Today, it is almost certain that global climate change will affect the frequency and severity of extreme meteorological and hydrological events. It is necessary to develop models and methodologies for the better understanding, forecasting, hazard prevention of weather induced extreme events and assessment of disaster risk. 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 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/or applications like insurance issues.

Lightning is the energetic manifestation of electrical breakdown, occurring after charge separation processes operating on micro and macro-scales, leading to strong electric fields within thunderstorms. Lightning is associated with 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 to 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 on:

Atmospheric electricity in fair weather and the global electrical circuit
Atmospheric chemical effects of lightning and LtNOx
Middle atmospheric Transient Luminous Events
Energetic radiation from thunderstorms and lightning.
Remote sensing of lightning from space and by lightning detection networks
Results from the Atmosphere-Space Interaction Monitor (ASIM) mission.
Thunderstorms, flash floods and severe weather
Lightning and climate
Modeling of thunderstorms and lightning
Now-casting and forecasting of thunderstorms
New airborne and ground-based observation techniques

High-impact climate and weather events typically result from the interaction of multiple hazards across various spatial and temporal scales. These events, also known as Compound Events, often cause more severe socio-economic impacts than single-hazard events, rendering traditional univariate extreme event analyses and risk assessment techniques insufficient. It is therefore crucial to develop new methodologies that account for the possible interaction of multiple physical drivers when analysing high-impact events. Such an endeavour requires (i) a deeper understanding of the interplay of mechanisms causing Compound Events and (ii) an evaluation of the performance of climate/weather, statistical and impact models in representing Compound Events.
The European COST Action DAMOCLES together with the EU H2020 ANYWHERE project will coordinate these efforts by building a research network consisting of climate scientists, impact modellers, statisticians, and stakeholders. This session creates a platform for this network and acts as an introduction of the work related to DAMOCLES and ANYWHERE to the research community. We therefore invite papers studying Compound Events and addressing the following topics representing the five working groups of DAMOCLES and Work Package 2 of ANYWHERE working on multi-hazard impacts..

Synthesis and Analysis: What are common features for different classes of Compound Events? Which climate variables need to be assessed jointly in order to address related impacts? How much is currently known about the dependence between these variables?
Stakeholders and science-user interface: Which events are most relevant for stakeholders? What are novel approaches to ensure continuous stakeholder engagement?
Impacts: What are the currently available sources of impact data? How can they be used to link observed impacts to climate and weather events?
Statistical approaches, model development and evaluation: What are possible novel statistical models that could be applied in the assessment of Compound Events?
Realistic model simulations of events: What are the physical mechanisms behind different types of Compound Events? What type of interactions result in the joint impact of the hazards that are involved in the event? How do these interactions influence risk assessment analyses?

Karst environments are characterized by distinctive landforms and unique hydrologic behaviors. Karst systems are commonly extremely complex, heterogeneous, and very difficult to manage because their formation and evolution are controlled by a wide range of geological, hydrological, geochemical and biological processes. Further, karst systems are extremely vulnerable due to the direct connection between the surface and subsurface compartments through conduit networks.
The great variability and unique connectivity may result in serious engineering problems: on one hand, karst groundwater resources are readily contaminated by pollution because of the rapidity of conduit flow; on the other hand, the presence of karst conduits that weakens the strength of the rock mass may lead to serious natural and human-induced hazards. The plan and development of engineering projects in karst environments thus require: 1) an enhanced understanding of natural processes that govern the initiation and evolution of karst systems through both field and modelling approaches, and 2) specific interdisciplinary approaches aiming at at better assessing the associated uncertainties and minimizing the detrimental effects of hazardous processes and environmental problems.
This session calls for abstracts on research related to geomorphology, hydrogeology, engineering geology, and/or hazard mitigation in karst environments in the context of climate change and increased human disturbance. It also aims to discuss various characterization and modelling methods applied in each specific research domain, with their consequences on the understanding of the whole process of karst genesis and functioning.

Flooding is the foremost natural hazard around the world, affecting human life and property (directly and indirectly). In the current era, many hydraulic and hydrologic modelling techniques are available for flood risk assessment and management as well as flood risk prevention and preparedness. They provide a platform for the scientific community to explore the causes of floods and to build up efficient methods for flood mitigation.

This session invites in-depth research work carried out through flood modelling including hydrological modelling, flood hydrodynamic modelling, flood inundation mapping, flood hazard mapping, risk assessment, flood policy, and flood mitigation strategy. It also welcomes studies dealing with various uncertainties associated with different stages of modelling and the exploration of modern techniques for model calibration and validation.

In addition, real-time flood inundation mapping is an important aspect for the evacuation of people from low-lying areas and reduction of the death toll. Real-time data gained through UAV-based flood inundation mapping and associated uncertainty in real-time aerial surveying are welcome in this special issue.

