Weather and climate models need to represent dynamical and physical processes on very different timescales, ranging from milliseconds to millennia and spatial scales ranging from global long waves down to sub-kilometric scales. The models are generally built around a dynamical core together with several physical parameterizations which represent sub-grid processes inside the atmosphere, land or ocean. Nowadays, the distinction between climate models and weather prediction models is becoming more and more artificial, as NWP centres keep on incorporating additional physical and chemical processes and atmosphere-ocean interaction to improve forecast skill in the medium range, while the climate community is on the edge of entering non-hydrostatic scales. Beyond that, in state-of-the-art unified model frameworks, NWP and climate setups share the same dynamical core and differ only w.r.t. the complexity of physical and chemical processes and component models coupled to the dynamical core. The community is facing an exciting future, as recent trends towards variable resolution horizontal grids and more local numerical methods open up new possibilities, but also pose a lot of new challenges.

This session aims to bring together scientists from the NWP and climate community who have an interest in discussing and improving the numerics of atmospheric dynamical cores and the coupling of physical parameterizations or component models (like ocean or chemistry models). Computer scientists are also invited, to share their expertise on optimization and algorithmic efficiency on current and future supercomputer architectures. Examples of specific topics of the session are:
• general aspects of numerical schemes in NWP and climate models (temporal and spatial discretization, conservation properties, consistency);
• the evaluation, validation, criticism and intercomparison of different numerical schemes;
• new test cases for model components or the fully fledged model, as well as testing strategies in general;
• variable-resolution modelling (static and/or adaptive mesh refinement);
• scale-aware parameterizations and evaluation of parameterizations in terms of their sensitivity to spatial and temporal resolution
• strategies regarding the coupling between physical parameterizations and dynamics, both in complex GCMs or idealized frameworks (numerics, evaluation of different splitting methods, different grids for physics and dynamics, etc.);
• investigations and improvements regarding the consistency between physical parameterizations and the dynamical core or amongst parameterizations, in terms of the underlying equations or numerical solvers;
• optimization and algorithmic efficiency on current and future supercomputer architectures;

This session presents and explores the increasingly sophisticated systems developed to aid, and often automate, the forecasting and warning process. The rapid proliferation of data available, including probabilistic and rapidly-updating NWP as well as a plethora of observations, combined with a growing appreciation of user needs and the importance of timely and relevant forecasts, has brought the development of these systems to the fore. The opportunities afforded by the WMO's HiWeather programme will also be discussed in this session.

Topics may include:
• Nowcasting systems
• Links to severe weather and severe weather impacts
• Automated first guess warning systems
• Post-processing techniques
• Seamless deterministic and probabilistic forecast prediction
• Use of machine learning and other advanced analytic techniques

The Sendai Framework for Disaster Risk Reduction 2015-2030 states that the implementation of effective disaster risk reduction measures should be based on an understanding of disaster risks, including all aspects of vulnerability, capacity, exposure of persons and hazard characteristics. Understanding these disaster risks establishes the basis for the development of impact models and impact-based warnings.
In recent years there has been increasing interest in multi-hazard impact-based warning systems to reduce the impact of natural disasters. For example, in 2015 the World Meteorological Organisation (WMO) published ‘WMO Guidelines on Multi-hazard Impact-based Forecast and Warning Services’ for National Meteorological and Hydrological Services and their partner agencies responsible for issuing warnings. Impact-based warnings use a combination of likelihood and potential impact to assess the risk of a hazard event and therefore which warning to issue. The whole decision chain from assessing the likelihood of a hazard and potential impacts to deciding which warning severity to issue to warning communication and verification is complex. For many hydrometeorological events the likelihood can be assessed by using ensemble prediction systems, but an assessment of impacts is often highly subjective. To aid the decision making process hazard impact models and methods for multi-hazard assessment have been developed. These are at the cutting-edge area of research and require a multi-disciplinary partnership approach. This research not only spans hydrometeorology but all natural hazards ensuring that people receive the best advice and information to build their resilience and prepare for natural hazard events.

This session invites presentations from all natural hazard areas on:
• Impact based warning systems that are being developed or have been implemented across the world
• The decision making process of meteorologists and other hazard specialists in issuing warnings
• Research and development of explicit hazard impact models and multi-hazard systems
• How impact models are used to aid the decision making process
• Verification of impacts and collection of impact data
• Communication of this information to governments, civil contingencies, the responder community and the public.

For both the atmosphere and the ocean, the quality of a model prediction depends on the accurate determination of the initial state and on a sophisticated model to simulate the subsequent evolution realistically. This session will focus on the use of observations, on data assimilation techniques which are developed or implemented in meteorology and oceanography, and on the observation impact.

Active research is carried out on algorithmic aspects of data assimilation such as :
• Intercomparison and study of the complementarity between different assimilation techniques : Kalman filtering, variational assimilation, nudging techniques for frequent analysis cycles used in nowcasting, etc...
• Variational techniques with longer assimilation windows and weak constraint methods to allow for the inclusion of model error estimates.
• Ensemble based assimilation systems and flow dependent estimation of background error statistics.
• Coupling strategies for the analysis of the atmosphere, the ocean and continental surfaces.
• Model error formulations in data assimilation.

This session will also accept papers about:
• The impact of observations assessed through Observing System Experiments (OSEs), Observing System Simulation Experiments (OSSEs), or sensitivity studies based on adjoint computations...
• Targeting strategies and observation network design.
• The impact of innovative observing systems on meso-scale NWP models, especially in the context of rapidly developing convective systems.

