Introduction. A major focus of SCOUT-O3 is the tropics and a key issue here is testing how well existing global 3D models perform in this region. This poster discusses experiments with the TOMCAT/SLIMCAT off-line 3D CTM which investigate how well this model captures aspects of tropical tracer transport and chemistry. The model was run with two different configurations: transport-only (i.e. idealised tracers) and full chemistry. We investigate tracer transport in the CTM tropical tropopause layer and compare results from different experiments which test the impact of different PBL parameterisations, the effect of vertical resolution and different methods for calculating the vertical advection. We also investigate the model performance in a full chemistry run. Evaluation of a 3D CTM in the Tropical Upper Troposphere/Lower Stratosphere Wuhu Feng and Martyn Chipperfield IAS, School of Earth and Environment, University of Leeds, U.K Acknowledgements. We are grateful for use of Darwin data and to BADC for ECMWF analyses. We thank W. Randel for HALOE data and B. Dinelli for MARSCHALS data. This work was supported by SCOUT-O3 (CTM). References Chipperfield, M.P., QJ, 132, , Fig 5. Observations (in-situ and remote (MARSCHALS) measurements) from the M55 aircraft data during SCOUT-O3 Darwin campaign compared with full chemistry runs of TOMCAT/SLIMCAT. Fig 7. Time series of observed and modelled tropical O 3, CH 4 and H 2 O at 100hPa and 46hPa from Nov 1992 until Dec 2004 Fig 2. Zonal mean MLS observed CO with the annual mean removed (Schoeberl et al., 2006) as a function of time and pressure compared with TOMCAT and SLIMCAT. Fig 1. Comparison of observed mean age of air and 3D CTM Fig 4. Modelled tropical CO evolutions in the TTL from TOMCAT/SLIMCAT using model different configurations. Idealised Tracers (SCOUT WP 6.2) SCOUT WP 6.2 is examining how well various CTMs capture transport to and within the TTL. The SCOUT CTMs will run idealised tracers, such as surface- emitted species with fixed lifetimes, to allow a direct comparison between the models. Tracers T5 (with 5 day lifetime) and T20 (with 20 day lifetime). Tracer T5w (5 day lifetime and loss due to large-scale precipitation). Boundary condition of 1 ppt at surface everywhere Idealised CO (prescribed OH loss and fixed surface mixing ratio). H 2 O (Marti and Mauersberger,1991) with climatological value below 422 hPa. TOMCAT/SLIMCAT 3D CTM 3D off-line chemical transport model (see Chipperfield [2006]).  -p (TOMCAT);  -  (SLIMCAT) vertical coordinate Forced by ECMWF analyses Variable horizontal/vertical resolution Vertical motion diagnosed from radiation scheme or divergence Includes convection, PBL mixing detailed chemistry: troposphere/ stratospheric chemistry See Preliminary Conclusions TOMCAT/SLIMCAT has a realistic deep tropical convection transport process into the TTL (Figs. 2, 3). SLIMCAT has more realistic diagnosed mean age of air (Fig. 1) and CO tape recorder (Fig. 2) than TOMCAT suggesting it gives better representation of stratospheric tracer transport than TOMCAT. Different vertical coordinate, vertical diffusion, advection and resolution in CTM will affect tracer transport process (Figs. 1, 2, 3, 4). Full chemistry SLIMCAT run overestimates observed CO cf TOMCAT (Fig. 5) possibly due to lower vertical resolution in the troposphere where the convection plays important role. SLIMCAT is able to simulate observed tropical O 3, H 2 O and HNO 3, CH 4 and HCl well (Figs. 5, 6 and 7). However, some distinguish still exists, ie. model underestimates observed HNO 3 and the tropopause height in SLIMCAT is lower than MARSCHALS (Fig. 5). Fig 3. Tropical CO profile comparison Fig 6. Profile of tropical ozone, CH 4, HCl and H 2 O climatologies comparison with a multiannual SLIMCAT full chemistry run