In weather prediction and climate modelling, numerical models of the Earth System are used extensively. For both the atmosphere and ocean components such models consist of a fluid dynamics solver (dynamical core) coupled to physical parameterizations to represent processes that occur below the grid scale (physics). Over time these models have become capable of sophisticated simulations. Research and development is constantly being undertaken to improve the accuracy and efficiency of the dynamical core, the physics, and their coupling.

This session encompasses the development, testing and application of novel numerical techniques for Earth system models, including governing equations, horizontal and vertical discretizations, structure preserving methods, time stepping schemes, advection schemes, adaptive multi-scale models, physics-dynamics coupling, regional and global models, classical and stochastic physical parameterizations (that are not covered in other sessions).

The session welcomes papers on:

1) Forecasting and simulating high impact weather events - research on improvement of high-resolution numerical model prediction of severe weather events (such as winter storms, tropical storms, and severe mesoscale convective storms) using data from various observational platforms, evaluation of the impact of new remote sensing data;

2) Development and improvement of model numerics - basic research on advanced numerical techniques for weather and climate models (such as cloud resolving global model and high-resolution regional models specialized for extreme weather events on sub-synoptic scales);

3) Development and improvement of model physics - progress in research on advanced model physics parametrization schemes (such as stochastic physics, air-wave-oceans coupling physics, turbulent diffusion and interaction with the surface, sub-grid condensation and convection, grid-resolved cloud and precipitation, land-surface parametrization, and radiation);

4) Model evaluation - verification of model components and operational NWP products against theories and observations, regional and global re-analysis of past observations, diagnosis of data assimilation systems;

5) Data assimilation systems - progress in the development of data assimilation systems for operational applications (such as reanalysis and climate services), research on advanced methods for data assimilation on various scales (such as treatment of model and observation errors in data assimilation, and observational network design and experiments);

6) Ensemble forecasts and predictability - strategies in ensemble construction, model resolution and forecast range-related issues, and applications to data assimilation;

7) Advances and challenges in high-resolution simulations and forecasting.

Forecasting the weather, in particular severe and extreme weather has always been the most important subject in meteorology. This session will focus on recent research and developments on forecasting techniques, in particular those designed for operations and impact oriented. Contributions related to nowcasting, meso-scale and convection permitting modelling, ensemble prediction techniques, and statistical post-processing are very welcome.
Topics may include:
 Nowcasting methods and systems, use of observations and weather analysis
 Mesoscale and convection permitting modelling
 Ensemble prediction techniques
 Ensemble-based products for severe/extreme weather forecasting
 Seamless deterministic and probabilistic forecast prediction
 Post-processing techniques, statistical methods in prediction
 Use of machine learning, data mining and other advanced analytical techniques
 Impact oriented weather forecasting
 Presentation of results from relevant international research projects of EU, WMO, and EUMETNET etc.

This session invites contributions that span all aspects of prediction and predictability in the 2 weeks to 2 months lead time range. The session welcomes contributions on physical processes, impacts, and climate services. In particular, we encourage studies of phenomena such as the Madden Julian Oscillation (MJO), tropical/extratropical waves, teleconnections, stratosphere - troposphere coupling, land - atmosphere coupling, ocean - atmosphere coupling, in addition to studies of predictability and skill of atmospheric or surface variables such as sea ice, snow cover, and land surface, and case studies of extreme or high impact weather events. Contributions regarding impact studies, applications, and climate services at the S2S time-scale are also highly welcome, including, but not limited to, the areas of hydrology, health, fire, agriculture, and energy. These can include modeling studies of the impacts and presentations of how S2S-derived information can be integrated into decision support systems at the local, regional and country level.

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.

This session investigates mid-latitude cyclones and storms on both hemispheres. We invite studies considering cyclones in different stages of their life cycles from the initial development, to large- and synoptic-scale conditions influencing their growth to a severe storm, up to their dissipation and related socioeconomic impacts.
Papers are welcome, which focus also on the diagnostic of observed past and recent trends, as well as on future storm development under changed climate conditions. This will include storm predictability studies on different scales. Finally, the session will also invite studies investigating impacts related to storms: Papers are welcome dealing with vulnerability, diagnostics of sensitive social and infrastructural categories and affected areas of risk for property damages. Which risk transfer mechanisms are currently used, depending on insured and economic losses? Which mechanisms (e.g. new reinsurance products) are already implemented or will be developed in order to adapt to future loss expectations?

The large-scale atmospheric circulation strongly influences Earth's climate, both locally and globally, via its transport of energy, moisture, and momentum. While our ability to simulate the global circulation is improving, large model biases and uncertainties in climate change projections persist. Our theoretical understanding of how atmospheric circulations respond to climate changes is also limited, particularly on regional scales and in the presence of zonal asymmetries. Advancing our knowledge of the underlying dynamics is therefore crucial for reliable climate projections and for correctly interpreting palaeoclimate records.

The objective of this session is to advance our mechanistic understanding of atmospheric circulation changes and to analyse their impacts at global and regional scales, specifically on precipitation in past, present, and future climates. We encourage theoretical, observational and modelling contributions on tropical (ITCZ, monsoons, Hadley & Walker circulations, MJO) and extratropical circulations (jet streams, storm tracks, blocking).

The variability in the stratosphere is important for many atmospheric phenomena. Examples include the dynamical two-way coupling between the stratosphere and troposphere during sudden stratospheric warming events, the transport of trace gases through the meridional circulation of the stratosphere, or the connection between the Quasi-Biennial Oscillation of the tropical stratosphere and the Madden-Julian Oscillation. This session is interested in all aspects of stratospheric circulation variability, including the mechanisms behind the vertical coupling between the stratosphere and troposphere in tropics and extratropics, the importance of stratospheric dynamics for explaining both short-term atmospheric weather and long-term climate variability, and the role of the stratospheric circulation for the chemical composition of the atmosphere. We welcome abstracts that study this problem from an observational, modelling, or theoretical viewpoint on all temporal and spatial scales.

Recent extreme weather and climate episodes, including the Siberian heatwave of early summer 2020 and the cold-air outbreak over the US in late summer 2020, highlight the need to further our understanding of linear and non-linear (quasi-stationary) planetary and synoptic-scale Rossby wave dynamics in the atmosphere, and their impacts on weather and climate events. Abstracts are solicited that are dedicated to:

i) the dynamics of linear wave propagation or quasi-stationarity, of wave breaking, atmospheric blocking, or jets as atmospheric Rossby waveguides. This includes the role of local and remote drivers (e.g., the tropics, Arctic, or stratosphere).

ii) exploring the links between extreme weather/climate events and linear and non-linear Rossby waves, including wave breaking and/or blocking.

iii) quantifying model representation of Rossby waves in climate and numerical weather prediction models, including wave propagation and breaking.

iv) exploring the role of Rossby wave trains on predictability at lead times from medium range (~2 weeks) to seasonal time-scales. This includes blocking and wave propagation.

v) analyzing projected future changes in planetary or synoptic-scale Rossby waves, or in their future impacts on weather and climate events.