The session will focus on the most recent developments in the field of ensemble techniques, ranging from its close connections with data-assimilation and nowcasting at short and medium ranges (up to 2 weeks) to their capacity to produce and deliver skilful and reliable forecasts of high-impact extreme events at sub-seasonal to seasonal (S2S) timescales.
As such it may provide a platform for exchanging ideas on how to create and use an ensemble system, techniques varying according to the forecast lead time. In particular, the forecaster perspective and the use of ensembles in predicting hazardous weather will be of interest.

The conveners invite papers on various issues associated with Ensemble Forecasting for weather prediction, such as:
• representation of initial uncertainties in Global and Limited-Area Ensemble Prediction Systems, including interlinks between data-assimilation and probabilistic forecasting;
• representation of model or boundary uncertainties in Global and Limited-Area Ensemble Prediction Systems;
• results from experiments including THORPEX Regional Campaigns, HyMeX, FROST-2014, etc.;
• results from recent studies using TIGGE and TIGGE-LAM databases;
• use, verification and calibration methods of Ensemble Prediction Systems;
• applications of probabilistic forecasts in the sectors of energy, health, transport, agriculture, insurance and finance;
• challenges tackled by the S2S WWRP/THORPEX-WCRP joint project (http://s2sprediction.net), including discussion on S2S sources of predictability, forecasts and socio-economic applications of high-impact climate services.

Participants are especially encouraged to present contributions and discuss strategies to bridge gaps between stakeholders and actionable S2S tailored products.

We welcome the submission of papers covering all verification aspects from research to applications to general verification practice and across all time and space scales, from nowcasts to climate projections. It is desirable that the focus of contributions be on forecast applications serving the society in one way or another and, consequently, assessing the potential adaptation of such applications in practice.

Highly welcome verification subjects and themes include high-impact, user oriented applications, warnings against adverse weather events or events with high risk or user relevance (for example energy, agriculture sectors or health issues) and multi-disciplinary approaches in various fields (for example hydrology, transport).

Papers on verification methods taking into account observation uncertainty, verification of probabilistic and ensemble forecasts, spatial verification techniques, methodologies to assess operational forecast products and services as well as communication of verification information to end-users and decision-makers are encouraged.

The sub-session on "Mesoscale Verification Inter-Comparison over Complex Terrain" (MesoVICT) is an integral part of the Forecast Verification session. The project, launched at the 2013 EMS/ECAM meeting, and which is likely to conclude by early 2019, focuses on the analysis and development of spatial verification methods using case studies from the MAP D-PHASE/COPS data sets over the Alps. For this sub-session we invite submissions on spatial verification techniques over complex terrain.

In every day work operational meteorologists face up interesting, challenging and even dangerous weather phenomena when nowcasting and monitoring short to medium range forecasts. Several meteorologists are also interested in climate and are following extended to seasonal range forecasts. Combining this knowledge, meteorologist are trying to find new ways to improve the predictability and developing new services for both nowcasting and longer range forecasts.
In this session we are focused in case studies on local hazards and how the warning process in these cases has worked, or could be improved. We are also interested in climate information's role in operational forecasting such as the use of climate information in impact warnings and extended-range forecasts. The session focuses firmly on the operational experience and is in association with the Working Group for Cooperation between European Forecasters (WGCEF).

Papers on the following topics are welcome:
· Case studies on local hazards and how the warning process has worked, or could be improved
· Climate information's role in operational forecasting
· Extended-range forecasts: applicability on operational forecasting
· Towards seamless forecasting

The session will handle various aspects of scientific and operational collaboration related to limited area numerical weather prediction models. It will include assessment of how limited area NWP consortia in Europe can work together and the common challenges in developing high resolution mesoscale models even at the sub-kilometre scale. The session will also discuss the collaboration between NWP developer and user (especially forecasters) communities and the best practices in communication between these groups.

More specifically, the session may include:
* Past and future activities related to the C-SRNWP Module of EUMETNET and other EUMETNET activities with a relevance to SRNWP (OPERA, E-ABO, E-ASAP, E-PROFILE, E-SURFMAR and EUCOS)
* Observation impact studies performed in order to assess the importance of different parts of the observing system for global and limited area NWP models
* Development of the Global Lake Database (currently supported by EUMETNET) and its applications in NWP and climate models
* The SRNWP Surface Data Pool, and its application for the verification of land surface models
* Possible collaboration on developing post-processing systems for probabilistic predictions at European level
* NWP user/developer interactions

The Weather Research and Forecasting model (WRF) is a widely used high-resolution meteorological model for operational weather forecasting, fundamental and applied research in meteorology, air quality, wind energy engineering, and consultancy studies. Its user’s community consists of universities, weather forecasters, and consultancy agencies world-wide. The goal of this session is to create a European forum to discuss research results concerning all aspects of the WRF modelling framework.
Papers are invited on:
• Initialization, and meteorological and land surface boundary conditions.
• Numerical and grid spacing aspects
• Studies concerning data assimilation.
• Development of physical parameterization schemes.
• Model evaluation and validation against a broad range of available observations.
• Future WRF development.
• Tailored WRF versions, e.g. polar WRF, WRF-LES, WRF-Chem, H-WRF, the WRF single-column model
• WRF applications in weather forecasting, air quality studies, wind energy engineering.
• Regional climate studies
• Mesoscale meteorological phenomena studied with WRF.