Mesoscale and severe convection are known to be important precipitation producing processes over land. They are often associated with hazardous weather (e.g. damaging winds, hail, lightning, tornadoes, extreme precipitation, and flooding), which we already see is becoming more frequent in many regions with climate change. At the same time, these storms remain difficult to predict throughout all lifecycle stages from initiation to upscale growth and dissipation.
The aim of this session is to gain an improved understanding of mesoscale and severe convective processes over land from a non-idealised perspective for current and future periods.
We invite contributions focussing on the underlying storm dynamics and microphysics, upscale effects, advances in modelling and predictability of these storm systems, and their impacts. We also invite contributions on the driving processes of the formation and evolution of severe convection, and how these factors explain spatio-temporal patterns of storm intensity, precipitation, and on-the-ground hazards. This includes contributions on land-convection interactions in connection with mesoscale and severe storms, e.g. effects of complex topography, soil moisture feedbacks, or land use / land use change including e.g. urbanisation, deforestation, or irrigation.
Contributions focussing on individual extreme events or giving climatological perspectives including future climates are welcome, as are studies relying on remote sensing data, in-situ observations, or high-resolution models, especially those that explicitly resolve convection.

Cold clouds (mixed-phase and ice) play an important role in the Earth’s radiation budget because of their high temporal and spatial coverage and their interaction with long wave and short wave radiation. Yet, the variability and complexity of their macro- and microphysical properties, the consequence of intricate ice particle nucleation and growth processes, make their study extremely challenging. As a result, large uncertainties still exist in our understanding of cold cloud processes, their radiative effects, and their interaction with their environment (in particular, aerosols).

This session aims to advance our comprehension of cold clouds by bringing observation- and modelling-based research together.

A diversity of research topics shall be covered, highlighting recent advances in cloud observation techniques, modelling, and subsequent process studies:

(1) Airborne, space borne, ground- or laboratory-based measurements and their derived products (retrievals), which are useful to constrain cloud properties like extent, emissivity, or crystal size distributions, to clarify formation mechanisms, and to provide climatology.

(2) Process-based, regional, and global model simulations that employ observations for better representation of cloud microphysical properties and radiative forcing under both current and future climate.

The synthesis of these approaches can uniquely answer questions regarding dynamical influence on cloud formation, life cycle, coverage, microphysical and radiative properties, crystal shapes, sizes, and variability of ice particles in mixed-phase as well as ice clouds. Joint observation-modelling contributions are therefore particularly encouraged.

Mountains cover approximately one-quarter of the total land surface on the planet, and a significant fraction of the world’s population lives within them, in their vicinity, and downstream. Orography critically affects weather and climate processes at all scales and, in connection with factors such as land-cover heterogeneity, is responsible for high spatial variability in mountain weather and climate. This session showcases research that contributes to improving our understanding of weather and climate processes in mountain and high-elevation areas around the globe, as well as their modification induced by global environmental change. This includes the interaction of mountain weather and climate with the terrestrial cryosphere.

We welcome contributions describing the influence of mountains on the atmosphere on meteorological and climate time scales, including terrain-induced airflow, orographic gravity waves, orographic precipitation, land-atmosphere exchange over mountains, forecasting, and predictability of mountain weather. We also encourage theoretical, modeling and observational studies on orographic gravity waves and their effects on the weather and the climate. Furthermore, we invite studies that investigate climate processes and climate change in mountain areas based on monitoring and modeling activities.

Particularly welcome are contributions that align with and address the interdisciplinary objectives of the Elevation-Dependent Climate Change (EDCC) working group of the Mountain Research Initiative, as well as the application and development of high-resolution (kilometer-scale) climate models over complex mountainous terrain, including advances in model design, challenges, and observational gaps needed for robust model evaluation and improvement.

The understanding of tropical phenomena and their representation in numerical models still raise important scientific and technical questions, particularly in the coupling between the dynamics and diabatic processes. Among these phenomena, tropical cyclones (TC) are of critical interest because of their societal impacts and because of uncertainties in how their characteristics (cyclogenesis processes, occurrence, intensity, latitudinal extension, translation speed) will change in the framework of global climate change. The monitoring of TCs, their forecasts at short to medium ranges, and the prediction of TC activity at extended range (15-30 days) and seasonal range are also of great societal interest.
The aim of the session is to promote discussions between scientists focusing on the physics and dynamics of tropical phenomena. This session is thus open to contributions on all aspects of tropical meteorology between the convective and planetary scale, such as:

- Tropical cyclones,
- Convective organisation,
- Diurnal variations,
- Local circulations (i.e. island, see-breeze, etc.),
- Monsoon depressions,
- Equatorial waves and other synoptic waves (African easterly waves, etc.),
- The Madden-Julian oscillation,
- etc.

