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

Abstract

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 Correspondence to: A. M. Aghedo (adetutu.m.aghedo@jpl.nasa.gov) 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.

Influence of model resolution on the atmospheric transport of <sup>10</sup>Be

“…The horizontal diffusion in the results from the T21L19 is probably overestimated smearing out the 10 Be gradients in the mid-and low latitudes. This is in agreement with previous findings by Aghedo et al (2010) who found weaker meridional transport at coarse resolutions, allowing for more diffusion towards low latitudes. Figure 11 shows the stratospheric 10 Be divided into the 30-degree bands.…”

“…The five bars at each latitude band show the model resolutions "T63L47", "T42L39", "T42L31", "T21L19" as well as the "T42L39old", published in Heikkilä et al (2009), respectively. ECHAM5 model has been reported by Aghedo et al (2010) who found that meridional transport was decreased at coarse resolution, leading to increased concentrations of surface tracers in the tropics.…”

“…They agree that, in general, increasing vertical resolution decreases the strength of stratosphere-troposphere exchange and increases residence time of stratospheric particles. In addition, Aghedo et al (2010) shows that increased vertical transport at a coarse resolution leads to a decreased meridional transport of surface tracers. Roeckner et al (2006) performed a careful validation of sensitivity of model climate to resolution against the ERA-40 reanalysis.…”

“…Gray (2003); Hack et al (2006); Richter et al (2008); van Velthoven and Kelder (1996), and in the ECHAM4/5 model specifically by Aghedo et al (2010); Land et al (2002); Roeckner et al (2006). These studies used idealised passive tracers in combination with the ECHAM atmospheric model only.…”

Estimating bacteria emissions from inversion of atmospheric transport: sensitivity to modelled particle characteristics

“…However, tracer transport in global chemistry-climate models can also be sensitive to many other aspects of the model that are beyond the scope of this study, such as model resolution, the use of prescribed meteorology to nudge the chemistry climate model towards the observed atmospheric state (Aghedo et al, 2010), numerical formulations of atmospheric dynamics (Rasch et al, 2006), and the formulation of the parameterization of deep convective transport (Mahowald et al, 1995;Luo et al, 2003;Tost et al, 2006b;Lawrence and Salzmann, 2008). The sensitivity of modelled transport to particle characteristics as simulated here could potentially change if these or other aspects of the model were changed.…”