An idealized study of fire emissions impact on pyroconvective clouds with WRF-Fire

Pyroconvective clouds can be directly linked to fire or volcanic eruptions. Two types of pyroconvective clouds are distinguished: pyrocumulus (pyroCu) or pyrocumulonimbus (pyroCb), depending on their vertical developments. These clouds are classified as extreme weather events that can happen during large fires. PyroCu/Cb clouds can significantly enhance the fire rate of spread due to vigorous updraft/downdraft speeds. They also increase the chance of long-range spotting due to the strong winds. Records from the past few decades show an increase in the occurrence of large wildfires in various regions of the world, including in North America. Therefore, understanding how these clouds form and evolve becomes more urgent. Prediction of pyroCu/Cb is challenging because various environmental factors such as meteorological conditions, fire behavior, fuel characteristics, and related emissions of heat, moisture, and aerosols can influence their formation and evolution. The aerosols emitted by fires can act as cloud condensation (CCN) and ice nuclei (IN) and change the microphysical and dynamical evolution of the clouds. Here, the impact of fire emissions on pyroCu/Cb clouds is investigated. This work will contribute to better predicting pyroCu/Cb clouds and improving the representation of fire emissions of aerosols in air quality models. Observation of pyroCu/Cb is limited and inherently dangerous. Therefore, numerical models can play a crucial role in their characterization. This study uses the Weather Research and Forecasting fire module (WRF-Fire) in large eddy simulation mode to simulate idealized pyroCu/Cb. A numerical domain with a 50 m grid spacing is used to resolve the convective motions explicitly. A simple emission model has been incorporated into WRF-Fire to estimate the aerosol emissions based on the burned fuel. For investigating the impact of fire aerosols as CCN/IN, the emissions are added to the base level aerosols in Thompson aerosol-aware microphysics (Thompson and Eidhammer, 2014). Simulations with and without aerosol emissions are conducted to investigate the impact of fire emitted aerosols on the pyroCu/Cb. Finally, simulations with different sounding profiles and fuel characteristics of recent pyroCu/Cb observations are considered to study the impact on convective cloud evolution.