Invited Speaker:
PD Dr. Heidi Kreibich (Helmholtz Centre Potsdam, GFZ German Research Centre for Geosciences)
Head of the Working Group Flood risk and climate adaptation

Prediction skill of hydro-meteorological forecasting systems has remarkably improved in recent decades. Advances in both weather and hydrology models, linked to the availability of more powerful and efficient computational resources, allowed the development of even more complex systems based on the combination of spatially distributed physically-based hydrologic- and hydraulic models with deterministic and/or ensemble meteorological forecasting systems. Coupled atmosphere-hydrological modeling aims at describing the full atmospheric-terrestrial regional water cycle, i.e. extending from the top of the atmosphere, through the boundary layer, via the land surface and subsurface till lateral flow in the groundwater and in the river beds. Fully two-way coupled model systems thereby give the possibility to study long range feedbacks between groundwater, soil moisture redistribution and precipitation. Via improved and completed process descriptions fully coupled modeling may also increase the performance of hydrometeorological predictions of various spatial and temporal scales.
The objective of the session is to create a valuable opportunity for the interdisciplinary exchange of ideas and experiences among atmospheric-hydrological modelers and members of both hydrology- and Earth System modeling communities. Contributions are invited dealing with the complex interactions between surface water, groundwater and regional climate, with a specific focus on those presenting work on the development or application of one-way (both deterministic and ensemble) or fully-coupled hydrometeorological prediction systems for floods/flash-floods, droughts and water resources. Presentations of inter-comparisons between one-way and fully-coupled hydrometeorological chains are encouraged, such as contributions on novel one-way and fully-coupled modeling systems that bridge spatial scales through dynamic regridding or upscaling/downscaling methodologies. Also, presentations addressing data assimilation in coupled model systems are welcome. Likewise abstracts are invited on field experiments and testbeds equipped with complex sensors and measurement systems allowing multi-variable validation of such complex modeling systems.

Heat extremes are already one of the deadliest meteorological events and they are projected to increase in intensity and frequency due to rising CO2 emissions. Thus the risk these events pose to society may increase dramatically and society will need to adapt if the worst impacts are to be avoided. However, uncertainties for understanding the development of extreme heat episodes and their impacts remain large. This session therefore aims to address this challenge, welcoming research which improves our understanding of extreme heat events and how to respond to them. Suitable contributions in this regard may: (i) assess the drivers and underlying processes of extreme heat in observations and models; (ii) explore the diverse socio-economic impacts of extreme heat events (for example, on aspects relating to human health or economic productivity); (iii) address forecasting of extreme heat at seasonal to sub-seasonal time scales; (iv) focus on societal adaptation to extreme heat, including (but not limited to) the implementation of Heat-Health Early Warning Systems.

Predicting current and future flood risk continues to be a major challenge for climatologists, hydrologists and hydraulicians. The complex nature of flood risk challenges established risk assessment methodologies and their modelling components, such as hydrologic and hydraulic simulation. Further, flood risk predictions are characterised by considerable uncertainty, which needs to be evaluated and clearly communicated to decision-makers. This session aims to review state-of-the-art flood risk assessment methodologies on different scales and experiences of recent flood events, the physical processes occurring during flood flows and uncertainties in measurement data and modelling. We welcome submissions in the areas of flood plain risk assessment and uncertainty analysis, floodplain management including new approaches to hydraulic and hydrologic modelling, model calibration and validation. Also, we are interested in contributions that show what kind of information is particularly helpful for reducing uncertainty, as well as measures for flood mitigation and the cost effectiveness of these measures. Since flood risk analyses have to include statements on extreme events, observation data are scarce. Therefore, we particularly invite contributions that address the issue of validation of flood risk analyses.

Globally, we are facing massive challenges on how we manage our catchments, in both rural and urban areas, in the next decades. With a changing climate and increased pressure on our land resources we need to ensure we manage the water in our catchments more sustainably and even more so during hydro-climatic extremes. Nature-based solutions (NBS) are 'living' solutions inspired by and continuously supported by nature or natural processes. NBSs are designed to address various societal challenges in a resource efficient and adaptable manner to provide simultaneously economic, social and environmental benefits (European Commission 2015). Therefore NBS can be used within both rural and urban areas to mitigate catchment flood risk, provide drought resilience, protect and enhance endangered freshwater ecosystems and reduce diffuse pollution. However, there are still challenges in implementing NBS for reasons such as lack of evidence surrounding the effectiveness (e.g. at larger scales) and for delivering multiple benefits.

Therefore this session focuses on key research and policy questions associated with NBS. For example, how do we develop locally adapted solutions in catchments and urban areas? What are the impacts of these measures at larger scales (e.g. sub-catchment/ catchment scale)? How can we address multi-disciplinary benefits? How can we do more for less? Importantly, how can we provide the evidence base around the concept of Nature Based Solutions for managing hydrological extremes and water resource management? Examples of studies that cover either the management of flooding, drought, water quality or ecology (both in the rural, peri-urban and urban context) using NBS approaches are at the heart of this session. Management measures could include techniques such as Green Infrastructure, Natural Water Retention Measures, Natural Flood Management, Catchment Restoration, Ecological Engineering or Blue-Green Infrastructure. We invite (but not limit to) abstracts that demonstrate good quality hydrological experiments around NBS; that develop new or improve existing modelling approaches/decision support tools; that investigate and quantify the multiple benefits; and which explore the challenges of implementation (e.g. stakeholder uptake/economics/cost benefit).

Geodesy is becoming increasingly important for observing the hydrological cycle and its effects on solid Earth shape. Signals in geodetic data have revealed water's influence on other geophysical processes including earthquakes, volcanos, land subsidence, mountain uplift, and other aspects of long- and short-term vertical land motion. GPS and InSAR measurements, for example, respectively provide high temporal and spatial resolution to study natural hydrologically-related deformation and monitor anthropogenic groundwater extraction and recharge, and GRACE is helping to track changes in the global terrestrial water storage. Signals of loading from changes in surface and groundwater storage are seen from basin to continental scale. Additionally, novel use of GPS reflectometry is operational for monitoring soil moisture and snow depth at continuous GPS stations in the western USA and Canada. We encourage contributions describing new observations and models of hydrological signals in geodetic time series and/or imaging. These include but are not limited to studies exploring deformation induced by loading, aquifer extraction/recharge, poroelastic deformation and stress changes, techniques for removing hydrological signals from geodetic datasets, monitoring water resources, or teleconnections between hydrologic and other geophysical phenomena.