The global monsoon system and its regional monsoon components have profound impacts on society and are among the most complex phenomena involving coupled atmosphere-ocean-land interactions. Monsoons can cause severe floods and droughts in the tropics as well as undergoing climate variability on subseasonal, interannual and decadal or longer time scales. In addition to its profound local effects, monsoon variability is also associated with global-scale impacts since the energy released by monsoon systems can influence the global circulation. However, it is notoriously difficult to simulate and forecast the monsoons on temporal scales from numerical weather prediction (NWP), subseasonal-to-seasonal and interannual-to-decadal predictions, and longer timescale climate projections. A better understanding of monsoon physics and dynamics, with more accurate simulation, prediction and projection of monsoon systems is therefore of a great importance to society.

The combination of modern- and palaeo-monsoon research can help us better understand the fundamental nature of the monsoon and its variability. Comparisons of monsoon responses to large-scale forcings found in the palaeoclimate record can help us to understand how the monsoon will respond to changes in forcings in the future, potentially allowing us to constrain estimates of climate change. Similarly, the wealth of observations, reanalysis products and modelling work in the contemporary period can help us piece together data from point-proxy records of the past.

This session therefore invites presentations on all aspects of monsoon research in contemporary, future and palaeoclimate periods (observational, modeling, attribution, prediction and projection) from the natural and anthropogenic variability and predictability of the monsoon systems on multiple time scales, to the impact of monsoons on extreme weather and climate events (floods, droughts, tropical cyclones, heat waves, etc.), as well as the links between monsoons and global climate change and feedbacks with the biosphere. Theoretical works based on idealized planetary and ITCZ frameworks are also invited.

As a weather pattern, the Asian monsoon impacts the lives of more than a billion people. With rapid population and economic growth across the monsoon region, it becomes a pressing concern that the convection coupled to surface emissions is playing a significant role in the region’s air quality. The uplift of pollutants also enhances aerosol–cloud interactions that may change the behaviour of the monsoon. The monsoon system is therefore relevant to scales and processes bridging regional air quality, climate change, and global
chemistry-climate interaction and the chemical transport effect of the monsoon system is seen from satellites as an effective transport path for pollutants to enter the stratosphere.
The session will focus on the dynamical, micro-physical, and chemical processes dominating transport, chemical transformations and particle and cloud formation throughout the Upper-Troposphere Lower-Stratosphere above the Asian Monsoon system as well their internal and ozone and climate couplings. Here we especially encourage experimental and modelling (process to global) studies from recent programmes (such as StratoClim, BATAL, OMO). Contributions addressing tropospheric processes within the Atmospheric Composition and Asian Monsoon (ACAM) Programme objectives, such as the coupling to local emission on air quality and aerosols, clouds, interactions with the Asian monsoon are also welcome.

Large-scale atmospheric circulation dynamics are the major driver of near surface climatic and environmental variability. Synoptic climatology examines atmospheric circulation dynamics and their relationship with near surface environmental variables. Within synoptic climatological analyses, a wide variety of methods is utilized to characterize atmospheric circulation (e.g., circulation and weather type classification, regime analysis, teleconnection indices). Various linear and non-linear approaches (e.g., multiple regression, canonical correlation, neural networks) are applied to relate the circulation dynamics to diverse climatic and environmental elements (e.g., air temperature, air pollution, floods).

The session welcomes contributions from the whole field of synoptic climatology. This includes application studies for varying regions, time periods (past, present, future) and target variables and in particular contributions on the development and the comparison of methods (e.g., varying circulation type classifications) and conceptual approaches (e.g., circulation types versus circulation regimes).

The infrasound field, the science of low-frequency acoustic waves, has developed into a broad interdisciplinary field encompassing academic disciplines of geophysics and recent technical and scientific developments. The infrasound network of the International Monitoring Network (IMS) for nuclear test ban verification and regional cluster arrays deployed around the globe have demonstrated their capacity for detecting and locating most of the disturbances in the atmosphere. Infrasound is capable of traveling up to thermospheric altitudes and over enormous ranges, where its propagation is controlled by the wind and temperature structure. Recent studies point out new insights on quantitative relationships between observables and atmospheric characteristics, and therefore opening a new field for atmospheric remote sensing.

New studies using lidar, radar, microwave spectrometer and mesospheric airglow observations complemented by satellite measurements help to better determine the interaction between atmospheric layers from the ground to the mesosphere and the influence of atmospheric waves on the mean flow. It is expected that further developing multi-instruments platforms would improve gravity wave parameterizations and enlarge the science community interested by operational infrasound monitoring. The ARISE project, funded by the European Commission, coordinates such studies. It proposes to design a novel infrastructure that integrates different atmospheric observation networks to infer a new 3D image of the atmosphere from the ground to mesosphere. In a higher frequency range, this monitoring system also offers a unique opportunity to provide in near-real time continuous relevant information about natural hazards with high societal benefits, like large volcanic eruptions, surface earthquakes or meteorites.

We invite contributions on current studies on sensors, characterization of different sources and large scale atmospheric phenomena, characterization of phenomena which affect acoustic propagation, utilization of acoustic waves to probe the atmosphere, contribution of gravity and planetary waves to the atmospheric dynamics and the coupling of atmospheric layers. In the session, we will also consider the role that infrasound and acoustic-gravity waves play in the coupled Earth’s crust – ocean – atmosphere system and, in particular, in ionospheric manifestations of physical processes in the ocean and in the solid Earth.

Internal gravity waves (IGWs) still pose major questions both to the atmospheric and ocean sciences, and to stellar physics. Important issues are IGW radiation from their various relevant sources, IGW reflection at boundaries, their propagation through and interaction with a larger-scale flow, wave-induced mean flow, wave-wave interactions in general, wave breaking and its implications for mixing, and the parameterization of these processes in models not explicitly resolving IGWs. The observational record, both on a global scale and with respect to local small-scale processes, is not yet sufficiently able to yield appropriate constraints. The session is intended to bring together experts from all fields of geophysical and astrophysical fluid dynamics working on related problems. Presentations on theoretical, modelling, experimental, and observational work with regard to all aspects of IGWs are most welcome, including those on major collaborative projects, such as MS-GWaves.

ENSO and its interactions with other tropical basins are the dominant source of interannual climate variability in the tropics and across the globe. Understanding the dynamics, predictability, and impacts of ENSO and tropical basins interactions, and anticipating their future changes are thus of vital importance for society. This session invites contributions regarding all aspects of ENSO and tropical basins interactions, including: dynamics, multi-scale interactions; low frequency, decadal and paleo variability; theoretical approaches; ENSO diversity; global teleconnections; impacts on climate, society and ecosystems; seasonal forecasting and climate change projections of ENSO and its tropical basins interactions. Studies aimed at understanding ENSO and its tropical basins interactions in models of a range of complexity are especially welcomed, including analysis of CMIP model intercomparisons.

The Arctic sea ice and high latitude atmosphere and oceans have experienced significant changes over the modern observational era. The polar climate is crucial for the Earth’s energy and water budget, and its variability and change have direct socio-economic and ecological impacts. Thus, understanding high-latitude variability and improving predictions of high latitude climate is highly important for society. Long-term variability in ocean and sea ice are the largest sources for predictability in high latitudes. Dynamical model predictions are not yet in the position to provide us with highly accurate predictions of the polar climate. Main reasons for this are the lack of observations in high latitudes, insufficient initialization methods and shortcomings of climate models in representing some of the important climate processes in high latitudes.

