An evaluation of the performance of chemistry transport models by comparison with research aircraft observations. Part 1: Concepts and overall model performance

Abstract

Abstract.A rigorous evaluation of five global ChemistryTransport and two Chemistry-Climate Models operated by several different groups in Europe, was performed. Comparisons were made of the models with trace gas observations from a number of research aircraft measurement campaigns during the four-year period 1995-1998. Whenever possible the models were run over the same four-year period and at each simulation time step the instantaneous tracer fields were interpolated to all coinciding observation points. This approach allows for a very close comparison with observations and fully accounts for the specific meteorological conditions during the measurement flights. This is important considering the often limited availability and representativity of such trace gas measurements. A new extensive database including all major research and commercial aircraft measurements between 1995 and 1998, as well as ozone soundings, was established specifically to support this type of direct comparison. Quantitative methods were applied to judge model performance including the calculation of average concentration biases and the visualization of correlations and RMS errors in the form of so-called Taylor diagrams. We present the general concepts applied, the structure and content of the database, and an overall analysis of model skills over four distinct regions. These regions were selected to represent various atmospheric conditions and to cover large geographical domains such that sufficient observations are available Correspondence to: D. Brunner (dominik.brunner@iac.umnw.ethz.ch) for comparison. The comparison of model results with the observations revealed specific problems for each individual model. This study suggests the further improvements needed and serves as a benchmark for re-evaluations of such improvements. In general all models show deficiencies with respect to both mean concentrations and vertical gradients of important trace gases. These include ozone, CO and NO x at the tropopause. Too strong two-way mixing across the tropopause is suggested to be the main reason for differences between simulated and observed CO and ozone values. The generally poor correlations between simulated and measured NO x values suggest that in particular the NO x input by lightning and the convective transport from the polluted boundary layer are still not well described by current parameterizations, which may lead to significant differences in the spatial and seasonal distribution of NO x in the models. Simulated OH concentrations, on the other hand, were found to be in surprisingly good agreement with measured values.

Trajectory matching of ozonesondes and MOZAIC measurements in the UTLS – Part 2: Application to the global ozonesonde network

“…Thouret et al, 1998;Schnadt Poberaj et al, 2009) or chemistry-transport models (e.g. Law et al, 2000;Brunner et al, 2003;Teyessè-dre et al, 2007). However, recent analyses cast some doubt on the long-term stability of the MOZAIC data; particularly the differences between MOZAIC and NOXAR in 1995-1997(Staufer et al, 2013 and the increase in bias between Frankfurt/Munich MOZAIC and the alpine surface site Zugspitze from 1994-2009 (Logan et al, 2012) raised some concern.…”

Evaluating the capability of regional-scale air quality models to capture the vertical distribution of pollutants

“…A possible reason for the difference might be related to the horizontal grid spacing of the AQ model (e.g. Brunner et al, 2003). As shown in Table 2, the DEHM model, which has the coarsest horizontal grid resolution, simulated the lowest mixing ratios, whereas the CAMx and CMAQ models, with much smaller horizontal grid spacing, simulated the highest mixing ratios.…”

“…This could be due to an underestimation of the exchange between the upper troposphere and the lower stratosphere and/or diffusive model errors related to coarse vertical gridding. By contrast, for the case of global-scale chemistry models, Brunner et al (2003) reported a tendency of models to overestimate ozone mixing ratios over NA between 5.5 and 8 km throughout the year (based on the period 1995-1998). However, while those results were averaged over the entire NA continent, here we are comparing regional-scale models at considerably smaller scales, and the different behaviour could simply be attributed to the different grid sizing as well as the different time period.…”

“…This is particularly true for long-lived species, whose concentrations are determined by the history of air parcels over long time periods during which meteorological errors may accumulate (e.g. Brunner et al, 2003;Blond and Vautard, 2004). Recent reports by the US National Research Council and the United Nations Task Force on Hemispheric Transport of Air Pollution (HTAP) (Szykman et al, 2012, and references therein) have urged the need for high-quality upper-air observational data to support model development and help address model deficiencies in the troposphere.…”

Evaluation of the MACC operational forecast system – potential and challenges of global near-real-time modelling with respect to reactive gases in the troposphere

“…The overestimation of O 3 mixing ratios during the summer months is a wellknown issue and has been described by various model validation studies (e.g. Brunner et al, 2003;Schaap et al, 2008;Ordoñez et al, 2010;Val Martin et al, 2014). Inadequate ozone precursor concentrations and aerosol induced radiative effects (photolysis) have been frequently identified as being the main factors.…”