Atmospheric Research 124 (2013) 1–20

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Atmospheric Research

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Amesoscale simulation of coastal circulation in the Guadalquivirvalley (southwestern Iberian Peninsula) using theWRF-ARW model

M.A. Hernández-Ceballos a,⁎, J.A. Adame b, J.P. Bolívar a, B.A. De la Morena b

a Department of Applied Physics, Faculty of Experimental Sciences, University of Huelva, Huelva, Spainb Atmospheric Sounding Station “El Arenosillo”, Atmospheric Research and Instrumentation Branch, National Institute for Aerospace Tecnology (INTA),Mazagón-Huelva, Spain

a r t i c l e i n f o

⁎ Corresponding author at: Department of AppliedCiencias Experimentales, Universidad de Huelva, Ca21071 Huelva, Spain. Tel.: +34 651698607.

E-mail address: miguelhceballos@gmail.com (M.A.

0169-8095/$ – see front matter © 2013 Elsevier B.V. Ahttp://dx.doi.org/10.1016/j.atmosres.2012.12.002

a b s t r a c t

Article history:Received 1 December 2011Received in revised form 4 December 2012Accepted 10 December 2012

Located in the southwest of the Iberian Peninsula, the Guadalquivir valley is a site of frequentproblems related to air pollution. The atmospheric dynamics of this region is poorly characterisedbut is fundamental to understanding the chemical and photochemical processes that contributeto the pollution problems. In this work, the atmospheric mesoscale Weather Research andForecasting (WRF-ARW)model was used to study the horizontal and vertical development of thetwo sea–land breeze patterns (pure and non-pure) that are identified in the coastal area as beingresponsible formanyof the air pollution events. In addition, data from fivemeteorological stationswithin the valley were used to validate and compare the model results.The FNL archives were used to define the initial and boundary conditions of the model. Fourdomains with a grid resolution of 81, 27, 9 and 3 km and 40 sigma pressure levels in eachdomain were defined. The Medium Range and Forecast (MRF) parameterisation scheme wasused with new values for both the bulk critical Richardson number and the coefficient ofproportionality. This new configuration was obtained from the sensitivity exercises.Several periods were modelled for both breeze patterns, focusing on the wind, the potentialtemperatures and the specific humidity fields. For the pure breeze, the horizontal movementalong the valley was conditioned by the arrival of a Mediterranean flow in the Guadalquivirvalley that limits the horizontal extension of the breeze to 20–40 km inland. In contrast, thenon-pure pattern was only identified in the coastal area; although motivated by the entranceof southwestern flows, a marine air mass transport along the valley was detected and reachedinland areas located approximately 200 km from the coast line.In both cases, the model results indicated the formation of a thermal internal boundary layer with avertical development of less than 500 m for the pure sea breeze while for the non-pure breeze canreach a vertical extension of 1 km. In the case of the non-pure pattern, the model forecast for theAtmospheric Boundary Layer (ABL) height distribution along the valley revealed a homogeneouspattern related to the entrance of the southwestern flows, in contrast with the clear division of thevalley observed for the pure pattern motivated by the arrival of Mediterranean flows.

© 2013 Elsevier B.V. All rights reserved.

Keywords:WRF modelSea–land breezeSouthwestern Iberian PeninsulaWind field

Physics, Facultad dempus de El Carmen,

Hernández-Ceballos).

ll rights reserved.

1. Introduction

Due to the relationship between the behaviour of airpollutants such as surface ozone or particulate matter(Oltmans et al., 2010; Marinoni et al., 2010) and the meteoro-logical conditions, the interpretation of data on atmospheric

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chemical species must be performed using meteorologicalinformation. Recently, the number of air quality studiescarried out in combination with atmospheric dynamics hasincreased in an attempt to understand the characteristicsand specific factors that determine the spatial and temporalvariability of the substances that mainly occur in the loweratmosphere (Helmig et al., 2008; Baker, 2010).

There are sites in which the relationship between theatmospheric dynamics and behaviour of pollutants is difficultto understand, and these sites are generally places with acomplex orography, such as coastal areas or valleys. In thissense, the sea–land breeze is a phenomenon widely studiedin many regions around the world due to its influence onatmospheric dynamics (Case et al., 2005; Lu et al., 2009) and onlocal air pollution (Ding et al., 2004; Levy et al., 2009). Thismesoscale circulation pattern occurs in coastal areas due todifferences in the surface temperatures of the land and the sea.This mechanism is characterised by a constant wind directionin diurnal periods, known as a sea breeze, which is generallyperpendicular to the coastline. After a transition period, with achange in thewind direction, in the nocturnal regime the flowscoming from the land are also perpendicular to the coastlineand are known as land breezes. Therefore, this circulationpattern is considered to be a recirculationmechanism (Garratt,1992) because it traps the pollutants emitted near the surfaceand limits a sufficient mixing with the air above. Hence, thedispersion processes in the lower atmosphere are limited.