We especially encourage contributions of observational analyses and modelling studies of tropical cyclones and other synoptic-scale tropical disturbances including the physics and dynamics of their formation, structure, and intensity, and mechanisms of variability of these disturbances on intraseasonal to interannual and climate time scales.

Findings from recent field campaigns are also encouraged.

Surface exchange fluxes of heat, momentum and mass above the global oceans and at the poles (snowpack, sea-ice, ocean and land) have significant impacts on atmospheric composition, biogeochemistry and climate at regional to global scales. Atmospheric boundary layer processes mediate these chemical and physical fluxes. This session is intended to provide an interdisciplinary forum to bring together researchers working in the areas of meteorology, atmospheric chemistry, air quality, biogeochemistry, stable isotope research, oceanography, and climate above the global oceans and in the polar regions.

The session focuses on new research in several areas which include: air-sea fluxes of climate-active trace gases (CO2, CH4, N2O) mediated by the atmospheric boundary layer above the oceans and in polar regions; regional emission and vertical mixing of aerosol, such as cloud-forming particles (CCN/INP) and their precursors (including dimethyl sulfide (DMS), marine organic compounds and halogenated species) and their impacts on atmospheric composition and climate; atmospheric deposition of nutrients (e.g., nitrogen, phosphorus, iron) and its impact on ocean biological systems; and biogeochemistry-climate feedback loops in the ocean-atmosphere system. We also welcome studies on how these surface fluxes may change in response to climate warming, as well as the local to large-scale influences on these exchanges. An adequate understanding and quantification of these processes is necessary to improve modeling and prediction of future changes above the oceans and in the polar regions, their teleconnections with mid-latitude weather and climate (including meridional transport of heat, moisture, chemical trace species, aerosols and isotopic tracers), and the coupling between local and large-scale dynamics.

The session has strong links to the Surface Ocean ̶ Lower Atmosphere Study (SOLAS) and the GESAMP Working Group 38 on atmospheric input of chemicals to the ocean. Submissions are encouraged from all areas covered by these programs, using a range of analysis approaches including field measurements, remote sensing, laboratory studies, and atmospheric and oceanic numerical models.

This year we particularly welcome studies on the impact of extreme events on air-sea gas exchange of climate-relevant compounds in marine systems. Here we invite contributions addressing physical drivers such as marine heatwaves, storms and tropical cyclones, circulation anomalies or sea ice changes; biogeochemical drivers such as hypoxic or anoxic conditions and acidification pulses; biological drivers such as harmful algal blooms; or compound events. Relevant studies may address impacts in all oceanic domains; e.g., open ocean, shelf waters and shallow (< 20 m depth) coastal ecosystems. The reporting on progress as well as critical knowledge gaps in polar regions will help define upcoming research programmes as part of Antarctica InSync and the International Polar Year 2032-33.

Along with the rapid economic development and accelerated urbanization process, many areas in the world are suffering from high levels of ozone and fine particle pollution. These air pollutants can affect weather and climate system through absorbing or scattering radiation. For example, ozone is a kind of greenhouse gases, while aerosols can not only directly affect solar radiation but serve as cloud ice nuclei or ice nuclei to modify microphysical processes of clouds and precipitation. On the other side, weather and climate are also closely linked to formation of air pollution. Monsoon climate, stagnant weather conditions and large-scale circulation patterns all play important roles in air pollution. Understanding how weather and climate interact with air pollution at present and in the future can help us in the field of air pollution prevention and mitigation of global warming.

This session aims to address the current challenges, methodological approaches and wider relevance of observing and modelling meteorology-atmospheric environment interactions around the world. We welcome contributions including, but not limited to, integrating multi-source data (such as, on-situ monitoring, remote sensing, etc.) with controlled experiments to identify the key processes, modelling of the interactions between climate change and air pollution as well as the future projections, assessment the impact of extreme weather on air pollution and future trends, and illustrating the development of cities and the effects of urban climate on regional air quality. Research that reveals the impact of air pollution on the environment, ecology and human health is also welcome.

Significant uncertainties remain in our understanding of Carbon Dioxide (CO2) and Methane (CH4) fluxes across land, ocean, and atmosphere on both regional and global scales. Remotely sensed CO2 and CH4 observations hold great potential for enhancing our understanding of the natural carbon cycle and monitoring anthropogenic emissions. Recent advances in remote sensing technologies for CO2 and CH4, spanning space, aircraft, and ground-based platforms, have delivered unprecedented accuracy and coverage. Moreover, upcoming next-generation platforms like CO2M, MicroCarb, Merlin, and TANGO promise to further enhance observational capabilities. When integrated with ground-based observation networks and modeling tools, these space-based observations can significantly improve our understanding of the carbon cycle at both local and global scales.

