Abstract
Abstract. Wildfires in the United Kingdom (UK) pose a threat to people, infrastructure and the natural environment. During periods of particularly fire-prone weather, wildfires can occur simultaneously across large areas, placing considerable stress upon the resources of fire and rescue services. Fire danger rating systems (FDRSs) attempt to anticipate periods of heightened fire risk, primarily for early-warning and preparedness purposes. The UK FDRS, termed the Met Office Fire Severity Index (MOFSI), is based on the Fire Weather Index (FWI) component of the Canadian Forest FWI System. The MOFSI currently provides daily operational mapping of landscape fire danger across England and Wales using a simple thresholding of the final FWI component of the Canadian FWI System. However, it is known that the system has scope for improvement. Here we explore a climatology of the six FWI System components across the UK (i.e. extending to Scotland and Northern Ireland), calculated from daily 2km × 2km gridded numerical weather prediction data and supplemented by long-term meteorological station observations. We used this climatology to develop a percentile-based calibration of the FWI System, optimised for UK conditions. We find this approach to be well justified, as the values of the "raw" uncalibrated FWI components corresponding to a very "extreme" (99th percentile) fire danger situation vary by more than an order of magnitude across the country. Therefore, a simple thresholding of the uncalibrated component values (as is currently applied in the MOFSI) may incur large errors of omission and commission with respect to the identification of periods of significantly elevated fire danger. We evaluate our approach to enhancing UK fire danger rating using records of wildfire occurrence and find that the Fine Fuel Moisture Code (FFMC), Initial Spread Index (ISI) and FWI components of the FWI System generally have the greatest predictive skill for landscape fire activity across Great Britain, with performance varying seasonally and by land cover type. At the height of the most recent severe wildfire period in the UK (2 May 2011), 50 % of all wildfires occurred in areas where the FWI component exceeded the 99th percentile. When all wildfire events during the 2010–2012 period are considered, the 75th, 90th and 99th percentiles of at least one FWI component were exceeded during 85, 61 and 18 % of all wildfires respectively. Overall, we demonstrate the significant advantages of using a percentile-based calibration approach for classifying UK fire danger, and believe that our findings provide useful insights for future development of the current operational MOFSI UK FDRS.
Highlights
In this study we investigate how the use of the Canadian Forest Fire Weather Index (FWI) System (Van Wagner, 1987) for forecasting fire danger in the United Kingdom (UK) can be enhanced, using a percentile-based approach, and explorePublished by Copernicus Publications on behalf of the European Geosciences Union.M
(1) A spatially and temporally detailed long-term UK record of the FWI components – a so-called “FWI climatology” (Sect. 3.1) – was used to define the extremes of each component for each 2 km × 2 km grid cell and season across the country. This data set formed the foundation of the percentilebased Fire danger rating systems (FDRSs). (2) A record of fire incidence across Great Britain extracted from the UK fire and rescue services (FRS) Incident Recording System (IRS) database (Sect. 3.2) and enhanced by land cover data (Sect. 3.3) was used to examine percentiles of the FWI components during past wildfire periods
To ensure that the percentile values of the FWI System components were based upon sound statistics, a data set capturing the long-term intra-seasonal variability of each FWI component was required, because UK weather conditions that appear to lead to exceptional wildfire danger, and “extreme” values of the FWI components, seem to be relatively infrequent
Summary
In this study we investigate how the use of the Canadian Forest Fire Weather Index (FWI) System (Van Wagner, 1987) for forecasting fire danger in the United Kingdom (UK) can be enhanced, using a percentile-based approach, and explore. UK fires are almost exclusively anthropogenic, and generally result from accidental ignitions, arson or escaped burns conducted for land management purposes, e.g. for the maintenance and improvement of moorland grouse habitat (Davies et al, 2006; Albertson et al, 2009) The impact of these fires can be greatly exacerbated when periods of low fuel moisture coincide with wind speeds conducive to fire spread. A large number of sustained ignitions may result in many landscape-scale fires burning near simultaneously across the UK, as happened most recently for example in 2003, 2006 and 2011 Such episodes place extreme stress upon the resources of fire and rescue services (FRS), in terms of both personnel and firefighting response assets (Davies and Legg, 2008). We believe that there remains considerable potential for its adaptation in the UK, as it has been successfully adapted in a number of other fire-affected environments around the world (e.g. de Groot et al, 2007; Fogarty et al, 1998; Taylor and Alexander, 2006)
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