Fluvial systems cover much of the Earth’s surface; they convey water, sediments, and essential nutrients from the uplands to the sea, intermittently transferring these materials from the river channel to the adjacent floodplain. The routing of sediment and water through the channel network initiates complex process-form interactions as the river bed and banks adjust to changes in flow conditions. Despite their ubiquity, little is known about the landform-driven morphodynamic interactions taking place within the channel that ultimately determine patterns of sedimentation and changes of channel form. Furthermore, an understanding of how these process-form interactions scale with the size of the fluvial system is also currently lacking. Recent technological advances now afford us the opportunity to study and to quantify these process-form interactions in detail across a range of spatial and temporal scales. This session aims to bring together interdisciplinary researchers working across field, experimental, and numerical modelling approaches who are advancing methods and providing new insights into: (i) sediment transport and morphodynamic functioning of fluvial systems, (ii) evaluating morphological change at variable spatial and temporal scales, such as at event vs. seasonal scales, and (iii) investigating the sedimentology of these river systems. We particularly welcome applications which investigate the morphodynamic response of fluvial systems in all types and sizes and we specifically would like to encourage submissions from early career researchers and students.

Invited speakers:
- Lina Polvi Sjöberg (Umeå University): "Streams frozen in time? Particle- to catchment- scale dynamics of high-latitude post-glacial streams."
- Anette Eltner (TU Dresden): "Unmanned aerial and water vehicle data for hydro-morphological river
monitoring"

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 such as YMC and PISTON are also encouraged.

Numerical atmospheric dispersion models are an essential tool for assessment of emergency situations related to airborne particles or gases released into the atmosphere by natural or man-made hazards. They are used complementary to observational data in order to fill-in e.g. temporal- or spatial gaps and to conduct forecasts facilitating the planning of mitigation strategies.
The focus of this session will be on environmental emergency scenarios (airborne hazards) which can have extremely high impact on society and environment: volcano eruptions, nuclear accidents, as well as more localised emergencies, such as dust storms and strong vegetation fires or other occasions when hazardous pollutants are injected into the atmosphere.

The International Monitoring System (IMS) of the Comprehensive Nuclear-Test-Ban Treaty (CTBT) senses the solid Earth, the oceans and the atmosphere with a global network of seismic, infrasound, and hydroacoustic sensors as well as detectors for atmospheric radioactivity. The primary purpose of the IMS data is for nuclear explosion monitoring regarding all aspects of detecting, locating and characterizing nuclear explosions and their radioactivity releases. On-site verification technologies apply similar methods on smaller scales as well as geophysical methods such as ground penetrating radar and geomagnetic surveying with the goal of identifying evidence for a nuclear explosion close to ground zero. Papers in this session address advances in the sensor technologies, new and historic data, data collection, data processing and analysis methods and algorithms, uncertainty analysis, machine learning and data mining, experiments and simulations including atmospheric transport modelling. This session also welcomes papers on applications of the IMS and OSI instrumentation data. This covers the use of IMS data for disaster risk reduction such as tsunami early warning, earthquake hazard assessment, volcano ash plume warning, radiological emergencies and climate change related monitoring. The scientific applications of IMS data establish another large range of topics, including acoustic wave propagation in the Earth crust, stratospheric wind fields and gravity waves, global atmospheric circulation patterns, deep ocean temperature profiles and whale migration. The use of IMS data for such purposes returns a benefit with regard to calibration, data analysis methods and performance of the primary mission of monitoring for nuclear explosions.

As the societal impacts of hazardous weather and other environmental pressures grow, the need for integrated predictions which can represent the numerous feedbacks and linkages between physical and chemical atmospheric processes is greater than ever. This has led to development of a new generation of high resolution multi-scale coupled prediction tools to represent the two-way interactions between aerosols, chemical composition, meteorological processes such as radiation and cloud microphysics.

Contributions are invited on different aspects of integrated model and data assimilation development, evaluation and understanding. A number of application areas of new integrated modelling developments are expected to be considered, including:

i) improved numerical weather prediction and chemical weather forecasting with feedbacks between aerosols, chemistry and meteorology,

ii) two-way interactions between atmospheric composition and climate variability.

This session aims to share experience and best practice in integrated prediction, including:

a) strategy and framework for online integrated meteorology-chemistry modelling;
b) progress on design and development of seamless coupled prediction systems;
c) improved parameterisation of weather-composition feedbacks;
d) data assimilation developments;
e) evaluation, validation, and applications of integrated systems.

This Section is organised in cooperation with the Copernicus Atmosphere Monitoring Service (CAMS), the "Pan-Eurasian Experiment" (PEEX) multidisciplinary program and the WMO Global Atmosphere Watch (GAW) Programme, celebrating its 30 years anniversary in 2019.

Precipitation is the main driver for a number of hydrologic and geomorphic hazards (such as floods, landslides and debris flows), which pose a significant threat to modern societies on a global scale. The continuous increase of population and urban settlements in hazard-prone areas in combination with evidence of changes in precipitation patterns lead to a continuous increase of the risk associated with precipitation-induced hazards. To improve resilience and to design more effective mitigation strategies, we need to better understand the aspects of vulnerability, risk, and triggers that are associated with these hazards.