This session aims for a better understanding and better representation of the mechanisms that control high latitude variability and predictability of climate in both hemispheres from sub-seasonal to multi-decadal time-scales in past, recent and future climates. Further, the session aims to discuss ongoing efforts to improve climate predictions at high latitudes at various time scales (as e.g. usage of additional observations for initialization, improved initialization methods, impact of higher resolution, improved parameterizations, novel verification approaches) and potential teleconnections of high latitude climate with lower latitude climate. We also aim to link polar climate variability and predictions to potential ecological and socio-economic impacts and encourage submissions on this topic.

The session offers the possibility to present results from ongoing projects and research efforts on the topic of high-latitude climate variability and prediction, including, but not limited to, the WMO Year of Polar Prediction (YOPP), NordForsk-project ARCPATH, MOSAiC, and the H2020-projects APPLICATE, INTAROS, BlueAction, and KEPLER.

The North Atlantic exhibits a high level of natural variability from interannual to centennial time scales, making it difficult to extract trends from observational time series. Climate models, however, predict major changes in this region, which in turn will influence sea level and climate, especially in western Europe and North America. In the last years, several projects have been focused on the Atlantic circulation changes, for instance OVIDE, RACE, OSNAP, and ACSIS. Another important issue is the interaction between the atmosphere and the ocean as well as the cryosphere with the ocean, and how this affects the climate.

Please note that while we hope to hold a session in the traditional format, we anticipate that some part or all of the session may be held online.

We welcome contributions from observers and modelers on the following topics:

-- climate relevant processes in the North Atlantic region in the atmosphere, ocean, and cryosphere
-- response of the atmosphere to changes in the North Atlantic
-- atmosphere - ocean coupling in the North Atlantic realm on time scales from years to centuries (observations, theory and coupled GCMs)
-- interpretation of observed variability in the atmosphere and the ocean in the North Atlantic sector
-- Comparison of observed and simulated climate variability in the North Atlantic sector and Europe
-- Dynamics of the Atlantic meridional overturning circulation
-- variability in the ocean and the atmosphere in the North Atlantic sector on a broad range of time scales
-- changes in adjacent seas related to changes in the North Atlantic
-- role of water mass transformation and circulation changes on anthropogenic carbon and other parameters
-- linkage between the observational records and proxies from the recent past

Clouds and aerosols play a key role in climate and weather-related processes over a wide range of spatial and temporal scales. An initial forcing due to changes in the aerosol concentration and composition may also be enhanced or dampened by feedback processes such as modified cloud dynamics, surface exchange or atmospheric circulation patterns. This session aims to link research activities in observations and modelling of radiative, dynamical and microphysical processes of clouds and aerosols and their interactions. Studies addressing several aspects of the aerosol-cloud-radiation-precipitation system are encouraged. Contributions related to the EU project “Constrained aerosol forcing for improved climate projections (FORCeS) are also invited.

Topics covered in this session include, but are not limited to:
- Cloud and aerosol macro- and microphysical properties, precipitation formation mechanisms
- The role of aerosols and clouds for the radiative energy budget
- Observational constraints on aerosol-cloud interactions
- High-resolution modelling, including large-eddy simulation and cloud-resolving models
- Parameterization of cloud and aerosol microphysics/dynamics/radiation processes
- Use of observational simulators to constrain aerosols, clouds and their radiative effects in models
- Experimental cloud and aerosol studies
- Aerosol, cloud and radiation interactions and feedbacks on the hydrological cycle and the climate system
- Interactions between aerosols and regional circulation systems and precipitation patterns
- Influence of aerosols on the distribution and intensity of climate extremes

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

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

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

(1) Airborne, spaceborne, ground- or laboratory-based measurements and their derived products (retrievals), which are useful to constrain ice 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 ice-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 ice 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.

While clouds are a crucial component of the global climate system, their representation in climate models is subject to considerable uncertainty. Predicting cloud phase, the amount of liquid and ice in the cloud, is critical for the correct representation of radiation processes and precipitation patterns. Yet, much of the uncertainty stems from the cloud ice phase, as ice formation processes are poorly understood and largely not described in the case of secondary ice production (SIP). In this session, we invite contributions from several fields (theoretical physics, laboratory experiments, field observations, remote sensing and multiscale modeling) that aim to: (i) understand secondary ice mechanisms, (ii) identify the conditions favorable for SIP, (iii) quantify SIP in relation to primary ice production, (iv) explore the impact of SIP on cloud life-cycle, radiation and/or precipitation, (v) investigate the interactions between SIP and atmospheric electricity, (vi) evaluate SIP representation in models and (vii) improve relevant parameterizations. Studies that link the importance of SIP within the context of deep convective systems, ground-cloud interactions (e.g. blowing snow), mountain, polar and marine systems are strongly encouraged.

This session deals with atmospheric convection, being dry, shallow, or deep convection. Contributions on these aspects resulting from the use of large-eddy simulations, convection-permitting simulations, coarser-resolution simulations using parameterised convection and observations are welcome. Studies that investigate the organization of convection, being in idealized set-ups (radiative convective equilibrium and self-aggregation) or in observations, as well as studies that investigate the importance of organization for climate are particularly welcome. Besides this, studies that investigate general aspects of convection such as processes controlling the lifecycle of convection, interactions of convection with other physical processes and representation of convection in numerical weather prediction and climate models are also welcome.

The uncertain response of clouds to global warming is a major contributor to the spread in climate sensitivity across climate models. Cloud feedback uncertainty is related to a limited understanding of the coupling between clouds, convection and the large-scale circulation across various spatial and temporal scales. This session focuses on efforts to advance our understanding of the cloud-circulation coupling and its role in climate change. Contributions from dedicated field campaigns such as EUREC4A, from various observing platforms like ground-based and satellite remote sensing or in situ measurements, as well as modelling and theoretical studies are welcomed. We also invite abstracts focusing on the role of convective organization and precipitation in modulating the cloud-circulation coupling and cloud feedbacks.

Mesoscale convective systems (MCSs) and meso-scale storms/disturbances are known to be important precipitation producing/triggering systems in high-mountain environments and on high-altitude plateaus. These meso-scale convective systems and disturbances can lead to severe weather locally and affect lower lying downstream regions.

The aim of this session is to gain an improved understanding of meso-scale systems and the associated processes leading to (extreme) precipitation in mountain regions and/or their downstream areas. We invite contributions on the dynamics of meso-scale storms/disturbances and meso-scale convective systems (including their formation and evolution) as well as smaller-scale convection in connection to atmospheric meso-scale features and how these factors explain spatio-temporal patterns of precipitation and precipitation dynamics. Contributions focussing on individual extreme events or giving climatological perspectives are welcome. Due to the nature of high-mountain environments it is difficult to directly observe their meso-scale atmospheric features and link these to the occurrence and spatio-temporal variability of precipitation. Therefore, contributions integrating remote sensing data, in-situ observations, and high-resolution models, especially those that explicitly resolve convections are particularly welcome.

This session is connected to the recently launched WRCP-CORDEX flagship pilot study “High resolution climate modelling with a focus on mesoscale convective systems and associated precipitation over the Third Pole region”.

