Departmento de Impacto Ambiental de la Energía, Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas, Madrid, Spain

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1 MAY 2001 MARTÍN ET AL. 905 Simulations of Mesoscale Circulations in the Center of the Iberian Peninsula for Thermal Low Pressure Conditions. Part II: Air-Parcel Transport Patterns FERNANDO MARTÍN, MAGDALENA PALACIOS, AND SYLVIA N. CRESPÍ Departmento de Impacto Ambiental de la Energía, Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas, Madrid, Spain (Manuscript received 27 September 1999, in final form 10 August 2000) ABSTRACT This paper discusses the variability of air-parcel transport under similar summer thermal low pressure conditions over the Iberian Peninsula. Three-dimensional trajectories were estimated by means of the Topography Vorticity- Mode Mesoscale model. Four cases of the 1992 summer thermal low pressure system have been considered, covering most of the variability of this synoptic meteorological situation. Transport patterns were very different among the four studied cases. Results indicate that synoptic wind forcing can influence trajectories under thermal low conditions. It was found that a remarkable difference exists between the transport of air parcels released at low levels, which are affected by thermally driven flows, and those released at higher levels, which are more influenced by the synoptic wind. Moreover, potentially polluted air parcels (released in the daytime) travel farther than the potentially nonpolluted ones (released at night) in the case of lower-level releases. In addition, important differences between the transport patterns of polluted morning air parcels and polluted afternoon air parcels are detected and discussed. 1. Introduction Recent pollutant emission inventories suggest that the Greater Madrid Area (5 million inhabitants) acts as a large source of pollutants, with road traffic as the major contributor to the total nitrogen oxide, volatile organic compound, and carbon monoxide emissions (Palacios and Martín 1996). Weather conditions in the Iberian Peninsula connected to the transport and dispersion of pollutants have been widely and experimentally studied in the framework of the Mesometeorological Cycles of Air Pollution in the Iberian Peninsula (MECAPIP) European Union (EU) project during (Millán et al. 1996). In addition, experimental campaigns have been conducted to investigate the spatial and temporal variability of pollutant concentrations in the Madrid airshed and to characterize the meteorological processes associated with pollution episodes [Regional Cycles of Air Pollutants in the Mediterranean Area (RECAPMA) EU project, ; South European Cycles of Air Pollution (SE- CAP) EU project, ]. The Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas Corresponding author address: Fernando Martín, Grupo de Modelizacion de la Contaminacion Atmosferica, Dpto. Impacto Ambiental de la Energía, CIEMAT, Avda. Complutense 22, Madrid, Spain. fernando.martin@ciemat.es (CIEMAT) was also involved in a national project that was aimed at the experimental characterization and modeling of the formation and distribution of photochemical pollutants in the Madrid air basin (sponsored by the Spanish Government, ). Two main kinds of atmospheric pollutant episodes exist in the center of the Iberian Peninsula. The first one usually occurs in wintertime. In this case, atmospheric conditions are usually stable with weak winds, as a result of the influence of a deep anticyclone that can remain stationary over the area for more than a week at a time. Under these conditions, the subsidence inversion can drop more than 150 m per day, reaching m above ground level (AGL) by the end of an episode. In addition, a strong surface radiative inversion can develop during the night, reaching up to 400 m AGL, which sometimes does not even disappear during the day. Air pollution measurements (mobile units and monitoring stations) suggested that a nocturnal accumulation occurs in the southern part of the Madrid airshed, and, moreover, a recirculation of the polluted air mass takes place during the episodes. Observed flow supports this behavior, as well as depicting the overall diurnal wind cycle, including drainage zones (Artíñano et al. 1994). The second kind of atmospheric pollutant episode is typical of summertime. In this case, weather conditions in the Iberian Peninsula are dominated by thermal low pressure systems. Experimental studies have shown that strong ground heating in summertime enhances ther American Meteorological Society

