Weather prediction has improved tremendously over the last decades. Ultimately, however, there are limits in predictability due to the multi-scale, non-linear nature of atmospheric dynamics.
To further improve our mechanistic understanding of atmospheric dynamics and to further improve numerical weather forecasting it is increasingly important to better understand the physical and dynamical processes connecting atmospheric motions across temporal and spatial scales. This includes, for example, the identification of the limits of predictability of different weather systems. For longer time scales, identifying and understanding sources of predictability and variability is of crucial importance. This session will therefore discuss the current understanding of physical and dynamical processes determining predictability and variability on different scales.

Contributions are invited that attempt to improve our understanding of atmospheric dynamics or that link process-based, dynamical understanding and predictability aspects. Atmospheric phenomena on all spatial and temporal scales are of interest. Particularly welcome are contributions that focus on high-impact weather, the sub-seasonal to seasonal (S2S) timescale, and related extremes. This may include, but is not limited to, the influence of remote factors (e.g., the stratosphere, the Artic, or the tropics) on the midlatitudes, predictability in the tropics and polar regions, stationary and recurrent systems (e.g. associated with heat waves, cyclone clustering, heavy precipitation), or processes driving seasonal or interannual variability.

Atmospheric Boundary-Layer processes on a variety of temporal and spatial scales strongly influence weather, air quality and climate. These processes include atmospheric turbulence, atmosphere-soil-vegetation interactions, gravity waves, boundary layer interactions with dry and moist convection, mesoscale flows, et cetera. These processes should be well-understood to enable accurate and skilled weather and air quality forecasting, as well as reliable climate model and scenario studies to serve society with high quality weather and climate information.

This session welcomes contributions on research into the understanding of boundary-layer processes and turbulence from either a conceptual, or modelling or observational perspective, their role and interactions and finally their implementation in atmospheric modelling.
Papers dealing with the following topics are invited:
• Theoretical and experimental studies of the turbulence-closure problem with emphasis on very stable stratification and convection, accounting for interactions between the mean flow, turbulence, internal waves and large-scale self-organized structures
• Boundary-layer clouds and marine, cloud-topped boundary layers: physics and parameterization within NWP and climate models
• Orographic effects: form drag, wave drag and flow blocking, gravity waves
• Challenges on the surface exchange processes, flux aggregation in atmospheric boundary layers over heterogeneous terrain
• Representation of boundary layers in atmospheric models
• Organization of deep convection across differing atmospheric scales
• Large-eddy simulation and direct numerical simulation of turbulent flows.

This session will welcome all technical and scientific contributions devoted to increasing our understanding of atmospheric phenomena that might represent a hazard for people, property and environment. Studies devoted to enhance physical understanding of severe weather phenomena (for example deep convection or intense straight lines winds) are of particular interest even if the severe weather phenomena are not linked directly to a specific hazard. Embracing the proposal given by the organizers for this year, particularly welcome will be the contributions dealing (directly or indirectly) with the Artic area or underlining aspects connected to artic drivers of atmospheric hazards.
Moreover, in line with the suggestions given by EMS 2019 meeting committee, we would encourage contributors to underline intercultural aspects of their methods and findings, and to point their attention not only to the physical and meteorological characteristics of atmospheric hazards, but also to their relevance in a changing climate including possible impacts on human activities and the environment.
Contributions dealing with studies of specific episodes (case studies) will be welcome, provided they further increase physical understanding and are representative at least for the area where these events took place.
Particularly welcome will be contributions incorporating both numerical and conceptual modelling to improve our understanding of severe weather phenomena.
In general we will encourage the exchange of expertise and experiences related to the various topics connected to hazardous atmospheric phenomena and severe weather events. For this reason an interdisciplinary approach will be particularly welcome.
Potential topics for this session include i.a.:
• Flash-floods and heavy rain events;
• Hail;
• Freezing rain, icing and intense snow falls;
• Cold/heat events, even those occurring at small time scales;
• Fog;
• Tornadoes, waterspouts, derechos and downbursts;
• Severe wind storms;
• Intense Mediterranean cyclones;
• Tropical like cyclones;
• Lightning;
• Polar lows, their evolution and impacts;
• Severe katabatic or foehn winds;
• Gap and orographic flows;
• Breaking of gravity waves, as well as severe turbulence;
The above-listed topics are of course not exclusive and the session’s Conveners eagerly anticipate papers on new ideas and approaches and novel understanding covering all aspects of atmospheric hazards and severe weather events.

