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Volume 18, issue 12
Atmos. Chem. Phys., 18, 8727-8744, 2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Special issue: Global and regional assessment of intercontinental transport...

Atmos. Chem. Phys., 18, 8727-8744, 2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 21 Jun 2018

Research article | 21 Jun 2018

Two-scale multi-model ensemble: is a hybrid ensemble of opportunity telling us more?

Stefano Galmarini1, Ioannis Kioutsioukis2, Efisio Solazzo1, Ummugulsum Alyuz3, Alessandra Balzarini4, Roberto Bellasio5, Anna M. K. Benedictow6, Roberto Bianconi5, Johannes Bieser7, Joergen Brandt8, Jesper H. Christensen8, Augustin Colette9, Gabriele Curci10,11, Yanko Davila6, Xinyi Dong12, Johannes Flemming13, Xavier Francis14, Andrea Fraser15, Joshua Fu12, Daven K. Henze16, Christian Hogrefe17, Ulas Im8, Marta Garcia Vivanco18, Pedro Jiménez-Guerrero19, Jan Eiof Jonson6, Nutthida Kitwiroon20, Astrid Manders21, Rohit Mathur17, Laura Palacios-Peña19, Guido Pirovano4, Luca Pozzoli3,1, Marie Prank22, Martin Schultz6, Rajeet S. Sokhi14, Kengo Sudo23, Paolo Tuccella11, Toshihiko Takemura24, Takashi Sekiya23, and Alper Unal3 Stefano Galmarini et al.
  • 1European Commission, Joint Research Centre, JRC, Ispra (VA), Italy
  • 2Physics Department, Laboratory of Atmospheric Physics, University of Patras, 26504 Rio, Greece
  • 3Eurasia Institute of Earth Sciences, Istanbul Technical University, Istanbul, Turkey
  • 4Ricerca sul Sistema Energetico (RSE SpA), Milan, Italy
  • 5Enviroware srl, Concorezzo, MB, Italy
  • 6Norwegian Meteorological Institute, Oslo, Norway
  • 7Institute of Coastal Research, Chemistry Transport Modelling Group, Helmholtz-Zentrum Geesthacht, Hamburg, Germany
  • 8Department of Environmental Science, Aarhus University, Frederiksborgvej 399, 4000 Roskilde, Denmark
  • 9INERIS, Institut National de l'Environnement Industriel et des Risques, Parc Alata, 60550 Verneuil-en-Halatte, France
  • 10CETEMPS, University of L'Aquila, L'Aquila, Italy
  • 11Dept. Physical and Chemical Sciences, University of L'Aquila, L'Aquila, Italy
  • 12Department of Civil and Environmental Engineering, The University of Tennessee, Knoxville, TN 37919, USA
  • 13European Centre for Medium-Range Weather Forecasts, Reading, UK
  • 14Centre for Atmospheric and Instrumentation Research (CAIR), University of Hertfordshire, Hatfield, UK
  • 15Ricardo Energy & Environment, Gemini Building, Fermi Avenue, Harwell, Oxon, OX11 0QR, UK
  • 16Department of Mechanical Engineering, University of Colorado, 1111 Engineering Drive, Boulder, CO, USA
  • 17Computational Exposure Division – NERL, ORD, U.S. EPA, Raleigh, NC, USA
  • 18CIEMAT, Av. Complutense, 40, 28040 Madrid, Spain
  • 19Department of Physics, Physics of the Earth, Facultad de Química, Campus de Espinardo, University of Murcia, 30100 Murcia, Spain
  • 20Environmental Research Group, Kings' College London, London, UK
  • 21Netherlands Organization for Applied Scientific Research (TNO), Utrecht, the Netherlands
  • 22Finnish Meteorological Institute, Atmospheric Composition Research Unit, Helsinki, Finland
  • 23Japan Agency for Marine-Earth Science and Technology, Yokohama, Japan
  • 24Research Institute for Applied Mechanics, Kyushu University, Fukuoka, Japan

Abstract. 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 Hemispheric Transport of Air Pollutants phase 2 (HTAP2)–Air Quality Model Evaluation International Initiative phase 3 (AQMEII3) activity and focuses on surface ozone concentrations over Europe for the year 2010. Observations from 405 monitoring rural stations are used for the evaluation of the ensemble performance. The analysis first compares the modelled and measured power spectra of all models 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. This study has been conducted in the attempt to identify that the improvements obtained by the hybrid ensemble relative to the mono-scale ensembles can be attributed to its hybrid nature. The improvements are visible in a slight increase of the diversity (4% for the hourly time series, 10% for the daily maximum time series) and a smaller improvement of the accuracy compared to diversity. Root mean square error (RMSE) improved by 13–16% compared to G and by 2–3% compared to R. Probability of detection (POD) and false-alarm rate (FAR) show a remarkable improvement, with a steep increase in the largest POD values and smallest values of FAR across the concentration ranges. The results show that the optimal set is constructed from an equal number of global and regional models at only 15% of the stations. This implies that for the majority of the cases the regional-scale set of models governs the ensemble. However given the high degree of redundancy that characterizes the regional-scale models, no further improvement could be expected in the ensemble performance by adding yet more regional models to it. Therefore the improvement obtained with the hybrid set can confidently be attributed to the different nature of the global models. The study strongly 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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Short summary
An ensemble of model results relating to ozone concentrations in Europe in 2010 has been produced and studied. The novelty consists in the fact that the ensemble is made of results of models working at two different scales (regional and global), therefore contributing in detail two different parts of the atmospheric spectrum. The ensemble defined as a hybrid has been studied in detail and shown to bring additional value to the assessment of air quality.
An ensemble of model results relating to ozone concentrations in Europe in 2010 has been...