2 906 JOURNAL OF APPLIED METEOROLOGY VOLUME 40 mally driven circulations and, along with topographical features, contributes to a stronger flow pattern. Moreover, intensive summer field campaigns have revealed some features about the horizontal and vertical distribution of the polluted air masses as well as their evolution within the planetary boundary layer (which can be up to 3000 m deep). The transport and mixing become more intense than in the winter conditions, as was shown by the detection of ozone plumes in areas located 100 km away from the city, usually mixed in a layer that reaches a height of m in the afternoon (Plaza et al. 1997). The study of the atmospheric circulations influencing the dispersion of pollutants is important. These circulations are the result of complex interactions of several factors (topography, land type, solar heating, the earth s rotation, upper-level scale forcing, etc.) working on different scales that produce a notable spatial and time variability of the three-dimensional meteorological and turbulence fields. Numerical atmospheric modeling provides a useful and cost-effective tool to investigate the atmospheric phenomena. It complements the field experiments and aids in the understanding of the air pollution processes. In addition, meteorological models provide meteorological and turbulence fields needed for use in dispersion and/or chemical transformation models. This paper provides an analysis of the variability of the polluted air-parcel trajectories computed with the Topography Vorticity-Mode Mesoscale (TVM) model in the Greater Madrid Area under thermal low pressure conditions. It is a continuation of the work carried out in Part I (Martín et al. 2001) on the evaluation of the TVM model s performance in simulating the mesoscale flows in the center of the Iberian Peninsula for the same synoptic cases. 2. Air-parcel trajectories Three-dimensional air-parcel trajectories were computed for four cases of a summer thermal low pressure system over the Iberian Peninsula. These cases were the same as those considered in the simulations of mesoscale atmospheric flows done in Part I (Martín et al. 2001). The geographical domain corresponds to an area of 340 km 310 km in the center of the Iberian Peninsula, roughly centered on the Greater Madrid Area and with the same grid specification and inputs as the simulations of Martín et al. (2001). The TVM model (Thunis 1995; Schayes et al. 1996; Bornstein et al. 1996) was used in the computations. It is a prognostic mesoscale meteorological model [a short description of this model is given in Part I (Martín et al. 2001)]. Three-dimensional air-parcel trajectories were computed by assuming that air parcels are transported by the three-dimensional wind field computed by the TVM model over a period of 12 h. The period was selected to account for the cycle of formation of tropospheric ozone during the daylight hours. This period is also onehalf of the sulfur dioxide lifetime [24 h; see Fig. 1 in Dennis et al. (1996)]. Taking into account dispersion, the time in which the atmospheric sulfur dioxide concentration decreases below the thresholds of atmospheric effects (10 g m 3 ) can be much lower than its lifetime, depending on the emission rates and advection [see Table 1.5 in Zanetti (1990)]. Air parcels were emitted every 3 h from the four levels starting at 0000 UTC. The TVM model was initialized at 1200 UTC in the same way as the simulations of Part I. The synoptic conditions corresponding to each of the specific studied cases were kept constant throughout the simulation. The TVM model was run for a simulation period of 48 h, which is the needed period for completing the 12-h trajectories. The computation of the first trajectory starts after 12 h of model time, corresponding to 0000 UTC (midnight), to guarantee computational stability. There are no significant differences between the wind fields computed by TVM for time separated by 24 h. For instance, the wind fields estimated for the 15th simulation hour (0300 UTC of the first simulation day) are almost equal to those for the 39th hour (0300 UTC of the second simulation day). Air parcels were assumed to be emitted from a location in Madrid City at two heights: 25 and 400 m AGL. These heights were selected to investigate the transport of pollutants that sometimes could be lifted by thermal convection throughout the mixing layer during the daytime, as well as transport tracks that can result from the vertical flow separations expected at night. Trajectory computations were performed by a subroutine implemented in TVM by the authors. This subroutine uses the TVM three-dimensional wind field at every time step as input for computation of the threedimensional movement of air parcels. Bear in mind that the computational time step of the TVM model is variable, and thus for the simulations shown in this paper, the maximum time step was fixed at 30 s. Because the aim was to estimate average transport patterns, air-parcel movement was assumed to be completely driven by the computed wind vector without any stochastic contribution representing the turbulence intensity. To take into account the turbulence effects, a stochastic term should be added to the transport equation. The mean value of this term would be zero because of its stochastic features. Then, no contribution or bias in an averaged transport pattern is expected. In addition, if it is assumed that the stochastic term follows a Gaussian distribution with zero mean and standard deviation being the function of the turbulence intensity, the transport pattern computed without the stochastic term corresponds to the averaged transport trajectory and is also the most probable. Hence, the resulted trajectories are representatives of average transport patterns. Estimates of the dispersion of the location of the air parcel at every time due to the stochastic contribution of the turbulence requires the computation of a huge number of trajectories or to use