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. 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. Thunderstorms and lightning 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 ASIM and GLM
Thunderstorms, flash floods, tropical storms and severe weather
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

Wind and solar power are the predominant new sources of electrical power in recent years. Portugal’s renewable production in March 2018 was 104% of its electricity demand, while Scotland reached 109% in November. By their very nature, wind and solar power, as well as hydro, tidal, wave and other renewable forms of generation are dependent on weather and climate. Modelling and measurement for resource assessment, site selection, long-term and short term variability analysis and forecasting for horizons ranging from minutes to decades are important topics.

The success of wind power means that wind turbines are increasingly put in sites with complex terrain or forests, with towers extending beyond the strict logarithmic profile, and in offshore regions that are difficult to model and measure. Major challenges for solar power are accurate measurements and the short-term prediction of the spatiotemporal evolution of the effects of cloud field and aerosols. Planning and meteorology challenges in Smart Cities are common for both.

For both solar and wind power, the integration of large amounts of renewable energy into the grid is another critical research problem due to the uncertainties in their forecast and to their spatio-temporal variabilities.

We invite contributions on all aspects of weather dependent renewable power generation, e.g.:

• Wind conditions (both resources and loads) on short and long time scales for wind power development, especially in complex environments (e.g. mountains, forests, coastal or urban).
• Long term analysis of inter-annual variability of solar and wind resource
• Typical Meteorological Year and probability of exceedance for wind and solar power development,
• Wind and solar resource and atlases.
• Wake effect models and measurements, especially for large wind farms and offshore.
• Performance and uncertainties of forecasts of renewable power at different time horizons and in different external conditions.
• Forecast of extreme wind events and wind ramps.
• Local, regional and global impacts of renewable energy power plants or of large-scale integration.
• Dedicated wind measurement techniques (SODARS, LIDARS, UAVs etc.).
• Dedicated solar measurement techniques (pyranometric sensors, sun-photometer, ceilometer, fish-eye cameras, etc.) from ground-based and space-borne remote sensing.
• Tools for urban area renewable energy supply strategic planning and control.
Other related topics will be considered by the conveners.

The AMANOM session will focus on observations and NWP model applications related to fog, clouds, contrails, precipitation, and short range forecasting of weather conditions associated with aviation operations. Abstracts for all areas of aviation meteorology, including Polar region, high altitude conditions, as well as airport environments, can be submitted to this session. Work on aviation meteorology parameters such as visibility, icing, gusts and turbulence, as well as fog and precipitation, will be considered for this session. Topics related to In-situ observations obtained from aircraft, Uncrewed Aerial Vehicles (UAVs), balloons, and supersites, remote sensing retrievals of meteorological parameters from satellites, radars, lidars, and MicroWave (MW)/InfraRed (IR) Radiometers, as well as other emerging technological platforms, and predictions of meteorological parameters from the numerical weather prediction models will be considered highly related to the goals of this session.

Extreme convective events are increasing in northern and eastern Europe in frequency and intensity accounting for major economic damages related to natural disasters in several countries. Forecasting the land convection locally developed in a short time range is very challenging since the models are not able to resolve them and this is an issue for the air traffic management.

In recent years, attention was paid to the detection and monitoring of volcanic clouds as their impact on the European air traffic control system was unprecedented (e.g. Eyjafjallajökull 2010 eruption). Volcanic clouds are very dangerous for the aviation operations as they can cause damage of the aircraft systems and engines not only close to active volcanoes but also at large distance from the eruption.

The Convective and Volcanic Clouds (CVC) detection and estimation of their physical parameters is a highly multidisciplinary and challenging topic since the same techniques and instruments can be used for meteorology, volcanic monitoring, atmospheric physics and climate purposes. There is an urgent need to develop new techniques and instruments for monitoring, detecting, forecasting and modeling the CVC, to develop early warning systems and to support end-users (such as air traffic managers and pilots) and policy makers. Furthermore, there is a need for improved information exchange regarding the impact of the CVC on daily aviation operations. In this regard, we will draw on the work of the Single European Sky ATM Research (SESAR) Joint Undertaking, with special focus on the latest funded exploratory research projects dealing with these topics.

The objective of the session is to connect different communities in touch with the CVC and to promote discussions between scientists working in remote sensing, modelers, meteorologists, physicists, sensors engineers, engines manufacturers, pilots and aviation managers, allowing the researchers to understand the end-users’ needs and allowing the end-users to understand the research capabilities.

This session solicits the latest studies from the spectrum of:
- detection, monitoring and modeling of CVC with novel techniques and new sensors,
- forecasting and nowcasting extreme weather events,
- study of the CVC structure,
- understanding the impact of the CVC on air traffic management,
- proposal of new products, tools or services focused on the end-users prospective.

With the launch of the novel Aeolus Doppler Wind Lidar mission in 2018, a wealth of direct wind and atmospheric backscatter profile observations have become available across the globe in the troposphere and lower stratosphere. This unique data set is revealing new information on atmospheric dynamics, clouds and aerosol optical properties to the benefit of various application areas. This includes Numerical Weather Prediction (NWP), climate modelling, Global Circulation Model dynamics and parameterized processes, e.g., stratosphere-troposphere interaction and atmospheric waves, land and ocean drag, convection, as well as air quality monitoring and air pollution transport. In the second year of the mission an important milestone was reached with the implementation and deployment of a very effective wind bias correction, much improving the wind product quality. This milestone initiated the public release of the Aeolus NRT (Near Real Time) wind products in May 2020, allowing all NWP centers to use Aeolus winds for their daily weather forecasts. It also marked the kick-off of scientific activities addressing the intended application areas and to develop further mission products.

In this session, we aim at discussing results using the Aeolus data for application in meteorology, atmospheric and Earth science process studies, air quality and climate studies, along with data product updates, their validation and reprocessing. Topics can cover Aeolus observation interpretation, atmospheric model diagnostics, model parameterizations, data assimilation experiments and many more. We welcome all presentations on the scientific analysis of Aeolus mission data, including inter-comparisons to independent measurements or atmospheric models, as well as on other data exploitation activities.

Traditionally, hydrologists focus on the partitioning of precipitation water on the surface, into evaporation and runoff, with these fluxes being the input to their hydrologic models. However, more than half of the evaporation globally comes back as precipitation on land, ignoring an important feedback of the water cycle if the previous focus applied. Land-use and water-use changes, as well as climate variability and change alter, not only, the partitioning of water but also the atmospheric input of water as precipitation, related with this feedback, at both remote and local scales.

This session aims to:
i. investigate the remote and local atmospheric feedbacks from human interventions such as greenhouse gasses, irrigation, deforestation, and reservoirs on the water cycle, precipitation and climate, based on observations and coupled modelling approaches,
ii. investigate the use of hydroclimatic frameworks such as the Budyko framework to understand the human and climate effects on both atmospheric water input and partitioning,
iii. explore the implications of atmospheric feedbacks on the hydrologic cycle for land and water management.

Typically, studies in this session are applied studies using fundamental characteristics of the atmospheric branch of the hydrologic cycle on different scales. These fundamentals include, but are not limited to, atmospheric circulation, humidity, hydroclimate frameworks, residence times, recycling ratios, sources and sinks of atmospheric moisture, energy balance and climatic extremes. Studies may also evaluate different sources of data for atmospheric hydrology and implications for inter-comparison and meta-analysis. For example, observations networks, isotopic studies, conceptual models, Budyko-based hydro climatological assessments, back-trajectories, reanalysis and fully coupled earth system model simulations.

