Ascending smoke vortices in the stratosphere – ASTuS

The paroxysmal stages of wildfires generate pyro-cumulonimbus that deposit large quantities of smoke in the stratosphere, comparable to a moderate volcanic eruption. It was discovered in 2020 by the coordinator and collaborators that this smoke self-organizes as synoptic scale anticyclonic vortices that rise under the heating due to the absorption of solar radiation by black carbon aerosols. These structures persist for several months in the stratosphere rising by 10 to 20 km. The consequence is the persistence of the released smoke for several years with a much longer climatic impact than expected so far, and is expected to increase in the future. The vortices also carry an intense ozone mini hole, likely to travel over continents in mid-summer, increasing ground UV. A second paper published in 2021 established that smoke vortices were also ubiquitous after the British Columbia fires of 2017 and they are probably present in many other similar events. Although volcanic aerosols are, in principle, much less absorbing than black carbon, there are indications of compact rising similar structures after some recent volcanic eruptions.

The project assembles a team of experts in geophysical fluid dynamics, atmospheric remote sensing and modellers to document and understand these very new atmospheric structures, their distribution and their impact. The project is divided in three main work packages. The first one is observation oriented and is devoted to exploring the data of the past. The second part is oriented towards fluid dynamics and radiative properties. The third one is oriented towards realistic modelling and impact studies.

In the first package, we investigate the past cases from the archive data by distinguishing a recent period after 2006 when the space lidar CALIOP is available and aerosol plumes can be localized and characterized with high precision from the earlier periods when less satellite data are available and more modelling work is required. We investigate in this work package how the observed heating is related to the aerosol properties and how the observed ozone hole affects the UV radiation at the ground level. In this package, we also get prepared to respond to future events with dedicated observation and modelling in a real time framework.

The second package is devoted to the basic understanding of stable rising heated vortices in a rotating fluid, which have never been described in the literature. The realistic observed conditions that are on the verge of inertial instability are highly nonlinear and present some theoretical and numerical challenge for the available methods. We will use a hierarchy of models and theoretical concepts starting from the well-mastered low Rossby number framework to get to the realistic cases. We will experiment numerically with idealized fluid dynamics model and a state-of-the-art non-hydrostatic model adapted to the stratosphere. An important component of this second package will be a laboratory experiment in a rotating tank, which is expected to provide a demonstrator and also a flexible model to question the theory and numerical simulations.

The third package will be devoted to reproduce the observed events by first performing detailed radiative calculation to estimate the heating rates. This information will be used in a full chemical-climate model that will be trained, to begin, on the cases of 2020 and 2017 and to estimate the impact of the events in terms of radiative budget and atmospheric composition.