3 MAY 2001 MARTÍN ET AL. 907 FIG. 1. Map of the modeling domain showing the regions affected by the airmass trajectories of each case. Here, S denotes the release point. dispersion models, such as particle-in-cell models, applied to a point release. However, it is out of the scope of this paper. Wind vectors at the location of an air parcel are computed from TVM grid values by bilinear interpolation. Air-parcel trajectories were computed taking into account topography, by assuming that air parcels can never be below 20 m AGL. This height corresponds to the first computational atmospheric level assumed in the TVM simulations. This level is the first one for vertical wind velocity computations, whereas the horizontal wind components are computed at 10 m AGL. Air parcels are forced to move at 20 m AGL whenever the estimated transport moves them below that height. It is expected that the computed trajectories will be reasonably realistic, given the agreement between model simulation results and observations for the meteorological fields in the four cases of summer thermal low pressure conditions (Martín et al. 2001). However, the resulting trajectories must be evaluated assuming the inherent uncertainty of the computations. Trajectories have been split also into two categories depending on their potential for pollution. This splitting was done by accounting for the typical time activity cycle of vehicle traffic in Madrid City (Palacios and Martin 1996). The first category is for potentially polluted parcels that correspond to the following release times: 0600, 0900, 1200, 1500, and 1800 UTC (local standard time in Spain in summer is equal to UTC plus 2 h). The second category is for potentially nonpolluted air parcels and corresponds to parcels emitted at 2100, 0000, and 0300 UTC. Air-parcel trajectories have been plotted on maps corresponding to the specific regions affected by them (Fig. 1). 3. Results a. 14 July The main feature of the trajectories in this case is the general trend toward an efficient northward transport at higher levels driven mostly by the synoptic wind (150 and4ms 1 ), whereas more variability is estimated at lower levels (Fig. 2). As expected, low-level trajectories were more affected by local circulations than those at upper levels. All the 25-m-starting-level trajectories are trapped in the mixing layer or below the thermal inversion. The longest transport (up to 140 km away from the release point) occurs for the air parcels emitted between 1800 and 0000 UTC in a west-southwest direction over the Tajo Valley. These air parcels are efficiently transported by wind speeds of about 4 5 m s 1 when the parcels are between 25 and 400 m AGL during the night, but also in the daytime leg of travel, when TVM

4 908 JOURNAL OF APPLIED METEOROLOGY VOLUME 40 FIG. 2. Air-parcel trajectories simulated for 14 Jul Left and right plots correspond to potentially clean and polluted air parcels, respectively, released at (top) 25 and (bottom) 400 m AGL. The size of the symbols is proportional to the elevation of the air parcel above the ground. Here, S denotes the release point. simulated persistent easterly flows over the Tajo Valley [see Fig. 6 of Martín et al. (2001)]. Air parcels emitted at 0300 and 1500 UTC also show an efficient transport in the same direction. However, the former is curved northwestward at the end when it is affected by upslope winds in the morning and early afternoon. For the 1500 UTC release, the air parcel is initially transported in a northwest direction until nighttime, when it is driven by the northeasterly drainage flows. Air parcels emitted during the morning (0600, 0900, and 1200) are transported far in the north-northwest direction, especially the last one, which traveled about 150 km away from the release point. For these three release times, the air parcels are efficiently moved to the mountains via transport by upslope winds in late afternoon or evening. They are then trapped by the downslope wind in the northern slopes of mountains and hence moved efficiently to the Northern Plateau. In this case, the potentially polluted air parcels emitted during the afternoon tend to be transported toward the west-southwest, whereas those emitted in the morning affect the mountain range and the Northern Plateau. The air parcels launched at 400 m AGL between 1800 and 0000 UTC show a tendency to be moved toward

