Ascending smoke vortices in the stratosphere – ASTuS

The project is based on a diagnostic, theoretical and experimental approach.

The diagnosis is based on data collected by satellite instruments as well as dedicated campaigns with small balloon instruments. There are a large number of instruments for monitoring stratospheric plumes and we have the ability to combine their complementary capabilities to provide the most comprehensive diagnosis possible from space. A very useful instrument was the CALIOP lidar on the CALIPSO mission. CALIOP is about to be replaced by the lidar of the Earth-Care mission launched on May 28, 2024, the expected performance of which is higher. Other particularly useful instruments are those that measure at the limb such as MLS, SAGE-III, ACE-FTS or OMPS-LP. We are also able to process IASI data to jointly derive a measurement of the SO2 and sulfate columns. However, not everything can be measured from satellites and in situ measurements are essential. Due to logistical constraints, only light instrument flights under small balloons can be organized with a fast response time by measuring size distribution and absorption/scattering properties. This concept was applied to the Hunga eruption with measurements taken from Reunion around ten days after the eruption which led to the first international publication on the characterization of the plume.
Some of the satellite data that can be used for our detailed analysis have existed since 2004 and therefore make it possible to go back into the recent past to investigate important events.

Understanding the evolution of plumes, the dynamics of anticyclonic eddies, and the climate impact is carried out by combining simplified theory with detailed and realistic simulations. Laboratory studies in rotating tanks are also carried out. Interest in heated eddies had until now been limited to tropical cyclones and the case of heated (or cooled) eddies moving vertically had not been considered. This is therefore a new subject in fluid dynamics. These vortices present significant challenges for understanding their structure, their stability, their long-term evolution and the conditions of their appearance. The dynamic observations available are currently based on meteorological analyzes which manage to remarkably follow the aerosol cloud seen by satellites but can present significant biases because they do not use any information on these aerosols. The information comes exclusively from the thermal signature of the vortex, in the form of a vertical temperature dipole which is recorded by satellites and used in the analysis. The conditions of this detection, its limits and the biases generated are an important element for analyzing the available data.

Due to the coincidence between the start of the project and the eruption of Hunga in January 2022, observational capabilities were reoriented towards this exceptional case during the first phase of the project. We were thus the first to highlight the effects of the very large quantity of water in the plume which, after causing the disappearance of the ashes in the first hours, carried along with the precipitating ice, caused a rapid conversion of SO2 into sulfates, and a rapid descent of the plume of 3 to 4 km in one week due to cooling by the emission of water vapor. We then described the continuation of the conversion using IASI simultaneously measuring SO2 and sulfates and showed the confinement in several anticyclonic structures induced by the descent. We were also the first to provide an evaluation of the radiative effect of the plume, showing that in the first weeks it exerted, because of the effect of the water, a positive effect unlike the cooling which is the norm for volcanic eruptions. We also showed the separation of aerosols and water in the months that followed and estimated the size of the aerosols from multi-frequency measurements of extinction by SAGE III. Here too, Hunga is exceptional with aerosols having a large size of 0.4µm in effective radius from the first month and remaining stable over time.
The study of cyclonic vortices theoretically and in a simplified axisymmetric framework with the non-hydrostatic WRF model revealed initially unexpected characters. The dependence of heating on aerosol concentration means that the densest regions rise the fastest. A front is thus formed at the top of the cloud limited only by particle diffusion, followed by a long tail behind. The dynamic impact is to create another frontal zone for the potential vorticity which quickly drops to zero at the top, which in principle leads to instability but not here. Inside this zone, stratification is reduced and absolute vorticity is almost zero. In compensation, a cyclonic vorticity is induced in the tail. In steady state, this helps to move the vortex upward and leave behind a cyclonic tail.
A study of the chemical composition of the main vortex caused by the 2020 Australian fires indicates that little outside air enters the plume which maintains significant ozone depletion relative to standard photochemical equilibrium due to favored destruction reactions by organic aerosols.

In the second part of the project, the nature of anticyclonic vortices will be investigated in more detail to understand their evolution in an environment disturbed by shear, their stability and the conditions of their formation. This will be done again with WRF but also with a WACAM model able to take into account the realistic environment.
Laboratory experiments which are complex to develop and have given preliminary results in the first phase will make it possible to explore a wide range of parameters linked to rotation and heating.
Depending on new events that may appear during the summer of 2024, diagnostic analyzes will focus on these events or investigate past events. In particular, we will try to exploit the CALIOP data in relation to limb measurements to obtain extinction measurements.

Kloss et al : Aerosol Characterization of the Stratospheric Plume From the Volcanic Eruption at Hunga Tonga 15 January 2022, Geophysical Research Letters, 49, e2022GL099394, doi.org/10.1029/2022GL099394, 2022.

Sellitto et al.: Radiative impacts of the Australian bushfires 2019–2020 – Part 1: Large-scale radiative forcing, Atmos. Chem. Phys., 22, 9299–9311, doi.org/10.5194/acp-22-9299-2022, 2022.

Sellitto et al.:The unexpected radiative impact of the Hunga Tonga eruption of 15th January 2022, Commun Earth Environ, 3, 288, doi.org/10.1038/s43247-022-00618-z, 2022.

Legras et al.: The evolution and dynamics of the Hunga Tonga–Hunga Ha’apai sulfate aerosol plume in the stratosphere, Atmos. Chem. Phys., 22, 14957–14970, doi.org/10.5194/acp-22-14957-2022, 2022.

Khaykin et al.: Global perturbation of stratospheric water and aerosol burden by Hunga eruption, Commun Earth Environ, 3, 316, doi.org/10.1038/s43247-022-00652-x, 2022.

Sellitto et al.: Volcanic Emissions, Plume Dispersion, and Downwind Radiative Impacts Following Mount Etna Series of Eruptions of February 21–26, 2021, Journal of Geophysical Research: Atmospheres, 128, e2021JD035974, doi.org/10.1029/2021JD035974, 2023.

Sellitto et al.: Radiative impacts of the Australian bushfires 2019–2020 – Part 2: Large-scale and in-vortex radiative heating, Atmos. Chem. Phys., 23, 15523–15535, doi.org/10.5194/acp-23-15523-2023, 2023.

Duchamp et al.: Observation of the Aerosol Plume From the 2022 Hunga Tonga—Hunga Ha’apai Eruption With SAGE III/ISS, Geophysical Research Letters, 50, e2023GL105076, doi.org/10.1029/2023GL105076, 2023.

Randel et al: Stratospheric Water Vapor from the Hunga Tonga–Hunga Ha’apai Volcanic Eruption Deduced from COSMIC-2 Radio Occultation, Remote Sensing, 15, 2167, doi.org/10.3390/rs15082167, 2023.

Dumelié et al.: Toward Rapid Balloon Experiments for sudden Aerosol injection in the Stratosphere (REAS) by volcanic eruptions and wildfires, Bulletin of the American Meteorological Society, 2023, doi.org/10.1175/BAMS-D-22-0086.1.

Podglajen et al.: Dynamics of diabatically forced anticyclonic plumes in the stratosphere, doi.org/10.1002/qj.4658, 2024.

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.