Meteorological conditions favouring the development of mega-fires

Abstract

In recent years, the development of clouds due to the convection created by the fire has been reported in several wildfires. These clouds can be of 2 types: pyro-cumulus (PyroCu) or pyro-cumulonimbus (PyroCb). In 2017, Portugal recorded 11 mega-fires, of which 8 occurred on October 15, 2017. As a case study of pyro-convection, the Quiaios wildfire was chosen. In this study, two numerical simulations were made using the MesoNH atmospheric model: a simulation coupled with the fire propagation model, ForeFire, with 3 nested domains (2000m, 400m and 80m of horizontal resolution) and 300 × 300 grid points each; and a non-coupled simulation, with a single domain of 15km horizontal resolution of 300 × 250 grid points. From the first simulation, significant pyro-convective activity was confirmed from the moment that some type of hydrometeor was simulated inside the smoke plume. The coupled simulation showed convection created by the fire allowing the vertical transport of water vapor and the development of PyroCu cloud. The large-scale simulation allowed to explore the entire development of the hurricane Ophelia. On October 15th, Ophelia brought to the continent strong southwest winds in lower troposphere, which were essential for increasing the rate of spread of the active fires and mega-fire development. The approach of the hurricane also contributed to the increase of precipitable water over the continent, which may have been an essential factor for the formation of PyroCb clouds. The study revealed the importance of using very high spatial and temporal resolution coupled simulation to study pyro-convective phenomena and how a largescale system can affect the fire behaviour and the development of strong pyro-convective activity. This study is funded by national funds through FCT - Foundation for Science and Technology, I.P. under the PyroC.pt project (Ref. PCIF/MPG/0175/2019) and ICT project (Refs. UIDB/04683/2020 and UIDP/04683/2020).