The sea–land breeze characterisation has frequently beencarried out using surface meteorological observations (Oliphantet al., 2001; Papanastasiou and Melas, 2009). However, withonly surface data, there is a lack of information on aspectsrelated to the structure and dynamics of the pattern, includingthe inland penetration of the sea breeze front, the compensatingreturn flow and the features of the thermal internal boundarylayer (TIBL) (Garrat, 1990). This layer is formed when a stablemarine air mass under the development of sea breeze circu-lation is transported to a warm land surface during the day; thebottom portion of the marine air mass (land surface) becomesunstable due to the heat gained from the warm land surface.This layer is important because it controls the vertical mixingin a coastal region during the development of the sea breezecirculation.

Due to its orography and weather conditions, the westernMediterranean Basin exhibits development of both sea–landand valley breezes. The valleywind is an anabatic flow formedduring the day by ground heating from the valley in the ab-sence of cyclonic or anti-cyclonic winds (Briggs and Smithson,1986). Several experimental efforts have been made in thelast few years to study these phenomena at rather fine spatialand temporal scales to improve the understanding of theirbehaviours and the parameterisations of the physical processesimplemented in the meteorological models. For example, theMECAPIP (Meso-meteorological Cycles of Air Pollution in theIberian Peninsula) described the sea–land breeze recirculationsystem in the eastern area of the Iberian Peninsula (Millanet al., 1996). In the southeast of France, a campaign known asESCOMPTE (Expérience sur Site pour Contraindre les Modèlesde Pollution atmosphérique et de Transport d'Emissions)focused on the study of the atmospheric dynamics and theinfluence of sea–land breezes on air pollutants (Kalthoff et al.,2005).

Adame et al. (2010) identified two patterns of sea–land breezes on the Huelva coastline (southwestern IberianPeninsula) based on surface wind observations. This area ischaracterised by the presence of the Guadalquivir valley,which ends in the Gulf of Cadiz (Atlantic Ocean). This areapresents elevated levels of temperature and solar radiationduring the warm season of a typical Mediterranean climate.With a population close to a million inhabitants, the metro-politan area of Seville is located in the low–medium portionof the Guadalquivir valley, while the Cadiz and Huelva urbanareas are situated on the coast. Furthermore, three importantindustrial complexes are located close to Huelva city, andDoñana National Park, known for its elevated ecologicalinterest, is also located in the mouth of the valley. Withrespect to orography, both the biogenic and anthropogenicatmospheric emissions and the weather conditions make thisregion ideal for the occurrence of air pollution events. Inrecent years, several studies focused in this topic have beenperformed in the Guadalquivir valley (Querol et al., 2002;Toledano et al., 2007; Adame et al., 2008). However, there is alack of meteorological studies specifically linked with meso-scale processes in the southwestern Iberian Peninsula.

The aim of this paper was to analyse the sea–land breezecirculation in the southwestern Iberian Peninsula and itsdevelopment along the Guadalquivir valley. As a startingpoint, the two patterns of breezes identified in the coastalarea by Adame et al. (2010) were taken into account. Thehigh-resolution mesoscale meteorological model, the NCARAdvanced Research WRF-ARW model (WRF version 2.1.2)(Skamarock et al., 2005), was applied in combination withthe experimental meteorological data.

The description and setup configuration of the WRF-ARWmodel, the features of the investigated area and the char-acteristics of the sea–land breeze patterns are covered inSection 2. Section 3 presents the results, beginning with acomparative study between the simulated and observed dataand followed by the horizontal and vertical evolution of thesea–land breeze. In addition, a study of the mixing heightbehaviour under both breeze patterns is reported. Finally, theconclusions are summarised in Section 4.

2. Site characterisation, model aspects and sea–landbreeze patterns

2.1. Area description and experimental stations

The southwestern Iberian Peninsula is characterised bythe presence of the Guadalquivir valley, with an approximatetriangular area of 35,000 km2 oriented in the southwest–northeast direction, a length close to 350 km and a widthgreater than 90 km at its mouth. The terrain elevation ischaracterised by a uniform height along the valley (less than100 m above sea level; hereafter, asl), which makes possiblea clear identification of the boundary of the valley, with theSierra Morena mountain chain to the north and the Beticmountain system to the south (Fig. 1a).

Data from several meteorological stations were employedin this work to validate and compare the observations withmodelled data. Two types of experimental records were used:surface data (five stations) and vertical profiles (two stations).With respect to the profiles, the soundings were obtained from

a) b)

Puebla del Rio

El Arenosillo

La LuisianaCórdoba

Linares D01

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Fig. 1. a) Orography of Guadalquivir valley (Domain 4) and location of meteorological stations. Line AB represents the cross section along the valley b) WRFmodelling domains (Lambert Conformal projection): domains D1 and D2 are coarse with resolutions 81 km and 27 km respectively. The D3 has a resolution of9 km while the finer, D4 has a resolution of 3 km. c) Wind roses of meteorological sites taken as reference.

