Pyro-cumulonimbus injection of smoke to the stratosphere: Observations and impact of a super blowup in northwestern Canada on 3–4 August 1998

Abstract

We report observations and analysis of a pyro-cumulonimbus event in the midst of a boreal forest fire blowup in Northwest Territories Canada, near Norman Wells, on 3–4 August 1998. We find that this blowup caused a five-fold increase in lower stratospheric aerosol burden, as well as multiple reports of anomalous enhancements of tropospheric gases and aerosols across Europe 1 week later. Our observations come from solar occultation satellites (POAM III and SAGE II), nadir imagers (GOES, AVHRR, SeaWiFS, DMSP), TOMS, lidar, and backscattersonde. First, we provide a detailed analysis of the 3 August eruption of extreme pyro-convection. This includes identifying the specific pyro-cumulonimbus cells that caused the lower stratospheric aerosol injection, and a meteorological analysis. Next, we characterize the altitude, composition, and opacity of the post-convection smoke plume on 4–7 August. Finally, the stratospheric impact of this injection is analyzed. Satellite images reveal two noteworthy pyro-cumulonimbus phenomena: (1) an active-convection cloud top containing enough smoke to visibly alter the reflectivity of the cloud anvil in the Upper Troposphere Lower Stratosphere (UTLS) and (2) a smoke plume, that endured for at least 2 hours, atop an anvil. The smoke pall deposited by the Norman Wells pyro-convection was a very large, optically dense, UTLS-level plume on 4 August that exhibited a mesoscale cyclonic circulation. An analysis of plume color/texture from SeaWiFS data, aerosol index, and brightness temperature establishes the extreme altitude and “pure” smoke composition of this unique plume. We show what we believe to be a first-ever measurement of strongly enhanced ozone in the lower stratosphere mingled with smoke layers. We conclude that two to four extreme pyro-thunderstorms near Norman Wells created a smoke injection of hemispheric scope that substantially increased stratospheric optical depth, transported aerosols 7 km above the tropopause (above ∼430 K potential temperature), and also perturbed lower stratospheric ozone.