Large-scale weather types, forest fire danger, and wildfire occurrence in the Alps

Abstract

In the Alps forest fires have burnt around 14,500ha 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 large-scale 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.