This session invites contributions on all aspects of remote sensing of CO2 and CH4, covering current missions (e.g., GOSAT/2/GW, OCO-2/3, S5P/S5, IASI-NG, Carbon Mapper, GHGSat), upcoming and planned missions (e.g., CO2M, MicroCarb, Merlin, TANGO), as well as ground-based (e.g., TCCON, COCCON), aircraft, and other remote sensing instruments. We welcome advances in retrieval techniques, instrumental concepts, and validation activities, with a particular emphasis on interpreting observations related to natural fluxes or anthropogenic emissions.

The session focuses on the variability of the tropospheric and stratospheric chemical composition on the timescales from diurnal to decadal. It discusses the processes driving this variability and attribution of changes to specific drivers. Special emphasis is put on the value of high-quality long-term measurement data sets both from scientific and societal perspective, including science-policy applications, and their sustainability. Supporting model simulations on different scales that utilize observational data will also be discussed. Contributions related to emerging constituents, new data sources and approached to atmospheric composition measurements (e.g. low cost sensor, emerging measurement techniques), measurement campaign that addresses specific processes and long-term projections of the atmospheric chemical composition are also welcome in the session.
Researchers are invited to present novel scientific results from mid- and long-term observational time series from various programmes and networks such as the Global Atmosphere Watch (GAW) Programme, European Monitoring, and Evaluation Programme (EMEP), Network for the Detection of Atmospheric Composition Change (NDACC), Southern Hemisphere Additional Ozonesondes (SHADOZ), Advanced Global Atmospheric Gases Experiment (AGAGE), National Oceanic and Atmospheric Administration (NOAA), regular airborne (e.g. CARIBIC, IAGOS, CONTRAIL) and other campaigns as well as satellite data and model simulations. Data relevant to tropospheric and stratospheric composition, in particular, related to climate change, ozone depletion, ecosystems and health impacts, and air quality as well as firn data on past atmospheric composition are welcome. We welcome contributions from multi-year modeling studies and inter-comparison exercises that address past and future tropospheric or stratospheric composition changes, carried out in the framework of international projects and initiatives.

This session aims to bring together the scientific community within air pollution modelling, focusing on modelling the atmospheric transport and transformation of air pollutants and precursors on global, regional and local scales.

Downscaling aims to process and refine global climate model output to provide information at spatial and temporal scales suitable for impact studies. In response to the current challenges posed by climate change and variability, downscaling techniques continue to play an important role in the development of new services and products. While the refinement of downscaling techniques proceeds at an unprecedented pace, users of climate information are facing the novel challenge of how to select amongst the choice of available datasets or how to assess their credibility with respect to a particular application. In this context, model evaluation and verification is growing in relevance and advances in the field will likely require close collaboration between various disciplines.

Recent developments, including the integration of AI and machine learning applications, the emergence of kilometre-scale simulations, and the widespread availability of open-source downscaling products, add new dimensions to this challenge. These advances raise important questions about the ‘added value’ of downscaling, especially in light of the cascade of uncertainty and the need for robust evaluation frameworks.

In our session, we aim to bring together scientists from the various geoscientific disciplines interrelated through downscaling: atmospheric modeling, climate change impact modeling, machine learning and verification research. We also invite philosophers of climate science to stimulate our discussion about the novel challenges that arise from evaluating complex models and modelling chains in the face of the increasingly heterogeneous needs of the growing user communities.

Contributions to this session may address, but are not limited to:
- newly available downscaling products,
- applications relying on downscaled data and impact assessments,
- downscaling method development and machine learning,
- bias correction and statistical postprocessing,
- challenges in the data management of kilometer-scale simulations,
- verification, uncertainty quantification and the added value of downscaling,
- downscaling approaches in light of computational epistemology.

Rainfall is a “collective” phenomenon emerging from numerous drops. It reaches the ground surface with varying intensity, drop size and velocity distribution. Understanding the relation between the physics of individual drops and that of a population of drops remains an open challenge, both scientifically and for practical implications. This remains true also for solid precipitation. Hence, it is much needed to better understand small scale space-time precipitation variability, which is a key driving force of the hydrological response, especially in highly heterogeneous areas (mountains, cities). This hydrological response at the catchment scale is the result of the interplay between the space-time variability of precipitation, the catchment geomorphological / pedological / ecological characteristics and antecedent hydrological conditions. Similarly to the small scales, accurate measurement and prediction of the spate-time distribution of precipitation at hydrologically relevant scales still remains an open challenge.