With global climate change affecting the frequency and severity of extreme meteorological and hydrological events, it is particularly necessary to develop models and methodologies for a better understanding and forecasting of present day weather induced hazards. Future changes in the event characteristics as well as changes in vulnerability and exposure are among the further factors for determining risks for infrastructure and society, and for the development of suitable adaptation measures. This session considers extreme events that lead to disastrous hazards induced by severe weather and climate change. These can, e.g., be tropical or extratropical rain- and wind-storms, hail, tornadoes or lightning events, but also floods, long-lasting periods of drought, periods of extremely high or of extremely low temperatures, etc. Papers are sought which contribute to the understanding of their occurrence (conditions and meteorological development), to assessment of their risk and their future changes, to the ability of models to reproduce them and methods to forecast them or produce early warnings, to proactive planning focusing to damage prevention and damage reduction. Papers are also encouraged that look at complex extreme events produced by combinations or sequences of factors that are not extreme by themselves. The session serves as a forum for the interdisciplinary exchange of research approaches and results, involving meteorology, hydrology, hazard management and applications like insurance issues.

Precipitation, both liquid and solid, is a central element of the global water/energy cycle through its coupling with clouds, water vapor, atmospheric motions, ocean circulation, and land surface processes. Precipitation is also the primary source of freshwater, while it can have tremendous socio-economical impacts associated with extreme weather events such as hurricanes, floods, droughts, and landslides. Accurate and timely knowledge of precipitation characteristics at regional and global scales is essential for understanding how the Earth system operates under changing climatic conditions and for improved societal applications that range from numerical weather prediction to freshwater resource management. This session will host papers on all aspects of precipitation, especially contributions in the following four research areas: Precipitation Measurement: Precipitation measurements (amount, duration, intensity etc) by ground-based in situ sensors (e.g., rain gauges, disdrometers); estimation of accuracy of measurements, comparison of instrumentation. Precipitation Climatology: Regional and global climatology; areal distribution of measured precipitation; classification of precipitation patterns; spatial and temporal characteristics of precipitation; methodologies adopted and their uncertainties; comparative studies. Precipitation Remote Sensing: Remote sensing of precipitation (spaceborne, airborne, ground-based, underwater, or shipborne sensors); methodologies to estimate areal precipitation (interpolation, downscaling, combination of measurements and/or estimates of precipitation); methodologies used for the estimation (e.g., QPE), validation, and assessment of error and uncertainty of precipitation as estimated by remote sensors. A special focus will be on international contributions to the exploitation of the international Global Precipitation Measurement (GPM) mission that provides state-of-the-art precipitation estimates (including solid precipitation) from space with unprecedented accuracy, time-space coverage, and improved information for microphysics.

The polar climate system is strongly affected by interactions between the atmosphere and the cryosphere. Processes that exchange heat, moisture and momentum between land ice, sea ice and the atmosphere, such as katabatic winds, blowing snow, ice melt, polynya formation and sea ice transport, play an important role in local-to-global processes. Atmosphere-ice interactions are also triggered by synoptic weather phenomena such as cold air outbreaks, polar lows, atmospheric rivers, Foehn winds and heatwaves. However, our understanding of these processes is still incomplete.

This session aims at showcasing recent research progress and augmenting existing knowledge in polar meteorology and climate and the atmosphere-land ice-sea ice coupling in both the Northern and Southern Hemispheres. It will provide a setting to foster discussion and help identify gaps, tools, and studies that can be designed to address these open questions. It is also the opportunity to convey newly acquired knowledge to the community.
We invite contributions on all observational and numerical modelling aspects of Arctic and Antarctic meteorology and climatology, that address atmospheric interactions with the cryosphere. This may include but is not limited to studies on past, present and future of:
- Atmospheric processes that influence sea-ice (snow on sea ice, sea ice melt, polynya formation and sea ice production and transport) and associated feedbacks,
- The variability of the polar large-scale atmospheric circulation (such as polar jets, the circumpolar trough and storm tracks) and impact on the cryosphere (sea ice and land ice),
- Atmosphere-ice interactions triggered by synoptic and meso-scale weather phenomena such as cold air outbreaks, katabatic winds, extratropical cyclones, polar cyclones, atmospheric rivers, Foehn winds and heatwaves,
- Role of clouds in polar climate and impact on the land ice and sea ice through interactions with radiation,
- Teleconnections and climate indices and their role in land ice/sea ice variability.
Presentations including new observational (ground and satellite-based) and modelling methodologies specific to polar regions are encouraged. Contributions related to results from recent field campaigns in the Arctic and in the Southern Ocean/Antarctica are also welcomed.

Driven by atmospheric turbulence, and integrating surface processes to free atmospheric conditions, the Atmospheric Boundary Layer (ABL) plays a key role not only in weather and climate, but also in air quality and wind/solar energy. It is in this context that this session invites theoretical, numerical and observational studies ranging from fundamental aspects of atmospheric turbulence, to parameterizations of the boundary layer, and to renewable energy or air pollution applications. Below we propose a list of the topics included:

- Observational methods in the Atmospheric Boundary Layer
- Simulation and modelling of ABL: from turbulence to boundary layer schemes
- Stable Boundary Layers, gravity waves and intermittency
- Evening and morning transitions of the ABL
- Convective processes in the ABL
- Boundary Layer Clouds and turbulence-fog interactions
- Micro-Mesoscale interactions
- Micrometeorology in complex terrain
- Agricultural and Forest processes in the ABL
- Diffusion and transport of constituents in the ABL
- Turbulence and Air Quality applications
- Turbulence and Wind Energy applications

Solicited contribution:

- "Surface energy balance closure: the role dispersive fluxes induced by submesoscale secondary circulations", by Dr. Matthias Mauder , Karlsruhe Institute of Technology KIT/IMK-IFU, Institute for Meteorology and Climatology - Atmospheric Environmental Research Germany.

The multitude of processes of various scales occurring simultaneously under strong winds in the air and sea boundary layers presents a true challenge for nonlinear science. We want to understand the physics of these processes, their specific role, their interactions and how they can be probed remotely, how these processes differ from their counterparts under moderate/weak winds. We welcome theoretical, experimental and numerical works on all aspects of processes in turbulent boundary layers above and below the ocean surface. Although we are particularly interested in the processes and phenomena occurring under strong wind conditions, the works concerned with similar processes under weaker winds which might provide an insight for rough seas are also welcomed. We are also very interested in works on remote sensing of these processes.
The areas of interest include the processes at and in the vicinity of the interface (nonlinear dynamics of surface water, wave-turbulence interactions, wave breaking, generation and dynamics of spray and air bubbles, thermodynamics of the processes in the boundary layers, heat and gas exchange), all the processes above and below the aIr/water interface, as long as they are relevant for strong wind conditions (such as, e.g. inertial waves generated by changing winds). Relevant nonlinear biological phenomena are also welcomed.
The main aims of the session is to initiate discussion of the multitude of processes active under strong winds across the narrow specializations as a step towards creating an integrated picture. Theoretical, numerical, experimental and observational works are welcomed.

