The simulation of the coupled evolution of atmospheric dynamics, pollutant transport, chemical reactions and atmospheric composition is one of the most challenging tasks in environmental modelling, climate change studies, and weather forecasting for the next decades as they all involve strongly integrated processes. Weather strongly influences air quality (AQ) and atmospheric transport of hazardous materials, while atmospheric composition can influence both weather and climate by directly modifying the atmospheric radiation budget or indirectly affecting cloud formation. Until recently, however, due to the scientific complexities and lack of computational power, atmospheric chemistry and weather forecasting have developed as separate disciplines, leading to the development of separate modelling systems that are only loosely coupled.

The continuous increase in computer power has now reached a stage that enables us to perform online coupling of regional meteorological models with atmospheric chemical transport models. The focus on integrated systems is timely, since recent research has shown that meteorology and chemistry feedbacks are important in the context of many research areas and applications, including numerical weather prediction (NWP), AQ forecasting as well as climate and Earth system modelling. However, the relative importance of online integration and its priorities, requirements and levels of detail necessary for representing different processes and feedbacks can greatly vary for these related communities: (i) NWP, (ii) AQ forecasting and assessments, (iii) climate and earth system modelling. Additional applications are likely to benefit from online modelling, e.g.: simulation of volcanic ash or forest fire plumes, pollen warnings, dust storms, oil/gas fires, geo-engineering tests involving changes in the radiation balance.

The COST Action ES1004 – European framework for online integrated air quality and meteorology modelling (EuMetChem) – aims at paving the way towards a new generation of online integrated atmospheric chemical transport and meteorology modelling with two-way interactions between different atmospheric processes including dynamics, chemistry, clouds, radiation, boundary layer and emissions. As its first task, we summarise the current status of European modelling practices and experience with online coupled modelling of meteorology with atmospheric chemistry including feedback mechanisms and attempt reviewing the various issues connected to the different modules of such online coupled models but also providing recommendations for coping with them for the benefit of the modelling community at large

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This chapter describes a new World Meteorological Organisation (WMO) approach to the provision of science-based services to assist the planning of safe, healthy, resilient and climate-friendly cities. The approach is outlined in full in the WMO Guidance on Integrated Urban Hydrometeorological, Climate and Environmental Services (Volume I) publication (WMO 2019), which has been approved for promotion and communication to WMO Members. Highlighted here are the underlying concepts and methods that underpin the services to help manage cities on a day-to-day basis, responding to short-term hazards but also to prepare for climate change are described. Heterogeneous dense observation networks, highresolution forecasts, multi-hazard early warning systems, long-term climate prediction and service applications all working in combination, are at the heart of such services, the implementation of which will support mitigation and adaptation strategies for achieving thriving climate resilient cities and promotion of the United Nations Sustainable Development Goals (WMO, 2021). To meet the needs of urban areas, the concept of Integrated Urban Hydrometeorological, Climate and Environmental Services (IUS) is bedded in a multidisciplinary approach that identifies the requisite partnerships, including research agencies, city governments, international organizations, and private sector stakeholders, to establish and sustain urban services. As this is an emerging and diverse service, the requirements are broad with considerable research, development, capacity building and cross-service integrations yet to be done. In addition to providing an overview of the concept, we set out the challenges and recommended path forward to develop and implement IUS. Keywords Integrated urban services; Multi-hazard early warning systems; Weather; Climate; Environment; Air Quality; Hydrology; World Meteorological Organization; Challenges and the way forward; Research and development; Seamless earth system prediction; Scales Key Messages • WMO's participation in the UN New Urban Agenda includes developing both the concept and guidance for the provision of multi-hazard early warning to long term urban planning services for cities. • Integration of weather, hydrology, environment and climate urban services (IUS) will lead to accurate, efficient and consistent services. • National Hydrometeorological Services are positioned to lead the promotion and development of Integrated Urban Services (IUS). • Understanding capabilities, capacities and roles and responsibilities amongst different levels of government, universities, agencies and decision-makers is the first challenge followed closely by data access and the co-design of the IUS. • Research and science will play a significant role to develop multi-disciplinary scientists to bridge the scientific gaps and to demonstrate the effectiveness of IUS.