What is a pyroCb?
Wildfires can become so intense that they produce thunderstorms. This phenomenon of fire-triggered thunderstorms is called pyroCbs, which is short for pyrocumulonimbus clouds or cumulonimbus flammagenitus. Cumulonimbus flammagenitus stems from the Latin words meaning “flame” and “created from.”
While the exact physics behind these fire-triggered thunderstorms is still unclear, pyroCbs are triggered by the uplift of ash, smoke, and burning materials via super-heated updrafts. Wildfire flames release enough heat and moisture into the atmosphere to produce thunderstorms. As these materials cool, clouds are formed that behave like traditional thunderstorms but without the accompanying precipitation.
StratoFIRE produced this short video teaser…
Recognition of pyroCBs
Over the past decade there has been increasing recognition that: (a) individual or clustered pyroCb events is a surprisingly common phenomenon in the middle latitude summer, (b) extremely strong pyroCbs inject smoke, ice and greenhouse-relevant gases deep in the stratosphere of total burden comparable, in many cases, to mid-sized volcanoes, (c) the smoke in the stably- stratified stratosphere stays for months and spreads over the globe by the prevailing winds, and (d) shifting patterns of precipitation and temperature in a warming climate create conditions that favor increasingly frequent and severe pyroCb outbreaks . The latest report of the Intergovernmental Panel on Climate Change (IPCC) identifies global aerosols as one of the key challenges in narrowing uncertainty to the estimates and interpretations of the Earth’s changing energy budget. On the same note, the research objectives of StratoFIRE are dictated by the accumulated evidence that the lower stratosphere (LS, <25 Km) may act as a mediator connecting extreme wildfires, smoke and global climate.
Why we study pyroCBs?
Although the stratosphere constitutes only a small fraction of the atmospheric mass, extensive research in the recent years have provided solid evidence about its key role on impacting circulation and climate on seasonal to decadal time scales. Aerosols in the stratosphere are crucial for understanding the long-term aerosol-chemistry interactions, troposphere-stratosphere coupling and the radiative forcing contributing to climate change. As key removal mechanisms of particulate matter are not effective in the stratosphere, the stratospheric smoke particles have a particularly long lifetime, allowing for long-range transport under the influence of the meridional Brewer-Dobson circulation and fast zonal wind jets. The lack of a detailed characterization of the composition and variability of non-volcanic sources in the stratosphere, is one of the key uncertainties in estimating direct global radiative forcing.
Smoke is largely composed of organic and black carbon, numerous reactive gases, and other aerosol precursors. Black carbon (BC) is of particular interest as it is the strongest absorber of shortwave light. Absorption by BC heats the plume causing shelf-lofting to higher altitudes , which prolongs the smoke lifetime and amplifies radiative and chemical perturbations. The global and regional radiative effects and feedbacks associated with stratospheric smoke are uncertain, partly because smoke ageing in the stratosphere is not well constrained. Apart from radiative perturbations, smoke may also accelerate ozone destruction via several mechanisms. These mechanisms are known through studies of volcanic aerosols , but their chemical reactivity might differ for smoke, given the different nature of smoke aerosols.
The science of StratoFIRE
StratoFIRE will provide the necessary breakthroughs in our understanding on the optical and microphysical properties of stratospheric smoke: Particles originating from biomass burning activities present a large variability in terms of their optical properties, primarily related to atmospheric ageing . One parameter that shows an extreme variability is the particle linear depolarization ratio (PLDR; the ratio of the cross to the co-polarized backscattered radiation), characteristic of particle shape. In StratoFIRE we aim to modify the widely used inversion algorithm GRASP/GARRLiC to also include the nearly-spherical shape model and reproduce the unique optical properties of stratospheric smoke. These new optical properties of smoke will update the look-up tables used by the global climate model to provide a more accurate representation of smoke/radiation interactions.
Fusion of observations with modeling
StratoFIRE will fuse observations with modelling to significantly reduced uncertainties related to smoke emissions: To date, only indirect methods are available to estimate smoke emission rates. Space-bone and ground-based observations provide information generally with some temporal and spatial distance from the source of the fire. In StratoFIRE we aim to apply an innovative inversion algorithm to largely improve the quantitative predictions of smoke emissions and injection heights near the tropopause.
Climate modelling
There is a shortage of studies with global climate models. StratoFIRE makes use of a comprehensive climate-aerosol-chemistry model (EMAC/MESSy), that participated in the CMIP6 Intercomparison, to assess with tailor-make simulations the significance of the plethora of proposed climatic impacts. We go beyond the conventional understanding of particles shape and we put into a thorough examination global effects related to the hypothesis of near-sphericity. For the first time, we quantify the semi-direct effect, atmospheric circulation and surface fluxes changes in relation to smoke injection so as to provide a complete assessment of the short-term climate signatures.