haute pression, associés à des conditions atmosphériques sèches et à un ensoleillement important.

DMC :

Les différences observées, entre les saisons estivales et hivernales, sont plus importantes que dans le cas du FFMC, ce qui peut être attribués aux couches du sol plus profondes, prises en compte dans le cas du DMC.
En comparant les différents types de climat, on observe que les valeurs les plus élevées du DMC sont associées aux climats de types anticycloniques (EA, WA, NA, H), alors que les situations cycloniques EC, WC et NC sont à l’origine des valeurs du DMC les plus faibles.

DC :

De manière générale, une corrélation importante entre quantité de précipitations et part des moisissures dans les couches profondes du sol peut être mise en évidence.
Les différences entre les climats de type anticyclonique et cyclonique sont négligeables. Les conditions météorologiques en Europe Centrale connaissent une variabilité trop importante, associée à des changements trop rapides pour influencer les conditions de moisissures dans les couches profondes du sol.

Feux de forêts et types de climats :

La plupart des feux de forêts ont été observés lors de la prévalence de conditions anticycloniques (EA, NA, H). Ces observations ne sont pas surprenantes, étant donné que les conditions anticycloniques sont généralement associées à des conditions atmosphériques sèches et à un danger potentiel de déclenchement de feux de forêt important. Cependant, on observe également de nombreux déclenchements de feux de forêts dans des conditions cycloniques de types NC et L.
Les conditions météorologiques de type cycloniques sont généralement associées à des conditions humides et à un faible danger de déclenchement de feux de forêt. Cependant, en été, les conditions cycloniques ne sont pas toujours associées par des précipitations à large échelle. Etant donné que la convection dans de telles conditions est généralement très importante, les précipitations sur les zones localement définies peuvent être très importantes, pendant que les zones en périphérie restent sèches. Avec une forte convection, la fréquence de d’apparition d’éclairs augmente, ce qui peut également être une source importante de déclenchement de feux de forêts dans les régions alpines. De plus, dans le cas conditions cycloniques, la vitesse du vent augmente, entraînant ainsi un développement du danger et de la propagation des feux de forêt.

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.)
 
 

Our conclusions were based on three different sub-indices of the Canadian Forest Fire Danger Rating System.
The whole Canadian system consists of three subindices for describing the fuel moisture conditions (FFMC – Fine Fuel Moisture Code, DMC – Duff Moisture Code, DC – Drought Code) and three sub-indices for fire behavior (ISI – Initial Spread Index, BUI – Build Up Index, FWI – Fire Weather Index).

The FFMC describes the moisture content of litter and other dried fuels and is an indicator of the relative ease of ignition of a layer of dry weight of about 0.25 kg m−2.
The DMC is a numerical rating of the average moisture content of a layer of about 7 cm depth or 5 kg m−2. It is also an indicator of the likelihood of fuel being ignited by lightning.
The DC is the slowest reacting code and is an indicator for the average moisture content of a deep, organic layer weighing around 25 kg m−2.

EA = Easterly Anticyclonic
EC = Easterly Cyclonic
SA =Southerly Anticyclonic
SC = Southerly Cyclonic
WA = Westerly Anticyclonic
WC = Westerly Cyclonic
NA = Northerly Anticlyclonic
NC = Northerly Cyclonic
H = High
L = Low

Nos conclusions sont basées sur 3 sous-indices du système de classement du danger relative au feux de forêts du Canada différents. Le système canadien se compose de 3 sous-indices décrivant les conditions de moisissures (FFMC – Fine Fuel Moisture Code, DMC – Duff Moisture Code, DC – Drought Code) et de 3 sous-indices décrivant le comportement du feu (ISI – Initial Spread Index, BUI – Build Up Index, FWI – Fire Weather Index).

La FFMC décrit la moisissure contenue dans la litière et est un indicateur de la facilité relative de déclenchement de la couche sèche (dont le poids est environ 0.25kg m-2).
La DMC est un classement numérique de la moisissure moyenne contenue dans une couche située environ 7 cm sous la surface (dont le poids est environ 5kg m-2). C’est également un indicateur qui décrit un carburant facilement inflammable par le biais d’éclairs ou de sources d’énergie.
La DC est associée à la couche la plus lente à réagir aux déclenchements de feux de forêt. Elle décrit une couche organique située en profondeur dont le poids est environ 25kg m-2

EA = Easterly Anticyclonic
EC = Easterly Cyclonic
SA =Southerly Anticyclonic
SC = Southerly Cyclonic
WA = Westerly Anticyclonic
WC = Westerly Cyclonic
NA = Northerly Anticlyclonic
NC = Northerly Cyclonic
H = High
L = Low


(4) - Remarques générales
 

(5) - Syntèses et préconisations
 

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