Headline: European WRF-CHEM workshop in Potsdam, 2017

On June 1-2, 2017 a meeting of users of the WRF-CHEM regional chemistry-climate model based in and around Europe was held at IASS Potsdam, Germany. This meeting built on previous meetings of a German WRF-CHEM users group which had been held since 2013 at IASS Potsdam, the Max Planck Institute for Chemistry in Mainz, the Institute of Meteorology and Climate Research - Atmospheric Environmental Research in Garmisch-Partenkirchen, and the Max Planck Institute for Meteorology in Hamburg, and a similar meeting organized in 2016 at Wageningen University, The Netherlands. These meetings serve to bring together experts and users based in and near Europe with varying degrees of experience using the WRF-CHEM modelling system in order to connect with each other, share information, exchange ideas, and discuss potential opportunities for collaboration.

An opening keynote presentation was given by Ravan Ahmadov (NOAA) summarizing the current state of WRF-CHEM development, best practices for using the model, and the future development roadmap. Following this, a number of contributed oral presentations were given:

• Marcus Hirtl: Overview of the WRF-Chem applications at the ZAMG
• Carlos Silveira: Testing WRF-Chem model with two European emission inventories
• Christoph Knote: Updates to the WRF-Chem MOZ* mechanism family
• Aurelia Lupascu: Tagged Ozone Mechanisms for WRF-Chem
• Alba Badia: Importance of reactive halogens in the tropical marine atmosphere using WRF-chem
• Amarachi Oghaego: Numerical simulation of harmattan dust over West Africa using the fine resolution weather model
• Quing Mu: Regional modelling of polycyclic aromatic hydrocarbons: WRF/Chem-PAH model development and East Asia case studies
• Auke Visser: Simulating ozone deposition in WRF-Chem: is there a need for a revised Wesely scheme?
• Renate Forkel: Ultrafine particles from power plants: Evaluation of WRF-Chem simulations with airborne measurements
• Ying Chen: Sea salt emission, transport and influence on size-segregated nitrate simulation: a case study in northwestern Europe by WRF-Chem
• Friderike Kuik: Modeling NO2 concentrations in the urban area of Berlin/Brandenburg with WRF-Chem: model evaluation and sensitivity to traffic emissions

As well as these oral presentations, several groups contributed poster presentations:

• Scott Archer-Nicholls: Radiative effects and health burden of residential combustion emissions in China
• Andreas Hilboll: Simulating air quality in the Netherlands with WRF-Chem 3.8.1 at high resolution
• Jan Karlický: Validation of gas phase chemistry in WRF/chem model over Europe
• Huidong Li: Comparison of different urban canopy modules (UCM&BEP) in simulating urban climate and air pollution for Berlin using WRF-Chem
• Yanhui Liu: Impact of Isoprene Emissions on Air Quality in China
• Noelia Otero: A model comparison of meteorological drivers of surface ozone concentrations over Europe
• Laura Palacios-Peña: Aerosol vertical distribution in WRF-Chem: Sensitivity to dry deposition
• Leidy Johana Romero Alvarez: Modelling an ozone pollution event over Southeast England using WRF-Chem
• Huseyin Toros: Air pollution Modelling studies in Ankara
• Lindsey Weger: The Impact of a Potential Shale Gas Development in Germany and the United Kingdom on Emissions and Air Quality

A large amount of time during the workshop was set aside for free, open discussion both in plenary and in small groups. Much of this discussion followed on directly from several of the presentations. Additionally, during the course of the workshop, several broad topics of interest to the attendees were identified, collected, and discussed towards the end of the meeting. These topics are briefly summarized here.

The planetary boundary layer

Many groups identified problems with the representation of mixing in the planetary boundary layer, especially the nocturnal boundary layer in urban areas. Modelled pollutants were seen to build up to unrealistically high concentrations in the lowest model layer during the night time. Several possible solutions to this problem were identified:

• Making sure that urban emissions follow a realistic vertical distribution.
• Activating an undocumented option in the dry deposition driver code, which introduces additional mixing in locations where CO emissions exceed a certain threshold.
• Investigating possible additional sources of heat and momentum fluxes in urban areas  and improving their physical representation in urban and boundary layer parameterisations.

The use of multi-layer urban canopy models instead of single layer models was discussed as a possible improvement, but several groups reported problems running these multi-layer urban models in combination with chemistry.

Emission processing

The processing of emission data is an issue with which many users of WRF-CHEM seem to struggle. There are existing tools already available to the community such as prep_chem_sources and anthro_emis, but nevertheless, users still need to do a significant amount of additional processing in order to make commonly available emission inventories useable in WRF-CHEM. Issues which were identified include the vertical distribution of emissions, NOx split, VOC speciation, accounting for different temporal emission profiles in different countries, and combining emission data from heterogeneous sources. A need was identified for a more general purpose emission processing tool for use with WRF-CHEM. Four groups agreed to continue communication on this, with a view to developing such a tool: The University of Bremen; The University of Cambridge; LMU Munich; and IASS Potsdam.

Choice and comparison of chemical mechanisms

Several participants were interested in an intercomparison exercise of the different chemical mechanisms available in WRF-CHEM. Due to time limitations this topic was not discussed in detail by the group.

User support

Participants in the workshop were unanimous in their appreciation of Ravan Ahmadov’s attendance. Dr. Ahmadov is one of very few WRF-CHEM model developers whose tasks include supporting users of the model. WRF-CHEM is growing in popularity worldwide due to its status as a freely-available, state-of-the-art regional chemistry-climate model, and there is a clear need for user support beyond the current capacity which WRF-CHEM development groups can currently provide. Unfortunately it is difficult to obtain funding for model support and knowledge transfer. Possible avenues for such funding may be Future Earth or the WMO.

Future meetings

Participants agreed that the IASS Potsdam meeting served a useful purpose for the European WRF-CHEM modelling community. It provided a geographically closer alternative than the WRF users workshop in Boulder for those with limited travel budgets, and connected allowed European users to network with each other and establish collaborations.
Hüseyin Toros (Istanbul Technical University) offered to host another similar meeting in April 2018. In addition to the workshop character of the Potsdam meeting, Dr. Toros also expressed a desire to include a tutorial component in the Istanbul meeting, which would allow relatively new users of WRF-CHEM to learn the basics. Participants agreed that the Istanbul meeting should be structured in such a way that these two components of the meeting (tutorial and user workshop) could be attended independently. Dr. Toros will draw on the expertise of NOAA in order to help organize the tutorial, and established WRF-CHEM modelling groups in Europe to help organize the user workshop.