A Conceptual Model for Development of Intense Pyroconvection in Western North America

Fire-triggered thunderstorms, known as pyrocumulonimbus (pyroCb), can alter fire behavior, influence smoke plume trajectory, and hinder fire suppression efforts. Intense pyroCb activity can also inject a significant quantity of aerosol mass into the lower stratosphere. One of the primary ingredients for pyroCb development is a large release of latent heat within the smoke plume. However, recent modeling studies have supported conflicting hypotheses of the moisture source and thermodynamics driving pyroCb activity. By incorporating a geostationary satellite-based pyroCb detection algorithm in conjunction with reanalysis data, this study describes the first observationally-based conceptual model for pyroCb development in western North America. Results are focused on 41 large wildfires observed within the United States and Canada during 2013, which produced more than 50 intense pyroCb events. 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 rising motion and 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 activity. The specific amount of mid-level moisture and instability required is shown to be strongly dependent on the surface elevation of the contributing fire. While surface-based fire weather indices are a useful indicator of intense fire activity, they have limited capability for characterizing pyroCb development. The intense radiant heat emitted by large wildfires can serve as a potential trigger, suggesting pyroCb may occasionally develop in the absence of traditional triggering mechanisms when an otherwise favorable thermodynamic and synoptic 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. The results of this work will improve our ability to detect, monitor, and predict pyroCb, and will help us to gain understanding of the role of this phenomenon in the climate system.