This session brings together scientists and practitioners who aim to measure and understand precipitation variability from drop scale to catchment scale as well as its hydrological consequences. Contributions addressing one or several of the following topics are encouraged:
- Novel techniques for measuring liquid and solid precipitation variability at hydrologically relevant space and time scales (from drop to catchment scale), from in-situ measurements to remote sensing techniques, and from ground-based devices to spaceborne platforms. Innovative comparison metrics are welcomed;
- Drop (or particle) size distributions, small scale variability of precipitation, and their consequences for precipitation rate retrieval algorithms for radars, commercial microwave links and other remote sensors;
- Novel modelling or characterization tools of precipitation variability from drop scale to catchment scale from various approaches (e.g. scaling, (multi-)fractal, statistic, deterministic, numerical modelling);
- Novel approaches to better identify, understand and simulate the dominant microphysical processes at work in liquid and solid precipitation.
- Applications of measured and/or modelled precipitation fields in catchment hydrological models for the purpose of process understanding or predicting hydrological response.
- Rainfall simulators developed to investigate the accuracy of disdrometer measurements in assessing drop size and fall velocity.

Fire is the primary terrestrial ecosystem disturbance globally and a critical Earth system process. Its frequency and intensity are expected to increase across most regions in the future, posing significant challenges for ecosystems, the carbon cycle, and society. Fire research is rapidly expanding across disciplines, underscoring the need to advance our understanding of fire's interactions with climate, the biosphere, and human systems. This session invites contributions investigating the role of fire in the Earth system at any spatiotemporal scale, using statistical (including AI) or process-based models, remote sensing, field and laboratory observations, proxy records, and data-model fusion techniques. We strongly encourage abstracts on fire's interactions with: (1) weather, climate, atmospheric composition, chemistry, and circulation, (2) vegetation composition and structure and biogeochemical cycle, ocean ecosystems; (3) cryosphere elements and processes (such as permafrost, sea ice), and (4) human health, land management, conservation, and livelihoods. Moreover, we welcome submissions that address: (5) spatiotemporal changes in fire (especially extreme fires) in the past, present, and future, 6) fire products and models, and their validation, error/bias assessment and correction, as well as (7) analytical tools designed to enhance situational awareness for fire practitioners and to improve fire early warning systems.

This session focuses on volatile organic compounds (VOCs) at the biosphere-atmosphere interface, encompassing innovative analytical methods, laboratory and field studies, and emission modelling approaches.

We invite contributions on plant and other biogenic VOC emissions sources (e.g., from soil, litter, and freshwater) under environmental changes and welcome contributions on methodological advances in sampling and analysis techniques, and modelling frameworks that bridge experimental observations with atmospheric processes.

Land–atmosphere interactions often play a decisive role in shaping climate extremes. As climate change continues to exacerbate the occurrence of extreme events, a key challenge is to unravel how land states regulate the occurrence of droughts, heatwaves, intense precipitation and other extreme events. This session focuses on how natural and managed land surface conditions (e.g., soil moisture, soil temperature, vegetation state, surface albedo, snow or frozen soil) interact with other components of the climate system – via water, heat and carbon exchanges – and how these interactions affect the state and evolution of the atmospheric boundary layer. Moreover, emphasis is placed on the role of these interactions in alleviating or aggravating the occurrence and impacts of extreme events. We welcome studies using field measurements, remote sensing observations, theory and modelling to analyse this interplay under past, present and/or future climates and at scales ranging from local to global but with emphasis on larger scales.

This session invites contributions focused on the understanding, modeling, and prediction of extreme events in weather, climate, and broader geophysical systems, from both theoretical and applied perspectives. We aim to bring together researchers from the traditional geophysical sciences with those working in mathematical, statistical, and dynamical systems approaches, fostering an interdisciplinary dialogue and discussions.
By highlighting the complementary nature of physical intuition and mathematical formalism, this session seeks to advance our understanding of the processes that give rise to extremes, improve predictive capabilities, and assess the extremes' societal and environmental impacts.
Topics of interest include, but are not limited to:
- Variability and projected changes in extremes under climate change
- Representation and performance of climate models in simulating extreme events
- Attribution of extreme events
- Emergent constraints on extreme behavior
- Predictability of extremes across meteorological to climate timescales
- Connections between extremes in dynamical systems and observed geophysical extremes
- Theoretical and applied studies of extremes in nonlinear and chaotic systems
- Downscaling techniques for extreme events
- Linking the physical dynamics of extreme events to their impacts on society and ecosystems.
We particularly encourage submissions that bridge disciplines, propose novel methodologies, or offer new insights into the mechanisms and consequences of extreme geophysical phenomena. We encourage submissions from the "Transdiscipinary Newtork to bridge Climate Science and Impacts on Society" (FutureMED) and the "Seasonal-to-decadal climate predictability in the Mediterranean: process understanding and services" (MEDUSSE) COST action communities.