5 MAY 2001 MARTÍN ET AL. 909 the west-southwest, but the first stages of their travels were northwestward. This result is because, in the initial stages, these air parcels were located above the nocturnal thermal inversion layer and were transported by the southeasterly synoptic flow toward the mountains while they were reducing their height above ground as the terrain elevation increased. These air parcels were then captured by the katabatic winds on the southern slopes of the mountains and were transported toward the westsouthwest. A similar phenomenon was observed in the 0300 trajectory but the trapping by the katabatic winds occurred very late, just before the sunrise when the katabatic winds started to disappear. Then, the air parcels were moved by the upslope winds during the morning and crossed the mountains. The remaining air parcels also crossed the mountains because of the southeasterly upslope flows and then were mainly transported by the modeled southeasterly flow over the Northern Plateau. In summary for the 14 July case, there is evident pollution transport from the urban sources toward the mountain range after drainage flows have ended, also affecting northern areas of the domain. b. 16 July The almost opposite daytime and nighttime flows in the lower layer are the reason that 25-m-starting-level trajectories are sited almost along a straight line running from the west-southwest to the east-northeast (roughly parallel to the mountain range; Fig. 3). The nocturnal northeasterly flows probably is effective in moving pollutants emitted between 1800 and 0000 UTC toward the west-southwest direction up to distances of 100 km from the release point for the 2100 trajectory. Morning ( ) air parcels have a clear transport toward the north driven by upslope winds. However, in contrast to the 14 July case, the air parcels do not cross the mountain range in spite of the fact that the winds apparently are as intense as in the 14 July case. For the 16 July case, the distance to the mountain range in the transport (north) direction is greater than for the 14 July case (northwest), and the air parcels reach the summit too late (at the end of the 12-h period). Trajectories computed for a longer period (not shown here) do cross the mountain range during the night for the early-morning releases when the air parcels were trapped by the downslope winds on the northern slopes of the mountains. For the 1200 trajectory, however, the air parcels are moved to the south in the last stage during the evening when they are trapped by the katabatic winds. The 0300 and 1500 air parcels show trajectories clearly affected by the daily cycle of the low-level flows. For the 400-m-starting-level trajectories, the air parcels are initially moved northward by the upslope wind when emitted during the day and by the synoptic wind (190 and3ms 1 ) when emitted during the night. In the releases, the air parcels cross the mountain range, reaching distances of up to 120 km away from the release point. The remaining air parcels are trapped at some stage of their travel by the katabatic flows on the southern slopes and are then transported in a southwest direction, except for the 0000 release, when the air parcel is trapped in the final stage of katabatic flow before sunrise but then is moved northward. As in the former case, differences between low-level and high-level trajectories are marked. In this case, the northeast southwest oscillation and loops of the lowerlevel trajectories are notable. It must also be pointed out that polluted air parcels emitted during the morning are transported toward the mountains, whereas those released in the afternoon are more efficiently moved toward the west-southwest. Last, the area potentially affected by the polluted air parcels is smaller than for the 14 July case. c. 15 September In this case, the transport at the lowest level is the shortest of the four cases (Fig. 4). The effect of the synoptic (4 m s 1 ) wind blowing from 320 modulates the mesoscale circulations so as to produce a dominant transport no further than 80 km to the area located northeast and southeast of the Greater Madrid Area. Nocturnal flows from the northeast direction are weak in contrast to the scenarios of July [see Fig. 7 in Martín et al. (2001)]. The 25-m-starting-level air parcels emitted between 0600 and 1200 UTC are moved northeast, driven by the daytime winds, and the remaining air parcels are transported to the south, including the potentially polluted air parcels released at 1500 and 1800 UTC, because of the nighttime winds. The transport length is similar for both groups. Some trajectories are curved because they are affected by the nighttime and daytime wind regimes. The behavior of trajectories of air parcels released at 400 m AGL also has a remarkable southwest northeast variation, but, in some cases, air parcels are at levels above the nocturnal inversion layer and hence are efficiently transported southward by the synoptic