The Hunga Tonga–Hunga Ha’apai volcano erupted on 15 January 2022, producing the largest stratospheric aerosol perturbation of the last 30 years. Stratospheric volcanic aerosols usually produce a transient climate cooling; these impacts depend on volcanic aerosol composition/size, due to size-dependent interactions with solar/terrestrial radiation. We demonstrate that the Hunga Tonga–Hunga Ha’apai stratospheric aerosols have a larger cooling potential per unit mass than the past climate-relevant El Chichón (1984) and Pinatubo (1991) eruptions.

We reformulate multi-model ensembles by treating ensemble forecasts as discrete probability distributions and combining them using barycenters. We compare the L₂ barycenter (equivalent to pooling) with the Wasserstein barycenter (more precisely its Gaussian approximation). Both have the same ensemble mean but differ in how they represent forecasts uncertainty. In terms of Continuous Ranked Probability Score, the Wasserstein barycenter outperforms more often while performing similarly on average.

Our study finds that the air mass fractions (AMFs) from the Asian boundary layer (ABL) to the polar regions are about 1.5 times larger than those from the same latitude band in the Southern Hemisphere. The transport of AMFs from the ABL to the polar vortex primarily occurs above 20 km and over timescales exceeding 2 years. Our analysis reveals a strong correlation between the polar pollutants and the AMFs from the ABL. About 20 % of SF₆ in the polar stratosphere originates from the ABL.

Upper tropical clouds have a strong impact on Earth's climate but are challenging to observe. We report the first long-duration observations of tropical clouds from lidars flying on board stratospheric balloons. Comparisons with spaceborne observations reveal the enhanced sensitivity of balloon-borne lidar to optically thin cirrus. These clouds, which have a significant coverage and lie in the uppermost troposphere, are linked with the dehydration of air masses on their way to the stratosphere.

Record-breaking wildfires ravaged south-eastern Australia during the fire season 2019–2020. These fires injected a smoke plume in the stratosphere, which dispersed over the whole Southern Hemisphere and interacted with solar and terrestrial radiation. A number of detached smoke bubbles were also observed emanating from this plume and ascending quickly to over 35 km altitude. Here we study how absorption of radiation generated ascending motion of both the the hemispheric plume and the vortices.

The role of gravity waves on tropical cirrus clouds and air-parcel dehydration was studied using the combination of Lagrangian observations of temperature fluctuations from superpressure balloons and a 1.5D model. The inclusion of the gravity waves to a reference simulation of a slow ascent around the cold-point tropopause drastically increases ice-crystal density, cloud fraction, and air-parcel dehydration, and it produces a crystal size distribution that agrees better with observations.

Atmospheric waves that carry momentum from tropospheric weather systems into the equatorial stratosphere modify the winds there. The Strateole-2 2019 campaign launched long-duration stratospheric superpressure balloons to measure these equatorial waves. We deployed a GPS receiver on one of the balloons to measure atmospheric temperature profiles beneath the balloon. Temperature variations in the retrieved profiles show planetary-scale waves with a 20 d period and 3–4 d period waves.

The long-duration atmospheric impact of the Tonga eruption in January 2022 is a plume of water and sulfate aerosols in the stratosphere that persisted for more than 6 months. We study this evolution using several satellite instruments and analyse the unusual behaviour of this plume as sulfates and water first moved down rapidly and then separated into two layers. We also report the self-organization in compact and long-lived patches.

This work has identified characteristic spatial and temporal scales for non-Gaussian outbreaks in forecasts, specifically, bimodality. Methodology is introduced which allows one to connect meteorological phenomena to bimodal outbreaks. Large-scale circulation interacting with local processes is uncovered as a frequent ingredient to such outbreaks. These insights not only provide a deeper understanding of the dynamical processes involved, but also have drastic implications for forecast skill.

While the assumption of Gaussianity leads to many simplifications, ensemble forecasts often exhibit non-Gaussian distributions. This work has systematically identified the presence of a specific case of non-Gaussianity, bimodality. It has been found that bimodality occurs in a large portion of global 2 m temperature forecasts. This has drastic implications on forecast skill as the minimum probability in a bimodal distribution often lies at the maximum probability of a Gaussian distribution.

We study the role of different processes in setting the lower stratospheric water vapour. We find that mechanisms involving ice microphysics and small-scale mixing produce the strongest increase in water vapour, in particular over the Asian Monsoon. Small-scale mixing has a special relevance as it improves the agreement with observations at seasonal and intra-seasonal timescales, contrary to the North American Monsoon case, in which large-scale temperatures still dominate its variability.