Representation of the Community Earth System Model (CESM1) CAM4-chem within the Chemistry-Climate Model Initiative (CCMI)

Abstract

Abstract. The Community Earth System Model (CESM1) CAM4-chem has been used to perform the Chemistry Climate Model Initiative (CCMI) reference and sensitivity simulations. In this model, the Community Atmospheric Model version 4 (CAM4) is fully coupled to tropospheric and stratospheric chemistry. Details and specifics of each configuration, including new developments and improvements are described. CESM1 CAM4-chem is a low-top model that reaches up to approximately 40 km and uses a horizontal resolution of 1.9 • latitude and 2.5 • longitude. For the specified dynamics experiments, the model is nudged to ModernEra Retrospective Analysis for Research and Applications (MERRA) reanalysis. We summarize the performance of the three reference simulations suggested by CCMI, with a focus on the last 15 years of the simulation when most observations are available. Comparisons with selected data sets are employed to demonstrate the general performance of the model. We highlight new data sets that are suited for multimodel evaluation studies. Most important improvements of the model are the treatment of stratospheric aerosols and the corresponding adjustments for radiation and optics, the updated chemistry scheme including improved polar chemistry and stratospheric dynamics and improved dry deposition rates. These updates lead to a very good representation of tropospheric ozone within 20 % of values from available observations for most regions. In particular, the trend and magnitude of surface ozone is much improved compared to earlier versions of the model. Furthermore, stratospheric column ozone of the Southern Hemisphere in winter and spring is reasonably well represented. All experiments still underestimate CO most significantly in Northern Hemisphere spring and show a significant underestimation of hydrocarbons based on surface observations.

Tropospheric Ozone Assessment Report: Assessment of global-scale model performance for global and regional ozone distributions, variability, and trends

“…However, the ensemble mean is biased high throughout the troposphere (by 5-20%) with biases strongest in fall and winter, consistent with evaluations against ozonesondes over NH extratropics described above and previous evaluations ). Recent individual model evaluations against observations have also found similar biases in the free troposphere (e.g., Zhang et al, 2010;Kim et al, 2013;Tilmes et al, 2016).…”

“…An approach to overcome this complication is to compare long-term ozone observations with model hindcasts forced (or nudged) with observed meteorology. This better isolates model-observation disagreements, and could potentially suggest model improvements (e.g., Pozzoli et al, 2011;Koumoutsaris and Bey, 2012;BrownSteiner et al, 2015;Lin et al, 2015b;Strode et al, 2015;Tilmes et al, 2016;Lin et al, 2017). Overall, such hindcast simulations capture observed decreases in summertime surface ozone in the populated regions of North America and Europe during 1990-2010, but have difficulties simulating the ozone increases measured at remote baseline sites.…”

“…Based on comparisons against ozonesonde observations and other in situ measurements, particular regional features of free tropospheric ozone can be generally captured by current global chemistry models (Zhang et al, 2010;Tilmes et al, 2016;Hu et al, 2017), although these have not been systematically investigated in all models. Such features include the ozone maximum west of southern Africa over the South Atlantic Ocean (e.g., Jonquières et al, 1998;Sauvage et al, 2006), the mid-Pacific minimum, which describes the well-characterized "wave-1" pattern in the tropics (Thompson et al, 2003;Ziemke et al, 2010), and the summertime free tropospheric ozone maximum over the Eastern Mediterranean (e.g., Kalabokas et al, 2013;Zanis et al, 2014).…”

“…Models differ in the complexity of representation of stratospheric chemistry, ranging from a passive ozonelike tracer or linearized ozone chemistry (McLinden et al, 2000), to detailed stratospheric chemistry coupled with the troposphere (e.g, Lin et al, 2012a;Neu et al, 2014;Iglesias-Suarez et al, 2016;Tilmes et al, 2016). There are also differences in the location and magnitude of stratosphere-to-troposphere transport among models that lead to model ozone discrepancies in the upper troposphere as well as for episodic ozone events near the surface (Fiore et al, 2014a).…”

The Met Office HadGEM3-ES chemistry–climate model: evaluation of stratospheric dynamics and its impact on ozone

“…Akiyoshi et al (2016) present a case study of the evolution of chemical species during the stratospheric sudden warming of winter 2010 using both a nudged model and observations to study the structure in the chemical fields. A more general overview of the impact of nudging on chemistry-climate models is given in Jöckel et al (2006Jöckel et al ( , 2015, Telford et al (2013), and Tilmes et al (2016).…”