Pyroconvection Classification Based on Atmospheric Vertical Profiling Correlation With Extreme Fire Spread Observations

Pyrocumulus (pyroCu) formation during convective fire-atmosphere interaction is a driving factor causing extreme and unpredictable wildfires. Here, by investigating several cases of pyroCu occurrence, we have monitored fire-induced changes in atmospheric boundary layer (ABL) vertical profiles of state variables (temperature, humidity, and wind). Particular emphasis is placed in relating them to observed versus modeled fire spread biases. To this end, during the 2021 fire season in the Iberian Peninsula, we conducted a pyroconvection monitoring campaign. We gathered data on hourly fire spread, plume surface, deepening and penetration stages, and in-situ radiosoundings within wildfire plumes. European Centre for Medium-Range Weather Forecasts ERA5 weather data completed the analysis as reference modeling information to characterize the meso and synoptic scales on the thermodynamic profiles. We propose a novel classification of pyroconvection-type events based on (a) differences in ABL thermodynamic stability, (b) regions characterized by high turbulence in the sub or pyroCu-layers, and (c) the reinforced entrainment of free-tropospheric air on top of every convective plume. This classification defines four types: (a) convective plumes, (b) overshooting pyroCu, (c) resilient pyroCu, and (d) deep pyroCu/pyroCb. Those prototypes change ABL conditions (temperature, humidity, height) differently depending on the dry or moist convection enhancement that drives them. Using this distinct behavior, we find correlations between observed ABL thermodynamic changes after fire-atmosphere interaction and fire spread biases. Our findings pave the way to quantify the pyroconvection effect on fire spread and facilitate safer and more physically sound decisions when analyzing extreme fires.

Automated summary

The formation of pyrocumulus plays a crucial role in extreme and unpredictable wildfires. By examining several cases of pyrocumulus occurrence, this study investigates the changes in atmospheric boundary layer variables and their relationship with observed fire spread biases. A classification of pyroconvection events based on atmospheric stability, turbulence, and entrainment of free-tropospheric air is proposed. The findings provide insights into the impact of pyroconvection on fire spread and can help in making safer decisions during extreme fires.