Improving the Representation of Precipitation Scavenging of Aerosols in the NOAA GFDL Coupled Chemistry-Climate Model AM2-CHEM Using Constraints From Radionuclide Tracers

Precipitation scavenging (wet deposition) is the dominant loss process for many aerosols but model parameterizations of this process are highly uncertain and vary in their extent of coupling to the hydrological cycle, substantially contributing to large uncertainties in the simulated loadings of and radiative forcings from aerosols. The International Global Atmospheric Chemistry Project (IGAC) has recently identified the improvement of wet deposition and scavenging parameterizations in large-scale models as a priority. A reasonable representation of precipitation scavenging is critical for coupled chemistry-aerosol-climate modeling and its application to climate change assessments. In this study we simulate the atmospheric distributions of the aerosol tracers 210Pb and 7Be with the NOAA GFDL AM2 coupled chemistry-climate model (AM2-CHEM). Because of their contrasting (and relatively well-known) sources at low and high altitudes, together with wet deposition as their principal sink, 210Pb and 7Be are a useful combination for testing wet deposition and vertical transport in global three-dimensional models. Our objective is to develop an improved representation of the wet deposition process in AM2-CHEM tied closely to the AM2 hydrological cycle and evaluated by comparing simulated 210Pb and 7Be against the long-term climatologies at the surface and in the upper troposphere / lower stratosphere (UT/LS). Our simulation of wet deposition includes, in particular, scavenging within convective updrafts in the Relaxed Arakawa-Schubert (RAS) cumulus parameterization. It also includes first- order rainout and washout from stratiform precipitation. We will show that 210Pb and 7Be concentrations at the surface and UT/LS and total deposition fluxes simulated by AM2-CHEM agree reasonably with observations and are comparable to those from other major global models. The developed framework for wet deposition will be applied to simulations of soluble trace gases and aerosols in AM2-CHEM. We will also discuss the use of 7Be to diagnose cross-tropopause transport in global models.