Sensitivity of tracer transport to model resolution, prescribed meteorology and tracer lifetime in the general circulation model ECHAM5

摘要

Atmospheric transport of traces gases and aerosols plays an important rolein the distribution of air pollutants and radiatively active compounds. Formodel simulations of chemistry-climate interactions it is important to knowhow the transport of tracers depends on the geographical resolution of thegeneral circulation model. However, this aspect has been scarcely investigateduntil now. Here, we analyse tracer transport in the ECHAM5 general circulationmodel using 6 independent idealized tracers with constant lifetimes, which arereleased in two different altitudes at the surface and in the stratosphere,respectively. Model resolutions from T21L19 to T106L31 were tested by performingmulti-annual simulations with prescribed sea surface temperatures and sea icefields of the 1990s. The impacts of the tracer lifetime were investigated byvarying 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 modeldue to enhanced vertical mixing, with the exception of the advection into thetropical region, which shows an inconsistent trend between the resolutions.Whereas, the meridional transport of tracers released in the stratosphere wasenhanced with higher model resolutions, except in the transport from tropicalstratosphere to the Southern Hemisphere, which exhibits an increase trend withincreasing model resolution. The idealized tracers exhibit a seasonal cycle,which is modulated by the tracer lifetime. In comparison to the run withprescribed sea surface temperature and sea ice fields, the simulation withprescribed meteorological fields did not exhibit significant change in themeridional transport, except in the exchange of stratospheric tracers betweenboth hemispheres, where it causes about 100% increase. The import of thesurface 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 upto 60% when prescribed meteorological fields is used. The ERA40 simulationalso showed larger interannual variability (up to 24% compared to 12% inthe standard simulations). Using our surface tracers released in either thenorthern or Southern Hemisphere, respectively, we calculate inter-hemispherictransport times between 11 and 17 months, consistent with values reported inthe literature. While this study cannot be used to relate differences in modelresults to specific changes in transport processes, it nevertheless providessome insight into the characteristics of tracer transport in the widely usedECHAM5 general circulation model.