Forecasting wind gusts may become the next major challenge in numerical weather prediction. With increasing computer power, operational NWP systems just entered the convective scale, allowing the model physics to simulate convective processes more explicitly. While this is very beneficial for precipitation forecasting, wind gusts are still a sub-grid scale phenomena relying on crude parametrizations. Furthermore, wind gusts cause large socio-economic damages every year. Wind gust predictions are getting higher relevance and load, e.g. for transport, aviation, urban development or public weather warnings. Yet forecast verification exhibits exceptionally low skill for wind gust predictions compared to other meteorological variables, which might also be impacted by a very sparse observational network. The spatial variability of wind gusts is probably as large as that of precipitation, but the observational network is much less dense and no equivalent to the spatial coverage of radar derived precipitation exists.

This session welcomes contributions which lead to a better understanding of the physical processes that determine wind gusts, and novel ideas/methods to improve wind gust forecasting and warnings in the future. More specifically, contributions on the following topics are welcome:

- Observations: The development of novel measurement tools for wind gusts (e.g. WindLIDARS) and suggestions for an optimized observational network in the future. Descriptions of the spatio-temporal variations of gusts.

- Explicit modelling: Small-scale model simulations (e.g. LES simulations) are a prerequisite to explicitly resolve the processes leading to wind gusts. Beside a better understanding of the physical processes they can be utilized to improve empirical approaches to approximate wind gusts more accurately.

- Wind gust forecasting and warnings: Methods to obtain guidance for wind gusts forecasts and warnings from operational weather forecasts, e.g. using historical observations by statistical postprocessing or forecast assimilation techniques. Prediction uncertainty of wind gusts.

- Climate monitoring: Long-term data sets for wind gusts as well as techniques for spatial wind gust analysis which are necessary for climate change adaptation and mitigation strategies

- Evaluation: The high-resolution model simulations on the one side and a sparse observational network on the other side require novel ideas in the verification of wind gusts simulations and warnings.

Measurements are essential to provide information on the actual state of the atmosphere for nowcasting purposes, for climate monitoring, for assimilation into numerical weather prediction (NWP) systems and to improve our understanding of atmospheric processes and their role in the climate system. In particular, there is a strong need for complex observations suitable to develop, improve and validate parameterizations used in NWP and climate models and to provide ground-truth against which to compare atmospheric parameters derived from satellite data. With a new generation of high-resolution forecast models (1-3 km) used for the prediction of high-impact weather, dense observational networks focusing on measurements in the lower few kilometers of the atmosphere are required.
This session is intended to give a forum to discuss recent developments and achievements in local to regional measurement concepts and technology. There will be a special emphasis on measurements which seek to improve our understanding of complex atmospheric processes – especially those characterizing interactions in the climate system – through obtaining comprehensive data sets. The focus is on measurements of atmospheric dynamics and thermodynamics, energy and water cycle components, and on the interaction of the atmosphere with the underlying surface. With respect to the special conference focus on the Arctic, contributions on measurements from that region are specifically invited.
The session will also include consideration of novel measurement approaches and networks under development for future operational use, e.g., within the frame of the Eumetnet observations program, and the performance of new measurement techniques. Manufacturers of hydro-meteorological instruments and system solutions are thus explicitly invited to present news on sensor development, sensor performance and system integration. Reports on metrological aspects of meteorological measurements are equally welcome.
Techniques may cover in-situ and remote sensing measurements from various platforms. Special attention will be given to the creation of a new generation of reliable unmanned instrument networks across Europe that provide calibrated and controlled data on the boundary layer structure in near-real time. Contributions are also invited that make use of advanced data sets for satellite data validation.

