Abstract
There is growing evidence that the climatic conditions favorable to the occurrence of large fires (LFs) might not be unique within a homogeneous biogeographic area. But the identification of these coexistent multi-scalar climatologies often relies on empirical observations. Here we classify summer LFs (>120 ha) in Mediterranean France for the period 1973 to 2012, according to their local-scale weather conditions (i.e. temperature, relative humidity, wind speed and fuel moisture proxies). Three distinct climatologies were identified, and were referred as fire weather types (FWTs). (i) One of them is associated with near-normal atmospheric conditions. (ii) A heat-driven (HD) type is mostly discriminated by warm anomalies. (iii) A wind-driven (WD) type is mostly discriminated by faster winds, but cooler anomalies than usual. The frequency of WD and near-normal LFs sharply decreased in southern France over the last decades while the frequency of HD fires remained unchanged. In addition the current increase in HD potential fire days indicates a potential shift in the dominant FWT for this region. This approach offers a better understanding of the variations in fire activity and fire spread patterns in the context of contemporaneous global changes.
Highlights
Large fires (LFs) are responsible for the majority of burned area in most regions of the world (Stocks et al 2003, San-Miguel-Ayanz et al 2013) and are, as such, key drivers of vegetation dynamics (Bond and Keeley 2005), global carbon cycle (Van der Werf et al 2010), and generate most of socioeconomic fire costs (Gill et al 2013).The analyses of historical meteorological data and fire records highlighted the prominent role of weather in driving the occurrence of these infrequent, but critical, LF events (Meyn et al 2007) through several processes intervening at different space-time scales (Swetnam and Betancourt 1990)
We propose an objective definition of fire weather types (FWTs) in Mediterranean France, where different fire types have been previously described (Lahaye et al 2014, Ruffault et al 2016)
Taking into account the similitude of results obtained for different fire sizes, we considered only fires >120 ha in the following, as in Ruffault et al (2016)
Summary
Large fires (LFs) are responsible for the majority of burned area in most regions of the world (Stocks et al 2003, San-Miguel-Ayanz et al 2013) and are, as such, key drivers of vegetation dynamics (Bond and Keeley 2005), global carbon cycle (Van der Werf et al 2010), and generate most of socioeconomic fire costs (Gill et al 2013).The analyses of historical meteorological data and fire records highlighted the prominent role of weather in driving the occurrence of these infrequent, but critical, LF events (Meyn et al 2007) through several processes intervening at different space-time scales (Swetnam and Betancourt 1990). At annual to seasonal scales, antecedent atmospheric conditions (i.e. years or seasons prior to the wildfire season) can limit or promote the growth of fine fuels. Prior and during the fire season, monthly to daily atmospheric variability controls the moisture content of vegetation. Formal relationships between the incidence of LFs and these multi-scalar meteorological factors have been explored in a variety of locations (Abatzoglou and Kolden 2011, 2013, Barbero et al 2011, 2014, Stavros et al 2014, Ruffault and Mouillot 2015). Isolating the temporal scales and processes through which top-down atmospheric processes influence the spread of fires remains, a scientific challenge, due to a series of interacting biophysical and anthropogenic factors controlling this relationship
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