REGIONAL CIRCULATIONS WITHIN THE IBERIAN PENINSULA EAST COAST

Transcription

1 REGIONAL CIRCULATIONS WITHIN THE IBERIAN PENINSULA EAST COAST Oriol Jorba, Santiago Gassó and José M. Baldasano* 1. INTRODUCTION The Iberian Peninsula is located at middle latitudes in the north hemisphere, in the most occidental part of Europe. It is limited at west by the Atlantic Ocean and by the Mediterranean Sea at east. Because these particularities and its complex orography, the development of intricate circulations within the Peninsula are common. The more usual synoptical situations affecting this region are westerly and northwesterly flows, and typical summertime barometric swamps (Martín, 1987; Jorba et al., 23). Photochemical air pollution episodes in the Iberian Peninsula are usually produced under summertime barometric swamps (MMAM, 21). The Mediterranean lands normally experience higher temperatures, greater amounts of sunshine and fewer rain days than western and central Europe during summer (Salvador et al., 1997). The pressure system that dominates the region is the Azores anticyclone, contributing to the development of large thermally driven convective systems over the Peninsula and land-sea breezes over the coasts. The complex orography of the Iberian Peninsula east coast, characterized by a first large range of mountains with heights between 5 to 3 m at few kilometres inland, contributes to an intricate circulation development. A combination of sea breezes with upslope winds contribute to the advection and injection inland of the sea-coastal air masses. In this sense, the topography of the region plays an important role inducing the orographic forcing of the flows that are injected aloft. Furthermore, the location along the east coast of several urban regions contributes to the pollution of the zone, with the advection of primary pollutants inland that react photochemically, producing a complex pattern of air pollution within the east coast. High levels of O 3 along the Iberian Peninsula have been reported during summer (Martín et al., 1991; Millán et al., 1991; Toll and Baldasano, 2; Barros et al., 23). Millán et al. (1997) generally described the recirculations processes occurring along the Western Mediterranean Basin. During the day the sea breezes combine with upslope winds to transport coastal pollutants inland, while at the leading edge of the breeze front a large fraction of these pollutants are injected in their return flows aloft at heights ranging from 2 to 3 km. Once in those upper layers the pollutants move back toward the sea, and *Oriol Jorba and José M. Baldasano, Laboratory of Environmental Modeling, Department of Engineering Projects, Universitat Politècnica de Catalunya (UPC), Barcelona, SPAIN. Santiago Gassó, Department of Engineering Projects, Universitat Politècnica de Catalunya (UPC), Terrassa, SPAIN.

2 2 O. JORBA ET AL. the compensatory subsidence creates stratified reservoir layers of aged pollutants, stacked up to 2-3 km high, along the coast over the sea. These layers act as reservoirs and retain ozone from one day to the following days. The next morning the lowermost layers are drawn inland by the sea breeze, and the aged pollutants can react with new coastal emissions. With the aim to deep into the knowledge of these recirculation processes, a numerical weather prediction model (NWP) has been used. A synoptical situation affecting the Iberian Peninsula has been analysed and results are presented in this contribution. 2. MODEL CONFIGURATION The NWP used in this work is MM5, the Pennsylvania State University (PSU)/National Center for Atmospheric Research (NCAR) Mesoscale Model 5, version 3, release 4 modeling system (Dudhia, 1993; MMD/NCAR, 21), it is a non-hydrostatic mesoscale primitive equation model. Figure 1. Left: Domains definition, radiosonde site (black square), cross section location (black line). Right: Surface streamlines for August 14 th at 12 UTC. The physics options used for the simulations were: the Mellor-Yamada scheme as used in the Eta model (Janjic, 1994) for the PBL parameterization, the Anthes-Kuo and Kain-Fritsch cumulus scheme (Kain and Fritsch, 1993), the Dudhia simple ice moisture scheme, the cloud-radiation scheme, and the five-layer soil model. Three nested grids were defined (figure 1 left): a coarse grid having 72 km grid spacing with 35 x 49 cells covering the major part of Europe, a medium grid of 24 km with 49 x 61 cells covering the Iberian Peninsula, and finally a fine grid of the Iberian Peninsula east cost with 6 km of resolution and 141 x 15 cells. A one-way nesting approach was used. Initialization and boundary conditions for the mesoscale model were introduced with analysis data of the ECMWF global model. Data were available at a 1-degree resolution (1-km approx. at the working latitude) at the standard pressure levels every 6 hours. 3. METEOROLOGICAL SITUATION An intense episode of photochemical air pollution was produced during August 1 th to 19 th of 2 in the north zone of the east Iberian coast. The highest ozone level of the

