Changes in the large fire seasons induced by climate variability may have implications in several sectors of modern society. This communication aims to investigate possible changes in the behaviour of active fires during the wintertime and document an event that occurred in the transboundary mountainous region in the north-western Iberian Peninsula between Portugal and Spain on 28 January 2022. The VIIRS active fire data, a satellite product, were analysed for the period between December 2012 and February 2022. The Meso-NH model was used to explore the atmospheric conditions during the event that burned almost 2400 ha. It was configured in a single domain with a horizontal resolution of 1500 m (300 × 300 grid points). The study highlights an increase in fire occurrence during the winter of 2021/22 and indicates that climate variability may create atmospheric conditions propitious for fire development even during the winter. The mild temperatures, dry air, and easterly flow affecting northern Portugal played an important role in the fire that occurred on 28 January 2022. Local orographic effects associated with downslope flow favoured fire propagation. Given the lack of knowledge about large winter fires, this study can be a starting point for future research on this subject.

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<p>Mega-fires are wildfires that burn an area greater than 10.000 hectares. Despite being a minority in relation to the total number of fires, they are the one with the greatest negative impact on society and the environment. Associated with this wildfire type, the phenomenon of pyro-convection has been reported in several cases. Strong pyro-convective activity can lead to the formation of clouds within the smoke plume, also known as pyro-cumulus (PyroCu) or pyro-cumulonimbus (PyroCb). In 2017, Portugal recorded 11 mega-fires, of which 8 occurred on the 15th October. Since the photographic evidence of the formation of a PyroCu cloud, the chosen case study was the Quiaios mega-fire. The study aims to simulate the impact of a fire in the atmosphere, as well as the large-scale meteorological conditions that were affecting Portugal during the mega-fires. For this purpose, two numerical simulations were performed using the MesoNH atmospheric model: a coupled simulation with the ForeFire fire propagation model, with 3 nested domains with resolution of 2000m, 400m and 80m (300 by 300 grid points), and a large-scale non-coupled simulation, with a 15km resolution (300 by 250 grid points) to study the large-scale conditions. The coupled simulation allowed identifying the formation of a PyroCu cloud composed by different species of hydrometeors, namely graupel and rain droplets. The pyro-cloud developed inside the plume due the vertical transport of water vapor to higher levels. In the context of large scale, the simulation well represented the evolution of hurricane Ophelia, showing the change in wind direction from Southeast to Southwest in Portuguese territory, which created a favourable condition to the intensification of the active fires and the development of PyroCb clouds during the late afternoon. This study was funded by national funds through FCT-Foundation for Science and Technology, I.P. under the PyroC.pt project (Ref. PCIF/MPG/0175/2019).</p><p> </p>

<p>Over mountainous terrain, the atmospheric structure becomes much more complex than homogeneous terrain in terms of the Atmospheric Boundary Layer (ABL). In the context of interaction between fire and atmosphere, abrupt changes in the ABL wind often lead to erratic and turbulent flow in the fire environment and expose firefighters to dangerous conditions. The study aims to characterize the ABL conditions associated with the largest forest fire that occurred in Portugal in 2019. The fire event took place in Vila de Rei county, which is surrounded by hills and valleys with large differences in altitudes. In order to study the regional scale, a numerical simulation was performed using the Meso-NH atmospheric model, configured with 500 × 500 grid points at 2500 m horizontal resolution, between 19 July at 0000 UTC and 25 July 2019 at 0000 UTC. The simulation covered the Iberian Peninsula and corresponds to the period when the fire burned more than 9,000 hectares in Vila de Rei. Such a simulation helped to characterize the lower troposphere, which contributed to the evolution of the ABL height over the days. Results indicate that the simulated ABL evolution is characterized by the presence of a coastal low-level jet with a maximum wind speed of 10 meters per second at ~ 600 meters’ altitude (1800 UTC of 20 July).  ABL height calculated from Richardson number method depicted a growing in the morning that reached a peak height by mid-afternoon. The ABL height ranged from 500 to 900 m throughout the afternoon and evening during the entire study period. Besides the identification of the fire weather conditions, this study also highlights the factors that contributed to the lower values of the ABL height in the wildfire event. This study was funded by national funds through FCT-Foundation for Science and Technology, I.P. under the PyroC.pt project (Ref. PCIF/MPG/0175/2019).</p>

Intense heatwave periods increase the likelihood of extreme wildfires which can lead to negative impacts on many socio-economic sectors. Also, there is still a need to better understand how this phenomenon can impact specific regions in terms of fire danger. The present study aims to investigate three heatwave periods occurring in 2003, 2018 and 2019. All the periods were associated with significant wildfires, two of them linked to mega fires in southern Portugal. The MesoNH is a full-physics non-hydrostatic limited-area research model able to represent atmospheric motions in different scales and has been used to explore the structure and dynamics of the atmosphere during the events. The model was configured with 2.5 km grid spacing and performed for several days depending on the episode. All the experiments were designed with 50 vertical sigma levels unequally spaced, stretching gradually from 30 m (bottom) to 900 m (top). Initial and lateral boundary conditions were provided by European Centre for Medium-Range Weather Forecasts (ECMWF) analysis, with updates every 6 h. The Monchique wildfires in August 2003 and 2018 burned above 25,000 ha. In 2019, the Vila de Rei wildfire burned almost 10,000 ha. All these wildfires’ events occurred under a heatwave influence. The numerical experiment allowed identifying some areas with potential to be affected by wildfires, mainly the regions where orographic effects act to enhance the fire danger. The atmospheric conditions simulated helped to explain the intense fire activity for several days. The heatwave environment associated with extreme heat, very low humidity, and airflow interacting with the local topography favoured the behaviour and evolution of the fires. In this study, the findings show the benefits of the use of cloud-resolving models over large domains to assess the fire danger conditions and helped us to better understand the atmospheric dynamic influencing the development of wildfires. Furthermore, the results may help define firefighting strategies in specific areas, namely in Southern Portugal. This study was funded by national funds through FCT-Foundation for Science and Technology, I.P. under the PyroC.pt project (Ref. PCIF/MPG/0175/2019).