Extreme weather conditions and Fire Radiative Power in Portugal: a probabilistic analysis

Fire is a natural hazard that is dependent on climate, vegetation, and human activities, and climate conditions and fuel availability in Portugal make it a fire prone country. Moreover, extreme weather conditions have led to three severe fire seasons in this century, namely in 2003, 2005, and 2017, which caused large burned areas, and economical and human losses. The relationship between weather conditions and burned area has been extensively studied, but the effect on fire intensity is less understood, despite this being an important variable for fire propagation and suppression.

In this work, the bivariate relationship between Fire Radiative Power (FRP) and two weather variables was studied, namely temperature and wind speed, using copula functions. FRP was used as a measure of fire intensity and was retrieved from the Global Monthly Fire Location Product (MCD14ML), a part of the MODIS Active Fire product. Daily temperature at 15h and hourly wind components were obtained from ERA5-Land. The analysis was performed for all cases, and separately for two main wind directions: northerly/westerly (NW) winds, and southerly/easterly winds (SE). The maximum daily wind speed and temperature for each direction was used, on the day of the fire (lag 0), and on the previous 1 to 3 days (lags 1 to 3), to account for preexisting weather conditions. FRP values occurring on the same day were summed. Only pixels identified as forests were considered in this analysis, and the study area is the North and Centre of Portugal. The analysis was performed in the period 2001-2023, on the months of March to October.

Copula functions were fitted to the variables and used to compute the conditional probabilities of high FRP values, under extreme temperature or high wind speed. The results show that SE wind on the day of the fire is the largest driver of high fire intensity, although SE wind in the previous days also yields high probability of high FRP values in the hotter months. NW winds do not significantly increase the probability of high fire intensity, compared to the case of non-windy conditions. The effect of temperature is very similar for both wind directions, and the cumulative effect of temperature on the days before the fire is higher than if considering only lag 0 on almost all cases.

Acknowledgments: This work is supported by the Portuguese Fundação para a Ciência e Tecnologia, FCT, I.P./MCTES through national funds (PIDDAC): UID/50019/2025 and LA/P/0068/2020 https://doi.org/10.54499/LA/P/0068/2020). This work was performed under the scope of project DHEFEUS (10.54499/2022.09185.PTDC), supported by national funds through FCT, and LA/P/0068/2020 (https://doi.org/10.54499/LA/P/0068/2020), “Fundos próprios para desenvolvimento de projetos de I&D” Project MEDCEX - reference: 100SPID8106.