THE EFFECT OF MESOSCALE FLOWS ON REGIONAL AND LONG-RANGE ATMOSPHERIC TRANSPORT IN THE WESTERN MEDITERRANEAN AREA

Experimental evidence accumulated over the years indicates that the formation of the Iberian thermal low on summer days is intimately associated with complex dispersion behavior of atmospheric pollutants emitted within the whole of Spain and in particular for those emitted in the coastal areas. The project "Mesometeorological Cycles of Air Pollution in the Iberian Peninsula", sponsored by the CEC, is intended to study the origin and evolution of the atmospheric circulations responsible for the observed behaviour, and document their importance in Spain and in the Mediterranean basin as a whole. To characterize the processes involved, tall stack plumes from sites in Spain have been tracked with COSPEC remote sensors and used as tracers of opportunity of the flow at their level. This information is being combined with available historical meteorological data to compose a mosaic of atmospheric circulation patterns for the whole of the Iberian peninsula and surrounding regions. The analysis performed to date confirms that: (a) the formation of the thermal low on summer days forces an inward flow of the coastal emissions towards the central plateau, (b) this convergence occurs at the peninsular level, (c) the inflow is mostly ageostrophic and strongly channelled along natural mountain passes, and (d) the marine airmass pulled inland along the Spanish east coast is highly enriched with O3. The latter, (d), appears to indicate that a compensatory flow to the thermal low is provided by subsidence over the colder waters of the Gulf of Lyon and northwestern Mediterranean area. In turn, this favors the formation of northeasterly winds along the Spanish east coast during the night, and may explain the O3 levels as a result of regional transport of precursors from source regions in this general area.