Articles | Volume 5, issue 1
Geosci. Model Dev., 5, 193–221, 2012
Geosci. Model Dev., 5, 193–221, 2012

Model description paper 31 Jan 2012

Model description paper | 31 Jan 2012

A Lagrangian model of air-mass photochemistry and mixing using a trajectory ensemble: the Cambridge Tropospheric Trajectory model of Chemistry And Transport (CiTTyCAT) version 4.2

T. A. M. Pugh1, M. Cain2, J. Methven3, O. Wild1, S. R. Arnold4, E. Real5, K. S. Law6, K. M. Emmerson4,*, S. M. Owen7, J. A. Pyle2, C. N. Hewitt1, and A. R. MacKenzie1,** T. A. M. Pugh et al.
  • 1Lancaster Environment Centre, Lancaster University, Lancaster, UK
  • 2Department of Chemistry, University of Cambridge, Cambridge, UK
  • 3Department of Meteorology, University of Reading, Reading, UK
  • 4Institute for Climate and Atmospheric Science, School of Earth and Environment, University of Leeds, Leeds, UK
  • 5CEREA, Joint laboratory Ecole des Ponts ParisTech/EDF R&D, Université Paris-Est, 77455 – Champs sur Marne, France
  • 6UPMC Univ. Paris 06, Univ. Versailles St-Quentin, CNRS/INSU, LATMOS-IPSL, Paris, France
  • 7Centre for Ecology and Hydrology, Edinburgh, UK
  • *now at: CSIRO Marine and Atmospheric Research, Aspendale, VIC 3195, Australia
  • **now at: School of Geography, Earth and Environmental Science, University of Birmingham, Edgbaston, Birmingham, B15 2TT, UK

Abstract. A Lagrangian model of photochemistry and mixing is described (CiTTyCAT, stemming from the Cambridge Tropospheric Trajectory model of Chemistry And Transport), which is suitable for transport and chemistry studies throughout the troposphere. Over the last five years, the model has been developed in parallel at several different institutions and here those developments have been incorporated into one "community" model and documented for the first time. The key photochemical developments include a new scheme for biogenic volatile organic compounds and updated emissions schemes. The key physical development is to evolve composition following an ensemble of trajectories within neighbouring air-masses, including a simple scheme for mixing between them via an evolving "background profile", both within the boundary layer and free troposphere. The model runs along trajectories pre-calculated using winds and temperature from meteorological analyses. In addition, boundary layer height and precipitation rates, output from the analysis model, are interpolated to trajectory points and used as inputs to the mixing and wet deposition schemes. The model is most suitable in regimes when the effects of small-scale turbulent mixing are slow relative to advection by the resolved winds so that coherent air-masses form with distinct composition and strong gradients between them. Such air-masses can persist for many days while stretching, folding and thinning. Lagrangian models offer a useful framework for picking apart the processes of air-mass evolution over inter-continental distances, without being hindered by the numerical diffusion inherent to global Eulerian models. The model, including different box and trajectory modes, is described and some output for each of the modes is presented for evaluation. The model is available for download from a Subversion-controlled repository by contacting the corresponding authors.