Geophysical Fluid Dynamics (GFD) is a truly interdisciplinary field, including different topics dealing with rotating stratified fluids. It emerges in the late 50s, when scientists from meteorology, oceanography, astrophysics, geological fluid dynamics, and applied mathematics began to mathematically model complex flows and thereby unify these fields. Since then many new aspects were added and deeper insight into many problems has been achieved. New mathematical and statistical tools were developed, standard techniques were refined, classical problems were varied. In this session we primarily focus on contributions from dynamic meteorology and physical oceanography that model flows by mathematical analysis. However, it is also a forum for experimental GFD and for astrophysical and geological aspects of GFD as well.

The session is addressed to experimentalists and modellers working on land surface interactions from local to regional scales. The programme is open to a wide range of new studies in micrometeorology and related atmospheric and remote sensing disciplines. The topics include the development of new devices, measurement techniques, experimental design, data analysis methods, as well as novel findings on surface layer theory and parametrization, including local and non-local processes. The theoretical parts encompass soil-vegetation-atmosphere transport, internal boundary-layer theories and flux footprint analyses. Of special interest are synergistic studies employing experimental data, parametrizations and models. This includes energy and trace gas fluxes (inert and reactive) as well as water, carbon dioxide and other GHG fluxes. Specific focus is given to outstanding problems in land surface boundary layer descriptions such as complex terrain, effects of horizontal heterogeneity on sub-meso-scale transport processes, energy balance closure, stable stratification and night time fluxes, dynamic interactions with atmosphere, plants (in canopy and above canopy) and soils.

Tropical ecosystems are biomes of global significance due to their large biodiversity, carbon storage capacity, and their role in the hydrological cycle. Historic and recent human activities have, however, resulted in intensive transformation of the tropical ecosystems in the Amazon, Central America, Central Africa and in South East Asia impacting the cycling of nutrient, carbon, water, and energy. Understanding their current functioning at process up to biome level in its pristine and transformed state is elemental for predicting their response upon changing climate and land use, and the impact this will have on local up to global scale.
This session aims at bringing together scientists who investigate the functioning of the tropical ecosystems across spatial and temporal scales by means of remote and in-situ observational, modelling, and theoretical studies. Particularly welcome are presentations of novel, interdisciplinary approaches and techniques.

Evapotranspiration (ET) is the key water flux at the interface of soil, vegetation and atmosphere. In-situ measurements to estimate ET (and its individual components evaporation and transpiration) have been developed in different research disciplines and cover a range of scales, from the point scale of individual sap flow sensors in trees over pedon-scale lysimeters to eddy covariance footprints. Each estimate and each scaling step includes a method-specific set of uncertainties which are rarely communicated. This is problematic for connecting different methods and the effort to scale up to remote sensing products from satellites or to model resolutions.

This session will mainly focus on the variety of ET estimates from different in-situ devices such as lysimeters, sap flow sensors, eddy covariance stations, scintillometers, approaches like the Bowen ratio method and others, including reporting and comparing the respective uncertainties of the methods. Additionally, we want to address the scale dependency of the various approaches and the scale gap between in-situ ET data, remote sensing products and catchment- or landscape-scale modelled ET. We welcome contributions that (1) assess and compare established and new in-situ ET measurements, (2) address uncertainty in the respective methods, (3) analyse trends as well as spatial and temporal patterns in in-situ measured ET data, (4) include cross-scale comparisons and scaling approaches and (5) incorporate in-situ measurements into modeling approaches.

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.

As the most evident example of land use and land cover change, urban areas play a fundamental role in local to large-scale planetary processes, via modification of heat, moisture, and chemical budgets. With rapid urbanization ramping up globally it is essential to recognize the consequences of landscape conversion to the built environment. Given the capability of cities to serve as first responders to global change, considerable efforts are currently being dedicated across many cities to monitor and understand urban atmospheric dynamics and examine various adaptation and mitigation strategies aimed to offset impacts of rapidly expanding urban environments and influences of large-scale greenhouse gas emissions.

This session solicits submissions from both the observational and modelling communities examining urban atmospheric and landscape dynamics, processes and impacts owing to urban induced climate change, the efficacy of various strategies to reduce such impacts, and techniques highlighting how cities are already using novel science data and products that facilitate planning and policies on urban adaptation to and mitigation of the effects of climate change. Emerging topics including, but not limited to, compounding impacts with urban COVID-19 outbreaks or urban climate informatics, are highly encouraged.

This session aims at fostering discussions on the physical processes at work at the air-sea interface, including their observation and representation in coupled numerical models, as well as their impact of air-sea fluxes. Examples of such processes are solar radiation-induced diurnal warming, rain-induced cool and fresh lenses, and processes controlling the formation and properties of the surface microlayer. Additional focus is on gustiness associated with convection in the atmospheric boundary layer and evaporative cold pools. Further focus is on air-sea interactions in polar regions, in particular related to cold air outbreaks, including the role of sea ice and the effect of leads. Air-sea interaction related to surface temperature and salinity fronts, as well as oceanic meso- and sub-mesoscale dynamics, are also of great interest. Studies considering the variability of biogeochemical properties related to air-sea processes will also be considered.

This session is thus intended for (i) contributions presenting observational or theoretical aspects of the processes described above and their impact on energy, water, momentum, gas and aerosols exchanges at the interface; and (ii) contributions focusing on the mathematical and algorithmic methods used to represent these processes in coupled ocean-atmosphere models.

This session seeks observational studies based on recent field campaigns or satellite remote sensing. This session also aims to gather studies using numerical models of any level of complexity (from highly idealized to realistic) and any resolution from Large Eddy Simulation (LES) to global circulation models. Studies describing the impact of the air-sea interaction physical processes on the mean global or regional climates and variability representation are also welcome.

Observations and model simulations illustrate significant ocean variability and associated air-sea interactions in the tropical Atlantic basin from daily-to-decadal time scales. This session is devoted to the understanding of ocean dynamics in the tropical and subtropical Atlantic Ocean, its interaction with the overlying atmosphere from the equator to the mid-latitudes and its climate impacts on adjacent to remote areas.
Relevant processes in the ocean include upper and deep ocean circulation, eddies, tropical instability waves, warm pools, cold tongues and eastern boundary upwellings. We are interested in air-sea interactions related to both the seasonal cycle and the development of modes of variability from local to basin scale (e.g. the Meridional Mode, the Atlantic Niño, and the Benguela Niño). We welcome studies on wind variations related to the development of these modes, as well as studies on high-frequency events, such as marine heat waves, the Madden-Julian Oscillation, tropical cyclones and convective systems. Furthermore, we seek studies on climate change in the region, and also of the climatic impacts of change and variability on marine ecosystems. Finally, we are also interested in contributions examining the causes and impacts of systematic model errors in simulating the local to regional Atlantic climate.
Studies based on direct observations, reanalysis, reconstructions as well as model simulations are welcome.