3 REGIONAL CIRCULATIONS 3 year was reached in this region with a maximum of 273?g/m3 in August 15th. The more intense part of the episode covered the days 14th to 16th. The situation modelled for this contribution is the episode that covers August 13th to August 16th of 2. This episode was characterized by a meteorological situation with weak synoptic forcing. In this case mesoscale phenomena, induced by the particular topography of the region, would be dominant. Thus, the description of the regional circulations occurring along the east Iberian coast will deep into the knowledge of the atmospheric phenomenas that contributes to develop the photochemical air pollution episode Figure 2. Synoptic situation of August 13th to 16 th, surface analysis. Synoptic situation of the episode corresponds to a typical summertime barometric swamp over the Iberian Peninsula (figure 2). In height, a zonal flow blows aloft the Peninsula veering to the southeast having northwesterly winds affecting the northeast of the Peninsula. At surface, the high-pressure area is centred over south Atlantic Ocean, with the anticyclonic wedge affecting most part of the Iberian Peninsula, producing a typical barometric swamp along the easterly part of the region. Under this situation surface winds are low. This fact, and the strong daily solar heating, produced the development of mesoscale phenomena. These phenomena in the region are mainly seabreezes, up-slope and down-slope winds and valley channelled winds. The heating during August 14th was so intense that a thermal low started to develop in the southeast of the Iberian Peninsula. The behaviour of the four days of the episode was similar, with low pressure gradient at surface and weak westerly northwesterly flows in height. 4. RESULTS: SUMMERTIME BAROMETRIC SWAMP - AUGUST 13th TO 16th OF MM5 simulation Results of the simulation with MM5 for the meteorological situation will be described. In order to validate the model behaviour, comparisons with surface measurements of wind and with the radiosonde launched at Barcelona site (see figure 1, left) have been done, and will be presented. As explained before, under this situation the Iberian Peninsula was dominated by the Azores anticyclone, with very low pressure gradient. The sea-breeze regime, developed within all the coast of the Western Mediterranean Basin, induces an anticyclonic circulation over all the Western Mediterranean, with general subsidence over the region

4 4 O. JORBA ET AL. as noted by Millán et al. (1997). This anticyclonic circulation extends aloft reaching heights even of 4 km, the middle troposphere. In figure 1 (right) the surface streamlines are shown at 12 UTC for August 14 th. The anticyclonic circulation is clearly developed. The formation of this circulation begins at August 13 th, where the barometric swamp establishes over the Iberian Peninsula, and the centre of the circulation moves over the western Mediterranean in function of the sea-breeze strength. All the Western Mediterranean coasts present inland sea-breeze flows. Only the canalisation of the Pyrenees and the Central Massif blows into the Mediterranean, but the flow turns to the coasts by the thermal forcing. In low levels, this canalisation plays an important role, because it is the only pass to bring new air into the western Mediterranean basin. Figure 3. Domain 3 surface wind field for August 16 th at UTC (left) and 12 UTC (right). A well developed sea-breeze regime is established along all the Iberian east coast for the analysed period, with breeze circulation cells up to 2 km height. This regime covers the central hours of the day, starting around 8-1 UTC and changing to a land-sea flow around 19 UTC. Figure 3 shows the surface wind field for August 16 th at and 12 UTC. It is remarkable the maintenance of the anticyclonic circulation over the sea at UTC induced by the see-breeze regime of the previous days. At night, an offshore flow is established along the coastal zone. Inland, the downslope winds regime is dominant. At noon, the onshore winds are well developed along all the east coast. At 12 UTC, the breeze has reached the first mountainous chain producing important orographic injections. A common characteristic of this situation is the permanent entrance of air masses by the Pyrenees-Massif Central canalisation and the anticyclonic circulation during all day. The strength of the sea-breeze and the complex orography of the east Iberian coast produce several injections of air due to orographic forcing. A cross section has been preformed in order to describe the recirculations in height (figure 4). A first plane coastal zone, a mountain chain, and a second higher mountainous chain characterize the orography of the cross section. Figure 1 shows the location of the cross section. During August 13 th, the development of the breeze produces three return flows around, 8, 12 and 23 m MSL. The major injection is produced when the breeze reaches the second mountainous chain. Figure 4 shows the major injections produced specifically at this point for August 13 th, 14 th and 16 th. The convergence of the onshore flow and the inland westerly flow produces a chimney injecting air up to 39 m MSL