This session provides a platform for contributions on high-resolution precipitation measurements, analyses, and applications in real-time as well as climate studies. Monitoring and statistical analyses of precipitation at small spatial and temporal scales are challenging. Therefore, special focus is placed on documenting the benefit of highly spatially and temporally resolved observations of different measurement platforms, e.g. satellites, radar networks, or opportunistic sensing, e.g. retrieving rainfall from microwave links. Papers on monitoring and analyzing extreme precipitation events including extreme value statistics, multi-scale analysis, and event-based data analyses are especially welcome, comprising definitions and applications of indices to characterize extreme precipitation events, e.g. in public communication. Contributions on long-term observations of precipitation and correlations to meteorological and non-meteorological data with respect to climate change studies are cordially invited. In addition, contributions on the development and improvement of gridded reference data sets based on in-situ and remote sensing precipitation measurements (e.g., GPCC, OPERA) are welcome.
High-resolution measurements and analyses of precipitation are crucial, especially in urban areas with high vulnerabilities, in order to describe the hydrological response and improve water risk management. Thus, this session also addresses contributions on the application of high-resolution precipitation data in hydrological impact and design studies.
Summarizing, one or more of the following topics shall be addressed:
• Precipitation measurement techniques
• High-resolution precipitation observations from different platforms (e.g., gauges, disdrometers, radars, satellites, microwave links) and their combination
• Precipitation reference data sets (e.g., GPCC, OPERA)
• Statistical analysis of extreme precipitation (events)
• Multi-scale analysis, including sub-kilometer scale statistical precipitation description and downscaling methods
• Definition and application of indices to characterize extreme precipitation events
• Climate change studies on extreme precipitation (events)
• Urban hydrology and hydrological impact as well as design studies
• New concepts of adaptation to climate change with respect to extreme precipitation in urban areas

This session invites contributions to discuss the present meteorological conditions in the Polar Regions (Arctic and Antarctic) and changes of these induced by climatic drivers.
Since 1980, near-surface climate warming in the Arctic has proceeded at approximately twice the global rate. Simultaneously with the warming, the Arctic has experienced notable changes such as decrease in sea ice extent and thickness and glacier retreat. These changes have been accompanied by changes in precipitation and in the large‐scale atmospheric circulation patterns.
The Polar atmosphere has very specific features such as shallow boundary layers, stratified conditions with low mixing and extreme insolation characteristics during large part of the year. These flow and turbulence patterns are important for exchange of heat, GHG and cloud condensation nuclei between the surface and the atmosphere, as well as for dispersion and mixing into higher elevation.
Changes in surface temperature and sea ice and glacier extent affect the atmospheric flow, turbulence and stratification, as well as snow properties, which in turn affects radiation processes and balance. In order to estimate future implications of climate change in the Polar Regions, it is important to understand the present meteorology as well as its development due to surface and temperature changes. This has to be achieved through combining measurements and modelling of local to regional scale meteorology. More specifically but not exclusively the session will address:
• Past and present Polar meteorological conditions
• Processes contributing to Polar amplification of climate warming
• Extremely shallow and stratified boundary-layers,
• Physics and occurrence of Arctic clouds, precipitation and haze
• Observations in Polar regions: challenges, experiences, networks and demand
• GHG concentrations development and GHG exchange with the cryosphere (e.g. permafrost thawing)
• Polar climate change trends and their local and remote impacts on different time scales
• Meso- and micro scale modelling for Polar predictions
• Long-range transport routes and emissions in Polar Regions
• Parameterization of specific processes for modelling the Polar atmosphere.
• Contribution to and support from international programmes: WMO Year of Polar Prediction, Arctic Monitoring and Assessment Programme, Sustaining Arctic Observing Networks, MOSAiC drifting station, International Arctic Systems for Observing the Atmosphere, Pan-Eurasian Experiment, etc.