PREDICTIONS OF PLUME DISPERSION IN COMPLEX TERRAIN - EULERIAN VERSUS LAGRANGIAN MODELS

Simulations of dispersion from an elevated point source in complex ter rain and non-stationary flow are presented using the Lagrangian atmosp heric dispersion model (LADM, Physick et al., 1992, Air Pollution Mode ling and its Applications, Vol. IX, pp. 725-729, Plenum Press, New Yor k; 1994 CSIRO Division of Atmospheric Research Technical Paper No. 24) and the Eulerian grid-based model (CALGRID, Yamartino ct al., 1989, C ALGIRD: a mesoscale photochemical grid model, Vol, I; model formation document, Report, Sacarmento, California), Both models use the same pr edicted windfields. We find that the different algorithms used for rel ease of pollutants into the model domains lead to initial concentratio ns at the release height in LADM one-third higher than in CALGRID. The CALGRID plume spreads laterally over a larger region than does the LA DM plume due to the finite-difference approach of CALGRID. The polluta nt mass in the extra volume occupied by the CALGRID plume is less than 10% of that released. The essentials of morning fumigation are simula ted more realistically under the Lagrangian approach. In LADM the elev ated plume is mixed down to the ground rapidly, causing a sharp increa se in ground-level concentrations (glc), whereas, in CALGRID glc incre ase more gradually over a few hours. The use of hourly averaged windfi elds in CALGRID compared to 10 min windfields in LADM leads to a relat ive separation of the two modelled plumes of 5 km at a distance of rou ghly 6 km downwind from the sources at 1500 LST. Consequently in compl ex terrain and non-stationary conditions, the plumes are subjected to different three-dimensional wind regimes. For the particular terrain s tudied, roughly 3% of the pollutant mass emitted into CALGRID during t he day is transported above 3000 m after 1700 LST whereas only 0.8% is transported above this height in LADM. During the daytime the CALGRID simulation produces maximum glc which are about 40% smaller than thos e predicted by LADM. Copyright (C) 1997 Elsevier Science Ltd.