Variability in Pyroconvection During the Bootleg Fire in 2021
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Authors
Winters, Braeden
Issue Date
2024
Type
Thesis
Language
en_US
Keywords
Forecasting , Meteorology , Pyroconvection , Wildfires
Alternative Title
Abstract
We examine the factors controlling the depth and vigor of pyrocumulus (pyroCu) and pyrocumulonimbus (pyroCB) clouds during the Bootleg Fire in Oregon, USA (July 2021). A combination of Next Generation Weather Radar (NEXRAD) and Geostationary Operational Environmental Satellite number 17 (GOES-17) observations are used to characterize the wildfire’s plume depth and the presence of pyroCu/Cb. These data are contextualized with High Resolution Rapid Refresh (HRRR) analysis fields to examine the thermodynamic structure of the atmosphere, and with GOES-17 Fire-Radiative Power (FRP) to characterize the fire’s forcing for plume development. Results show a range of plume depths and pyroconvective outcomes, spanning dry convective columns with maximum depths <8 km, to deep pyroCb (~13 km MSL) with precipitation and lightning. We find that the plume height relative to the convective condensation level (CCL) discriminates between pyroCu and non-pyroCu periods. For pyroCb formation, we examine FRP relative to the PyroCb Firepower Threshold (PFT), which measures how much energy is needed to initiate moist convection. We find that the FRP’s exceedance of the PFT adequately predicts pyroCb initiation, albeit not necessarily its longevity or spatial extent. We also explore the relationship between the plume rise geometry and pyroCb formation, finding that plume uprightness is associated with its ability to cross the CCL and form pyroCu/Cb. Lastly, we explore the terms Fire Convective Available Potential Energy (Fire CAPE) and Fire Convective Inhibition (Fire CIN) to describe the instability of the environment around the fire, providing insights into the fire’s proclivity to initiate pyroCu/Cb. Ultimately our results will help forecasters identify when a fire is likely to produce pyroCb.