Development of Intense Pyroconvection and Ensuing Stratospheric Smoke Injection in Western North America

Intense fire-triggered thunderstorms, known as pyrocumulonimbus (or pyroCb), often inject a significant quantity of aerosol mass into the lower stratosphere.  PyroCb can also alter fire behavior, influence smoke plume trajectories, and hinder fire suppression efforts.  This study describes the first observationally-based conceptual model for pyroCb development and ensuing stratospheric smoke injection by applying reanalyzed meteorological model output to an inventory of 26 intense pyroCb events from June-August 2013.  A control inventory containing periods of intense fire activity without pyroCb is also examined.  Results are based on 88 intense wildfires observed within the western United States and Canada.  While surface-based fire weather indices have been highlighted as a useful indicator of intense fire activity, they are not a skillful predictor of intense pyroCb.  Development occurs 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 or under the influence of a monsoonal anticyclone.  Upper-tropospheric dynamics are conducive to rising motion and vertical convective development.  Mid- and upper-tropospheric conditions therefore resemble those that produce traditional dry thunderstorms. The specific quantity of mid-level moisture and instability required is shown to be strongly dependent on the surface elevation of the contributing fire.  The intense radiant heat emitted by large wildfires can serve as a potential trigger, implying that pyroCb occasionally develop in the absence of traditional meteorological triggering mechanisms.  This conceptual model suggests that meteorological conditions favorable for pyroconvection and stratospheric smoke injection are observed regularly in western North America.  PyroCb may therefore be a significant and endemic feature of regional summer climate.  Results from study are essential for improved detection, monitoring, and prediction of pyroCb, which will ultimately enable improved understanding of this phenomenon’s role in the climate system.