5 REGIONAL CIRCULATIONS 5 for August 14 th. This injection is more impressive for August 16 th, where a chimney reaches heights over 5 m MSL, and breaks the northwesterly circulation aloft. Important subsidence over the Mediterranean Sea is produced Figure 4. Cross section of wind field and the mixing ratio at the central part of the east Iberian coast (Left: August 13 th at 15 UTC, centre: August 14 th at 15 UTC, right: August 16 th at 13 UTC). m Figure 5 shows two trajectories extracted from the MM5 simulation. It is noticeable the low speed of both trajectories, due to the low baric gradient dominant within the region. The subsidence over the sea and the anticyclonic circulation manage the evolution of both trajectories. The air at middle troposphere is forced to go down by the subsidence over the east coast, once in low levels the air masses recirculate over the sea with a possible return to the seaboard 3 days later. 13 th 33 m 13 th 29 m 14 th 2 m 16 th 1 m 14 th 335 m 14 th 256 m 14 th 15 th 142 m 1 m 15 th 59 m 16 th 37 m 16 th 7 m 15 th 1875 m 15 th 15 m Figure 5. MM5 trajectories for August 13 th to 16 th (top view). The combination of these circulations over the sea, and the orographic injections showed in figure 4 produces a recirculation of pollutants along the east coast that contributes to the development of episodes of photochemical air pollution in this region under barometric swamp conditions. Moreover, thermal injections of air in central parts of the Peninsula are common in summer (Salvador et al., 1997). These air masses are

6 6 O. JORBA ET AL. advected to the coast where the subsidence can introduce them to the recirculation processes described above. The accumulation of polluted air masses contributes to the degradation of the air quality of the east Iberian coast in summer Comparison with measurements With the aim to validate the results and the explanation exposed above, comparisons with surface and aloft measurements have been done. 52 surface stations located at the northeast of the Iberian Peninsula have been used for the surface comparisons, and a radiosonde launched at the Barcelona city (North of the Iberian east coast) contributes to the verification of the model results. The BIAS and the root mean square error (RMSE) of the wind velocity at 1 m AGL have been evaluated. In figure 6 the reader can see the evolution of these statistics for the period August 13 th to 16 th _ _ _ _ Figure 6. Evolution of the RMSE and BIAS of the wind velocity (ms -1 ) at 1 m (Solid line: RMSE, dotted line: BIAS). The absolute error remains below 2.5 ms -1 for all the simulation with an increase at the evening of the last day. The error decreases after the cold start of the simulations and stabilizes around 2 ms -1. The model tends to overestimate the nocturnal winds, and presents some problems to simulate the strength of the diurnal winds. The behaviour of the error has a daily cycle that tends to increase in magnitude as the simulation progress. Hence, the higher errors appear during august 16 th Figure 7. Vertical profile of the horizontal wind at Barcelona site for August 13 th to 16 th twice daily, and 12 UTC (Left panel: MM5 results, right panel: Barcelona radiosonde data).

7 REGIONAL CIRCULATIONS 7 Data of the radiosonde routinely launched twice daily ( UTC, 12 UTC) in Barcelona city has been used to validate the model results for the upper layers. Figure 7 shows the radiosonde and MM5 data of the period analysed for this site. The vectors represent the horizontal wind at a specific height. The model results at noon present a marked sea-breeze cell structure. In most cases, the model simulates a narrow surface layer of see-breeze flow blowing southerly inland. The return of the flow is located between 5 and 1 m AGL for the simulated days with a northwesterly direction. However, the structure is not so clear with the data measured by the radiosondes. In this sense, the inland flow blows more southwesterly than the model results and presents an extended layer reaching heights over 15 m AGL, except for august 15 th. This day has a more complex vertical structure with a first return at low levels, above 2 m AGL, and a second inland flow at 5m AGL with the return around 2 m AGL. These profiles show the complexity of the circulations induced over eastern coast of the Iberian Peninsula due to its intricate orography MM5 Figure 8. Potential temperature (ºC) profiles of the radiosonde data launched at Barcelona site, and MM5 mixing layer height (m AGL) estimation (Diamonds: data of August 13 th at 12 UTC, squares: data of 14 th at 12 UTC, triangles: data of 15 th at 12 UTC, dashes: data of 16 th at 12 UTC). Finally, in figure 8 we present the comparison of the mixing layer (ML) height between the model diagnostic at 12 UTC with the Eta PBL scheme, and the potential temperature profiles of the radiosonde data. The differences between the model prediction and the measurements are around 1 m, and for well developed ML the model tends to subestimate the height. 5. CONCLUSIONS Results from a simulation of a summertime barometric swamp over the Iberian Peninsula have been shown in order to describe the circulations that are produced along the Iberian east coast. The complex orography of the seaboard and the influence of the warm Mediterranean Sea develop these circulations with the low synoptical forcing of the day. The relevant phenomenas that drive all the circulations within the east Iberian coast and the western Mediterranean basin under a situation of barometric swamp have been described. The results of the simulation have shown the importance of the orographic features of the coastal region in the development of the recirculations observed.

