Detection and Monitoring of Intense Pyroconvection in Western North America using Remote Sensing and Meteorological Data

Abstract:

Fire-triggered thunderstorms, known as pyrocumulonimbus (pyroCb), can alter fire behavior, influence smoke plume trajectory, and hinder fire suppression efforts. Intense pyroCb can also inject a significant quantity of aerosol mass into the lower stratosphere. Systematic detection and monitoring of these events is important for wildfire response and aviation applications, as well as understanding climate and air quality implications. The United States Naval Research Laboratory (NRL) recently developed a near-real-time pyroCb detection algorithm using geostationary satellite observations, currently focused on GOES-West. The algorithm is tuned to the microphysics of fire-perturbed thunderstorms over elevated terrain in western North America. By incorporating reanalysis data, NRL has also developed the first observationally-based conceptual model for pyroCb development. Results are focused on 41 large wildfires observed in the United States and Canada during 2013, which produced more than 50 intense pyroCb. The majority of these develop when a layer of increased moisture content and instability is advected over a dry, deep, and unstable mixed layer, typically along the leading edge of an approaching disturbance. The upper-tropospheric dynamics and synoptic pattern must also be conducive for vertical development of convection. Mid- and upper-tropospheric conditions similar to those that produce traditional dry thunderstorms are therefore paramount for development and maintenance of pyroCb. The amount of mid-level moisture and instability required is strongly dependent on the surface elevation of the contributing fire. Surface-based fire weather indices have limited capability for predicting pyroCb development. The intense radiant heat emitted by large wildfires can serve as a potential trigger, suggesting pyroCb may develop in the absence of traditional triggering mechanisms when an otherwise favorable meteorological environment is in place. This conceptual model suggests that pyroCb, traditionally considered a niche phenomenon, are in fact a significant and endemic feature of the regional summer climate. Results of this work will improve our ability to detect, monitor, and predict pyroCb, providing increased understanding of this phenomenon’s role in the climate system.