Millán

Photooxidant dynamics in the Mediterranean basin in summer: Results from European research projects

Most of the Mediterranean Sea is surrounded by mountains 1500 m or higher. Their east and south facing slopes favor the early formation of upslope winds, reinforcing the sea breezes. These slopes also act as orographic chimneys which link the surface winds directly with their return flows aloft, creating recirculations. To characterize the dynamics of pollutants in the Mediterranean basin and to compose a mosaic of the atmospheric circulations involved, the European Commission (EC) supported the following projects: (1) meso-meteorological cycles of air pollution in the Iberian Peninsula (MECAPIP), 1988–1991, intended to document the atmospheric circulations over the Iberian Peninsula; (2) regional cycles of air pollution in the west central Mediterranean area (RECAPMA), 1990–1991, which extended the characterization from the Atlantic coast of Portugal to Italy; and (3) south European cycles of air pollution (SECAP), 1992–1995, for the whole of the basin. The level of interpretation of the data and the elaboration and validation of working hypotheses across the basin have followed, in turn, with the corresponding lags in space and time. The purpose of this paper is to present a summary (to 1995) of the documented, as well as the postulated, processes involved. The MECAPIP and RECAPMA projects have shown that stacked layer systems form along the Spanish Mediterranean coasts, 2–3 km deep and more than 300 km wide, with the most recent layers at the top and the older ones near the sea. These act as a reservoir for aged pollutants to reenter land the next day, and tracer experiments have shown that turnover times are from 2 to 3 days. During the night, part of this system drifts along the coast. Under strong insolation these circulations become “large natural photochemical reactors,” where most of the NOx emissions and other precursors are transformed into oxidants, acidic compounds, aerosols, and O3 (exceeding some EC directives for several months). Finally, the preliminary analysis of the data obtained in the SECAP project supports the hypothesis that pollutants emitted in the Mediterranean basin could be transported toward the Intertropical Convergence Zone, located along northern Africa in summer, and pumped directly into the upper troposphere. If this is verified, the Mediterranean basin could be one place where all the links from the local to the global scales could be identified and documented.

Meteorology and photochemical air pollution in Southern Europe: Experimental results from EC research projects

The MECAPIP project of EC has served to document the summer atmospheric circulations and the related air pollution dynamics over Spain and the Western Mediterranean. The first includes surface wind convergence over the Iberian peninsula, large-scale compensatory subsidence over the surrounding coastal areas, and the formation of re-circulatory cells as a result of the sea breezes combining with up-slope winds and their return and compensatory flows. With respect to the second, experimental evidence indicates that, (a) during the day the sea breezes can transport ozone 60 to 100, or more, km inland, (b) during the afternoon, photo-oxidants are injected 3 to 5 km into the mid-troposphere over the Spanish Central plateau, and (c) stratified reservoir layers, stacked up to ≈2–3 km high, build up along the Mediterranean coast. These retain ozone from one day to the next and following days. The lowermost layers are drawn inland by the sea breeze during the day, and the aged pollutants can react with new coastal emissions. All of these processes have a marked diurnal cycle and can recur for many consecutive days or weeks. Tracer experiments show that the average return times for pollutants can be of the order of 2–3 d. This includes first emission on the coast in the morning, travel inland with sea breeze, convective and/or orographic injection in return flows followed by layering and subsidence over the sea, and their final re-entry with the sea breeze a few days later. All together, those mechanisms can be considered to form a large “photochemical reactor” which operates almost every day from spring to fall. It can generate ozone levels 2 to 3 times higher than EC directives for damages to vegetation 65 μg m−3 (≈32 ppbv), 24 h avg. This work presents a summary of results illustrating some of the above-mentioned processes. Similar situations are also expected to occur in other Mediterranean regions.

Ozone Cycles in the Western Mediterranean Basin: Interpretation of Monitoring Data in Complex Coastal Terrain

Abstract In summer, the complex layout of the coasts and mountains surrounding the western Mediterranean basin favors the development of mesoscale atmospheric recirculations and the formation of ozone reservoir layers above the coastal areas and the sea. Tropospheric ozone cycles vary here according to location and exposure of the monitoring station in relation to the flows and reservoir layers, and large differences can be encountered within tens of kilometers. The basic premise for this work is that the representativeness of any station is determined by the (fore)knowledge of the processes affecting the site, at the proper timescales and space scales within its region. Thus, available data have been combined with mesoscale analysis and modeling to interpret the observed summer ozone cycles for the monitoring network at Castellon, on the Spanish east coast. The area is approximately 120 km by 120 km, is backed by coastal mountains, and includes the following: a conurbation, industries, and a densely trav...

Rainfall Components: Variability and Spatial Distribution in a Mediterranean Area (Valencia Region)

Abstract In the last half of the twentieth century, the precipitation regime on the Spanish east coast showed an overall pattern of reduction in precipitation totals. This work presents the results from a novel procedure to disaggregate the precipitation series in the Valencia region on the basis of their meteorological origin. Important differences are found between specific trends and changes in the contributions from 1) Atlantic frontal systems, 2) convective–orographic storms, and 3) easterly advections over the Mediterranean Sea (backdoor cold fronts). The series for the three components are then used to analyze the evolution, variability, and spatial distribution of the region’s rainfall and to determine the correlations with the North Atlantic Oscillation (NAO) index. The results establish significant differences between inland areas, with a trend toward decreasing precipitation and thus increasing aridity, and coastal areas, with increasing precipitation that tends to be progressively more torrent...

