A tropospheric chemical‐transport model: Development and validation of the model transport schemes

Abstract

AbstractWe describe a 3D off‐line chemical‐transport model (CTM) for studies of tropospheric chemistry. the tropospheric CTM is a development of the existing TOMCAT stratospheric CTM to which we have added a parametrized convection scheme and treatment of vertical diffusion. the CTM diagnoses the occurrence of convection during the model simulations from the forcing analysis fields of winds, temperature and humidity.The spatial distributions of the vertical mass fluxes calculated by these schemes appear realistic. the convective mass fluxes agree well with values derived from global analyses, having maximum values at low latitudes and at the intertropical convergence zone, although the TOMCAT cloud‐top height may be too low.We have used the model's convection scheme to diagnose the convective precipitation when forced by European Centre for Medium‐Range Weather Forecasts (ECMWF) initialized analyses. Despite the off‐line approach, the CTM‐diagnosed convective precipitation is in very good spatial agreement with global observations and output from a general‐circulation model (GCM), showing that not only does convective venting of tracers occur in the correct locations, but also that the convection scheme can be coupled to wet‐deposition schemes in full chemistry simulations. TOMCAT exhibits a low‐latitude bias in cloud amount, due to the model's neglect of mid‐level convection, and underestimates convective precipitation and cloud amount, the latter by at least 20% compared with GCM results. This underestimate may be due to excessive stability of the ECMWF analyses used in the off‐line model or to differences in the formulation of convection in the ECMWF forecast model and in TOMCAT.We have evaluated the CTM's transport processes by performing a range of simulations with a surface‐emitted radon (Rn) tracer. the agreement with radon observations improves with an increase in horizontal resolution from 7.5° × 7.5° to 2.8° × 2.8° and with the inclusion of vertical diffusion and convection in the model formulation. When run at a horizontal resolution of 2.8° × 2.8° TOMCAT captures the tracer transport and seasonal evolution in tracer transport associated with major meteorological features and with convective processes. TOMCAT reproduces the observed seasonality in radon at surface sites, but overestimates the surface radon concentration, partly due to insufficient vertical diffusion in the model. However, transient systems and their tracer transport within the boundary layer are simulated realistically. Compared with observations, the modelled radon profiles are too ‘C‐shaped’ (the modelled profile has a mid‐tropospheric minimum which is not present in the observations) which may be caused by the model's neglect of convective downdraughts and/or entrainment and detrainment rates that are too low in the cloud column, resulting in insufficient mixing within the cloud column. the radon concentration in the upper troposphere is underestimated by TOMCAT; this may be due to a number of factors: the underestimate in convective‐cloud amount, the way in which the base mass flux is calculated, the limitation of organized entrainment to subcloud levels, and a possible static stability of the analyses.