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General background

Although atmospheric chemical transport (ACT) models can be coupled to numerical weather prediction (NWP) models either off-line or on-line at present, a scientific perspective of chemical weather forecasting (CWF) would argue for an eventual migration from off-line modelling (where the ACT model is run after the NWP model is completed) to on-line modelling, allowing coupling and integration of the physical and the chemical components of CWF systems. Specifically, better and more complete representations of physical and chemical processes and interactions in the models are needed. One of the key outcomes of the finalizedCOST-728 is to suggest a European strategy for integrated atmospheric Chemical Transport (ACT) / Numerical Weather Prediction (NWP) and Climate (CLIM) modelling that would result in a European framework (see: Based on this and recent discussions with interested researchers at workshops held in Lecce, Copenhagen, Reading, Geneva and at ECMWF and WMO a new Action is suggested. The Action focus will be on the online coupled ACT/NWP-CLIM modelling with two-way feedbacks and development of a European framework for integrated modelling systems.

Historically Europe has not adopted a community approach to modelling and this has led to a large number of model development programmes, usually working independently. However, a strategic framework (within a COST Action) will help to provide a common goal and direction to European research in this field while having multiple models.

This COST Action seems to be the best approach to integrate, streamline and harmonize the interaction between atmospheric chemistry modellers, weather modellers and end users. It will lead to strongly integrated and unified tools for a wide community of  cientists and users. Previously, air quality and numerical weather prediction (NWP) were developed separately. With more advanced NWP models and air quality models it became easier to study urban-scale air pollution and therefore allow the integration of NWP and air quality (AQ) (Baklanov et al., 2002). The strategy of integrated modelling and on-line coupling of air quality and meteorology modelling has a good perspective in future, therefore this Action will help building wide useful information for the community based on the new generation of integrated 'environment - weather' forecasts.

The COST Action funds will be used only for networking and capacity-building activities, the main research developments will be funded by relevant collaborating EU FP7 research projects (e.g. MACC, MEGAPOLI, CityZen, PEGASOS, PBL-PMES, see more in Sec. B4) and by national programs of the participating countries.

Current state of knowledge

The prediction and simulation of the coupled evolution of atmospheric transport and chemistry will remain one of the most challenging tasks in environmental modelling over the next decades. Many of the current environmental challenges in weather, climate, and air quality involve strongly coupled systems (see overview in Zhang, 2008; Baklanov et al., 2008a). It is well accepted that weather is of decisive importance for air quality, or for the aerial transport of hazardous materials. It is also recognized that chemical species will influence the weather by changing the atmospheric radiation budget as well as through cloud formation. Until recently however, because of the complexity and the lack of appropriate computer power, air chemistry and weather forecasts have developed as separate disciplines, leading to the development of separate modelling systems that are only loosely coupled (offline). In NWP, the dramatic increase in computer power enables us to use higher resolution to explicitly resolve fronts, convective systems, local wind systems, and clouds, or to increase the complexity of the numerical models. Additionally we can now directly couple air quality forecast models with numerical weather prediction models to produce a unified modelling system – online – that allows two-way interactions. While climate modelling centres have gone to an Earth System Modelling approach that includes atmospheric chemistry and oceans, NWP centres as well as entities responsible for Air Quality forecasting are only beginning to discuss whether an online approach is important enough to justify the extra cost (IFS, 2006; Grell, 2008; Baklanov et al, 2008b, 2010). NWP and AQ forecasting centres may have to invest in additional computer power as well as additional man power, since additional expertise may be required. We are in favour of integrating weather and chemistry together, for both NWP and air quality and chemical composition forecasting.

For NWP centers, an additional attractiveness of the online approach is its possible usefulness for meteorological data assimilation (Hollingsworth et al., 2008), where the retrieval of satellite data and direct assimilation of radiances will likely improve – assuming that the modelling system can beat climatology when forecasting concentrations of aerosols and radiatively active gases.

Reasons for the Action

The focus on integrated systems is timely, since recent research has shown that meteorology/climate and chemistry feedbacks are important in the context of many research areas and applications, including NWP, climate modelling, air quality forecasting, climate change, and Earth system modelling. Potential impacts of aerosol feedbacks include (Jacobson et al., 2007; Zhang, 2008; Baklanov et al., 2008; Baklanov, 2010; Grell and Baklanov, 2010; Zhang et al., 2010a, b):

  • a reduction of downward solar radiation (direct effect);
  • changes in surface temperature, wind speed, relative humidity, and atmospheric stability (semi-direct effect);
  • a decrease in cloud drop size and an increase in drop number by serving as cloud condensation nuclei (first indirect effect);
  • an increase in liquid water content, cloud cover, and lifetime of low level clouds, and suppression or enhancement of precipitation (second indirect effect).

Traditionally, aerosol feedbacks have been neglected in NWP and air quality modelling mostly due to an historical separation between the meteorological and air quality communities as well as a limited understanding of the underlying interaction mechanisms. Such mechanisms may, however, be important on a wide range of temporal and spatial scales, from days to decades and from global to local. Field experiments and satellite measurements have shown that chemistry-atmosphere feedbacks exist among the Earth systems including the atmosphere (e.g., Kaufman and Fraser, 1997; Rosenfeld, 1999; Rosenfeld and Woodley, 1999; Givati and Rosenfeld, 2004; Grell et al., 2005; Lau and Kim, 2006; Rosenfeld et al., 2007, 2008).

Current overviews showed that there exist a number of online coupled ACT/NWP-CLIM modelling systems in Europe (see an overview of existing online coupled models in Europe in Baklanov et al., 2008 and in Table 1 of Part II). However, many of such models were not built for local to regional scales, but they are global-scale systems including only relatively simplified chemistry components (e.g., MESSy). Some meso/regional-scale online integrated modelling systems are developing today (WRF-Chem, Enviro-HIRLAM, RegCM-Chem, etc.) which consider feedbacks with direct and indirect effects of aerosols and radiatively active gases. However, more research remains to be done to develop and evaluate advanced integrated coupled models covering the full range of spatial and temporal scales and applications from NWP to climate change simulation. The Action plans to build a network of European and non-European groups involved in the development of coupled ACT/NWP-CLIM models, ranging from global to regional and local, and from meteorology to climate time scales. Such a network would facilitate the intercomparison of models, collection of observations suitable to evaluate the models and sharing of knowledge on chemistry-meteorology/climate feedbacks.

Complementarity with other research programmes

This COST Action will be closely linked with, and complementary to many current and planned European research FP7 projects: MACC, MEGAPOLI, CityZen, PEGASOS, PBL-PMES, TRANSPHORM; International programmes: WMO GAW GURME, IGBP/IGAC, GEMES, COSMOS, AQMEII initiative; ongoing COST Action ES0602 Chemical Weather and proposed COST Action ‘Air Quality and Climate Interactions and Implications for European Air Quality’. All the European NWP communities: the ALADIN, COSMO, HIRLAM and UK UM are actively participating in this COST Action.

Most of the active participants of the COST Action are already deeply involved in the integrated modeling activities, e.g., in the scope of ongoing above mentioned EU FP7 research projects, EU educational projects TEMPUS 26005 “COMBAT METEO”, and TEMPUS 159352 “QUALIMET, Nordic Network projects NetFAM and MUSCATEN, and a number of national research projects. The importance of integrated systems and such COST Action is fully understood and supported by the research community. The COST Action is not duplicating the aims of these research projects, but focusing on the European framework, networking and capacity-building activities.