The Unknown Story of Pyrocumulonimbus

Global and regional warming trends have been identified and associated with exacerbated wildfire occurrence and impact. Attention to this topic has only been heightened with growing concern regarding anthropogenic climate forcing and fire's apparent increase in the wildland/urban interface. Superimposed on this important topic is a relatively new discovery, forest fire smoke in the stratosphere. The cause is a particularly energetic form of blowup called pyrocumulonimbus (pyroCb). While pyroconvection and pyrocumulus are well known, the peculiar vertical extent of its impact potential escaped our attention until 1998 when it was discovered that Canadian pyroCbs had injected smoke (and certainly other related emissions) well beyond the tropopause and into the stratospheric “overworld.” Reports of pyroCb are increasing in the science literature but are still rare. Moreover, the literature contains several cases of mystery clouds in the stratosphere or layers attributed to volcanoes when the volcanic evidence is lacking.

Here we attempt to characterize the occurrence frequency of pyroCb and identify individual cases in which pyroCb impact on the stratosphere has been missed or mis-identified. For this we use data from nadir-viewing polar orbiter and geosynchronous satellites, satellite-based profiles, and ground-based lidar profiles. PyroCb frequency in the northern extratropical fire season of 2002 is established. In addition, we highlight the curious and surprising interconnection between volcanic and pyroCb plumes in the satellite era with three cases in which we reinterpret literature reports of stratospheric volcanic clouds as pyroCb smoke.