This contribution analyzes the circulatory patterns of air pollutants in Barcelona (Spain), an area with strong coastal and orographic influence, during two typical summertime and wintertime situations from the meteorological point of view. Barcelona is located on the northeastern corner of the Iberian Peninsula by the Mediterranean Sea, and presents an opportunity to study the air-water-land interface. Barcelona's location, together with the orography surrounding the region, contributes to the complexity of the dispersion of pollutants emitted in the region.
Numerical simulations have been performed with the meteorological non-hydrostatic mesoscale model MEMO. The domain considered, and modeled, covers an area of 80x80 km2, which includes the main orographic features thought to determine the mesoscale circulation in the region.
Typical meteorological situation for summertime and wintertime were extracted from the study of series of synoptic maps for the region. Since the intention was to simulate the mesoscale flows in the area, the synoptic situation in both cases was chosen to be weak, so that no important large-scale forcings were introduced. The lack of large scale forcings allowed the development of mesoscale circulations resulting from the differential heating of the sea and land; this differential heating establishes sea breezes which generate a general inshore flow, from sea to land during daytime, and offshore flows during nighttime. At the same time, mountains, arranged parallel with the coast, cause up-slope flows during day and down-slope flows during night that have the same orientation as the sea breeze winds. The particular orientation of two river valleys present in the modeled region also provoke up-valley and drainage flows in the direction of the sea breeze. These orographic features, therefore, enhance the inflow/outflow regime of the sea breeze.
Results have shown that the main differences between the mesoscale circulations developed in summertime and wintertime are found in the intensity and duration of the sea-breeze inland flow during the afternoon. The stronger temperature gradient between sea and land during summertime provoke a very intense onshore flow that starts early in the afternoon and spans for 8 hours or more. On the contrary, during wintertime, wind speeds are much more uniform along the day and sea breeze blows later in the afternoon and only for a few hours (3 to 4 hours). Drainage from the river valleys is reproduced in the two cases and, together with the land breeze typical of the nocturnal period, transports air pollutants that are produced over the terrain towards the sea, were they are incorporated to large scale circulations.
Evaluation of wind speed and direction simulated with the model with measurements from surface stations (and with remotely-measured winds with lidar in the summer case) showed that the model was able to reproduce the mesoscale regimens that occur during a typical diurnal cycle. The general lower velocities occurring during the wintertime situation makes this scenario more difficult to simulate by the model, which shows a slight tendency to overemphasize the afternoon onshore flow.