wind during the night. Potentially polluted air parcels are transported to the east area of the domain, reaching distances of up to 100 km away from the release point. The effect of the daytime and nighttime regimes is also notable in many trajectories. d. 17 September Trajectories (Fig. 5) are clearly affected by the synoptic (4 m s 1 ) wind blowing from 260, which favors mesoscale upvalley flows and almost inhibits downvalley winds [see Fig. 8 in Martín et al. (2001)]. Transport toward northeast is dominant at every level but is more intense for trajectories starting at 400 m AGL. In these cases, many trajectories cross the eastern domain boundary. Once the air parcel leaves the domain, it cannot come back in computationally but could return in real life. However, it is very unlikely under the flow regimes

6 910 JOURNAL OF APPLIED METEOROLOGY VOLUME 40 FIG. 3. Same as Fig. 2 but for 16 Jul estimated for this case. The air parcels are transported northeastward by the upvalley flows, but the 0000 and 0300 trajectories have a clear easterly direction due mainly to the synoptic wind. Only the air parcels emitted between 1800 and 2100 are trapped by the nocturnal thermal inversion layer during the night and have a short transport driven by weak drainage flows. Some air parcels launched at 25 m AGL, that is, those emitted between 0600 and 1200 UTC, cover long distances, traveling 100 km or more in the northeast direction due because of the strong southwesterly flows developed at the daytime. Other air parcels move mostly in the same direction but no farther than 60 km from the release point. 4. Ensemble trajectories Results of the last section demonstrate that trajectories under thermal low pressure conditions have a wide

7 MAY 2001 MARTÍN ET AL. 911 FIG. 4. Same as Fig. 2 but for 15 Sep variability. This variability can be influenced by synoptic wind forcing. However, an analysis of the ensemble of trajectories provides some interesting results. In Fig. 6, the trajectories for the four days studied are shown split into three main groups. The first one corresponds to nighttime release, that is, those parcels emitted between 2100 and 0300 UTC, which match the period of minimum pollutant emission from the Greater Madrid Area as mentioned in section 2. The second group corresponds to morning releases, that is, those air parcels emitted between 0600 and 1200 UTC. The third one corresponds to afternoon releases (1500 and 1800 UTC). The last two groups correspond to potentially polluted air parcels. The transport patterns show stronger differences between the 25-m-level and 400-m-level releases in the afternoon and nighttime groups. The nighttime 25-mlevel releases are mainly transported to the southwest or to the northeast. Southward transport is less frequent. The longest distances correspond to the southwestward transport, reaching 150 km away from the release location (14 July case). The strong drainage flows in the lower tropospheric levels are mainly the cause of the observed southwestward or southward transport patterns. Despite some trajectories being curved northward (because of the upslope flows in the daytime leg), they do not cross the mountain barrier. Nevertheless, the combination of weak drainage flows in the nighttime

8 912 JOURNAL OF APPLIED METEOROLOGY VOLUME 40 FIG. 5. Same as Fig. 2 but for 17 Sep leg and strong upvalley flows in the daytime leg explains why the effective transport of the nightime air parcels is toward northeast in some cases. The transport pattern of the nighttime 400-m-level releases does not show a dominant path, because the air parcels are frequently above the surface inversion layer at nighttime and then are transported by the flows above the inversion. However, in some stages of the transport, the air parcel can be trapped by katabatic or drainage flows in the southern slopes of mountain barrier. In addition, the air parcels are affected by the mixing layer flows in the daytime leg. The air parcel can be transported in any direction to distances longer than 100 km in some cases. The trajectories cross the mountain barrier in some cases. The 25-m-starting-level trajectories corresponding to the afternoon releases are similar to those of the nighttime releases, but the northeastward transport is shorter. The trajectories are very affected by the nighttime flows in the surface inversion layer. In contrast, the trajectories of the afternoon 400-m-level releases cover a wide area. The influence of the afternoon regimes is more intense than in the trajectories of the 25-m-level releases, giving rise to important northward or northeastward transport. Some of these trajectories are frequently deflected when

9 MAY 2001 MARTÍN ET AL. 913 FIG. 6. Air-parcel trajectories for the four studied cases grouped by (top) nighttime, (middle) morning, and (bottom) afternoon releases. Left and right plots correspond to air parcels released at 25 and 400 m AGL, respectively. Here, S denotes the release point.

