Citation:

Galmarini, S., I. Kioutsioukis, E. Solazzo, U. Alyuz, A. Balzarini, R. Bellasio, A. Benedictow, R. Bianconi, J. Bieser, J. Brandt, J. Christensen, A. Colette, G. Curci, Y. Davila, X. Dong, J. Flemming, X. Francis, A. Fraser, J. Fu, D. Henze, C. Hogrefe, U. Im, M. Garcia Vivanco, P. Jimenez-Guerrero, J. Jonson, N. Kitwiroon, A. Manders, R. Mathur, L. Palacios-Pena, G. Pirovano, L. Pozzoli, M. Prank, M. Schultz, R. Sokhi, K. Sudo, P. Tuccella, T. Takemura, T. Sekiya, AND A. Unal. Two-scale multi-model ensemble: is a hybrid ensemble of opportunity telling us more? Atmospheric Chemistry and Physics. Copernicus Publications, Katlenburg-Lindau, Germany, 18:8727-8744, (2018). https://doi.org/10.5194/acp-18-8727-2018

Impact/Purpose:

Previous work has demonstrated that combining outputs from multiple air quality models into an ensemble estimate increases the usefulness of such outputs for forecasting and impact assessment applications. While a number of studies have focused on developing approaches for optimizing the construction of such ensembles, there has been no research on quantifying to which extent, if any, the scale at which air quality models are operated (global or regional) affects the characteristics of the resulting ensemble. Leveraging on the coordinated modeling activities of the Task Force on Hemispheric Transport of Air Pollution (TF-HTAP) and the Air Quality Model Evaluation International Initiative (AQMEII), this study compares ozone ensembles constructed from only global models, only regional models, and both global and regional models. Results show that the combined ensemble improves upon the global-only and regional-only ensembles, suggested that results from both groups of models complement each other and combining them leads to an improved characterization of air quality.

Description:

In this study we introduce a hybrid ensemble consisting of air quality models operating at both the global and regional scale. The work is motivated by the fact that these different types of models treat specific portions of the atmospheric spectrum with different levels of detail and it is hypothesized that their combination can generate an ensemble that performs better than mono-scale ensembles. A detailed analysis of the hybrid ensemble is carried out in the attempt to investigate this hypothesis and determine the real benefit it produces compared to ensembles constructed from only global scale or only regional scale models. The study utilizes 13 regional and 7 global models participating in the HTAP2/AQMEII3 activity and focuses on surface ozone concentrations over Europe for the year 2010. Observations from 405 monitoring stations are used for the evaluation of the ensemble performance. The analysis first compares the modelled and measured spectra and then assesses the properties of the mono-scale ensembles, particularly their level of redundancy, in order to inform the process of constructing the hybrid ensemble. The main conclusion of this study is that the improvements obtained by the hybrid ensemble relative to the mono-scale ensembles can be attributed to its hybrid nature. Moreover, the optimal set is constructed from an equal number of global and regional models at only 15% of the stations. Finally, the study reaffirms the importance of an in-depth inspection of any ensemble of opportunity in order to extract the maximum amount of information and to have full control over the data used in the construction of the ensemble.

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