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On the projection of future fire danger conditions with various instantaneous/mean-daily data sources

Journal: Climatic Change
Year: 2013   Volume: 118
Initial page: 827   Last page: 840
Status: Published
In this status since: 27 May 2013
PDF file: CLIM-D-12-00184_rev2.pdf
DOI: 10.1007/s10584-012-0667-2

Fire danger indices are descriptors of fire potential in a large area, and combine a few variables that affect the initiation, spread and control of forest fires. The Canadian Fire Weather Index (FWI) is one of the most widely used fire danger indices in the world, and it is built upon instantaneous values of temperature, relative humidity and wind velocity at noon, together with 24 hourly accumulated precipitation.
However, the scarcity of appropriate data has motivated the use of daily mean (DM) values, as surrogates of the instantaneous ones, in several studies that aimed to assess the impact of global warming on fire. In this paper we test the sensitivity of FWI values to both instantaneous and DM values, analyzing the impact in mean seasonal fire danger (SSR) and extreme fire danger conditions (90th percentile [FWI90] and FWI>30 [FOT30]). To this aim, we analyzed different data sources (local observations, reanalysis, and simulations with a regional climate model [RCM WRF model]), and compared the resulting instantaneous and DM versions both in present climate and in a future scenario. In particular, we were interested in determining the effect of these datasets on the projected changes obtained for the mean and extreme seasonal fire danger conditions in future climate scenarios, as given by RCMs. Overall, our results warn against the use of DM data for the computation of present and future fire danger conditions. DM data lead to systematic negative biases of fire danger calculations.
Although the mean seasonal fire danger indices might be corrected to compensate this bias, fire danger extremes (FWI90 and specially FOT30) cannot be reliably transformed to accommodate the spatial pattern and magnitude of their respective instantaneous versions, leading to inconsistent results when projected into the future. As a result, we advocate caution when using DM data and strongly recommend the application of the standard definition for its calculation as closely as possible. Threshold-dependent indices derived from FWI are not reliably represented by the DM version and thus can neither be applied for the estimation of future fire danger season length and severity, nor for the estimation of future extreme events.