This session invites contributions on the latest developments and results in lidar remote sensing of the atmosphere, covering • new lidar techniques as well as applications of lidar data for model verification and assimilation, • ground-based, airborne, and space-borne lidar systems, • unique research systems as well as networks of instruments, • lidar observations of aerosols and clouds, thermodynamic parameters and wind, and trace-gases. Atmospheric lidar technologies have shown significant progress in recent years. While, some years ago, there were only a few research systems, mostly quite complex and difficult to operate on a longer-term basis because a team of experts was continuously required for their operation, advancements in laser transmitter and receiver technologies have resulted in much more rugged systems nowadays, many of which are already operated routinely in networks and several even being fully automated and commercially available. Consequently, also more and more data sets with very high resolution in range and time are becoming available for atmospheric science, which makes it attractive to consider lidar data not only for case studies but also for extended model comparison statistics and data assimilation. Here, ceilometers provide not only information on the cloud bottom height but also profiles of aerosol and cloud backscatter signals. Scanning Doppler lidars extend the data to horizontal and vertical wind profiles. Raman lidars and high-spectral resolution lidars provide more details than ceilometers and measure particle extinction and backscatter coefficients at multiple wavelengths. Other Raman lidars measure water vapor mixing ratio and temperature profiles. Differential absorption lidars give profiles of absolute humidity or other trace gases (like ozone, NOx, SO2, CO2, methane etc.). Depolarization lidars provide information on the shapes of aerosol and cloud particles. In addition to instruments on the ground, lidars are operated from airborne platforms in different altitudes. Even the first space-borne missions are now in orbit while more are currently in preparation. All these aspects of lidar remote sensing in the atmosphere will be part of this session.

In the past years, the analysis of compound events has emerged as an essential step to enhance our knowledge of and response to multi-hazard high-impact events that occur simultaneously or sequentially, causing interconnected or aggravated impacts. Compound events involve two (or more) events happening together. These can be independent events (in which the outcome of one event has no effect on the probability of the other), or dependent events (when the outcome of one event affects the probability of another). Compound weather, climate or hydrological events refer to combinations of multiple drivers or hazards that may lead to large impacts and disasters. These events can be related to extreme conditions (e.g. storms, heatwaves, floods and droughts), or to combinations of events that are not themselves extremes but lead to an extreme event or significant impact when combined.
In this Short Course, we will introduce compound events, their types (preconditioned, multivariate, temporally compounding, and spatially compounding events), and the methods used to detect and characterize them. We will highlight the advantages and limitations of statistical methods (regression, multivariate statistics, and classification), empirical approaches based on large datasets, high-dimension approaches such as copulas, and complex network-based techniques that help to identify non-trivial spatio-temporal patterns of extreme events.
The Short Course will focus on sharing experience from a wide range of applications worldwide, state-of-the-art methodological approaches, open access code and datasets, and will allow participants to discuss their own challenges in detecting, characterizing and assessing the risk of compound events in diverse contexts (climate, atmospheric, hydrologic, ocean and natural hazards sciences).

This session welcomes contributions on atmospheric convection, including dry, shallow, or deep convection. A particular session focus is the organization of convection, such as mesoscale convective systems, convectively-coupled waves, idealized studies of self-aggregation, or research on the importance of organization for climate sensitivity. Additionally, submissions that address other aspects of convection like the convective lifecycle and structures including cold pools, interactions of convection with other physical processes or the representation of convection in numerical weather prediction and climate models are strongly encouraged. The research can use any tool, from idealized theoretical models, large-eddy simulations, convection-permitting simulations, to coarser-resolution simulations using parameterised convection, machine learning techniques, or observations and field campaigns.

Anthropogenic and natural aerosols play key roles in driving climate change over a range of spatial and temporal scales, both close to emission sources and also remotely through teleconnections. Aerosols can directly interact with radiation by scattering and absorption and indirectly through modulating cloud properties, and thereby modify the surface and atmospheric energy balance, cloud dynamics and precipitation patterns, and the atmospheric and oceanic circulation. Changes in regional aerosol emissions accelerate greenhouse gas-driven climate changes in some regions, counteract them in others, and may interact with natural variability to further stress human and ecological systems. However, our understanding of these impacts still lags those due to greenhouse gases. The poor aerosol integration in many climate risk and impact studies currently leads to potentially dangerous omissions in projections of near-term climate change impacts.

This session addresses: the strong and spatially complex trends in temperature, hydroclimate, air quality, and extreme events driven by aerosol changes over the historical era, and those expected in the near future; the interplay between aerosol-driven changes and those induced by other forcing factors; and their extensions to climate risk and impact studies. We encourage contributions based on model and observation-based approaches to investigate the effects of aerosols on regional decadal climate variability and extremes, tropical-extratropical interactions and teleconnections, and the interactions with modes of variability such as the NAO, ENSO, AMV, and PDO. This year we especially welcome studies focusing on the climate effects of African air pollution, notably how absorbing aerosols influence Sub-Saharan precipitation, and any analyses using the RAMIP dataset. We also welcome focused studies on aerosol influences on monsoon systems, midlatitude and Arctic responses, extreme temperature and precipitation, atmospheric and oceanic circulation changes, tropical cyclones, and daily variability, using for example CMIP6 projections, large ensemble simulations, or specifically designed experiments. We also encourage studies focusing on climate risk and concrete regional impacts on nature and society resulting from changes in anthropogenic and natural aerosol emissions.

