SIMULATION OF TRACER DISPERSION FROM ELEVATED AND SURFACE RELEASES INCOMPLEX TERRAIN

A new version of an advanced mesoscale dispersion modeling system for simulating passive air pollutant dispersion in the real atmospheric pl anetary boundary layer (PBL), is presented. The system comprises a dia gnostic mass-consistent meteorological model and a Lagrangian particle dispersion model (LADISMO). The former version of LADISMO, developed according to Zannetti (Air pollution modelling, 1990), was based on th e Monte Carlo technique and included calculation of higher-order momen ts of vertical random forcing for convective conditions. Its ability t o simulate complex Bow dispersion has been stated in a previous paper (Hernandez et al. 1995, Atmospheric Environment, 29A, 1331-1341). The new version follows Thomson's scheme (1984, Q. Jl Roy. Met. Soc. 110, 1107-1120). It is also based on Langevin equation and follows the idea s given by Brusasca et al. (1992, Atmospheric Environment 26A, 707-723 ) and Anfossi et al. (1992, Nuovo Cemento 15c, 139-158). The model is used to simulate the dispersion and predict the ground level concentra tion (g.l.c.) of a tracer (SF6) released from both an elevated source (case a) and a ground level source (case b) in a highly complex mounta inous terrain during neutral and synoptically dominated conditions (ca se a) and light and apparently stable conditions (case b). The last ca se is considered as being a specially difficult task to simulate. In f act, few works have reported situations with valley drainage flows in complex terrains and real stable atmospheric conditions with weak wind s. The model assumes that nearly calm situations associated to strong stability and air stagnation, make the lowest layers of PBL poorly dif fusive (Brusasca et al., 1992, Atmospheric Environment 26A, 707-723). Model results are verified against experimental data from Guardo-90 tr acer experiments, an intensive held campaign conducted in the Carrion river valley (Northern Spain) to study atmospheric diffusion within a steep walled valley in mountainous terrain (Ibarra, 1992, Energia, No. 1, 74-85). (C) 1997 Elsevier Science Ltd.