THE PYROCONVECTIVE PATHWAY FOR STRATOSPHERIC WATER VAPOR AND AEROSOL

A detailed analysis of pyrocumulonimbus (pyroCb) cases is presented that explores their convective dynamics, stratospheric plume characteristics and down-stream radiative effects. Satellite observations in conjunction with ground station data and radiative transfer models are used to quantify the impact that pyroCbs have on localized stratospheric aerosol and water vapor. The initial meteorological and fire conditions are explored using a cloud- and aerosol-resolving model to determine the dominant mechanics driving the convection and their effects on microphysics. Results show that intense sensible heat fluxes are the dominant convection trigger over a wildfire in an unstable atmosphere. Direct observations by cloud profiling radar of the active convective stage of a pyroCb are analyzed for the first time, and comparisons with non-pyro meteorological deep convection in the same vicinity and season show that the pyroCb has an extreme delay in the growth of precipitation-sized cloud droplets to altitudes above the homogeneous freezing level.Stratospheric aerosol plume morphology is analyzed for several cases, and an empirical heat accumulation efficiency model is developed to describe observed radiatively-induced self-lofting in the stratosphere. The model results suggest pyroCb aerosol plumes are ∼ 30% efficient at converting shortwave radiative heating into sensible heating, thereby driving buoyant uplift once injected into the stratosphere. PyroCbs directly inject H2O vapor into the stratosphere, which is shown to be significantly large for two separate cases. The cloud-resolving model confirms a previous hypothesis that uniquely small ice particle microphysics can enhance stratospheric H2O in detrained convective anvils. Satellite retrieval evidence suggests plume water vapor anomalies are a result of inefficient removal of small ice particles within the detrained pyroCb anvil. Model-injected total water—represented as the sum of all ice and absolute humidity—shows at least 30% of H2O survives the convective detrainment stage, and diminishes within the evolving plume over the observation period when using satellite observations of H2O as a benchmark. In the plumes presented herein, pyroCb H2O anomalies are as large as 4±3 ppmv above the background in the lower stratosphere. Detailed line-by-line radiative transfer simulations suggest that these anomalies produce an instantaneous longwave radiative forcing up to +1.0 W m −2 at the tropopause

Microwave Limb Sounder (MLS) observations of biomass burning products in the stratosphere from Canadian forest fires in August 2017

Forest fires in British Columbia in August 2017 caused a pyrocumulonimbus event that injected a polluted air mass into the lower stratosphere. The Microwave Limb Sounder (MLS) on the Aura satellite first observed the polluted air mass on 14 August 2017 and continued to observe it for 60 d (100 d in water vapour). We estimate the mass of CO injected into the stratosphere to be 2400 Gg. Events in which a fire injects its burning products directly into the stratosphere are rare: this is the third of four such events in the 16 years since the launch of Aura, the second largest of the four events, and the only one in the Northern Hemisphere. The other three events occurred in Australia in December 2006, February 2009 and from December 2019 to January 2020. Unlike the 2006 and 2009 events, but like the 2019–2020 event, the polluted air mass described here had a clearly elevated water vapour content: between 2.5 and 5 times greater than that in the surrounding atmosphere. We describe the evolution of the polluted air mass, showing that it rose to an altitude of about 24 km (31 hPa) and divided into several identifiable parts. In addition to CO and H2O, we observe enhanced amounts of HCN, CH3CN, CH3Cl and CH3OH with mixing ratios in the range to be expected from a variety of measurements in other biomass burning plumes. We use back trajectories and plume-dispersion modelling to demonstrate that the pollutants observed by MLS originated in the British Columbia fires, the likeliest source being at 53.2∘ N, 121.8∘ W at 05:20 UTC on 13 August 2017.</p

Connections of climate change and variability to large and extreme forest fires in southeast Australia

The 2019/20 Black Summer bushfire disaster in southeast Australia was unprecedented: the extensive area of forest burnt, the radiative power of the fires, and the extraordinary number of fires that developed into extreme pyroconvective events were all unmatched in the historical record. Australia’s hottest and driest year on record, 2019, was characterised by exceptionally dry fuel loads that primed the landscape to burn when exposed to dangerous fire weather and ignition. The combination of climate variability and long-term climate trends generated the climate extremes experienced in 2019, and the compounding effects of two or more modes of climate variability in their fire-promoting phases (as occurred in 2019) has historically increased the chances of large forest fires occurring in southeast Australia. Palaeoclimate evidence also demonstrates that fire-promoting phases of tropical Pacific and Indian ocean variability are now unusually frequent compared with natural variability in preindustrial times. Indicators of forest fire danger in southeast Australia have already emerged outside of the range of historical experience, suggesting that projections made more than a decade ago that increases in climate-driven fire risk would be detectable by 2020, have indeed eventuated. The multiple climate change contributors to fire risk in southeast Australia, as well as the observed non-linear escalation of fire extent and intensity, raise the likelihood that fire events may continue to rapidly intensify in the future. Improving local and national adaptation measures while also pursuing ambitious global climate change mitigation efforts would provide the best strategy for limiting further increases in fire risk in southeast Australia