This session aims to gather contributions dealing with various precipitation induced hazards that address the aspects of vulnerability analysis, risk estimation, impact assessment, mitigation policies and communication strategies. Specifically, we aim to collect contributions from the academia, the industry (e.g. insurance) and government agencies (e.g. civil protection) that will help identify the latest developments and ways forward for improving the resilience of communities at local, regional and national scales, and proposals for improving the interaction between different entities and sciences.

Contributions focusing on, but not limited to, novel developments and findings on the following topics are particularly encouraged:

- Physical and social vulnerability analysis and impact assessment of precipitation-related hazards.
- Advances in the estimation of socioeconomic risk from precipitation-induced hazards.
- Characteristics of precipitation patterns leading to high-impact events.
- Evidence on the relationship between precipitation patterns and socioeconomic impacts.
- Hazard mitigation procedures.
- Communication strategies for increasing public awareness, preparedness, and self-protective response.
- Impact-based forecast and warning systems

Hydroclimatic conditions and the availability of water resources in space and time constitute important factors for maintaining an adequate food supply, the quality of the environment, and the welfare of inhabitants, in the context of sustainable growth and economic development. This session is designed to explore the impacts of hydroclimatic variability, climate change, and the temporal and spatial availability of water resources on: food production, population health, the quality of the environment, and the welfare of local ecosystems. We particularly welcome submissions on the following topics:

Complex inter-linkages between hydroclimatic conditions, food production, and population health, including: extreme weather events, surface and subsurface water resources, surface temperatures, and their impacts on food security, livelihoods, and water- and food-borne illnesses in urban and rural environments.

Quantitative assessment of surface-water and groundwater resources, and their contribution to agricultural system and ecosystem statuses.

Spatiotemporal modeling of the availability of water resources, flooding, droughts, and climate change, in the context of water quality and usage for food production, agricultural irrigation, and health impacts over a wide range of spatiotemporal scales

Intelligent infrastructure for water usage, irrigation, environmental and ecological health monitoring, such as development of advanced sensors, remote sensing, data collection, and associated modeling approaches.

Modelling tools for organizing integrated solutions for water, precision agriculture, ecosystem health monitoring, and characterization of environmental conditions.

Water re-allocation and treatment for agricultural, environmental, and health related purposes.

Impact assessment of water-related natural disasters, and anthropogenic forcings (e.g. inappropriate agricultural practices, and land usage) on the natural environment; e.g. health impacts from water and air, fragmentation of habitats, etc.

The assessment of precipitation variability and uncertainty is crucial in a variety of applications, such as flood risk forecasting, water resource assessments, evaluation of the hydrological impacts of climate change, determination of design floods, and hydrological modelling in general. Within this framework, this session aims to gather contributions on research, advanced applications, and future needs in the understanding and modelling of precipitation variability, and its sources of uncertainty.
Specifically, contributions focusing on one or more of the following issues are particularly welcome:
- Novel studies aimed at the assessment and representation of different sources of uncertainty versus natural variability of precipitation.
- Methods to account for different accuracy in precipitation time series, e.g. due to change and improvement of observation networks.
- Uncertainty and variability in spatially and temporally heterogeneous multi-source precipitation products.
- Estimation of precipitation variability and uncertainty at ungauged sites.
- Precipitation data assimilation.
- Process conceptualization and modelling approaches at different spatial and temporal scales, including model parameter identification and calibration, and sensitivity analyses to parameterization and scales of process representation.
- Modelling approaches based on ensemble simulations and methods for synthetic representation of precipitation variability and uncertainty.
- Scaling and scale invariance properties of precipitation fields in space and/or in time.
- Physically and statistically based approaches to downscale information from meteorological and climate models to spatial and temporal scales useful for hydrological modelling and applications.

Over the last decades, a significant body of empirical and theoretical work has revealed the departure of statistical properties of hydrometeorological processes from the classical statistical prototype, as well as the scaling behaviour of their variables in general, and extremes in particular, in either state, space and/or time. This PICO session (i.e., a 2-minute oral presentation, nicknamed "2-minute madness", followed by an interactive poster presentation on dedicated touch-screens) aims at presenting the latest developments on:
- Coupling stochastic approaches with deterministic hydrometeorological predictions, in order to better represent predictive uncertainty;
- Stochastic-dynamic approaches that are more consistent with the hydrometeorological reality than both deterministic and statistical models separately;
- Variability at climatic scales and its interplay with the ergodicity of space-time probabilities;
- Linking underlying physics and scaling stochastics of hydrometeorological extremes;
- Development of parsimonious representations of probability distributions of hydrometeorological extremes over a wide range of scales and states;
- Understanding and using parsimonious parametrizations of extremes in risk analysis applications and hazard prediction.
The suggested session description is submitted to the HS division of EGU and is sponsored by the International Commission on Statistical Hydrology of the International Association of Hydrological Sciences (ICSH-IAHS, former STAHY).

