Sensitivity of tracer transport to model resolution, prescribed meteorology and tracer lifetime in the general circulation model ECHAM5 A. M. Aghedo1,*, S. Rast1, and M. G. Schultz1,** 1Max Planck Institute for Meteorology, Hamburg, Germany *now at: Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, USA **now at: ICG-2, Research centre, Jülich, Germany
Abstract. Atmospheric transport of traces gases and aerosols plays an important role
in the distribution of air pollutants and radiatively active compounds. For
model simulations of chemistry-climate interactions it is important to know
how the transport of tracers depends on the geographical resolution of the
general circulation model. However, this aspect has been scarcely investigated
until now. Here, we analyse tracer transport in the ECHAM5 general circulation
model using 6 independent idealized tracers with constant lifetimes, which are
released in two different altitudes at the surface and in the stratosphere,
respectively. Model resolutions from T21L19 to T106L31 were tested by performing
multi-annual simulations with prescribed sea surface temperatures and sea ice
fields of the 1990s. The impacts of the tracer lifetime were investigated by
varying the globally uniform exponential decay time between 0.5 and 50 months.
We also tested the influence of using prescribed meteorological fields (ERA40)
instead of climatological sea surface temperature and sea ice fields.
Meridional transport of surface tracers decreases in the coarse resolution model
due to enhanced vertical mixing, with the exception of the advection into the
tropical region, which shows an inconsistent trend between the resolutions.
Whereas, the meridional transport of tracers released in the stratosphere was
enhanced with higher model resolutions, except in the transport from tropical
stratosphere to the Southern Hemisphere, which exhibits an increase trend with
increasing model resolution. The idealized tracers exhibit a seasonal cycle,
which is modulated by the tracer lifetime. In comparison to the run with
prescribed sea surface temperature and sea ice fields, the simulation with
prescribed meteorological fields did not exhibit significant change in the
meridional transport, except in the exchange of stratospheric tracers between
both hemispheres, where it causes about 100% increase. The import of the
surface tracers into the stratosphere is increased by up to a factor of 2.5,
and the export from the stratosphere into the troposphere was increased by up
to 60% when prescribed meteorological fields is used. The ERA40 simulation
also showed larger interannual variability (up to 24% compared to 12% in
the standard simulations). Using our surface tracers released in either the
northern or Southern Hemisphere, respectively, we calculate inter-hemispheric
transport times between 11 and 17 months, consistent with values reported in
the literature. While this study cannot be used to relate differences in model
results to specific changes in transport processes, it nevertheless provides
some insight into the characteristics of tracer transport in the widely used
ECHAM5 general circulation model.
Citation: Aghedo, A. M., Rast, S., and Schultz, M. G.: Sensitivity of tracer transport to model resolution, prescribed meteorology and tracer lifetime in the general circulation model ECHAM5, Atmos. Chem. Phys., 10, 3385-3396, doi:10.5194/acp-10-3385-2010, 2010.