Atmosphere and Cryosphere are closely linked and need to be investigated as an interdisciplinary subject. Most of the cryospheric areas have undergone severe changes in last decades while such areas have been more fragile and less adaptable to global climate changes. This AS-CR session invites model- and observational-based investigations on any aspects of linkages between atmospheric processes and snow and ice on local, regional and global scales. Emphasis is given on the Arctic, high latitudes and altitudes, mountains, sea ice, Antarctic regions. In particular, we encourage studies that address aerosols (such as Black Carbon, Organic Carbon, dust, volcanic ash, diatoms, bioaerosols, bacteria, etc.) and changes in the cryosphere, e.g., effects on snow/ice melt and albedo. The session also focuses on dust transport, aeolian deposition, and volcanic dust, including health, environmental or climate impacts at high latitudes, high altitudes and cold Polar Regions. We include contributions on biological and ecological sciences including dust-organisms interactions, cryoconites, bio-albedo, eco-physiological, biogeochemical and genomic studies. Related topics are light absorbing impurities, cold deserts, dust storms, long-range transport, glaciers darkening, polar ecology, and more. The scientific understanding of the AS-CR interaction needs to be addressed better and linked to the global climate predictions scenarios.

The rapid decline of the Arctic sea ice in the last decade is a dramatic indicator of climate change. The Arctic sea ice cover is now thinner, weaker and drifts faster. Freak heatwaves are common. On land, the permafrost is dramatically thawing, glaciers are disappearing, and forest fires are raging. The ocean is also changing: the volume of freshwater stored in the Arctic has increased as have the inputs of coastal runoff from Siberia and Greenland and the exchanges with the Atlantic and Pacific Oceans. As the global surface temperature rises, the Arctic Ocean is speculated to become seasonally ice-free by the mid 21st century, which prompts us to revisit our perceptions of the Arctic system as a whole. What could the Arctic Ocean look like in the future? How are the present changes in the Arctic going to affect and be affected by the lower latitudes? What aspects of the changing Arctic should observational, remote sensing and modelling programmes address in priority?
In this session, we invite contributions from a variety of studies on the recent past, present and future Arctic. We encourage submissions examining interactions between the ocean, atmosphere and sea ice, on emerging mechanisms and feedbacks in the Arctic and on how the Arctic influences the global ocean. Submissions with a focus on emerging cryospheric, oceanic and biogeochemical processes and their implications are particularly welcome.
The session promotes results from current Arctic programmes and discussions on future plans for Arctic Ocean modelling and measurement strategies, and encourages submissions on the first results from CMIP6 and the recently completed Multidisciplinary drifting Observatory for the Study of Arctic Climate (MOSAiC). This session is cosponsored by the CLIVAR /CliC Northern Ocean Regional Panel (NORP) that aims to facilitate progress and identify scientific opportunities in (sub)Arctic ocean-sea-ice-atmosphere research.

This session is intended to provide an interdisciplinary forum to bring together researchers working in the areas of high-latitude meteorology, atmospheric chemistry, stable isotope research, oceanography, and climate. The emphasis is on the role of boundary layer processes that mediate exchange of heat, momentum and mass between the Earth's surface (snow, sea-ice, ocean and land) and the atmosphere 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 in the polar regions and their teleconnections with mid-latitude weather and climate, including meridional transport of heat, moisture, chemical trace species, aerosols and isotopic tracers (indicating airmass origins and atmospheric processes); and regional emission and vertical mixing of climate active trace gases and aerosol, such as cloud-forming particles (CCN/INP) and their precursors. It is expected that the recent implementation of new measurements such as those from pan-Arctic water vapor isotope networks, observations such as those obtained during the MOSAiC field program, and data from existing networks will help diagnose long-range moisture and aerosol sources and the coupling between local and large-scale dynamics. We encourage submissions such as (but not limited to):
(1) External controls on the boundary layer such as clouds, radiation and long-range transport processes
(2) Results from field programs, such as MOSAiC, and routine observatories, insights from laboratory studies, and advances in modeling and reanalysis,
(3) Use of data from pan-Arctic and Antarctic observing networks,
(4) Surface processes involving snow, sea-ice, ocean, land/atmosphere chemical and isotope exchanges, and natural aerosol sources
(5) The role of boundary layers in polar climate change and implications of climate change for surface exchange processes, especially in the context of reduced sea ice, wetter snowpacks, increased glacial discharge and physical and chemical changes associated with an increasing fraction of first year ice and increasing open water.

Aerosol particles are key components of the earth system important in radiative balance, human health, and other areas of key societal concern. Understanding their formation, evolution and impacts relies on developments from multiple disciplines covering both experimental laboratory work, field studies and numerical modelling. In this general session all topics of Aerosol Chemistry and Physics are covered. Contributions from aerosol laboratory, field, remote sensing and model studies are all highly encouraged.

As in previous years, this year the session will dedicate some of its time to focus on a hot topic which this year is on the use of statistical methods and machine learning across a range of scales in aerosol research: from studies designed to decipher source and/or process contributions to measured aerosol signatures through to the role of aerosols in climate projections. Regarding the latter, we will be presenting contributions related to the H2020 project FORCeS which aims at reducing the uncertainty related to aerosols in climate projections.

Why focus on such a broad scale of activities? The aerosol community has spent significant effort developing and deploying a range of instruments to capture the evolving chemical and physical properties of laboratory and ambient systems. This information in turn should be used to better constrain source and process contributions in mechanistic and/or impact driven models. As the resolving power of such instruments to provide us with a 'snapshot' of a chemical signature increases, through a given form of instrument response, so to must we evaluate the usefulness of a range of analytical methods to convert those signatures into information about source and process contributions. Likewise, as the complexity of information on aerosol evolution increases, we must consider alternative ways for improving such descriptions in large-scale impact driven models. As the availability of various machine learning and statistical packages increases, we are now able to start conducting such research. With this in mind, aside from general submissions on aerosol research, we encourage contributions from this emerging area of research.

A better understanding of the role of natural aerosols in the atmosphere is essential for assessing anthropogenic radiative forcing and the climate response. Our session explores primary aerosols and those formed from precursor gases emitted by natural sources, e.g. from wildfires, deserts, volcanoes, oceans, and vegetation. The session intends to bring together experts from different fields to assess the state-of-the-science knowledge on natural aerosols and to identify future directions to reduce uncertainty in their emissions and impacts. We encourage submissions that use models across different spatial scales and consider past, present or future perspectives, as well as measurements from remote sensing, field campaigns and laboratory experiments. Questions of particular interest are, but not limited to:

- How can we distinguish between truly natural aerosols and those whose emissions or formation are influenced by anthropogenic activities?
- Where are the missing links in our understanding of the lifecycle of natural aerosols?
- How does the contributions of natural aerosols to atmospheric composition and deposition change over time?
- What are the consequences of changes in natural aerosols, e.g., for photovoltaic power production?

The Aerosol Chemistry Model Intercomparison Project (AerChemMIP), part of the Coupled-Model Intercomparison Project 6 (CMIP6), builds on earlier Chemistry-Climate Model Intercomparison(CCMI), and Hemispheric Transport of Air Pollution (HTAP) experiments to quantify the climate and air quality impacts of aerosols and chemically reactive gases from the pre-industrial to present day, and projected into the future. Linked projects, the Radiative Forcing Model Intercomparison Project (RFMIP) and the Precipitation Driver Response Model Intercomparison Project (PDRMIP), examine radiative forcing estimates across the CMIP6 models. This session, with a focus on global scale atmospheric chemistry and aerosol modelling and radiative forcing in AerChemMIP, RFMIP and other CMIP6-era simulations, aims to bring together scientists with an interest in:

- quantifying the impact of emissions changes on atmospheric composition and radiative forcing over the period 1850-2100.

