METEOROLOGICAL AIR-POLLUTION POTENTIAL FOR SANTIAGO, CHILE - TOWARDS AN OBJECTIVE EPISODE FORECASTING

The geography and climate of the Santiago basin are, in general, unfav orable for the diffusion of air pollutants. Consequently, extreme even ts occur frequently during the high pollution season extending from Ap ril to August. The meteorological conditions concurrent with those ext reme events are mainly associated with the leading edges of coastal lo ws that bring down the base of the semipermanent temperature inversion reducing the diurnal growth of the surface mixed layer. Tn order to p roduce an objective 12 to 24-hour episode forecast, a two-way multivar iate discriminant analysis has been used in the definition of a meteor ological air-pollution potential index (MAPPI), separating high and lo w meteorological air-pollution potential days, The same procedure has been applied in the selection of the most efficient predictors for the MAPPI objective forecast, based on 12 and 24 UTC radiosonde data at Q uintero, about 100 km to the NW of Santiago. Results indicate about 70 % correctly forecasted days, with satisfactory skill-scores relative t o persistency. The strong persistency characterizing the most efficien t predictors in the 12-hour objective forecast scheme, makes the predi ction of the first and last days of any particular air-pollution poten tial episode particularly difficult. To overcome this problem, a new s et of predictors based on continuous measurements near the level of th e top of the temperature inversion layer (900 hPa during air-pollution episodes) is being tested. Preliminary results indicate that the time -integrated zonal wind component at that level is a reliable precursor for both the onset and the end of air-pollution potential episodes.