Climatic feedbacks and desertification: The Mediterranean model

Abstract Mesometeorological information obtained in several research projects in southern Europe has been used to analyze perceived changes in the western Mediterranean summer storm regime. A procedure was developed to disaggregate daily precipitation data into three main components: frontal precipitation, summer storms, and Mediterranean cyclogenesis. Working hypotheses were derived on the likely processes involved. The results indicate that the precipitation regime in this Mediterranean region is very sensitive to variations in surface airmass temperature and moisture. Land-use perturbations that accumulated over historical time and greatly accelerated in the last 30 yr may have induced changes from an open, monsoon-type regime with frequent summer storms over the mountains inland to one dominated by closed vertical recirculations where feedback mechanisms favor the loss of storms over the coastal mountains and additional heating of the sea surface temperature during summer. This, in turn, favors Medite...

Horizontal Grid Size Selection and its Influence on Mesoscale Model Simulations

The use of two-dimensional spectral analysis applied to terrain heights in order to determine characteristic terrain spatial scales and its subsequent use for the objective definition of an adequate grid size required to resolve terrain forcing are presented in this paper. In order to illustrate the influence of grid size, atmospheric flow in a complex terrain area of the Spanish east coast is simulated by the Regional Atmospheric Modeling System (RAMS) mesoscale numerical model using different horizontal grid resolutions. In this area, a grid size of 2 km is required to account for 95% of terrain variance. Comparison among results of the different simulations shows that, although the main wind behavior does not change dramatically, some small-scale features appear when using a resolution of 2 km or finer. Horizontal flow pattern differences are significant both in the nighttime, when terrain forcing is more relevant, and in the daytime, when thermal forcing is dominant. Vertical structures also are investigated, and results show that vertical advection is influenced highly by the horizontal grid size during the daytime period. The turbulent kinetic energy and potential temperature vertical cross sections show substantial differences in the structure of the planetary boundary layer for each model configuration.

Mesoscale circulations over complex terrain in the Valencia coastal region, Spain – Part 1: Simulation of diurnal circulation regimes

We collected ground-based and aircraft vertical profile measurements of meteorological parameters during a 2-week intensive campaign over the Valencia basin, in order to understand how mesoscale circulations develop over complex terrain and affect the atmospheric transport of tracers. A high-resolution version of the RAMS model was run to simulate the campaign and characterize the diurnal patterns of the flow regime: night-time katabatic drainage, morning sea-breeze development and its subsequent coupling with mountain up-slopes, and evening flow-veering under larger-scale interactions. An application of this mesoscale model to the transport of CO 2 is given in a companion paper. A careful evaluation of the model performances against diverse meteorological observations is carried out. Despite the complexity of the processes interacting with each other, and the uncertainties on modeled soil moisture boundary conditions and turbulence parameterizations, we show that it is possible to simulate faithfully the contrasted flow regimes during the course of one day, especially the inland progression and organization of the sea breeze. This gives confidence with respect to future applicability of mesoscale models to establish a reliable link between surface sources of tracers and their atmospheric concentration signals over complex terrain.

Torrential rains on the spanish Mediterranean coast: Modeling the effects of the sea surface temperature

Torrential rains are a frequent meteorological risk in the Mediterranean Basin, and the work reported here is part of a long-term study that includes the analysis of the synoptic conditions involved in their genesis. This paper studies the role of SST in torrential rain development. Two episodes were selected for simulation with the Regional Atmospheric Modeling System. Three runs of each were performed by progressively improving the SST input data sources: from monthly climatological averages, to data from the International Satellite Land Surface Climatology Project, to near-to-real-time data derived from National Oceanic and Atmospheric Administration (NOAA) satellite images. For the first episode, the maximum total precipitations calculated were 242, 301, and 496 mm, respectively, versus more than 550 mm measured. For the second event, the maxima were 316, 349, and 378 mm, respectively, versus more than 450 mm measured. The conclusion is that significant improvements in the modeling of peak precipitation can be expected when using SST derived from NOAA satellite data.

Atmospheric nitrogen deposition on the east coast of Spain: relevance of dry deposition in semi-arid Mediterranean regions.

Bulk deposition composition and pine branch washing were measured from April 1999 to March 2000 on the east coast of Spain. The main objective was to characterise N deposition patterns with special emphasis on dry deposition. Bulk deposition in the region is dominated by neutralisation processes by Ca2+ and HCO3-, ClNa of marine origin and a high correlation between NO3- and SO4(2-). SO4(2-) concentrations show a decrease with respect to previous studies in the region in agreement with generalized sulfur emission decreases while the remaining ions, including NO3-, are higher due to their general increase as well as to the inclusion of dry deposition in bulk collectors in the present study. An enrichment in NO3- has been observed in dry deposition composition branch washing) with respect to bulk deposition, while an impoverishment has been observed in the case of NH4+. Annual bulk deposition varies between 7.22-3.1 and 3.5-1.8 Kg ha(-1) year(-1) for S- SO4(2-) and N- NO3-, respectively. N total deposition goes from 9.78 to 6.8 Kg ha(-1) year(-1) at most stations, with the lowest deposition at the control station and Alcoi. The relative dry deposition with respect to the total was over 40% at most stations, going up to 75% at the southern station. N-deposition is expected to be higher considering that N-NH4+ deposition has been underestimated in this study.

The dynamics of aged airmasses and ozone in the Western Mediterranean : Relevance to forest ecosystems

Abstract EC supported projects have shown that atmospheric re-circulations develop along the Western Mediterranean coasts in summer. These lead to the formation of layers over the sea which act as reservoirs for aged pollutants to enter land with the seabreeze of the next or following days. Under strong insolation these circulations can be considered as “large natural photo-chemical reactors” where O3 is produced at levels which are potentially harmful to vegetation. The reach of these circulations within the coastal mountain ranges is also important to understand how, and where, forest ecosystems can be affected by pollutants in this region.