Abstract
Abstract. We discuss and evaluate the representation of atmospheric chemistry in the global Community Atmosphere Model (CAM) version 4, the atmospheric component of the Community Earth System Model (CESM). We present a variety of configurations for the representation of tropospheric and stratospheric chemistry, wet removal, and online and offline meteorology. Results from simulations illustrating these configurations are compared with surface, aircraft and satellite observations. Major biases include a negative bias in the high-latitude CO distribution, a positive bias in upper-tropospheric/lower-stratospheric ozone, and a positive bias in summertime surface ozone (over the United States and Europe). The tropospheric net chemical ozone production varies significantly between configurations, partly related to variations in stratosphere-troposphere exchange. Aerosol optical depth tends to be underestimated over most regions, while comparison with aerosol surface measurements over the United States indicate reasonable results for sulfate , especially in the online simulation. Other aerosol species exhibit significant biases. Overall, the model-data comparison indicates that the offline simulation driven by GEOS5 meteorological analyses provides the best simulation, possibly due in part to the increased vertical resolution (52 levels instead of 26 for online dynamics). The CAM-chem code as described in this paper, along with all the necessary datasets needed to perform the simulations described here, are available for download at www.cesm.ucar.edu.
Highlights
Atmospheric chemistry plays an integral role in the distribution of the non-CO2 radiatively active gases and aerosols (Forster et al, 2007)
Community Atmosphere Model (CAM)-chem can be used in three separate modes, all embedded within Community Earth System Model (CESM): (1) a fully coupled Earth System model, (2) with specified sea-surface and seaice distributions and (3) with specified meteorological fields
In the configurations described in this paper, atmospheric chemistry interacts with the climate only through radiation since no cloud-aerosol interaction is available in CAM4; the impact on temperature is overwritten in the case of specified dynamics
Summary
Atmospheric chemistry plays an integral role in the distribution of the non-CO2 radiatively active gases and aerosols (Forster et al, 2007). CAM-chem can be used in three separate modes, all embedded within CESM: (1) a fully coupled Earth System model (i.e., with all climate components active, which offers the possibility to connect the chemistry with biogeochemical processes in the land and ocean models), (2) with specified sea-surface and seaice distributions and (3) with specified meteorological fields. In particular it allows an incorporation of the biogeochemical algorithms in the land components; (3) it allows for the radiative algorithms incorporated into CAM4 to be fully exploited in the offline version This allows a calculation of the instantaneous radiative forcing within the offline model version, including a calculation of the instantaneous radiative forcing for specific events (e.g., forest fires) (Pfister et al, 2008; Randerson et al, 2006); (4) it allows for the strict conservation of tracer mass by accounting for changes in mixing ratio as the water vapor concentration changes within the atmosphere.
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