The role of mesoscale weather effects in simulating pyroconvection

Abstract

Extreme wildfire events are characterised by strong interactions between the convective wildfire plume and the atmosphere, often resulting in erratic and difficult-to-control fire behaviour. Previous (modelling) studies on pyroconvection have focused mostly on the plume's thermodynamics and induced circulation. The dynamic interactions between these convective plumes and mesoscale weather effects are less studied and are complex, due to the different scales involved. It therefore remains unclear how mesoscale effects, such as sea breezes, interact with a convective wildfire plume, and especially whether resolving mesoscale atmospheric motions is necessary to simulate wildfire plume dynamics. In this study, we present a set of large-eddy simulations (LESs), nested in a mesoscale simulation, of the Santa Coloma de Queralt wildfire in northeastern Spain (July 24, 2021). We study the interaction between a low-level moisture front and the wildfire plume and how this is dependent on the nesting of the LES, comparing our simulations with an in-plume radiosounding. We find that the LES nested in a mesoscale simulation produces a sharply defined density current of moisture, as opposed to a gradual moistening of the atmospheric boundary layer for the LES without the mesoscale nesting. This results in a different timing of pyrocumulus formation. Both LES setups produce wildfire plumes that are concurrent with the in-plume radiosounding. We also show how the front influences the circulations around the plume, resulting in a downwind rotor-like circulation. This case study therefore shows that the main added value from resolving the mesoscale explicitly is in the timing of pyrocumulus formation and circulation changes. The characteristics of the convective plume itself are not substantially different between the simulations.