Urban hydrological processes are characterised by high spatial variability and short response times resulting from a high degree of imperviousness. Therefore, urban catchments are especially sensitive to space-time variability of precipitation at small scales. High resolution precipitation measurements in cities are crucial to properly describe and analyse urban hydrological response. At the same time, urban landscapes pose specific challenges to obtaining representative precipitation and hydrological observations.
This session focuses on high resolution precipitation and hydrological measurements in cities and on approaches to improve modelling of urban hydrological response:
- Novel techniques for high resolution precipitation measurement in cities and approaches for merging remote sensing data with in situ measurements to obtain representation of urban precipitation fields;
- Novel approaches to hydrological field measurements in cities, including data obtained from citizen observatories;
- Novel approaches to modelling urban catchment properties and hydrological response, from physics-based models, fully and semi-distributed modelling to stochastic and statistical conceptualisation;
- Applications of measured precipitation fields in urban hydrological models to improve prediction of flood response and real-time control of stormwater systems for pollution load reduction;
- rainfall modelling for urban applications, including stochastic rainfall generators.

Extreme convective events are increasing in northern and eastern Europe in frequency and intensity causing many deaths, injuries and damage to property every year, and accounting for major economic damages related to natural disasters in several countries. Atlantic hurricanes become extra-tropical cyclones and, sometimes, reach northern Europe. Mediterranean hurricanes (Medicanes or tropical-like cyclones) are not that frequent as other convective systems or tropical cyclones, but these can still reach the intensity of tropical cyclones, causing severe damages in the Southern European region. Supercells and connected tornadoes are also becoming more frequent in central Europe.

In recent years, attention was paid to the detection and monitoring of volcanic ash clouds as their impact on the European air traffic control system was unprecedented. In 2010 the Eyjafjallajökull eruption caused the closure of the airspace of several countries generating the largest air traffic shutdown since the World War II. 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, and they affect economic, political and cultural activities. Europe is located between the chain of Icelandic and Italian/African volcanoes which are unpredictable, and could easily emanate the ash clouds throughout the skies of the continent.

The recent Anak Krakatau eruption (December 2018) highlighted the issue on different techniques to distinguish volcanic ash clouds than convective clouds and the unsolved problem to understand if the cloud top was tropospheric or stratospheric. Specific discussion on this topic will be very welcome to the session.

The extreme convective clouds and the volcanic ash clouds are types of “extreme clouds”. The “extreme clouds” 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. But there is an urgent need to develop new techniques and instruments for monitoring, detecting and modeling “extreme clouds” to develop early warning systems and to support users, decision makers and policy makers. Given the large uncertainties that still remain in the field, enlarging and coordinating the research community for developing new techniques and improving our knowledge is required. Furthermore, there is a need for improved information exchange regarding the impact of the extreme clouds on daily operations between the producers and intended uses of the information.

The objective of the session is to connect different communities in touch with the “extreme clouds”, such as scientists working in remote sensing, modelers, meteorologists, physicists, aviation managers. Thus, allowing the researchers to understand the end-users’ needs and for the end-users to understand the research capabilities.

This session solicits the latest studies from the spectrum of:
- detection, monitoring and modeling of extreme clouds,
- understanding the impact of extreme clouds on climate changes,
- proposal of new products or services focused on the end-users prospective,
- discussion on Anak Krakatau eruption (December 2018)

By considering studies over this range of topics we aim to identify new methods, detail current challenges, understand common techniques/methods and identify common discussions within the communities of atmospheric physicists, meteorologists, modelers, air traffic managers, pilots sensors engineers and engines manufacturers. We particularly welcome and encourage contributions connecting different fields such as:
- forecasting tools to support air traffic management improving the limits of the present science and new products/tools providing better services to the end-users,
- extreme clouds remote sensing with novel techniques and new sensors,
- novel techniques to detect overshooting and their impact on climate.

The aim of the session is to promote discussions between scientists on future developments, in understanding, monitoring and forecasting the extreme clouds, studying their impact and to extend the discussion with the end-users for improving air safety. This session is thus open to contributions on all aspects of remote sensing, forecast, and tools/services development such as:

- Extreme clouds remote sensing
- Extreme clouds modeling
- Extreme clouds forecasting and nowcasting
- Extreme clouds structure
- Extreme clouds and climate change
- Overshooting and Ice clouds
- Air traffic management issues related to extreme clouds
- Airport issues

The occurrence of extremes such as droughts, flash floods, hailstorms, storm surges and tropical storms can have significant and sometimes catastrophic consequences to society. However, not all low probability weather/climate events will lead to “high impacts” on human or natural systems or infrastructure. Rather, the severity of such events depend also intrinsically on the exposure, vulnerability and/or resilience to such hazards of affected systems, including emergency management procedures. Similarly, high impact events may be compounded by the interaction of several, e.g., in their own right less severe hydro-meteorological incidents, sometimes separated in time and space. Or they may similarly result from the joint failures of multiple human or natural systems. Consequently, it is a deep transdisciplinary challenge to learn from past high impact events, understand the mechanisms behind them and ultimately to project how they may potentially change in a future climate.

The ECRA (European Climate Research Alliance) Collaborative Programme on “High Impact Events and Climate Change” aims to promote research on the mechanisms behind high impact events and climate extremes, simulation of high impact events under present and future climatic conditions, and on how relevant information for climate risk analysis, vulnerability and adaptation may be co-created with users, e.g., in terms of tailored climate services. For this aim, this Interdisciplinary and Transdisciplinary Session invites contributions that will serve to (i) better understand the mechanisms behind high impact events from a transdisciplinary and interdisciplinary perspective, e.g. case studies and the assessment of past high impact events, including detection and attribution; (ii) project changes to high impact events through, e.g. high resolution climate and impacts modelling (including economic modelling); (iii) produce climate information at the relevant scales (downscaling); and co-create climate services with users to help deal with the risk and/or impacts of high-impact events, e.g. risk analysis and climate adaptation. Abstracts that highlight recent advances from a transdisciplinary perspective for example through the innovation of climate services will be particularly encouraged. Authors and contributors to this session will be offered to present their work in a Special Issue of the journal “Sustainability”.