We welcome contributions involving the use of stable isotopes of light elements (C, H, O, N, S) or novel tracers (such as COS) in field and laboratory experiments, the latest instrument developments, as well as theoretical and modelling activities, which advance our understanding of biogeochemical and atmospheric processes. We are particularly interested in the latest findings and insights from research involving:

- Isotopologues of carbon dioxide (CO2), water (H2O), methane (CH4), carbon monoxide (CO), oxygen (O2), carbonyl sulfide (COS), and nitrous oxide (N2O)
- Novel tracers and biological analogues
- Polyisotopocules including "clumped isotopes"
- Non-mass-dependent isotopic fractionation and related isotope anomalies
- Intramolecular stable isotope distributions ("isotopomer abundances")
- Quantification of isotope effects
- Analytical, methodological, and modelling developments
- Flux measurements

The Earth's middle atmosphere, mesosphere, and lower thermosphere (MLT) region provide a great platform for studying ionospheric dynamics, disturbances, eddy mixing, atmospheric drag effects, and space debris tracking. The thermal structure of these regions is influenced by numerous energy sources such as solar radiation, chemical, and dynamical processes, as well as forces from both above (e.g., solar and magnetospheric inputs) and below (e.g., gravity waves and atmospheric tides). Solar atmospheric tides, related to global-scale variations of temperature, density, pressure, and wind waves, are responsible for coupling the lower and upper layers of the atmosphere and significantly impact their vertical profiles in the upper atmosphere. With evidence of climate change impacts on the middle and upper atmosphere, monitoring and understanding trends through observational data is critical. There has been a contraction of the stratosphere and a decrease in the density of the upper atmosphere, which could impact the accumulation of space debris. This session invites presentations on scientific work related to various experimental/observational techniques, numerical and empirical modeling, and theoretical analyses on the dynamics, chemistry, and coupling processes in the altitude range of ~ 20 km to 180 km of the middle atmosphere and MLT regions, including long-term climatic changes.

Regional monsoons have profound impacts on water, energy, and food security. Monsoons cause severe floods and droughts as well as undergoing variability on subseasonal, seasonal-to-decadal and palaeoclimate time scales. In addition to their profound local effects, monsoon variability also causes global-scale impacts via teleconnections, and the monsoons are linked together as part of the global monsoon via the divergent circulation, with aspects of coherent variability and interactions with planetary scale transports of heat and moisture.

Monsoons are complex phenomena involving coupled atmosphere-ocean-land interactions and remain notoriously difficult to forecast at NWP, subseasonal and seasonal scales, casting doubt also on our future climate projections. A better understanding of monsoon physics and dynamics and their response to forcing, with more accurate simulation, prediction and projection of monsoon systems is therefore of great importance.

This session invites presentations on any aspects of monsoon research in present-day, future and palaeoclimate periods, involving observations, modelling, attribution, prediction and climate projection. Topics ranging from theoretical works based on idealized planets and ITCZ frameworks to the latest field campaign results are equally welcomed, as is work on impacts, extremes and compound weather events, NWP modelling, S2S and decadal forecasting, and the latest CMIP findings to help inform the IPCC AR7.

Cloud feedbacks remain the dominant source of uncertainty in estimates of global and regional climate sensitivity. Advancing our understanding of the key processes governing cloud formation, evolution, and radiative effects is therefore essential for improving their representation in climate models and reducing uncertainties in future climate projections.
Cloud condensation nuclei (CCN), ice-nucleating particles (INPs), and secondary ice production (SIP) are central to cloud microphysical processes and radiative feedbacks, with far-reaching influences on weather and climate dynamics. This session focuses on the interactions between aerosols, CCN, INPs, and SIP, and their impacts on cloud properties, based on both laboratory and field studies.
Particular attention will be given to the Southern Ocean (SO), one of the cloudiest regions on Earth. Its pristine aerosol environment offers a natural laboratory for disentangling fundamental aerosol- cloud -radiation interactions in the relative absence of anthropogenic pollution, thereby providing critical insights into cloud microphysical processes.
We invite contributions that address pressing open questions on the coupling between gas-phase chemistry, aerosol nucleation and growth, cloud development, precipitation, and radiative impacts, with an emphasis on the Southern Hemisphere. Special focus will also be placed on advancing our understanding of SIP mechanisms, their influence on cloud evolution, and their representation in weather and climate models.
Topics of interest include:
• Laboratory studies on INPs and secondary ice production
• Aerosol, CCN, and INP sources and characteristics from field measurements (e.g., in-situ flight campaigns)
• Modeling of secondary ice production processes
• Advances in parameterizations of cloud formation and development in models (e.g., deep convective clouds, mixed-phase clouds, mesoscale convective systems)

Solicited Speaker: Prof. Dr. Mira Pöhlker, Leibniz Institute for Tropospheric Research (TROPOS), Germany.
Solicited Presentation: " The interplay of Clouds, Aerosols and Radiation above the Southern Ocean".