10 914 JOURNAL OF APPLIED METEOROLOGY VOLUME 40 the air parcels are trapped by katabatic or drainage flows at nighttime. Other trajectories are driven by upper-level flows, because the air parcels remain above the surface inversion layer at nighttime. No air parcel is transported toward the southern area of the domain. The polluted air parcels emitted at both reference levels during the morning follow a very similar distribution of trajectories. They affect the mountain range, the Northern Plateau, and the northeastern area of the domain, reaching distances longer than 100 km away from the release point in many cases (especially for 400-mlevel releases). The air parcels are driven by the daytime flows in the deep mixing layer. The probability that polluted air parcels released in the morning affect areas located south of the Greater Madrid Areas is likely very low. The results shown here agree with the observations of pollutant concentrations made during the experimental campaigns carried out during these days, when higher levels of ground-level ozone were detected in the stations located close to the mountains or in the eastern area of the domain (Artíñano et al. 1994; Plaza et al. 1997). In addition, these results support the idea that the area potentially affected by the pollutants emitted from the Greater Madrid Area can be very wide, especially for photochemical pollution. In this case, secondary pollutants such as ozone are produced from primary pollutants such as nitrogen oxides and volatile organic compounds while the air mass is being transported. Taking into account the need for solar radiation for producing ozone, it is clear that estimated transport patterns for morning air parcels emitted at levels shown in this paper can provide a rough estimation of the extension of the area affected by ozone pollution. Preliminary results from photochemical pollution simulations carried out by the authors, which will be the subject of another paper, support this idea. 5. Conclusions The computed three-dimensional transport patterns of air parcels released from a location at Madrid City were very different among the four studied cases referred to as summer thermal low pressure systems over the Iberian Peninsula. Results indicate that synoptic wind forcing can influence trajectories under thermal low conditions. No significant statistical estimates are available about how frequent the discussed cases are. Nevertheless, the pollutant measurements obtained in experimental campaigns held under summer thermal low pressure conditions (about 15 cases in 4 yr) suggest that the cases similar to those of September in this paper with dominant transport toward northeast or east may be slightly more frequent than those with dominant transport toward the west or north (July cases in this paper). Two common aspects exist in the studied transport patterns. First, a remarkable difference exists between the transport of air parcels released at low levels (25 m AGL) and those released at higher levels (400 m AGL). The former are affected by thermally driven flows, and the latter are more influenced by the synoptic wind. Second, potentially polluted air parcels (launched in the daytime) travel farther than the potentially nonpolluted ones (released at night) for lower release levels. The polluted morning air parcels have a clear tendency to be moved toward the mountains or in the northeast direction and to travel long distances (longer than 150 km in some cases). In contrast, polluted afternoon air parcels emitted at lower levels are mostly transported to areas located parallel to the mountain range over the Greater Madrid Area, with west-southwest transport being more efficient. The probability that polluted air parcels affect areas located south of the Greater Madrid Area is very low under thermal low pressure conditions. Acknowledgments. The authors express their gratitude to Mercedes Gómez for her help in making the figures. The work shown in this paper has been made possible with the data from the experimental campaigns done under the EU RECAPMA (STEP PL890009/0006.C) and Spanish CICYT (NAT CE) projects. This work has been financially supported by the Spanish CI- CYT Project AMB REFERENCES Artíñano, B., M. Pujadas, J. Plaza, S. N. Crespí, H. Cabal, B. Aceña, and J. Terés, 1994: Air pollution episodes in the Madrid airshed. Transport and Transformation of Pollutants in the Troposphere: EUROTRAC Symposium 94, P. M. Borrell et al., Eds., SPB Academic Publishing, Bornstein, R. D., P. Thunis, P. Grossi, and G. Schayes, 1996: Topographic Vorticity-Mode Mesoscale- (TVM) model. Part II: Evaluation. J. Appl. Meteor., 35, Dennis, R. L., D. W. Byun, J. H. Novak, K. 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