- evaluating reactive gases and aerosols in CMIP6 models against observations.

- exploring chemistry-climate interactions in CMIP6 models, with a focus on climate response to changes in trace gas and aerosol concentrations.

- understanding components of radiative forcing.

We invite submission of abstracts in the following areas:

- Global and regional evaluation of CMIP, CCMI and HTAP models and variability across different spatial and temporal scales.

- Modelling studies to understand the cause and impact of changes in atmospheric composition in CMIP6 experiments.

- Quantification of radiative forcing in CMIP6 models, including the roles of chemistry and aerosols.

We particularly welcome contributions from the AerChemMIP, RFMIP and PDRMIP modelling communities to these discussions.

Anthropogenic aerosol plays a key role in driving climate anomalies over a range of spatial and temporal scales, both near the emission location and remotely through teleconnections. Aerosols can interact with radiation and clouds, directly and through absorption, microphysics and circulation, and thereby modify the surface and atmospheric energy balance, cloud dynamics and precipitation patterns, and the atmospheric and oceanic circulation. This session addresses progress in understanding the mechanisms and pathways by which aerosols affect regional climate features, overall, over the historical era, and in the near future. We encourage contributions on new 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 interplay with modes of variability such as the NAO, AMO, and PDO. Focus studies on monsoon, midlatitude, and Arctic responses, extreme precipitation, circulation changes, daily variability, CMIP6 projections of high and low aerosol futures, and investigations using large ensemble simulations are welcome.

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. Special emphasis is put on the scientific value of high-quality long-term measurement data sets and supporting model simulations. Both approaches contribute to improved understanding of the mechanisms that control the variability of atmospheric chemical composition (including multiple gaseous species). Presentations related to the projections of the atmospheric composition are welcome in this session as well.
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 ozone depletion, climate change, 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.

Anthropogenic emissions of greenhouse gases and ozone depleting substances have caused substantial changes in the chemical composition of the middle atmosphere that, in turn, can influence tropospheric processes. For example, increasing greenhouse gases are expected to modify the large-scale circulation of the stratosphere, the Brewer-Dobson circulation (BDC), and the stratospheric abundance of radiatively active gases, notably ozone and water vapour. Such changes in the BDC and composition affect the exposure of the biosphere to harmful UV radiation and can feed back on surface climate through their influence on radiative forcing and tropospheric dynamics. In addition, long-term changes in the ozone layer (e.g. ozone hole and recovery) strongly influence the tropospheric circulation in the Southern Hemisphere. These changes are further coupled to a variety of Earth system feedbacks. We welcome abstracts which explore middle-atmosphere composition changes, their impacts on tropospheric climate and composition, and resulting feedbacks. Abstracts may address these issues on time-scales encompassing inter-annual to centennial timescales, as well as impacts ranging from the tropics to poles and globally. Research might also concern long-term ozone trends (depletion and recovery), as well as changes in the upper troposphere and lower stratosphere (UTLS) region. We welcome contributions exploring these topics using chemistry-climate and Earth system models, Earth observations, as well as contributions on novel methodological approaches (as e.g. machine learning) to gain insights into composition changes, related feedbacks and theoretical studies.

A correct characterization of atmospheric transport is crucial for the understanding of environmental challenges such as acid rain, ozone depletion, and climate change. However, there are still significant gaps in our knowledge. The impacts of the recent large wildfires, the role of the atmosphere for the spread of microplastic particles, the transport of biological material such as bacteria, viruses, pollen, and fungal spores are topics of increasing concern. In additions, transport of dust, greenhouse gases, classical pollutants, and ozone-depleting substances continues to be an active research area.

For a realistic characterization of the atmospheric composition, the connection between local and global scales often plays an important role and present challenges for models, as well as the correct reconstruction of anthropogenic and natural emission. In this context, Lagrangian models, more robust with respect to scales, represent an important tool and find applications for many practical purposes including volcanic eruptions, nuclear accidents and extreme pollution events. In addition, in combination with observations, inverse modelling techniques can be used to assess emission sources, which are generally poorly constrained.

Similarly, the development of models with respect to numerical issues and parameterization of processes, such as dry and wet deposition for various kinds of trace substances, is ongoing. Case studies can help to estimate how good models perform and enable tuning of those parametrizations.

Contributions are therefore invited for the following topics:
• Applications of Lagrangian atmospheric transport models.
• Interpretation of measurements of atmospheric trace substances using atmospheric transport modelling, including novel applications such as biological or microplastic materials.
• Studies combining observations and models to infer information about emissions or transport characteristics.
• New developments and improvements in atmospheric transport modelling, including the improvement of parameterization of atmospheric processes, the quantitative assessment of uncertainties, improving model performance, and the proper coupling of Lagrangian models to Eulerian Numerical Weather Prediction and General Circulation models.

Accurate and precise atmospheric measurements of greenhouse gas (GHG) concentrations reveal the rapid and unceasing rise of global GHG concentrations due to human activity. The resulting increases in global temperatures, sea-level, glacial retreat, and other negative impacts are clear. In response to this evidence, nations, states, and cities, private enterprises and individuals have been accelerating GHG reduction efforts while meeting the needs of global development. The urgency, complexity and economic implications of GHG reductions demand strategic investment in science-based information for planning and tracking emission reduction policies and actions. In response, the World Meteorological Organization (WMO) Global Atmosphere Watch Program (GAW) and its partners have initiated the development of an Integrated Global Greenhouse Gas Information System (IG3IS). IG3IS combines atmospheric GHG concentration measurements and human-activity data in an inverse modeling framework to help decision-makers take better-informed action to reduce emissions of greenhouse gases and pollutants that reduce air quality. This service is based on existing and successful measurement and analysis methods and use-cases for which the scientific and technical skill is proven or emerging.

This session intends to gather presentations from researchers and decision-makers (user-community) on the development, implementation and use of atmospheric measurement-based “top-down” and data-driven “bottom-up” GHG emission inventory estimates, and the combination of both approaches, explicit in space and time, to deliver actionable emissions information at scales where human activity occurs and emission reduction is most effective. This session will also showcase the new projects and “good-practice” standards of the World Meteorological Organization (WMO) Integrated Global Greenhouse Gas Information System (IG3IS), which is part of WMO’s commitment to science-based services.

Significant uncertainties exist in our understanding of the CO2 and CH4 fluxes between land or ocean and atmosphere on regional and global scales. Remotely-sensed CO2 and CH4 observations provide a significant potential for improving our understanding of the natural carbon cycle and for the monitoring of anthropogenic emissions. Over the last few years, remote sensing technologies for measuring CO2 and CH4 from space, aircraft, and from the ground made great advances. New passive and active instruments from different platforms became available offering unprecedented accuracy and coverage.

This session is open to contributions related to all aspects of remote sensing of the greenhouse gases CO2 and CH4 from current, upcoming and planned satellite missions (e.g., OCO-2/3, GOSAT-2, Tansat, S5P, MERLIN, Microcarb, CO2M), as well as ground-based (e.g., TCCON, COCCON), aircraft, and other remote sensing instruments. This includes advances in retrieval techniques, instrumental concepts, and validation activities. We specifically encourage contributions that focus on the interpretation of observations with respect to natural fluxes or anthropogenic emissions.