Atmospheric Transport Simulation by a GCM Eulerian Model for a Pollutant Accidental Release

Eulerian models are generally not appropriate to simulate the atmospheric transport for a punctual source, which is instantaneously mixed over one grid box at the initial instant. However, for multi-day transport episodes, eulerian models are generally preferred since lagrangian models require thousands of particles to simulate accurate dispersion. The Global Climate Model LMDZ, developed at the Laboratoire de Météorologie Dynamique in Paris, is tested and evaluated for fictitious sources, punctual in time and space, released in equatorial and mid-latitude regions at different altitudes. This eulerian model calculates over the globe, large-scale advection based upon finite-volume methods and parameterization of turbulent mixing and convection. Simulations with 192x145 grid cells essentially distributed in the source vicinity and 19 levels vertically, are carried out in a "nudged" mode, where horizontal velocities and temperature are relaxed towards wind analyses ECMWF, available every 6 hours. Transport is based on mass conservation of the tracer in the atmosphere. Numerical results are compared with those of the HYSPLIT lagrangian model, developed by the Air Resources Laboratory of NOAA, which calculates 3D trajectories of particles. Concentration maps for transport episodes of 1 to 5 days show that the grid resolution of a few tens of kilometers is necessary in the eulerian model. Sensitivity tests are carried out by varying the spatial resolution of models, using different convection models and using different wind fields (NOAA winds and ECMWF winds).