Modelling pyro-convective activity and the meteorological conditions leading to mega-fires

Abstract

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).