One of the big challenges in Earth system science consists in providing reliable climate predictions on sub-seasonal, seasonal, decadal and longer timescales. The resulting data have the potential to be translated into climate information leading to a better assessment of multi-scale global and regional climate-related risks.
The latest developments and progress in climate forecasting on subseasonal-to-decadal timescales will be discussed and evaluated in this session. This will include presentations and discussions of predictions for a time horizon of up to ten years from dynamical ensemble and statistical/empirical forecast systems, as well as the aspects required for their application: forecast quality assessment, multi-model combination, bias adjustment, downscaling, etc.
Following the new WCPR strategic plan for 2019-2029, prediction enhancements are solicited from contributions embracing climate forecasting from an Earth system science perspective. This includes the study of coupled processes, impacts of coupling and feedbacks, and analysis/verification of the coupled atmosphere-ocean, atmosphere-land, atmosphere-hydrology, atmosphere-chemistry & aerosols, atmosphere-ice, ocean-hydrology, ocean-ice, ocean-chemistry and climate-biosphere (including human component). Contributions are also sought on initialization methods that optimally use observations from different Earth system components, on assessing and mitigating the impacts of model errors on skill, and on ensemble methods.
We also encourage contributions on the use of climate predictions for climate impact assessment, demonstrations of end-user value for climate risk applications and climate-change adaptation and the development of early warning systems.

A special focus will be put on the use of operational climate predictions (C3S, NMME, S2S), results from the CMIP5-CMIP6 decadal prediction experiments, and climate-prediction research and application projects (e.g. EUCP, APPLICATE, PREFACE, MIKLIP, MEDSCOPE, SECLI-FIRM, S2S4E).

Solicited talk:
Multi-year prediction of ENSO
By Jing-Jia Luo from the Institute for Climate and Application Research (ICAR), Nanjing University of Science Information and Technology, China

Drought and water scarcity are important issues in many regions of the Earth, requiring innovative hydro-meteorological monitoring, modelling and forecasting tools to evaluate the complex impacts on the availability and quality of water resources. While drought describes a natural hazard, water scarcity is related to long-term unsustainable use of water resources and associated socio-economic aspects. Both phenomena are, however, closely linked, with the complex interrelationship requiring careful attention.
While an increase in the severity and frequency of droughts can lead to water scarcity situations, particularly in regions that are already water stressed, overexploitation of available water resources can exacerbate the consequences of droughts. In the worst case, this can lead to long-term environmental and socio-economic impacts. Particular attention should, therefore, be paid to the feedbacks between these two phenomena, including the potential impacts of climate change. It is therefore necessary to improve both monitoring and sub-seasonal to seasonal forecasting for droughts and water availability, and to develop innovative indicators and methodologies that translate the information provided into effective drought early warning and risk management.
This session will address statistical, remote sensing and physically-based techniques, aimed at monitoring, modelling and forecasting hydro-meteorological variables relevant to drought and/or water scarcity. These include, but are not limited to, precipitation, snow cover, soil moisture, streamflow, groundwater levels and extreme temperatures. The development and implementation of drought indicators meaningful to decision making processes, and ways of presenting and explaining them to water managers, policy makers and other stakeholders, are further issues to be addressed.
The session aims to bring together scientists, practitioners and stakeholders in the fields of hydrology and meteorology, as well as in the field of water resources and/or risk management; interested in monitoring, modelling and forecasting drought and water scarcity, and in analyzing their interrelationships, hydrological impacts, and the feedbacks with society. Particularly welcome are applications and real-world case studies in regions subject to significant water stress, where the importance of drought warning, supported through state-of-the-art monitoring and forecasting of water resources availability is likely to become more important in the future.

Intense rainfall and/or orographic precipitation events in small and medium size catchments can trigger flash floods, which are characterized by very short response times and high specific peak discharges. Under appropriate topographic conditions, such rainstorms also cause debris flows or shallow landslides mobilizing large amounts of unconsolidated material. Although significant progress has been made in the last decade in the management of flash floods related risks, these events remain poorly understood and their predictability is limited by a high non-linearity in the hydrological response, related to threshold effects and structured heterogeneity at all scales. In addition, predicting the initiation and runout of rainfall-induced landslides and their interactions with hydrological and hydraulic processes is still affected by large uncertainties. Therefore, improving the flash floods understanding, forecasting and risk management capacities requires multi-disciplinary approaches, as well as innovative measurements and modelling approaches as these events often occur in ungauged basins.

This session welcomes contributions illustrating current advances and approaches in monitoring, modelling, forecasting and warning flash floods and associated geomorphic processes. Contributions documenting the societal responses and impacts, and analysing risk management systems are also welcome. The session will cover the following main scientific themes:
- Development of new measurement techniques adapted to flash floods monitoring and quantification of the associated uncertainties
- Use of remote sensing data, weather radar, and lightning for improving forecasting models input data
- Development of modelling tools for predicting and forecasting flash floods and/or rainfall-induced landslides in gauged and ungauged basins
- Use of new criteria such as specific “hydrological signatures” for model and forecast evaluation
- Identification of processes leading to flash flood events and/or rainfall-induced landslides from data analysis and/or modelling, and of their characteristic space-time scales
- Evolutions in flash-flood characteristics possibly related to changing climate.
- Observation, understanding and prediction of the social vulnerability and social response to flash floods and/or associated landsliding
- Flash flood and/or rainfall-induced landslide risk assessment using multi-disciplinary approaches and warning systems, and evaluation of the relevance of those systems.

