Observationally derived transport diagnostics for the lowermost stratosphere and their application to the GMI chemistry and transport model

Transport from the surface to the lowermost stratosphere (LMS) can occur on timescales of a few months or less, making it possible for short-lived tropospheric pollu- tants to influence stratospheric composition and chemistry. Models used to study this influence must demonstrate the credibility of their chemistry and transport in the upper tropo- sphere and lower stratosphere (UT/LS). Data sets from satel- lite and aircraft instruments measuring CO, O3, N2O, and CO2 in the UT/LS are used to create a suite of diagnostics for the seasonally-varying transport into and within the low- ermost stratosphere, and of the coupling between the tropo- sphere and stratosphere in the extratropics. The diagnostics are used to evaluate a version of the Global Modeling Initia- tive (GMI) Chemistry and Transport Model (CTM) that uses a combined tropospheric and stratospheric chemical mech- anism and meteorological fields from the GEOS-4 general circulation model. The diagnostics derived from N2O and O3 show that the model lowermost stratosphere has realis- tic input from the overlying high latitude stratosphere in all seasons. Diagnostics for the LMS show two distinct layers. The upper layer begins 30 K potential temperature above the tropopause and has a strong annual cycle in its compo- sition. The lower layer is a mixed region 30 K thick near the tropopause that shows no clear seasonal variation in the degree of tropospheric coupling. Diagnostics applied to the GMI CTM show credible seasonally-varying transport in the LMS and a tropopause layer that is realistically coupled to the UT in all seasons. The vertical resolution of the GMI CTM in the UT/LS, 1 km, is sufficient to realistically rep- resent the extratropical tropopause layer. This study demon- strates that the GMI CTM has the transport credibility re- quired to study the impact of tropospheric emissions on the stratosphere.

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