Pyro-cumulonimbus: a newly discovered mechanism for troposphere-to-stratosphere injection

In the last five years, a phenomenon now referred to as pyro-cumulonimbus has gone from the discovery stage to growing acceptance and appreciation. The “pyroCb” (for short) is an extreme combination of wildfire flaming and convection that has sufficient energy to inject smoke and other biomass burning emissions deeply into the stratospheric overworld. The first evidence for the stratospheric impact of the pyroCb came from satellite solar occultation and lidar profiles of aerosol extinction and backscatter, respectively, showing summertime aerosol layers three to seven kilometers above the tropopause. The solar occultation data, primarily the Naval Research Lab's Polar Ozone and Aerosol Measurement (POAM) III, recorded a hemispheric impact from the pyroCb—a quintupling of the zonal average stratospheric aerosol optical depth and a multi-month decay period in the boreal summer/fall of 1998. Back trajectories combined with visible and IR satellite imagery revealed the link with pyro-convection. A more detailed study of nadir-viewing and geostationary satellite imagery has since revealed several unique characteristics of the active pyroCb processes and the post-convection plume.

In this paper, the first of its type presented to an AMS Middle Atmosphere conference, we will present an overview of the pyroCb phenomenon. This will include a case study of a boreal forest fire in Alaska in 1990 that erupted into a pyroCb and generated a smoke plume in the stratosphere that was originally accounted for in the literature as the product of an unreported volcanic eruption. This case study will contain an analysis of the antecedent and triggering meteorology, multi-wavelength imager views of the explosive convection, and an analysis of NASA's Stratospheric Aerosol and Gas Experiment (SAGE) II aerosol extinction profiles in the summer and fall of 1990. Our presentation will also contain a study of long-term aerosol records such as Total Ozone Mapping Spectrometer (TOMS) aerosol index to assess the global frequency of the pyroCb phenomenon. Finally, this report will detail several peculiar manifestations of the pyroCb and its effects, using visible, near-IR, and IR imager wavelengths. For example, GOES and Defense Meteorological Satellite Program (DMSP) visible and IR imagery captured a pyroCb in August 1998 that exhibited a plume penetrating into the lowermost stratosphere that persisted for at least two hours.