Ensemble hydro-meteorological prediction systems have higher forecasting skills than their deterministic counterparts, which in turn can improve risk assessment decision-making in operational water management. Ensemble forecasts are now common many operational settings, such as flood and drought forecasting, and can be used in applications from forecasting extreme events to optimisation of water resources allocation. However, moving from deterministic forecasting systems to a probabilistic framework poses new challenges but it also opens new opportunities for the developers and users of ensemble forecasts to improve their systems.

This session brings together scientists, forecasters, practitioners and stakeholders interested in exploring the use of ensemble hydro-meteorological forecast techniques in hydrological applications: e.g., flood control and warning, reservoir operation for hydropower and water supply, transportation and agricultural management. The session will also explore new forecast products and systems in terms of their implementation and practice for real-time forecasting.

Contributions will cover, but are not restricted to, the following topics:
- The design of ensemble prediction systems
- Requirements and techniques to improve the skill of hydro-meteorological ensemble forecasting systems
- Methods to bias correct and calibrate ensemble forecasts
- Methods to assess the quality or benchmark the performance of ensemble forecasts
- Approaches to deal with forecast scenarios in real-time
- Strategies for balancing human expertise and automation in ensemble forecasting systems
- Challenges of the paradigm shift from deterministic to ensemble forecasts
- Methods and products that include forecaster knowledge to improve the interpretation of ensemble forecasts
-Use of cost/loss scenarios for optimising systems
- Approaches for efficient training (including role-playing games) on the use and value of ensemble predictions.

The session welcomes new experiments and practical applications showing successful experiences, as well as problems and failures encountered in the use of uncertain forecasts and ensemble hydro-meteorological forecasting systems. Case studies dealing with different users, temporal and spatial scales, forecast ranges, hydrological and climatic regimes are welcome.

Solicited speaker Niko Wanders from Utrecht University: From seasonal forecasting to water management decisions: challenges and opportunities

The session is part of the HEPEX international initiative: www.hepex.org

This session will address the understanding of sources of predictability and quantification and reduction of predictive uncertainty of hydrological extremes in operational hydrologic forecasting. Including uncertainty estimation in operational forecasting systems is becoming a more common practice. However, a significant research challenge and central interest of this session is to understand the sources of predictability and development of approaches, methods and techniques to enhance predictability (e.g. accuracy, reliability etc.) and quantify and reduce predictive uncertainty in general. Providing uncertainty estimates for integrated catchment models involving forecasting models, either as a cascade or as alternative models, can prove particularly challenging and are an issue of interest to the session. Data assimilation or pre-/post-processing in real-time can provide important ways of improving the quality (e.g. accuracy, reliability) and reducing the uncertainty of hydrological forecasts. Methods that help update forecasts in real-time to reduce bias and increase accuracy, and case study demonstrations of their use, are of interest to this session.
The models involved with the methods for predictive uncertainty, data assimilation, post-processing and decision-making may include catchment models, runoff routing models, groundwater models, coupled meteorological-hydrological models as well as combinations of these. Demonstrations of the sources of predictability and subsequent reduction in predictive uncertainty at different scales through improved representation of model process (physics, parameterization, numerical solution, data support and calibration) and error, forcing and initial state are of special interest to the session.
Contributions are expected to address the following issues:
(i) Sources of predictability (model, forcing, initial conditions)
(ii) Quantification and reduction of predictive uncertainty
(iii) Real-time data assimilation
(iii) Untangling sources of uncertainty in the meteorological-hydrological forecasting chain
(iv) Effect of (improved) representation of model process on forecast quality and predictive uncertainty
(v) Methods for preparing meteorological predictions as input to real-time hydrological probability forecasts
(vi) Verification (methods) of hydrologic forecasts
(vii) Case studies of the above

Solicited speaker is Maurizio Mazzoleni (from Uppsala University) who will give a talk about Real-time assimilation of crowdsourced observations in hydrological and hydraulic models.

This interactive PICO session aims to bridge the gap between science and practice in operational forecasting for different water-related natural hazards. Operational (early) warning systems are the result of progress and innovations in the science of forecasting. New opportunities have risen in physically based modelling, coupling meteorological and hydrological forecasts, and ensemble forecasting. However, once a system is operational, the development often continues more in the field of applied research or consultancy. Furthermore, development of these types of systems is usually performed within one field of expertise. Forecasting warning research can be more effective when these efforts and experiences are combined.

The focus of this session will be on bringing the expertise from different fields together as well as exploring differences, similarities, problems and solutions between forecasting systems for varying natural hazards. Real-world case studies of system implementations - configured at local, regional and national scales - will be presented, including trans-boundary issues. An operational warning system can include monitoring of data, analysing data, making forecasts, giving warning signals and suggesting response measures.

Contributions addressing the following topics are welcome:
- Applications of forecasting warning systems for water-related natural hazards, such as: flood, drought, tsunami, landslide, hurricane etc.
- Applications of forecasting warning systems for other hazards, such as: pollution
- Operational data validation and calibration
- Operational warning methods and procedures
- Real time control for hazards
- The operational system as a tool for improved risk management and decision making
- Performance of operational forecasts, event analysis
- Serious games and training with operational systems
- Structure of operational forecasting systems
- Techniques/applications to better communicate forecasts with users - such as visualization tools and impact assessments
- Impact-based forecasts for early action, response and control.