This session is organized for the second time at EGU General Assemblies and reflects the recent emerging trends of atmospheric monitoring: (i) realising multi-instrument synergy retrieval and (ii) bridging three branches of atmospheric research (modelling, in situ measurements, and remote sensing).

This session encourages discussions that explore the synergies of complimentary observations, such as synergies of passive imagery with active vertical profiling of the atmosphere, synergies of observations in different spectral ranges and at different time and/or spatial scales, and synergies of satellite observations with sub-orbital observations and chemical transport model simulations. Synergy is important because the quality of measurements cannot be radically improved once the instruments have been deployed, but algorithms can continuously evolve and notably improve results with data fusion and optimization of the joint sensitivity of multi-instrument datasets. The the session especially welcomes the ides and demonstrations of synergy methods and Interdisciplinary applications using novel observations from the , EPS-SG, MTG, PACE, Copernicus Sentinels, EarthCARE and other recent and forthcoming advanced satellite missions (e.g., CO2M) as well as field campaigns.

The session also invites presentations that demonstrate the benefits of collaboration amongst the three core fields of atmospheric aerosol studies outlined in the Models, In situ, and Remote sensing of Aerosols (MIRA) international working group, which was formed to facilitate collaborations and improve discussions amongst these fields of study and across regional boundaries. More information can be found at https://science.larc.nasa.gov/mira-wg/.

Methane is an important greenhouse gas that has contributed to ∼25% of the increase in radiative forcing experienced to date. Despite methane’s short atmospheric lifetime (~10 years), the global average methane mole fraction has increased three times faster than carbon dioxide since 1750. Rapid and severe reductions in methane emissions are required to lower peak warming, reduce the likelihood of overshooting warming limits and reduce reliance on net negative carbon dioxide emissions. In order to track mitigation efforts and ensure emission quantification required in legislation can be met, we must be able to accurately attribute and quantify emissions and are actively doing so through activities such as the UNEP International Methane Emissions Observatory (IMEO).

This session will highlight measurement studies at all scales and from ground-based to satellites, that focus on quantification and source attribution of methane emissions from human activities. We especially encourage submissions from both IMEO and non-IMEO funded work that focus on the following topics: (1) new technologies / methods to provide accurate and repeatable emissions measurements, (2) demonstration of affordable and reliable quantification methods for mitigation tracking, (3) attribution of emissions to specific sources and, (4) methods for upscaling measurements into inventories and creating policy relevant datasets.

The session focuses on the latest developments in spectroscopic and optical instrumentation and technologies from the UV to THz spectral regions and their use in atmospheric applications. These applications include observation of spatial and temporal changes in the concentrations and optical properties of atmospheric constituents, as well as the study of atmospheric processes in laboratories, atmospheric simulation chambers, and field deployments.

As the scattering and absorption of radiation by atmospheric particles are central to Earth’s radiation balance, this session facilitates an exchange of insights between the measurement and modeling communities. Accurately quantifying these radiative effects remains a challenge. Specifically, uncertainties in morphology and the computational cost of modeling complex particles necessitate novel and efficient approaches.

The session aims to be a forum for sharing information on the state-of-the-art and emerging developments in atmospheric sensing measurements and modeling. We welcome contributions from atmospheric scientists, engineers, and industry. Topics include developments and applications of novel spectroscopic methods (such as frequency comb, cavity-enhanced, and photoacoustic spectroscopies) dedicated to measuring aerosols, isotopologues, and trace gases, as well as advances in the measurement and modeling of atmospheric particle optics. Approaches using ground and airborne platforms, simulation chambers, and new data analysis tools (including deep learning) are also encouraged.

Lightning is the energetic manifestation of electrical breakdown in the atmosphere, occurring as a result of charge separation processes operating on micro and macro-scales, leading to strong electric fields within thunderstorms. Lightning is associated with tropical storms and severe weather, torrential rains and flash floods. Lightning is also responsible for a vast number of wildfires, burned area, and fire emissions to the atmosphere. It has significant effects on various atmospheric layers and drives the fair-weather electric field. It is a strong indicator of convective processes on regional and global scales, potentially associated with climate change. Lightning produces nitrogen oxides, which are a precursor to ozone production. Thunderstorms and lightning are essential parts of the Global Electrical Circuit (GEC) and control the fair weather electric field. They are also associated with the production of energetic radiation up to tens of MeV on time scales from sub-millisecond (Terrestrial Gamma-ray Flashes) to tens of seconds (gamma-ray glows).

