¹National Institute of Aerospace, Hampton, VA
²NASA Langley Research Center, Hampton, VA
³NOAA Geophysical Fluid and Dynamics Laboratory, Princeton, NJ
⁴University of Maryland, Baltimore County, MD
⁵NASA Goddard Space Flight Center, Greenbelt, MD
⁶Scientific Systems and Applications, Inc., Hampton, VA
⁷Harvard University, Cambridge, MA

Abstract

We assess within the Global Modeling Initiative (GMI) modeling framework the utility of cosmogenic beryllium-7 (⁷Be), a natural aerosol tracer, for evaluating cross-tropopause transport in global models. The GMI chemical transport model (CTM) was used to simulate atmospheric ⁷Be distributions using four different meteorological data sets (GEOS1-STRAT DAS, GISS II' GCM, fvGCM, and GEOS-4 DAS), featuring significantly different stratosphere- troposphere exchange (STE) characteristics. The simulations were compared with the upper troposphere / lower stratosphere (UT/LS) ⁷Be climatology constructed from ~25 years of aircraft and balloon data, as well as climatological records of surface concentrations and deposition fluxes. Comparison of the fraction of surface air of stratospheric origin estimated from the ⁷Be simulations with observationally-derived estimates (i.e., 23-27% of the annual mean ⁷Be in surface air at northern mid-latitudes originates from the stratosphere [Dutkiewicz and Husain, 1985, referred to as DH85]) indicates excessive cross-tropopause transport at middle latitudes in simulations using GEOS1-STRAT and at high latitudes using GISS II' meteorological data. These simulations also overestimate ⁷Be deposition fluxes at middle latitudes (GEOS1-STRAT) and at high latitudes (GISS II'), respectively. We show that excessive cross-tropopause transport of ⁷Be corresponds to overestimated stratospheric contribution to tropospheric ozone. Our perspectives on STE in these meteorological fields based on ⁷Be simulations are consistent with previous studies with the same meteorological fields using ozone as a tracer for STE. We further applied the DH85 constraint to other global models including GFDL AM2 and GEOS-Chem (driven by GEOS-3 DAS and GEOS-5 DAS). We conclude that the DH85 constraint for ⁷Be and observed ⁷Be total deposition fluxes can be used routinely as a first-order assessment of cross- tropopause transport in global models.