Ongoing climate change and a shorter return period of climate and hydrological extremes has been observed to affect the distribution and vitality of ecosystems. In many regions, available water is a crucial point of survival. Risk can be enhanced by the exposure and/or by the vulnerability of the affected ecosystem.
The session focuses on the complex assessment of all determining factors through a joint utilization of a broad spectrum of databases and methods (e.g. field and laboratory measurements, remote sensing, modelling and monitoring techniques) that can provide a suitable basis for developing long-term strategies for adaptation.
The session should provide a multidisciplinary platform for sharing experiences and discussing results of local and catchment scale case studies from a wider range of relevant fields such as
• observed impacts and damage chains in natural ecosystems induced by climate and hydrological extremes;
• correlation between the underlying environmental factors (e.g. climate, water holding capacity, soil characteristics) and the distribution/vitality of ecosystems;
• integrated application or comparison of databases and methods for the identification and complex assessment of ecosystem responses to abiotic stress factors;
• expected tendencies of abiotic risk factors affecting and limiting the survival of the vulnerable species.
Contributions are encouraged from international experiences, ongoing research activities as well as national, regional and local initiatives.

Fire is a global phenomenon influencing ecosystem functioning, carbon stocks and fluxes, and atmospheric composition, with large impacts on human health, safety and economy. The relative importance of climate, vegetation and humans as drivers of fire activity varies across spatial and temporal scales. Multiscale and interdisciplinary assessments of fire behavior are required to understand global climate-fire feedbacks, as well as regional interactions between vegetation and humans, and fire.
Fire influences the global carbon cycle among others through its carbon emissions and post-fire ecosystem carbon sequestration. In addition, black carbon (also known as pyrogenic carbon, charcoal, soot) is a crucial component in the carbon cycle, yet uncertainties remain regarding sizes, losses and fluxes between land, rivers, oceans and atmosphere.
Remote sensing provides baseline information for all stakeholders involved in monitoring of biomass burning at different scales and for understanding how ecosystems respond to fires. However, there are still large uncertainties in satellite-based active fire, burned area, and fire emissions estimates, in part due to the complexity and diversity of the ecosystems affected. Building on the environmental significance and scientific challenges described above, this session will bring together fire scientists working on biomass burning monitoring and early warning systems. The aim of this session is to improve the understanding of interactions between fire, vegetation, carbon, climate and humans. We invite contributions developing or using remote sensing datasets, in situ observations, charcoal records, laboratory experiments and modeling approaches. We welcome studies that help to improve our understanding of (1) the relative importance of climate, vegetation and humans on fire occurrence across spatial and temporal scales (2) the impacts of fire on vegetation, atmosphere and society, (3) feedbacks between fire, vegetation and climate, and (4) the role of fire in the carbon cycle, with special focus on the transfer of black carbon and other fire markers from terrestrial ecosystems to aquatic environments, and their biogeochemical fate in these environments, (5) innovative use of remote sensing technologies (LIDAR, infrared cameras, drones) for fuel characterization, fire detection and monitoring; (6) algorithms/models applicable to regional-to-global scale fire analyses exploring active fire detection and characterization (e.g., fire radiative power, area affected, combustion phase), burned area mapping, atmospheric emissions and smoke transport, (7) fire product validation and error assessment, (8) analytical tools designed to enhance situational awareness among fire practitioners and early warning systems, addressing specific needs of operational fire behavior modeling.

Invited speakers:
Emilio Chuvieco, University of Alcala
Elena Kukavskaya, Sukachev Institute of Forest, Russian Academy of Sciences

Estimating the impact of climate change on both the natural and socio-economic environment plays an important role in informing a range of national and international policies, including energy, agriculture and health. Understanding these impacts, and those avoided, has never been more pertinent since the adoption of the 2015 Paris Agreement, which sought to hold “the increase in the global average temperature to well below 2°C above pre-industrial levels and to pursue efforts to limit the temperature increase to 1.5°C, recognizing that this would significantly reduce the risks and impacts of climate change".

Policies may aim to mitigate (i.e. reduce emissions), counteract (i.e. negative emissions) and/or adapt to anthropogenic climate change and it is equally important to quantify the impact of implementing these options. While rapid, deep mitigation is clearly a pre-requisite to success, delays to such measures imply a greater reliance upon large scale negative emissions technologies. Those based on land are likely to face competing pressure from wide ranging economic activity, and knowledge of these interactions and synergies is limited. Similarly while adaptation options are wide ranging, the uses of nature-based solutions, which often provide mitigation co-benefits and are often highly cost effective, are under-researched and rarely integrated into overall natural hazard or climate change risk management strategies.

Furthermore, the methods used to evaluate impact in the climate context are many and varied, including empirical, econometric and process-based. These methods continue to evolve implying that the assessment of impact may depend upon the analytical approach chosen.

This inter- and transdisciplinary session aims to draw together scientists, developing climate-impact evaluation methods, evaluating the impact (or avoided impact) of anthropogenic climate change upon natural and socio-economic environments, investigating the potential for mitigation and counteraction options to reduce long term risk, and studying the value of multiple adaptation options to stakeholders when planning how to manage vulnerability.

Invited speaker: Sonia Seneviratne