8 8 O. JORBA ET AL. Comparisons with measurements have been done in order to evaluate the reliability of the model results, and to validate the model in situations of low barometric forcings over the Iberian Peninsula. Reasonably agreement is produced between model results and observations. Due to the characteristics of the high populated seaboard of the east Iberian Peninsula, and the particular circulations induced over the region several photochemical contamination episodes usually occur in summer under barometric swamp situations. This contribution helps to improve the knowledge of the processes responsibles of the high ozone levels in the region. 6. ACKNOWLEDGEMENTS The authors wish to thank the Spanish Meteorological Institute (INM) for providing data from the ECMWF, and the Catalan Meteorological Service (SMC) for providing surface station and radiosonde data for validation. Simulations were run on an HP Exemplar V25 belonging to the Supercomputing Center of Catalonia (CESCA). This work was developed under project IMMPACTE and REN2-12-C2-2/CLI. 7. REFERENCES Barros N., Toll, I., Soriano, I., Jiménez, P., Borrego, C., and Baldasano, J. M., 23, Urban photochemical pollution in the Iberian Peninsula: Lisbon and Barcelona airsheds, J. Air&Waste Manag. Ass. (in press). Dudhia, J., 1993, A non-hydrostatic version of the Penn State-NCAR mesoscale model: Validation tests and simulation of an Atlantic cyclone and cold front, Mon. Wea. Rev. 121: Janjic, Z. I., 1994, The step-mountain eta coordinate model: Further development of the convection, viscous sublayer, and turbulent closure schemes, Mon. Wea. Rev. 122: Jorba, O., Pérez, C., Baldasano, J. M., Rocadenbosch, F., and López, M. A., 23, Cluster analysis of backtrajectories arriving at Barcelona air basin, in 1 st EARLINET Symposium on the Structure and Use of the Database derived form Systematic Lidar Observations, Hamburg (Germany) February. Kain, J. S., and Fritsch, J. M., 1993, Convective parameterization for mesoscale models: The Kain-Fritsch scheme. The representation of cumulus convection in numerical models, K. A. Emanuel and D. J. Raymond, Eds., Amer. Meteor. Soc., p Martín, J., 1987, Característiques climatològiques de la precipitació en la franja costera mediterrània de la Península Ibèrica, PhD. Thesis, Barcelona, edited by Institut Cartogràfic de Catalunya, pp Martín, M., Plaza, J., Andrés, M. D., Bezares, J. C., and Millán, M. M., 1991, Comparative study of seasonal air pollutant behaviour in a Mediterranean coastal site: Castellón (Spain), Atmos. Environ. A 25: Mesoscale and Microscale Meteorology Division, National Center for Atmospheric Research, 21, PSU/NCAR Mesoscale Modeling System Tutorial Class Notes and User s Guide: MM5 Modeling System Version 3, June 21. Millán, M. M., Artíñano, B., Alonso, L. A., Navazo, M., and Castro, M., 1991, The effect of meso-scale flows on the regional and long-range atmospheric transport in the western Mediterranean area, Atmos. Environ. A 25: Millán, M. M., Salvador, R., Mantilla, E., and Kallos, G., 1997, Photooxidant dynamics in the Western Mediterranean basin in summer: Results from European research projects, J. G. R.. 12(D7): Ministerio de Medio Ambiente, 21, Medio Ambiente en España 2, Madrid. Salvador, R., Millán, M. M., Mantilla, E., and Baldasano, J. M., 1997, Mesoscale modelling of atmospheric processes over the western Mediterranean area during summer, Int. J. Env. Pollution 8: Toll, I., and Baldasano, J. M., 2, Modeling of photochemical air pollution in the Barcelona area with highly disaggregated anthropogenic and biogenic emissions, Atmos. Environ. A 34: