880 JOURNAL OF APPLIED METEOROLOGY VOLUME 40

Simulations of Mesoscale Circulations in the Center of the for Thermal Low Pressure Conditions. Part I: Evaluation of the Topography Vorticity-Mode Mesoscale Model

FERNANDO MARTIÂN,SYLVIA N. CRESPIÂ, AND MAGDALENA PALACIOS Departmento de Impacto Ambiental de la EnergõÂa, Centro de Investigaciones EnergeÂticas, Medioambientales y TecnoloÂgicas, ,

(Manuscript received 27 September 1999, in ®nal form 10 August 2000)

ABSTRACT The Topography Vorticity-Mode Mesoscale (TVM) model has been evaluated for four different cases of thermal low pressure systems over the Iberian Peninsula. These conditions are considered to be representative of the range of summer thermal low pressure conditions in this region. Simulation results have been compared with observations obtained in two intensive experimental campaigns carried out in the Greater Madrid Area in the summer of 1992. The wind ®elds are qualitatively well simulated by the model. Detailed comparisons of the time series of simulations and observations have been carried out at several meteorological stations. For wind speed and direction, TVM results are reasonably good, although an underprediction of the daily thermal oscillation has been detected. The model reproduces the observed decoupled ¯ow in the nighttime and early morning along with the evolution of mixing layer ¯ow during the day. In addition, the model has simulated speci®c features of the observed circulations such as low-level jets and drainage, downslope, upslope, and upvalley ¯ows. The model also simulates the formation of hydrostatic mountain waves in the nighttime in some cases.

1. Introduction mal low dominates summer atmospheric conditions. In spite of the high frequency of this mesoscale pressure The Greater Madrid Area is located in a 700-m high system in the south of Europe and in other parts of the plateau at the center of the Iberian Peninsula. It is bor- world (Junning et al. 1984; Barry and Chorley 1987), dered to the north-northwest by a high mountain range it has hitherto received little attention from the scienti®c (), 40 km from the city, and to the community, and a signi®cant lack of thermal low and northeast and east by lower mountainous terrain. The former and closest is about 200 km long and is aligned associated air circulation studies exist. Although the along the southwest±northeast axis, with a mean altitude thermal low in Spain is more frequent in summer, it has of 2000 m. The highest summit reaches up to 2400 m also been detected at the beginning of autumn and even and is located 50 km northwest of the city. The climate during the last days of winter near to the spring season in Madrid is somewhat extreme, typical of a continental (Font 1983). These infrequent cases are associated with area, with hot dry summers and cold winters, with most weak synoptic conditions, long dry periods, and strong days being under clear-sky conditions. These topograph- surface heat ¯uxes. ic and climatological features along with a heat island Surface heating and atmospheric convective motions effect contribute to complex mesoscale circulations and are the common characteristics in thermal low devel- mixing conditions, which have an important in¯uence opment. However, several peculiarities lead to a signif- on atmospheric pollution episodes. icantly different thermal low in the Iberian Peninsula, The geographical features of the Iberian Peninsula as compared with other countries. The particular hori- and its particular location in the Mediterranean area cre- zontal and vertical sizes (less than 1000 km and 3000 ate speci®c meteorological conditions in which the ther- m, respectively), as well as the high intensity and per- sistence of this mesoscale system, are also related to the geographical location of the Iberian Peninsula, which is almost completely surrounded by sea. Temperature dif- Corresponding author address: Fernando MartõÂn, Grupo de Mo- ferences between the air over the heated ground and air delizacion de la Contaminacion Atmosferica, Dpto. Impacto Am- over the sea, along with mountain range orientation, biental de la EnergõÂa, CIEMAT, Avda. Complutense 22, 28040 Ma- drid, Spain. produce strong air convergence, which is channeled by E-mail: [email protected] the main mountain valleys (MillaÂn et al. 1991). A strong

᭧ 2001 American Meteorological Society

Unauthenticated | Downloaded 09/28/21 09:22 PM UTC MAY 2001 MARTIÂ N ET AL. 881 reduction of the inland surface pressure reaches a max- Portela (1994) and Portela and Castro (1996) using the imum in the early afternoon, when heating of the ground PronoÂstico a Mesoscala (PROMES) model (Gaertner is most marked and when convective air cells are com- 1994) and by Ibarra et al. (1994) using the Regional pletely developed. When the solar energy begins to de- Atmospheric Modeling System (RAMS) model. In both crease, a slow dissipation of the thermal low takes place cases, working with almost the same spatial domain (the until it disappears during night hours. Therefore, the entire Iberian Peninsula), the main features of the ther- thermal low is a 24-h meteorological system clearly mal low were well simulated, but differences were found associated with intense solar radiation over the arid re- in predicting smaller-scale ¯ows, such as circulations in gions. Portela and Castro (1991) presented a climatic the plateaus and in areas near large mountain ranges. description of thermal lows in the Iberian Peninsula. In the PROMES simulation, the horizontal grid spacing They found the formation of thermal low pressure sys- was 20 km ϫ 20 km, and hence smaller-scale ¯ows tems over the Iberian peninsula is very related to the were better resolved than with the RAMS simulations, de®cit of evaporation in semiarid soils. It could explain for which the horizontal resolution was 32 km ϫ 32 why the thermal lows also can be observed in spring or km. early autumn but being less frequent than in summer- In contrast with these regional-scale modeling studies time. They also made a high-resolution analysis of the simulating the ¯ows over the entire Iberian Peninsula, pressure ®elds that allowed a classi®cation of the ther- this paper is focused on the meso-␤-scale modeling in mal low systems taking into account the location of the a smaller area of it. The current work is presented in area of maximum pressure gradient and the thermal low two parts. Part I is presented in this ®rst paper. The intensity. objective is to evaluate the performance of the Topog- Extensive experimental documentation about the be- raphy Vorticity-Mode Mesoscale (TVM) model in sim- havior of air convergence under summer thermal low ulating the evolution of the meso-␤-scale atmospheric conditions exists (MillaÂn et al. 1991; MartõÂn and Pal- conditions in the center of the Iberian Peninsula under omino 1995). These studies prove that the thermal low the forcing of a summer thermal low pressure system. and sea breezes force an inward ¯ow of coastal pollutant To do this, the TVM predictions in the Greater Madrid emissions toward the center of the Iberian Peninsula. Area are compared with wind and temperature obser- The strong links between the local air circulation of the vations from the surface and upper-air meteorological sea breeze and this mesoscale system are related to the stations for four different cases of thermal low pressure particular orientation of mountain valleys near the Span- situations. The paper includes a description of the TVM model, the cases selected for modeling, the data sources, ish coast that favors inland air motions. Moreover, the and the model con®guration, along with a discussion of strong heating of the Iberian Peninsula soils can inten- the model results compared with observations. sify the inland penetration of air masses. Experimental In Part II (Martin et al. 2001), the variability of pol- results have shown that sea-breeze penetration is sig- luted air parcel trajectories computed with the TVM ni®cantly greater when air convergence associated with model in the Greater Madrid Area under thermal low thermal low conditions interacts with the sea-breeze cir- pressure conditions is discussed. culation (MartõÂn and Palomino 1995). Furthermore, the The practical use of a mesoscale meteorological mod- thermal low can inject pollutant air masses to upper el requires understanding its characteristics and range atmospheric levels, where they can then be transported of application. For the quanti®cation of the accuracy of long distances. model results, it is also necessary to estimate input data Under thermal low conditions, the local air circulation accuracy and how it affects the results, to evaluate the over the central plateaus of the Iberian Peninsula, where uncertainties in model assumptions and parameteriza- the convergence zone is usually located, can be very tions, and to judge how the model represents reality. different. Extensive instrumentation deployment over The evaluation procedure will ensure that users can as- the Madrid area (Plaza et al. 1997) has allowed detection sess the degree of reliability and accuracy inherent in of signi®cant differences of air circulations when ther- the model (Moussiopoulos 1996). mal low conditions affect the Iberian Peninsula. These The mesoscale prognostic TVM model has been eval- differences could explain the levels of pollutants de- uated previously to simulate the atmospheric ¯ows for tected in the center of Spain. winter anticyclonic conditions in the center of the Ibe- Several works have been devoted to modeling the rian Peninsula. Model results agreed in signi®cant as- complete thermal low pressure system structure. In the pects with observed wind ¯ows over the Greater Madrid case of large tropical thermal lows, Leslie (1980) in- Area under anticyclonic conditions in wintertime, for corporated a simple surface heat balance scheme into a example, the daily cycle of thermally driven ¯ows, the large-scale numerical forecast model for Australia. The displacement of the surface wind convergence line to- Gaertner et al. (1993) study of the Iberian thermal low ward the south as a result of the in¯uence of the synoptic consisted of a two-dimensional simulation with a hy- ¯ow on the mesoscale ¯ow, and the model's prediction drostatic, high-resolution, primitive-equation model. of two layers with very different ¯ows (MartõÂn et al. Three-dimensional simulations of the structure and 1996). First simulations of the ¯ow in the center of the ¯ows of the Iberian thermal low system were done by Iberian Peninsula were carried out by Gaertner (1994)

Unauthenticated | Downloaded 09/28/21 09:22 PM UTC 882 JOURNAL OF APPLIED METEOROLOGY VOLUME 40 for several meteorological conditions with the PROMES Soil moisture is computed by a prognostic equation, model including two cases of the summer thermal low which depends on the SBL latent heat ¯ux and surface and a winter anticyclonic system (MartõÂn et al. 1996). latent heat ¯ux, computed by use of the Penman±Mon- teith formulation (Monteith 1981). This formulation ac- counts for dynamic and vegetation effects by use of 2. TVM model aerodynamic and surface resistances that are constant The TVM model is a prognostic mesoscale meteor- for each land use type. ological model. It is a three-dimensional mesoscale vor- Remaining boundary conditions are as follows. ticity-mode numerical model for atmospheric ¯ows in 1) At the upper boundary, the wind is geostrophic, vor- complex terrain based on the Urban Meteorology (URB- ticity is zero, and temperature and humidity values MET) model (Bornstein et al. 1987). The URBMET match synoptic-scale values. model is a three-dimensional, hydrostatic, shallow-con- 2) A ®lter is used in the uppermost computational levels vection, Boussinesq (incompressible) model to simulate to smooth all prognostic variables, except TKE, at urban in¯uences and sea-breeze fronts by use of the each time step to avoid re¯ections of vertically prop- vorticity equations with both hydrostatic horizontal vor- agating gravity waves (Schayes et al 1996). ticity components and two streamfunctions. Several ver- 3) At lateral boundaries, zero-gradient (open) boundary sions have been developed during the 1990s. Initially, conditions are assumed. the TVM model was an URBMET version for non¯at 4) At the surface, stream functions are zero and TKE topography. Schayes and Thunis (1990) made the ®rst is ®xed at 4u2 (Therry and LacarreÁre 1983), where reformulation by applying sigma-height coordinates. * u is friction velocity. TVM uses Cartesian coordinates horizontally and a ter- * rain-following coordinate in the vertical. A complete How TVMNH20a solves the PBL hydrodynamic and explanation of the formulation and descriptions of some thermodynamic transport equation at each time step is applications of this version of TVM can be found in described in Schayes et al. (1996). The elliptic equations Schayes et al. (1996) and Bornstein et al. (1996). The of streamfunctions and vorticity are solved via a mod- version of the TVM model used in the current study i®ed biconjugate gradient method, and advection is (TVMNH20a) was developed by Thunis (1995): it is a solved by the third-order parabolic piecewise method. nonhydrostatic, incompressible, and Boussinesq meso- The model is only applicable (i) to the meso-␤-scale, scale model. However, TVMNH20a can be run in a because horizontal synoptic-scale variations cannot be hydrostatic mode, which keeps the two horizontal hy- considered; (ii) to steady synoptic conditions; and (iii) drostatic vorticity equations but does not include the to clear-sky, light-wind, and weak pressure-gradient me- additional terms for the nonhydrostatic formulation. teorological cases, (iv) with no initial direction shear. The TVMNH20a model assumes two main layers: an Other versions of the TVM model have been evaluated atmospheric layer and a subsurface layer. The atmo- for a number of case studies and geographic locations spheric layer is further separated into two sublayers: a (e.g., Athenian Photochemical Smog Intercomparison of constant ¯ux surface that corresponds to the surface Simulations air quality study of Athens, Greece; Fos, boundary layer (SBL) and a transition layer in which France, experiment; and New York sea breezes, Boulder TVMNH20a uses a level-1.5 closure scheme in which windstorm, and Madrid, Spain, experiments), and com- turbulent kinetic energy (TKE) is computed from a pared with other mesoscale meteorological models such prognostic equation involving advection, TKE shear as RAMS, MEMO (Mesoscale Model), PROMES, and production, buoyancy destruction/production, vertical MAR (Model Atmospherique Regional) (e.g., Thunis et diffusion, molecular dissipation, and horizontal diffu- al. 1993; Bornstein et al. 1993, 1996; MartõÂn et al. 1996, sivity. Horizontal wind speed, potential temperature, 1997). and speci®c humidity in the SBL are assumed to obey Monin±Obukov similarity scaling via the Businger 3. Cases studied forced and mixed convective functions. The infrared radiative ¯uxes have been computed from the scheme In the framework of the Regional Cycles of Atmo- of Sasamori (1968), with carbon dioxide concentrations spheric Pollution in the Mediterranean Area (RECAP- ®xed at 320 ppm and with absolute humidity computed MA) project, the atmospheric pollution team of the Cen- as a function of temperature, pressure, and speci®c hu- tro de Investigaciones EnergeÂticas, Medioambientales y midity. TecnoloÂgicas (CIEMAT) carried out two experimental A soil model based on the ``force±restore'' method campaigns during the summer of 1992 to study the dy- (Deardorff 1978) is included in TVMNH20a to compute namics and chemistry of air pollution in the Greater surface temperature from the surface soil heat ¯ux and Madrid Area. RECAPMA ®eld campaigns lasted three the temperature of the lower soil layer, assumed constant days during July and September of 1992. Meteorolog- within each soil class. The surface soil heat ¯ux is ob- ical conditions during these periods consisted of thermal tained using a residual method from the surface energy low pressure systems over the Iberian Peninsula on ®ve balance equation (Schayes 1982; Schayes et al. 1996). days (14±16 July, and 15 and 17 September). During

Unauthenticated | Downloaded 09/28/21 09:22 PM UTC MAY 2001 MARTIÂ N ET AL. 883 the afternoon of 15 July, thunderstorms occurred, and use data were translated to the same mesh as the hence this day was rejected for simulations because the topography data. The eight different land use types TVMNH20a model assumes clear-sky conditions and and their associated physical parameters are shown lacks any treatment of clouds or moist processes. Hence, in Table 1. Surface and (10 cm) deep soil temper- the selected days for simulations were 14 and 16 July atures are listed in Table 2. Because observations and 15 and 17 September. were unavailable, deep soil data were obtained by Surface weather charts at 1200 UTC (Fig. 1) show trial and error, using values within climatological the thermal low pressure system in the ®nal phase of ranges. development. The center was located over the Iberian 3) Meteorology: Synoptic wind speed and direction Peninsula because of the strong heating of the semiarid must be imposed as top boundary conditions. In ad- land of the peninsula in contrast to the surrounding sea dition, initial potential temperature and wind speed and ocean. The low pressure system was clearly formed pro®les, along with surface speci®c humidity data, at 1200 UTC on 16 July and on 15 and 17 September. are needed. All of this information has been obtained On this last day, there was also an interaction with a from wind pro®les measured from free soundings frontal system that was going over the northern coast carried out by CIEMAT and those of the National of the peninsula. Synoptic charts at the 850- and 500- Institute of Meteorology (Instituto Nacional de Me- hPa levels only showed low pressure systems over or teorologõÂa: INM, hereinafter). Speci®ed synoptic close to the Iberian Peninsula on 16 July at the 500- winds need to account for the meteorological situ- hPa level (when a weak cold low pressure system from ation modeled. Deep anticyclonic or cyclonic situ- 15 July appeared) and 17 September (when a weak ations usually affect the entire troposphere, but the trough affected northern regions of the peninsula). The Iberian thermal low is usually embedded in a sub- anticyclone was well developed at 850 and 500 hPa on tropical anticyclone. Air masses affected by the 15 September. Although the thermal low pressure sys- strong buoyancy in the center of the Iberian Pen- tems over the Iberian Peninsula appeared to be similar, insula can be frequently injected up to 3000 m above there are signi®cant differences in the upper atmosphere ground level (AGL) under thermal low pressure con- among the four selected days. In this study, the selected ditions. Therefore, while the lowest 3000 m deep days thus cover a wide range of atmospheric conditions layer of atmosphere is affected by the thermal low under the label of a summer thermal low pressure sys- system, the anticyclonic conditions remain in the tem. middle and upper troposphere. In this case, it is clear that the synoptic wind data (which are used as upper boundary condition) have to be obtained from alti- 4. Input data and model con®guration tudes no higher than 3000 m AGL using soundings The geographical domain corresponds to an area of launched at noon or the early afternoon (see Table 340 km ϫ 310 km in the center of the Iberian Peninsula 3). The initial wind and temperature pro®les were that covers most of the Tajo Valley, the mountain range obtained by analyzing smoothed radiosonde sound- called Sistema Central (which includes ings from two locations close to Madrid to obtain in its western area, Sierra de Guadarrama in its center, an initial sounding for each simulated case. These and Sierra de AylloÂn in its northeastern area), Montes pro®les were extended to the entire domain to rep- de Toledo, and part of the Northern and Southern Pla- resent the initial state of the atmosphere for each teaus (Fig. 2). The Greater Madrid Area is close to the case. In Fig. 4, the initial meteorological pro®les are center of the domain, between the two ranges. It is a depicted up to 10 000 m in potential temperature and large conurbation extending about 50 km ϫ 50 km and 4500 m in wind to show clearly the initial conditions including the city of Madrid and several satellite towns. in the lower layers where the vertical changes are The main input data of TVMNH20a consist of: sharper. 4) Model con®guration: Although the model can use 1) Topography: Data for a mesh of 34 ϫ 31 cells with nonregular or stretched grids with variable resolu- a spacing of 10 km ϫ 10 km were obtained from a tion, for the current simulations a regular grid was digital terrain model (CNIG 1995) for Spain with a selected. Details about the miscellaneous aspects of spacing of 1 km ϫ 1 km (Fig. 2). These data were input data are given in Dutrieux (1997). The number smoothed by applying a Shapiro ®lter to reduce of grid points currently selected was 35 ϫ 32 in the slopes to a maximum of 6% and to ®lter out 2⌬x horizontal, with 24 vertical levels. The top of the waves, where ⌬x is the length of a grid cell. domain was set at 15 400 m above mean sea level 2) Land use data: These data have been obtained by (MSL). The vertical resolution was variable, with processing the CORINE land use data for Spain (I. the minimum separation at the bottom levels (20 m) RaÂbago and M. Sousa 1995, personal communica- increasing with altitude. The ®lter to smooth all the tion) with a 5 km ϫ 5 km spacing (Fig. 3). CORINE prognostic variables, except TKE, is applied to the is the acronym of coordination of the Collection of ®ve uppermost computational levels (i.e., above Information on the State of the Environment. Land 11 400 m MSL). The simulations cover 37 h starting

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FIG. 1. Meteorological charts from the Spanish National Institute of Meteorology for four selected cases.

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FIG. 2. Modeled area showing main geographical features.

FIG. 3. Land use types.

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TABLE 1. Physical parameters associated with land use types. Soil heat capacity per area unit was estimated assuming the depth of upper

soil layer is equal to 10 cm. Here, z0 is roughness length. Surface Soil heat resistance to capacity per

evaporation area unit z0 Land use type Albedo Emissivity (s mϪ1) (J K Ϫ1 mϪ2) (m) 1 Water reservoir 0.09 0.93 0.00 1.00 0.001 2 Urban area 0.20 0.92 480.0 3.00 ϫ 105 1.000 3 Herbaceous and shrubs 0.20 0.91 480.0 1.00 ϫ 105 0.060 4 Forest 0.16 0.97 480.0 1.80 ϫ 105 0.800 5 Olive trees and groves 0.19 0.94 480.0 1.00 ϫ 105 0.200 6 Orchards and vineyards 0.19 0.94 480.0 1.00 ϫ 105 0.200 7 Cropland 0.15 0.90 480.0 1.00 ϫ 105 0.500 8 Pasture 0.20 0.94 480.0 1.00 ϫ 105 0.500

at 1200 UTC [in Madrid, the difference between uni- sults and observations are presented from the analysis versal coordinated time (UTC) and local mean solar of surface (10 m) wind ®elds, wind and temperature time is about 12 min]. The TVMNH20a model com- time series from surface stations, and vertical pro®les putes the suitable time step for each resolution, but of wind and temperature at the Villanueva site. an upper limit is imposed; for these cases, 30 s was considered. a. Surface wind patterns

1) 14 JULY 5. Data used in model evaluation The wind ®eld is in¯uenced by a 150 synoptic wind Six meteorological 10-m towers were deployed in the Њ with a speed of 4 m sϪ1. By 0300 UTC (hereinafter, all region of Madrid (Fig. 5). Wind speed and direction at time references will be UTC), 15 h after the start of the 10 m AGL and temperature at 1.5 m AGL were auto- model simulation (Fig. 6), drainage ¯ows are notable matically recorded every 10 min at all stations except in many areas of the domain (e.g., southeasterly ¯ows the one installed at the CIEMAT headquarters (CI). This over the Northern Plateau and northeasterly ¯ows over one was equipped with temperature sensors at 10 and in the Tajo Valley and Greater Madrid Area). Another 80 m AGL and with wind speed and direction sensors important feature of the wind ®eld is the intense down- at 80 m AGL. Data (averages, standard deviations, ver- slope ¯ow observed in the northern slope of the moun- tical gradients, atmospheric stability, etc.) from this sta- tain range, which is related to the ¯ow acceleration ob- tion were recorded every hour. Observed wind data from served when air is ¯owing over the ridge in stable con- the 14 INM synoptic stations were recorded every 3 h, ditions, giving rise to the formation of mountain waves, but, unfortunately, gaps during the night are frequent. as can be seen in the vertical cross-sectional results (see Several free and tethered soundings were launched section 7). In addition, because the synoptic ¯ow has a daily by the CIEMAT team at the Villanueva site. De- northward component, southward-directed katabatic tails about the experimental deployment are in CrespõÂ ¯ows on southern slopes are weak, except those on the et al. (1995). These data were used to evaluate the per- steepest southern slopes of Sierra de Gredos. Most ob- formance of the model in its simulation of the vertical servations are in the Greater Madrid Area and its sur- structure of the lower troposphere, especially the bound- roundings. In this area, the simulated wind ®ts the ob- ary layer. servations well. The rest of the domain has sparse cov- erage at this hour. 6. Simulations By 0900, the weak land warming is enough to weaken The nonhydrostatic TVM model (version the strong southeasterly downslope ¯ows on the north- TVMNH20a) has been run to simulate the four cases ern slope, but weak southeasterly ¯ows remain in the described in section 3. Comparison between model re- Northern Plateau. In the Greater Madrid Area, winds are generally weak and start to rotate clockwise to east- erly or southeasterly ¯ows because of the heating of the TABLE 2. Data of soil temperature (K) for every scenario. Here, Tg southern slopes of the Sistema Central. These features is the soil surface temperature, T is the surface temperature of water g1 are con®rmed by the observations. areas, and T2 is the temperature at the bottom boundary of the soil model. By 1500, upslope winds are dominant on the southern slopes, but no clockwise rotation to westerly or south- 14 Jul 16 Jul 15 Sep 17 Sep westerly ¯ows (in contrast with the other cases simu-

Tg (K) 305 304 302 304 lated) is observed in the Greater Madrid Area and in Tg1 (K) 297.5 297.5 297.5 297.5 Tajo Valley. This situation probably is due to the forcing T (K) 302 302 297 299 2 of the southeasterly synoptic ¯ow produced by the lo-

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TABLE 3. Meteorological inputs. 14 Jul 16 Jul 15 Sep 17 Sep Surface potential temperature (K) 311 309 307 308 Surface speci®c humidity (10Ϫ3 kg kgϪ1) 8 8 8 8 Synoptic wind direction (Њ) 150 190 320 260 Synoptic wind speed (m sϪ1) 4 3 4 4 cation of the thermal low center over the southwestern very steep southern slope of Sierra de Gredos is almost area of the Iberian Peninsula (Fig. 1). On the northern inhibited, because the synoptic wind is nearly opposite slopes of the Sistema Central, the upslope wind is in- to the katabatic ¯ow. By 0900, almost no differences hibited, in agreement with the observations. It probably exist between the estimated wind ®elds for both days. is due to a downward momentum transfer from aloft in As for the 14 July case, a downward momentum transfer a well-mixed layer (Atkinson 1989). It can also be the in the mixing layer can be the reason that upslope ¯ows reason that southeasterly ¯ows are also dominant in the in the lee side of the Sistema Central do not appear and Southern and Northern Plateaus. southerly and southwesterly ¯ows are dominant in most By 2100, TVM shows that upslope ¯ows have dis- part of the domain at 1500. appeared, which is con®rmed by the observation at the By 2100, a weak ¯ow with some drainage in the station on the southern slope of the Sistema Central. Greater Madrid Area and in Tajo Valley exists. The TVM results also simulate weak winds in the west and strongest winds are blowing from a southerly direction north of the Greater Madrid Area, but not in the east on the northern slopes (downslope winds) and on the and southeast. This result agrees with the observations. Northern Plateau. Modeled drainage ¯ows in the south- On the northern slopes of the Sistema Central and North- ern slopes, just west and north of the Greater Madrid ern Plateau, southeasterly ¯ow is again dominant and Area, are stronger than those simulated for 14 July as shows a clear tendency toward the nighttime ¯ow shown con®rmed by the observations. at 0300.

3) 15 SEPTEMBER 2) 16 JULY Under the thermal low conditions of 15 September, Although some similarities exist between the simu- the synoptic wind is from the northwest with a speed lated wind ®elds for 16 July and those for 14 July, some of4msϪ1, in contrast to the two former cases. The differences exist (Fig. 7). On 16 July, the synoptic wind wind ®eld pattern is correspondingly different, espe- blew from 190Њ with a speed of 3 m sϪ1. The ®rst panel cially over the Sistema Central (Fig. 8), where north- of Fig. 7, at 0300, shows a considerable difference in westerly ¯ows are found during most of the entire day, the ¯ow over the southern slope of the Sistema Central although oscillations are detected. During the night, as compared with the former case. Drainage ¯ow on the downslope winds on the southern slope are strong (more

FIG. 4. Initial meteorological pro®les used in simulations.

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FIG. 5. Simulation domain showing locations of meteorological stations and topographic height contours. than in any other case) and can also be observed at the appears just over the peaks of the range in simulations Hoyo station. In this case, the effect of leeside accel- when a zero synoptic wind speed is assumed (not shown erations of downslope ¯ows was more signi®cant than here). Some similar features were observed in simula- in other cases. These are observed in the wind ®elds at tions of ¯ows for winter conditions (MartõÂn et al. 1996). 0300 and 2100. As demonstrated in section 7, mountain The speed of this movement may depend on the com- waves also are formed. Bear in mind that sunset in Sep- ponent of the synoptic wind vector perpendicular to the tember is about 1.5 h earlier than in July. Thus, wind Sistema Central and on the intensity of solar heating, ®eld features at 2100 are closer to nighttime ¯ow pat- as seen in other simulations (not shown here). terns than are those in July. In addition to the wind over the Sistema Central, the ¯ow over the Greater Madrid 4) 17 SEPTEMBER area and the Tajo Valley consists of weak northerly and northeasterly drainage ¯ows at night. A convergence The 260Њ synoptic wind blew with a speed of 4 m zone is detected in the narrower western area of the Tajo sϪ1 on 17 September. As in the former case, the north- Valley, where interaction of the downslope ¯ows is westerly downslope ¯ow is important on the southern strong. Unfortunately, no observations exist in this area slopes of the Sistema Central (Fig. 9), but the wind to con®rm this feature. speed is lower and wind direction has an important west By 0900, northwesterly downslope ¯ows on the component at 0300. The downslope wind on the north- southern slopes of the Sistema Central become weak, ern slopes is also signi®cant, but less so than on the drainage ¯ows almost disappear, and upslope winds start southern slopes. The effect of the direction of the syn- in both slopes of the Sistema Central with a convergence optic wind, almost parallel to the Sistema Central, is line sited over the southern slopes but close to the crests. clear in the simulated ¯ow. There are also important After midday, (by 1500), the strong daytime ¯ow pattern downslope winds on the northern slopes of the Montes is completely developed, and westerly and southwest- de Toledo, and signi®cant southwesterly ¯ows on the erly ¯ows are dominant in the Tajo Valley and the Great- Northern Plateau. In contrast, the wind is very weak in er Madrid Area. In contrast, the ¯ow over the mountains the Greater Madrid Area and in Tajo Valley, also con- and plateaus is northwesterly. The convergence line par- ®rmed by observations. allel to the Sistema Central was moved southward about Later in the morning, (by 0900), downslope ¯ows 20 km with respect to its position at 0900 by the forcing have weakened signi®cantly, upslope winds are starting, of the northwesterly synoptic ¯ow. This southward and the ¯ow in the Tajo Valley is clearly from the south- movement of the convergence line continues progres- west. By 1500, the southwesterly ¯ow is intense in the sively during the daytime. This convergence line usually entire domain, as evidenced by observations and in the

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FIG. 6. Simulated wind ®elds (gray arrows) and observed winds (black arrows) for 14 Jul at 10 m. simulated wind ®eld. This general southwesterly ¯ow remain in some areas, but they either weaken or their over the entire domain did not occur in the other cases. ¯ow direction changes, as on the southern slopes of the It is likely that the 260Њ synoptic wind has favored this Sistema Central. In this area, the wind blows from the ¯ow, but perturbation of the mesoscale ¯ows is also west or northwest (downslope). It produces a conver- important. On the one hand, the southwesterly upvalley gence area that moves southward, reaching the Greater ¯ow in the Tajo Valley and in the Greater Madrid Area Madrid Area and the Tajo Valley, as the daytime south- is stronger than in any other case. On the other hand, westerly ¯ow weakens and the downslope winds ac- the wind direction is disturbed by the Sistema Central, celerate. where a convergence area appears. However, the model does not ®t the observation in the northern slope of the b. Surface wind comparison Sistema Central where weak upslope ¯ows are observed in contrast with the modeled southwesterly ¯ows. In Time series comparison between observed and mod- this case, the model probably overestimates the down- eled (10 m) surface wind speed and direction at the six ward transfer momentum in the mixing layer, which stations deployed by CIEMAT (Fig. 5) during the ex- implies a destruction of the upslope ¯ows in the lee perimental campaigns was carried out to check the per- sides. At night, by 2100, intense southwesterly ¯ows formance of the model in the area surrounding Madrid.

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FIG. 7. Same as Fig. 6 but for 16 Jul.

All observed data are at 10 m AGL, except the CI sta- 1) 14 JULY tion, which is at 80 m AGL (80-m wind simulations were used for this comparison). The Hoyo (HY) station Wind direction is well simulated of most stations, es- (northern area of Madrid region) is representative of pecially the time of changes in ¯ow direction in the early ¯ows close to the slopes of the Sistema Central, but it morning (Fig. 10). The most important differences are is on the southern slope of a small hill. The San MartõÂn observed at the EN and SM stations, where effects caused (SM) station is located south of Madrid in a small valley by the local small-scale topography cannot be simulated with a north±south orientation, and the El EncõÂn (EN) by the smoothed topography of the simulations. station is east of the Greater Madrid Area close to a For wind speed, overprediction occurs at the EN, SM, small ridge oriented northeast±southwest. Last, the Vil- and VI stations, but the speed is well simulated at the lanueva (VI) station is west of the Greater Madrid Area, remaining three stations located close to the Sistema and the Majadahonda (MJ) and CI stations are repre- Central or in the Greater Madrid Area for the nighttime sentative of the ¯ows in¯uenced by the Madrid urban period (0000±0700). During morning, the most accurate area. The CI station is located close to the summit (500 speed predictions are at the EN, SM, and HY stations, m away) on the western slope of a 100-m-high hill with and an underprediction is observed at the remaining an inclination of about 4%. stations, especially CI. In the afternoon, the wind speed

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FIG. 8. Same as Fig. 6 but for 15 Sep. is well simulated at every station, but after 1800 it is wind through the day can only be done for three stations underpredicted at EN, MJ, and CI even when wind di- (Fig. 11). During this day, TVM results ®t the wind rection is simulated well. As seen in the evening wind speed and direction evolution relatively well at the HY ®elds (Fig. 6), an area of more-intense southeasterly station. Observed data for the CI station show a wide ¯ow is modeled just over and southeast of the Greater variability, especially during the daytime. In the early Madrid Area, where the EN and CI stations are located. morning, observed winds in this station are mostly from However, the strong winds observed at the CI station the northeast as modeled. Then, an abnormal variation can be due to the in¯uence of the 100-m-high hill on in the observed wind direction occurs, with some per- whose western slope it is located. This hill is relatively sistent northeast wind around noon, in contrast to the small and is not represented in the 10 km ϫ 10 km progressive evolution given by the model. In the after- topography of the simulations. noon, the observed wind is from the west or southwest, in contrast to the modeled south or southwest wind. Slight overprediction in the nocturnal wind speed also 2) 16 JULY occurs. A similar behavior, but less erratic in the morn- Unfortunately, because of the lack of observed data ing, is observed for the VI station. In spite of the lack records, comparisons between observed and simulated of observed data at the other stations, it might be con-

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FIG. 9. Same as Fig. 6 but for 17 Sep. cluded that the morning wind evolution is not well sim- during the morning and early afternoon at the CI station ulated by the model in the Greater Madrid Area, and it is not reproduced by the TVM simulations. At the HY seems that the northeasterly drainage ¯ow is also ov- station, TVM gives a clockwise rotation of the wind erpredicted in this area. However, the wind evolution direction from a 140Њ to 260Њ direction during the day- in areas close to the mountains is simulated well. time, but the observed wind blows from a 150Њ to 190Њ direction, as in the July simulations. In this case, the HY station is an area of strong spatial variability in the 3) 15 SEPTEMBER simulated wind direction, because it falls in the modeled Wind direction is simulated well by the TVM model convergence area (see wind ®eld discussion). for the nighttime (Fig. 12). The diurnal evolution is also Wind speed is generally simulated accurately, al- simulated well at the EN and VI stations, at the HY though underprediction of early morning wind speed at station in the early morning and late afternoon, and at all stations has been found. At that time, wind speeds the CI station in the afternoon. The main differences higher than 2 m sϪ1 were observed, but the model sim- are detected at the SM station, because the local cir- ulates almost calm wind. On the other hand, an over- culations in its small valley cause signi®cant distur- prediction was detected at Villanueva at nighttime and, bances on the mesoscale ¯ow. The variability observed to a lesser extent, during the day. During the late af-

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FIG. 10. Simulated temperature and wind velocity vs observed values for 14 Jul. ternoon or evening, some overprediction also exists at good ®t of model results to observations at the HY the SM station. The most important differences were station on the southern slope of Sistema Central is seen found at the CI station, where a signi®cant underpred- at night, when the northwesterly downslope ¯ow be- iction (about a factor of 2) is observed at night, and comes important. wind speed is overpredicted (by about a factor of 2) during the afternoon and evening. At midnight, the ob- 4) 17 SEPTEMBER served wind at CI is strongly accelerated, blowing from the northeast and reaching speeds of 7 m sϪ1. This ac- The TVM model simulates fairly well the complete celeration can be related to a local perturbation of the daily cycle of wind direction at every station (Fig. 13). northerly and northeasterly mesoscale drainage ¯ows. However, the modeled change of wind direction is ad- This local perturbation could result from the effect of vanced 2h at the VI, EN, and SM stations. At the EN the hill on which the station is located. In addition, a and SM stations, results are better for the daytime than

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FIG. 11. Same as Fig. 10, but for 16 Jul. at night, because of local drainage winds at night that extreme underprediction (about 7ЊC) is at the SM sta- were not simulated by the model. tion. In contrast, the nighttime temperature is generally Observed and modeled wind speeds generally coin- overpredicted (especially for the September cases), ex- cide with the general pattern of the daily cycle, which cept for the CI station. consists of very weak winds at night and strong accel- The amplitude of the daily oscillation of temperature erations during the day. Differences exist, however, in is mostly underpredicted, especially for the September the magnitude of the maximum wind speeds. In all cas- cases (Figs. 12 and 13). However, for the case of 15 es, the observed wind speed maximum occurs about September, the amplitude of measured daily thermal os- 1500 and it is slightly underpredicted. At the VI station, cillation is very high, reaching 33ЊC at the SM station the underprediction is higher (the maximum observed (much higher than at other stations), in contrast with wind speed was 9.4 m sϪ1 at the VI station, and the the average thermal oscillation observed for the other model result was 5.2 m sϪ1). studied cases. Best results are for the CI station, where observations and model results correspond to 10 m AGL. The cause of the more notable underprediction of c. Surface temperature the daily thermal oscillation for the September cases Simulated surface temperatures were compared with might be changes in the soil conditions after all the observations. Temperature was measured at 1.5 m AGL summer season. The speci®c results for the SM station at all of meteorological stations, except for the CI station (in which the daily thermal oscillation is always much where it was at 10 m AGL. Model results for the 1.5-m- higher than the other stations) might be due to the dif- AGL temperature were obtained by interpolating be- ferences between the local soil where the station were tween TVM results for the two lowest temperature lev- installed and the land use type selected as representative for their 10 km 10 km cells. Modeled and observed els, taking into account the formulation used in TVM ϫ daily cycle of temperature are synchronous for all sta- for the SBL. tions, except VI and CI, where maximum temperature Maximum temperature simulations ®t the observa- is observed later (about 1700). tions in many cases, especially the July cases (Figs. 10 and 11). Nevertheless, underprediction is notable at some stations, especially for the 15 September case (Fig. d. Vertical pro®les 12). In this case, the maximum temperature is under- Vertical TVM pro®les of potential temperature and wind predicted by 2ЊC at the VI, HY, and CI stations. The speed and direction have been compared with observed

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FIG. 12. Same as Fig. 10 but for 15 Sep. pro®les taken from free soundings launched at the Villa- However, the stable layer height is simulated well by the nueva station (Fig. 14). Two sets of pro®les have been model. The model overpredicts the mixing layer heating used to check the ability of the model to simulate the in early morning, except in the case of 17 September. mixing layer growth. The ®rst set of pro®les correspond This overprediction is connected to the slight overpre- to early morning, and the second set is representative of diction in the surface temperature at the Villanueva sta- noon or early afternoon. Observed pro®les represent local tion (Figs. 10±13). At this station, observed temperature atmospheric conditions at a certain moment of the day, growth is less intense than is modeled, giving a maximum but model results are a smooth representation of atmo- temperature about 2h after the simulated maximum. spheric features over a 10 km ϫ 10 km cell. Moreover, Early-morning wind pro®les show a more complex the lower vertical resolution of the TVM results as com- situation because of the ¯ow decoupling observed be- pared with the high detail of the observations increases tween the mixing layer and upper level. In pro®les of differences between observed and modeled variables. around 0700 or 0800 UTC (see pro®les for 14 and 16 Deepening of the mixing layer is simulated well by July in Fig. 14), some nocturnal characteristics remain, the model. Some discrepancies are detected in a datum- such as northeasterly drainage ¯ows in the mixing layer to-datum comparison, because sometimes 2- or 3-K dif- well distinguished from the ¯ow in the upper level. This ferences have been observed between observations and feature is simulated well by the model. In the pro®les model results, especially in the early morning pro®les. made later (0930 UTC, especially for the 17 September

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FIG. 13. Same as Fig. 10 but for 17 Sep. case), modeled wind pro®le shows the start of the south- wind direction have been computed for the four above erly upslope ¯ows, which ®ts well with the observations. scenarios. The plane runs from the south to the north The upper-level results also ®t the observations, but to and crosses the x Universal Transverse Mercator (UTM) a lesser extent for the 15 September case, when wind coordinate at 449 km (see Fig. 3). The selection of this direction change is simulated to occur in a lower level plane was done taking into account the Greater Madrid than that observed and wind speed is underpredicted. Area location and that it should be better to exclude In the late morning pro®le of the 14 July case, wind peaks of the mountain range in order to have an average speed and direction are simulated well by the model in representation of mountain range effects. The selected the mixing layer, but wind speed is underpredicted in the cross section is just on the east side of the Greater Ma- upper level. Best agreement between observations and drid Area and cross a saddle between peaks in the ridge. model results occurs in the early afternoon pro®les (see cases of 16 July and 15 and 17 September in Fig. 14), when the mixing layer is almost completely developed. a. 14 July The synoptic wind blows from 150Њ with a speed of 7. Cross-sectional results 4.0msϪ1. The initial vertical temperature gradient is The y±z plane distributions of potential temperature 2.5KkmϪ1 from 1600 to 4100 m MSL. During the perturbation, vertical wind speed, TKE, wind speed, and night, a hydrostatic mountain wave is observed in north-

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FIG. 14. Simulated and observed vertical pro®les of wind and potential temperature. ward direction, as is clearly seen in (i) the undulation because simulated stability is relatively strong in the of the isentropic lines with propagation vertically but layer between 1500 and 3000 m MSL, the synoptic not horizontally, (ii) the distribution of updrafts and winds are also weak but with a signi®cant perpendicular- downdrafts, and (iii) the acceleration of the ¯ow on the to-ridge component, and the ridge is very wide. Under northern slope (Figs. 15a,c). This result is expected, these conditions, buoyancy is dominant enough that the

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FIG. 15. Simulated values of perturbation (from initial surface state) of potential temperature (K; dotted lines) and vertical wind speed (m sϪ1; solid and dashed lines represent positive and negative values, respectively) at (a) 0300 and (b) 1500 UTC including (shaded) turbulent kinetic energy (J KgϪ1). Simulated (shaded) horizontal wind speed (m sϪ1) and direction (Њ) corresponding to (c) 0300 and (d) 1500 UTC. All information is in north±south cross-sectional plane corresponding to a UTM x coordinate of 449 km for 14 Jul. vertical accelerations are inhibited and the ¯ow is hy- energy can be strong, reaching almost 3.0 J kgϪ1, and drostatic. The maximum vertical wind speed is about the maximum mixing layer depth is about 2000 m (Fig. 0.1msϪ1. The amplitude of the isentropic undulation 15b). Flows are mainly from 120Њ to 150Њ in the Greater is about 300 m. The maximum horizontal wind speed Madrid Area and Tajo Valley. Maximum wind speed was 5.0 m sϪ1. (about 4.0 m sϪ1) is over the Sistema Central, as can In addition, katabatic ¯ows are observed on the south- be seen in Fig. 15d. ern slope and from the northeast direction in the Greater Madrid Area and Tajo Valley. In both cases, katabatic b. 16 July ¯ows are in a shallow layer, about 200 m thick, in agree- ment with depth of the surface inversion layer at 0300 The synoptic wind blows from 190Њ with a speed of UTC. 3.0msϪ1, and the vertical temperature gradient is 2.3 During the day, differential heating of the surface KkmϪ1 from 2000 to 4200 m MSL. In this simulation, produces thermally driven circulations with weak up- mountain waves are not detected, because neither per- drafts and downdrafts in the mountains. The nighttime turbation in the isentropic lines nor signi®cant vertical mountain waves have disappeared. The turbulent kinetic ¯ows are observed (Fig. 16a). In this case, the predicted

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FIG. 16. Same as Fig. 15, but for 16 Jul. stability between 1500 and 3000 m MSL is very weak. runs counter to observations, because the difference in Most circulations are katabatic ¯ows along mountain 1.5-m temperature between urban and rural regions in slopes and drainage ¯ows over the Greater Madrid Area the Greater Madrid Area is less during the late morning and the Tajo Valley. Katabatic ¯ows on the northern and early afternoon (CrespõÂ and ArtõÂnÄano 1995). How- slopes are favored by the synoptic wind (Fig. 16c). The ever, these updrafts may be speci®c to this case. Further northeasterly ¯ow over the Greater Madrid Area and studies must be done to determine the cause of this Tajo Valley is only in a 200-m-thick layer affected by effect. a thermal inversion, similar to that estimated for the 14 The TKE patterns are in¯uenced by many factors, July simulation at 0300 UTC. such as TKE production by buoyancy or wind shear and In the daytime, updrafts appear over the Greater Ma- TKE advection. Generally the effects of these factors drid Area (Fig. 16b). This could be related to some effect are superimposed, and it is dif®cult to distinguish one of either the high roughness or differential heating (i.e., from the others. However, for the 16 July case, some convection) in the urban area with respect to the rural of them can be distinguished. In Fig. 17, the evolution surroundings. The former hypothesis is related to the of the TKE patterns can be observed. The TKE pro- ¯ow deceleration observed in the surface horizontal duction by buoyancy is re¯ected by the general increase wind ®elds estimated by the model over the Greater of TKE during the morning and early afternoon before Madrid Area at 1500 (see Fig. 7). This latter hypothesis 1500. Then, the TKE destruction starts. At 1000, the

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FIG. 17. Simulated values of turbulent kinetic energy (J KgϪ1) for 16 Jul at (a) 1000, (b) 1200, (c) 1500, and (d) 1700 UTC in north±south cross-sectional plane corresponding to a UTM x coordinate of 449 km. maximum TKE values are over the downwind sides of plitude of about 500 m, with a wave length of about 50 the southern hills and the ridge and the mixing layer is km); (ii) acceleration of the horizontal wind speed on 500 m deep over these areas. At 1200, the TKE values the lee side (maximum northwest wind speed of 6.0 m and the mixing layer depth have increased notably. The sϪ1); and (iii) descending and ascending ¯ows on the maximum TKE values remain over the downwind sides lee side (in a tilted column about 50 km wide) with of the southern hills and the ridge. The cause of this maximum vertical wind speeds of about 0.15 m sϪ1 displacement can be the TKE advection taking into ac- (Figs. 18a,c). Note that the modeled stability is more count the dominant southwesterly ¯ow. However, a third intense than that simulated for the 14 July case. More- maximum value appears over the Tajo Valley and the over, northeasterly ¯ows in a layer of about 1500-m Greater Madrid Area, which can be related to the above- thick over the Greater Madrid Area and Tajo Valley also mentioned updrafts. At 1500, the mixing layer reaches occur. The predicted thermal surface inversion layer is the maximum development (3000 m deep), and the TKE similar to those of the other cases. production has ®nished (reaching maximum TKE values Although the thermal low pressure synoptic situation is higher than 3.0 J kgϪ1). The maximum TKE values are similar to those of July, diurnal heating is weaker, the being advected northward. It is more notable at 1700, predicted mixing layer is shallower, and the thermal tur- when the maximum TKE value over the downwind side bulence is weaker (maximum TKE of about 1.5 J kgϪ1 at of the Sistema Central has been advected toward the 1500 UTC; Fig. 18b). Updrafts related to the mountain northern boundary by the more intense southwesterly crest were simulated to shift southward as an effect of the ¯ow (about 4.0 m sϪ1) over the ridge (Fig. 16d.). northwest synoptic wind. However, isentropic lines also curved over the updraft area in the layer 2700±4000 m MSL. It could also be due to remaining hydrostatic moun- c. 15 September tain waves in that layer. The expected air convergence area A synoptic 320Њ wind blew with a speed of 4.0 m on the lee side is also compressed to a 25-km-wide column sϪ1, and the vertical temperature gradient is 3.5 K kmϪ1 with a vertical velocity of about 0.15 m sϪ1, and a down- from 1400 to 6000 m MSL. At nighttime, a mountain slope ¯ow exists from the mountain crest. Winds in the wave is simulated by the model, as evidenced by (i) mixing layer are from the southwest, with typical devia- strong perturbation in the isentropic lines (vertical am- tions due to mountain slopes with maximum wind speeds

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FIG. 18. Same as Fig. 15, but for 15 Sep. over the Sistema Central and minimum wind speeds that in the Greater Madrid Area, and the nighttime surface coincide with the updraft area. The ¯ow is from the north- inversion takes place as in the other cases (Figs. 19a,c). west above the mixing layer (Fig. 18d). During the day, as in the other September case, solar heating is less intensive, and it thus produces a less- developed mixing layer (about 1500 m deep) than for d. 17 September the July cases. The synoptic wind, blowing from the The synoptic 260Њ wind blows with a speed of 4.0 m southwest direction, was almost parallel to the mountain sϪ1, and the vertical temperature gradient is 5.0 K kmϪ1 range and therefore does not produce any north±south from 2000 to 2800 m MSL and 0.8 K kmϪ1 from 2800 deformation in the convective cells. They are thus wider to 5200 km MSL. As for the cases of 14 July and 15 and less intensive, with updrafts over the mountains. It September, a mountain wave is simulated at 0300 UTC, also gives rise to deformations in the boundary of the with a maximum vertical wind speed of about 0.15 m mixing layer and in the isentropic lines (Fig. 19b). The sϪ1. A northwesterly ¯ow with speeds higher than 6.0 southwest wind blows with a speed higher than 4.0 m msϪ1 is on the southern slope, whereas southwesterly sϪ1 in a 2000-m-deep layer over most of the southern ¯ows with similar wind speed values appear on the area of the domain. This wind could be related to a northern slope of the Sistema Central and over the Mon- synergistic effect of the imposed synoptic wind and the tes de Toledo. Weak northeasterly ¯ows are simulated diurnal upvalley wind (Fig. 19d).

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FIG. 19. Same as Fig. 15, but for 17 Sep.

8. Conclusions nocturnal leeslope winds, drainage and katabatic ¯ows, upvalley and upslope winds, diurnal convergence of Four cases of summer thermal low pressure systems over the Iberian Peninsula in 1992 have been used to ¯ow on the lee slopes, etc.). In some cases, the daytime study the ability of the TVM model in simulating me- upslope winds in the lee side of the Sistema Central are soscale circulations over the Greater Madrid Area. inhibited, probably because of a downward momentum These cases cover most of the variability of this synoptic transfer from aloft in a well-mixed layer. It agrees with meteorological situation. Mesoscale circulations are the observations in most of the cases except the case of mostly thermally driven (daily ¯ow cycle, upslope and 17 September in which the model overestimates the downslope winds, and upvalley, katabatic, and drainage downward transfer momentum. Some modeled aspects winds), but forcing by the synoptic ¯ow is also impor- could not be checked because of a lack of measured tant (modulation of the daily cycle, nocturnal winds in data. Especially good results were obtained for the case the mountains, etc.). of 14 July. Observed data obtained from meteorological and Comparison of observed and modeled time series of sounding stations, deployed during two experimental wind data in the stations showed that the simulated evo- campaigns in the summer of 1992, were used to check lution ®ts the general aspects of the observed evolution. the TVM results. In general, the TVM model reproduces The best simulations were for the 14 July and 17 Sep- the main features of the atmospheric circulations (strong tember cases. However, some differences are notable.

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Most of them could be due to the low resolution of the A thermal surface inversion layer is usually simulated simulations (10 km ϫ 10 km) as compared with local by the model. features (land use type and topography) especially at Daytime ¯ows are mostly thermally driven as a result the EN, SM, and CI stations. For the 16 July case, the of the strong diurnal surface heating. This heating pro- wind evolution in the morning is not simulated well by duced a thickening of the mixing layer, which reached the model in the Greater Madrid Area, and it may be a 3000-m depth in the simulation of 16 July; a clearly that the northeasterly drainage ¯ow is also overpredicted lower depth is seen in the September simulations. The in this area. In contrast, the evolution close to the moun- upper boundary of the mixing layer follows the topog- tains is simulated well by the model. However, the few raphy and has a wavy shape, with crests and valleys operational stations on this day provide too few obser- corresponding to updrafts and downdrafts, respectively. vational data for more de®nitive conclusions to be made. Updrafts may be related to the differential heating and For the 15 September case, the model probably over- ¯ow convergence in the mountain slopes and the Greater predicts the diurnal clockwise rotation in areas close to Madrid Area. Updrafts and downdrafts appear alterna- the southern slopes of Sierra de Guadarrama, which is tively, forming cells that are shifted by their interaction affected by a ¯ow convergence (as seen at Hoyo station), with the synoptic ¯ow. The TKE advection may be an but at most of the remaining stations the simulations ®t important factor to explain the modeled evolution of the the observation well. The nocturnal ¯ow is also simu- TKE ®elds in the daytime. lated well. An important ®nal conclusion is the strong sensibility Daily thermal oscillation was generally underpre- of the (modeled and real) atmospheric mesoscale cir- dicted, especially for the September cases and at the SM culations to the (input and real) synoptic conditions. It and EN stations. The best model results correspond to implies that careful attention must be paid in the input the CI station. The cause of the more notable under- speci®cation of atmospheric conditions to get a good prediction of the daily thermal oscillation for the Sep- forecast of mesoscale ¯ows, especially for real-time tember cases might be changes in the soil conditions forecasting. In addition, this sensibility of the mesoscale after the summer season. The daily thermal oscillation ¯ows to changes in the thermal low pressure forcing observed at the SM station was abnormally high with has important implications in the transport patterns of polluted air parcels, as will be shown in Part II (MartõÂn respect to other stations and contributes to make the et al. 2001). underprediction more notable. It might be due to the differences between the local soil where the station were installed and the land use type selected as representative Acknowledgments. The authors thank Robert Born- for the 10 km ϫ 10 km cells. The better temperature stein for his comments about the model results, and also simulations correspond to the daytime. Moreover, some Philippe Thunis and Alexis Dutrieux for the TVM code lags between model result and observation were de- and support. The authors express their gratitude to Mer- tected at the VI and CI stations. cedes GoÂmez for her help in making the ®gures. The For the meteorological pro®les, the results show that work shown in this paper has been made possible be- mixing layer deepening is simulated well by the model cause of the data from the experimental campaigns done in spite of some discrepancies detected in a datum-to- under the EU (European Union) RECAPMA STEP PL890009/0006.C and Spanish CICYT (ComisioÂn In- datum comparison, especially in the early morning pro- terministerial de Ciencia y TecnologõÂa) NAT91-1240- ®les. The modeled wind pro®les generally ®t well the CE projects. This work has been ®nancially supported ¯ow decoupling in the early morning, and also the mod- by the Spanish CICYT Project AMB96-1230. el results reproduce most of the characteristics of late- morning and afternoon wind pro®les. The results of the y±z planes show additional char- REFERENCES acteristics of the mesoscale ¯ows. During the night, ¯ows are mainly driven by the interaction of the syn- Atkinson, B. W., 1989: Mesoscale Atmospheric Circulations. Aca- optic wind with topography. Hydrostatic mountain demic Press, 495 pp. Barry, R. G., and R. J. Chorley, 1987: Atmosphere, Weather and waves appear in the simulations of 14 July and 17 Sep- Climate. Methuen, 460 pp. tember and are seen very clearly in the 15 September Bornstein, R. D., S. Klotz, U. Pechinger, R. Salvador, R. Street, L. simulation. In these cases, the simulated stability in the J. Shieh, F. Ludwig, and R. Miller, 1987: Application of linked layer between roughly 1500 and 3000 m MSL (the three-dimensional PBL and dispersion models to New York City. Air Pollution Modelling and its Application V, C. de Wispelaere, mountain range is about 1500 m MSL high in average F. A. Schiermeier, and N. V. Gillani, Eds., Plenum Press, 543± after applying a Shapiro ®lter to the topography) was 564. very strong, and there was a signi®cant perpendicular- , P. Thunis, and G. Schayes, 1993: Simulation of urban barrier to-ridge component of the wind vector. Katabatic ¯ows effects on polluted urban boundary layers using the three-di- mensional URBMET/TVM model with urban topographyÐnew are also simulated, especially in the 16 July simulation. results from New York City. Air Pollution, P. Zannetti et al., Moreover, the downvalley ¯ows along the Greater Ma- Eds., Computational Mechanics Publications, 15±34. drid Area and Tajo Valley are marked for the July cases. , , P. Grossi, and G. Schayes, 1996: Topographic Vorticity-

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Mode Mesoscale-␤ (TVM) model. Part II: Evaluation. J. Appl. er, and C. A. Brebbia, Eds., Computational Mechanics Publi- Meteor., 35, 1824±1834. cations. 637±646. CNIG, 1995: Modelo Digital del Terreno MDT-1000 (Terrain Digital , S. N. CrespõÂ, and M. Palacios, 1997: Simulations of air ¯ows Model). Centro Nacional de InformacioÂn Geogra®ca. in the center of the Iberian Peninsula under thermal low con- CrespõÂ, S. N., and B. ArtõÂnÄano, 1995: Estudio del fenoÂmeno de isla ditions. Third TVM Users Meeting, Louvain-La-Neuve, Belgium, teÂrmica en la ciudad de Madrid y anaÂlisis experimental de su Institut d'Astronomie et Geophysique Georges Lemaitre, 7.1± estructura vertical (Study of the heat island phenomenon in the 7.63. city of Madrid and experimental analysis of its vertical structure). , , and , 2001: Simulations of mesoscale circulations Resumenes de la XXV ReunioÂn Bienal de la Real Sociedad Es- in the center of the Iberian Peninsula for thermal low pressure panÄola de FõÂsica, Santiago de Compostella, Spain, Universidad conditions. Part II: Air-parcel transport patterns. J. Appl. Me- teor., 40, 905±914. de Santiago de Compostela, A3±23, 53±54. MillaÂn, M. M., B. ArtõÂnÄano, L. Alonso, M. Navazo, and M. Castro, , , and H. Cabal, 1995: Synoptic classi®cation of the mixed- 1991: The effect of mesoscale ¯ows on regional and long-range layer height evolution. J. Appl. Meteor., 34, 1666±1677. atmospheric transport in the western Mediterranean area. Atmos. Deardorff, J., 1978: Ef®cient prediction of ground surface tempera- Environ., 25A, 949±963. ture and moisture, with inclusion of a layer of vegetation. J. Monteith, J. L., 1981: Evaporation and surface temperature. Quart. Geophys. Res., 83, 1198±1903. J. Roy. Meteor. Soc., 107, 1±27. Dutrieux, A., 1997: TVMNH2.0a Users' Guide. Version 1.0. ATM- Moussiopoulos, N., 1996: State of the art of air pollution modellingÐ PRO, 35 pp. needs and trends. Monitoring, Simulation and Control, Vol. IV, Font, I., 1983: ClimatologõÂa de EspanÄa y (Climate of Spain Air Pollution, B. Caussade, H. Power, and C. A. Brebbia, Eds., and Portugal). Inst. Nacional de MeteorologõÂa. Ministerio de Computational Mechanics Publications, 47±56. Transportes y Comunicaciones de Madrid, 296 pp. Plaza, J., M. Pujadas, and B. ArtõÂnÄano, 1997: Formation and transport Gaertner, M. A., 1994: AplicacioÂn de un Modelo Numerico de Pred- of the Madrid ozone plume. J. Air Waste Manage. Assoc., 47, iccioÂn MeteoroloÂgica a la SimulacioÂn de Flujos AtmosfeÂricos a 766±774. Mesoscala en la Zona Centro de la PenõÂnsula Iberica (Application Portela, A., 1994: ClimatologõÂa SinoÂptica de las Depresiones TeÂr- of a numerical meteorological prediction model to the simulation micas en la PenõÂnsula Iberica (Synoptic climatology of thermal of mesoscale atmospheric ¯ows in the central zone of the Iberian lows in the Iberian Peninsula). Ph.D. thesis, Universidad Com- Peninsula). Ph.D. thesis, Universidad Complutense de Madrid, plutense de Madrid, Spain, 319 pp. Spain, 319 pp. , and M. Castro, 1991: Primera aproximacioÂn a una climatologõÂa , C. FernaÂndez, and M. Castro, 1993: A two-dimensional sim- de las depresiones teÂrmicas en la PenõÂnsula IbeÂrica (First ap- proximation to a climate description of thermal lows in the Ibe- ulation of the Iberian summer thermal low. Mon. Wea. Rev., 121, rian Peninsula). Rev. Geo®s., 47, 205±215. 2740±2756. , and , 1996: Summer thermal lows in the Iberian peninsula: Ibarra, J. I., R. L. Walko, and W. A. Lyons, 1994: Mesoscale dis- A three-dimensional simulation. Quart. J. Roy. Meteor. Soc., persion modeling of the Iberian thermal low in summer time. 122A, 1±22. Computer Simulations, Vol. I, Air Pollution II, J. M. Baldasano Sasamori, T., 1968: The radiative cooling calculation for application et al., Eds., Computational Mechanics Publications. 77±85. to general circulation experiments. J. Appl. Meteor., 7, 721±729. Junning, L., Q. Zhengan, and S. Fumin, 1984: An investigation of Schayes, G., 1982: Direct determination of diffusivity pro®les from the summer lows over the Qinghai±Xizang Plateau. Proc. Int. synoptic reports. Atmos. Environ., 16, 1407±1413. Symp. on the Qinghai±Xizang Plateau and Mountain Meteorol- , and P. Thunis, 1990: A three-dimensional mesoscale model in ogy, Beijing, China, Amer. Meteor. Soc., 369±389. vorticity mode. Instutt d'Astronomie et de Geophysique Contrib. Leslie, L. M., 1980: Numerical modeling of the summer heat ¯ow 60, Univ. Catholique de Louvain-la Neuve, Belgium, 42 pp. over Australia. J. Appl. Meteor., 19, 381±387. , , and R. D. Bornstein, 1996: Topographic Vorticity-Mode MartõÂn, F., and I. Palomino, 1995: AnaÂlisis de las brisas en la costa Mesoscale-␤ (TVM) model. Part I: Formulation. J. Appl. Me- atlaÂntico±andaluza y su penetracioÂn en el valle del Guadalquivir teor., 35, 1815±1823. (Analysis of the breezes in the Atlantic±Andalucian Coast and Therry, G., and P. LacarreÁre, 1983: Improving the eddy kinetic energy model for planetary boundary layer description. Bound.-Layer their penetration into the Guadalquivir valley). Resumenes de la Meteor., 25, 63±88. XXV ReunioÂn Bienal de la Real Sociedad EspanÄola de FõÂsica, Thunis, P., 1995: Development and implementation of the nonhy- Santiago de Compostella, Spain, Universidad de Santiago de drostatic Topographic Vorticity-Mode Mesoscale (TVM/NH) Compostela, A3±32, 71±72. model. Ph.D. dissertation, Univ. Catholique de Louvain-la Neu- , M. A. Gaertner, I. Palomino, M. Castro, and B. ArtinÄano, 1996: ve, Belgium, 116 pp. Simulation of winter mesoscale ¯ows in the center of the Iberian , P. Grossi, G. Graziani, H. GalleÂe, B. Moyaux, and G. Schayes, Peninsula by using two prognostic models. Monitoring, Simu- 1993: Preliminary simulations of the ¯ow ®eld over the Attica lation and Control, Vol. IV, Air Pollution, B. Caussade, H. Pow- Peninsula. Environ. Software, 8, 43±54.

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