COMPARISON OF CONSTANT VOLUME BALLOONS, MODEL TRAJECTORIES AND TRACERTRANSPORT DURING ETEX

In the field phases of the European Tracer EXperiment (ETEX), an inert tracer was released for 12 h into the atmosphere and samples taken at several locations downwind. During the same time, several Constant Vo lume Balloons (CVB) (10 and 6 for ETEX first and second release, respe ctively) were launched into different altitudes and followed as far:as 21-188 km, to indicate the initial dispersion directions of the trace r puff. A model simulating the CVB behaviour in hydrostatic mese-scale model forecasts is applied to ETEX data to demonstrate its capability to predict the tracer puff mean axis over long distances (- 2000 km). CVB model results are first compared to air parcels trajectories and 2D (i.e. isentropic, isobaric and isodensity) trajectories. Then they are compared to the measured CVB trajectories and finally to the trace r puff trajectories. As expected,the CVB model and isodensity model tr ajectories are found to be identical. The 16 CVBs calculated trajector ies nearly overlap the real ones over 21-188 km with mean absolute hor izontal transport deviations less than 20 km (average value of 8.2 km) . The corresponding relative transport deviations are less than 45% wi th an average value of 20.6%. Better predictions are obtained for the ETEX second release. During the 60 h following ETEX's first release st art, the simulated CVBs are mainly found in the area of the maximum su rface concentrations of the released tracer, up to 2000 km. Up to 36 h after ETEX second tracer release start, the simulated CVB trajectorie s predict well the mean axis of the tracer puff, but failed later. (C) 1998 Elsevier Science Ltd. All rights reserved.