A detailed evaluation of the global climate-chemistry model EMAC was carried out using the Earth System Model Validation Tool. Four EMAC simulations have been evaluated in order to identify differences in simulated ozone and selected climate parameters that resulted from (i) different setups of the EMAC model (nudged versus free-running) and (ii) different boundary conditions (emissions, sea surface temperatures and sea-ice concentrations). Quantitative performance metrics were calculated in a consistent manner for climate parameters and ozone. This is important for the interpretation of the evaluation results since biases in simulated climate can impact on biases in chemistry and vice versa. The observational datasets used for the evaluation include ozonesonde and aircraft data, meteorological reanalyses and satellite measurements.
Tropospheric column ozone in the EMAC ACCMIP simulation compared with a sensitivity simulation with an additional HNO3 formation channel (ACCMIP-S2) and MLS/OMI satellite observations. The values on top of each panel show the global average in Dobson units (DU)
With the exception of some specific features, no large differences that could be related to the different setups (nudged vs. free-running) of the EMAC simulations were found, which offers the possibility to evaluate and improve the overall model with the help of shorter nudged simulations. The main differences between the two setups is a better representation of the tropospheric and stratospheric temperature in the nudged simulations, which also better reproduce stratospheric water vapor concentrations, due to the improved simulation of the temperature in the tropical tropopause layer. Ozone and ozone precursor concentrations, on the other hand, are very similar in the different model setups, if similar boundary conditions are used. Different boundary conditions however lead to relevant differences in the four simulations. Biases which are common to all simulations are the underestimation of the ozone hole and the overestimation of tropospheric column ozone, the latter being significantly reduced when lower lightning emissions of nitrogen oxides are used.
In order to further investigate possible other reasons for such a bias, a sensitivity simulation with a newly proposed HNO3-formation channel of the HO2 + NO reaction was performed. The introduction of the new HNO3-formation channel significantly reduces this bias. Therefore, including the new reaction rate could potentially be important for a more realistic simulation of tropospheric ozone, although laboratory experiments and other model studies need to confirm this hypothesis. In addition, some modifications to the reaction rate, which have a strong dependence on water vapor, might be needed.
Reference: Righi, M., V. Eyring, K.-D Gottschaldt, C. Klinger, F. Frank, P. Jöckel, and I. Cionni, Quantitative evaluation of ozone and selected climate parameters in a set of EMAC simulations, Geosci. Model Dev., 8, 733-768, doi:10.5194/gmd-8-733-2015, 2015.
Contact: Dr. Mattia Righi