The Untold Story of Pyrocumulonimbus

Abstract

Wildfire is becoming the focus of increasing attention. It is now realized that changes in the occurrence frequency and intensity of wildfires has important significant consequences for a variety of important problems, including atmospheric change and safety in the urban–wildland interface. One important but poorly understood aspect of wildfire behavior—pyrocumulonimbus firestorm dynamics and atmospheric impact—has a curious history of theory and observation. The “pyroCb” is a fire-started or fire-augmented thunderstorm that in its most extreme manifestation injects huge abundances of smoke and other biomass-burning emissions into the lower stratosphere. The observed hemispheric spread of smoke and other biomass-burning emissions could have important climate consequences. PyroCbs have been spawned naturally and through anthropogenesis, and they are hypothesized as being part of the theoretical “Nuclear nuclear winter” work. However, direct attribution of the stratospheric aerosols to the pyroCb only occurred in the last decade. Such an extreme injection by thunderstorms was previously judged to be unlikely because the extratopical tropopause is considered to be a strong barrier to convection. Two recurring themes have developed as pyroCb research unfolds. First, some “mystery layer” events—puzzling stratospheric aerosol-layer observations—and other layers reported as volcanic aerosol, can now be explained in terms of pyroconvection. Second, pyroCb events occur surprisingly frequently, and they are likely a relevant aspect of several historic wildfires. Here we show that pyroCbs offer a plausible alternate explanation for phenomena that were previously assumed to involve volcanic aerosols in 1989–1991. In addition, we survey the Canada/U.S. fire season of 2002 and identify 17 pyroCbs, some of which are associated with newsworthy fires, such as the Hayman, Rodeo/Chediski, and Biscuit Fires. Some of these pyroCbs injected smoke into the lowermost stratosphere.