This session seeks contributions from research in atmospheric electricity with emphasis on:

Atmospheric electricity in fair weather and the global electrical circuit
Effects of dust and volcanic ash on atmospheric electricity
Thunderstorm dynamics and microphysics
Middle atmospheric Transient Luminous Events
Energetic radiation from thunderstorms and lightning
Experimental investigations of lightning discharge physics processes
Remote sensing of lightning and related phenomena by space-based sensors
Thunderstorms, flash floods, tropical storms and severe weather
Lightning-ignited wildfires and ecological effects of lightning
Connections between lightning, climate and atmospheric chemistry
Modeling of thunderstorms and lightning
Now-casting and forecasting of thunderstorms using machine learning and AI
Regional and global lightning detection networks
Lightning Safety and its societal effects
Planetary lightning in the solar system and beyond

The large-scale atmospheric circulation is an essential component of the climate system. Understanding the drivers, variability and the dynamical processes of this circulation is important for improving global and regional climate projections under anthropogenic climate change, and for predicting the associated impacts on extreme weather and climate events. This session encourages theoretical, modelling and observational research on the large-scale atmospheric circulation, including (but not limited to) the following topics:
-Response of the large-scale atmospheric circulation to climate change, including shifts and changes in intensity of the jet stream, Hadley and Walker cells, intertropical convergence zone, and monsoons;
-Changes in storm track intensity and structure in response to climate change and/or internal variability;
-Representation of the large-scale atmospheric circulation in climate models: inter-model variability, model biases, and methodologies for reducing uncertainty in model projections;
-Novel metrics and analysis methods for studying the large-scale atmospheric circulation;
-Interactions between the different components of the large-scale circulation, including tropical-extratropical interactions and teleconnection patterns;
-Role of moisture in the large-scale atmospheric circulation;
-Energy transport by the large-scale atmospheric circulation;
-Stratospheric-tropospheric interactions affecting the large-scale circulation.

This session explores forecasting in geosciences using statistical methods. Ranging from linear regression to the most advanced machine learning (ML) or artificial intelligence (AI) algorithms, the session welcomes all contributions developing and/or using these tools for various applications such as AI-based numerical weather prediction and nowcasting, time series forecasting in geosciences, forecast blending, statistical post-processing, and downscaling.
This session also welcomes contributions advancing the assessment of AI-based forecasts. Aiming at a proper and in-depth assessment of the strengths and weaknesses of AI-based models, the session will report on benchmarking activities, new verification methodologies, diagnostics of forecast realism, and progress in the interpretability of AI-based models.

This session is designed to foster interdisciplinary discussions among geoscientists from meteorology, climate, hydrology, and other related communities, promoting the use of statistical methods in forecasting, verification, and beyond.

In recent years, machine learning (ML) and artificial intelligence (AI) have emerged as powerful weather forecasting tools, including for weather and climate extremes and related events. Data-driven algorithms applied across different temporal and spatial scales have shown great promise in predicting phenomena such as hurricanes, floods, heatwaves, and droughts, while also improving the accuracy and timeliness of climate projections.

This session seeks contributions exploring the development and application of ML or ML-enhanced algorithms for forecasting weather and climate at multiple spatial and temporal scales and for detecting and anticipating extreme weather and climate events. We welcome studies that address the use of AI for short-and medium-range meteorological forecasts, extended-range forecasts, sub-seasonal to seasonal climate forecasts, or longer-term climate projections, spanning local to global spatial scales.

We particularly encourage submissions that connect extremes to their societal and environmental impacts, such as impacts on infrastructure, ecosystems, health, or energy systems.Contributions that integrate ML with physical mechanisms to advance the representation of climate variables in numerical models or climate datasets are also highly encouraged.

By bringing together experts from AI, data science, meteorology, climate science, and impact modelling, this session aims to foster interdisciplinary collaborations that push the boundaries of forecasting and understanding extreme weather and climate events, as well as their impacts. We encourage submissions from early-career scientists, established researchers, and industry professionals alike.

Data assimilation (DA) is widely used in the study of the atmosphere, the ocean, the land surface, hydrological processes, etc. The powerful technique combines prior information from numerical model simulations with observations to provide a better estimate of the state of the system than either the data or the model alone. This short course will introduce participants to the basics of data assimilation, including the theory and its applications to various disciplines of geoscience. An interactive hands-on example of building a data assimilation system based on a simple numerical model will be given. This will prepare participants to build a data assimilation system for their own numerical models at a later stage after the course.
In summary, the short course introduces the following topics:

(1) DA theory, including basic concepts and selected methodologies.
(2) Examples of DA applications in various geoscience fields.
(3) Hands-on exercise in applying data assimilation to an example numerical model using open-source software.

This short course is aimed at people who are interested in data assimilation but do not necessarily have experience in data assimilation, in particular early career scientists (BSc, MSc, PhD students and postdocs) and people who are new to data assimilation.