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the Gibraltar station (36.15 N, 5.35 W, 4 m asl), identified asa WMO (World Meteorological Organization) radiosonde sta-tion with daily soundings, and from the El Arenosillo station(37.1 N, 6.7 W, 42 m asl). In this last case, the meteorologicalprofiles were only used in the experimental campaigns. Inaddition, five surface meteorological stations were selectedalong the Guadalquivir valley, and the selection was based onthe optimal spatial coverage of the valley. In the coastal area,we selected El Arenosillo; Puebla del Río (37.2 N, 6.1 W, 23 masl) in the low valley, and La Luisiana (37.5 N, 5.2 W, 188 masl) and Córdoba (37.8 N, 4.8 W, 91 masl) in themiddle valley.Finally, Linares (38.06 N, 3.64 W, 443 m asl) was selected forthe high portion of the valley (Fig. 1a).

In Hernández-Ceballos (2012), a study of the wind regimein the Guadalquivir valley was performed by the calculationof wind roses using the hourly surface wind data from thepreviously mentioned five stations during the period from2000 to 2007. These results showed that the surface windsblow mainly from the southwest and northeast, showingthe influence of the axis direction of the valley (southwest–northwest); northwest flowswere also detected in the coastalarea. These observations ensure that the locations of the fivemonitoring sites along the valley are representative of itssurface wind behaviour.

2.2. The WRF model

The Weather Research and Forecasting (WRF-ARW) modelbased on the Eulerian mass solver was used to investigate thesea breeze development and characteristics in the study area.This mesoscale model is a state-of-the-art atmospheric simu-lation system based on the Fifth-Generation Penn State/NCARMesoscale Model (MM5) (Grell et al., 1994) and is one ofthe most widely designed systems for meteorological research(Kirkwood et al., 2010; Jiménez et al., 2010). The model con-sists of fully compressible non-hydrostatic equations, and theprognostic variables include the three-dimensional wind data,perturbation quantities of potential temperature, geo-potential,surface pressure, turbulent kinetic energy and various scalars

(water vapour mixing ratio, cloud water, etc.). The model'svertical coordinate corresponds to the terrain following thehydrostatic pressure (Eta coordinates), and the Arakawa-Cgrid staggering is used as the horizontal grid. Third-orderRunge–Kutta time integration is used in the model. Thismodel incorporates the last advances and a complete set ofphysics options for representation of the sub-grid scale physicalprocesses for convection, explicit microphysics, atmosphericradiation, boundary layer turbulence and surface temperature/moisture treatment.

Created and maintained by the Centre for EnvironmentalPrediction (NCEP) (Kalnay et al., 1996), the FNL global analysiswith a spatial resolution of 1°×1° (longitude–latitude) and avertical resolution of 27 pressure levels, was used to definethe initial and the boundary conditions. These meteorologicalfiles have a temporal coverage of 6 h (00, 06, 12, 18 UTC). Thetopography information was obtained from the U.S. GeologicalSurvey (USGS) global 30 arc-s elevation (GTOPO30) dataset(Bliss and Olsen, 1996; Gesch and Larson, 1996). In addition,the USGS land use/land cover system (Anderson et al., 1976)was used to determine the surface physical properties and thelower boundary conditions. Information on the physics optionsand the grid definition setup in theWRF model for this work isshown in Table 1.

The fourmodelling domainswere centred at 37.5 N, 4.4 W(Fig. 1b). The mother domain (D1) was built with 81 km ofspatial resolution, covering theWesternMediterranean Basin,southern Europe and northern Africa. The purpose of thisdomain was the simulation of the synoptic features that in-fluence the region. The first nested domain (D2) had a spatialresolution of 27 km and covered the Iberian Peninsula andits surroundings. The third domain (D3), with a resolution of9 km, was situated over the south half of the Iberian Peninsula,with the finer domain (D4) (grid resolution of 3 km) centredover the region of Andalusia and covered the Guadalquivirvalley and its surroundings. The four domains interacted witheach other through a two-way nesting strategy, all with avertical structure that included 40 sigma levels, 23 of whichwere located in the first 2000 m with the objective of an

Table 1Details of the grids and the physics options used in the WRF Model.

Domains Domain 1 Domain 2 Domain 3 Domain 4

Horizontal resolution 81 km 27 km 9 km 3 kmGrid points 40×40 52×52 91×70 151×91Domains of integration (22.73 N; 19.00 W)

(50.69 N; 11.66 E)(32.65 N; 11.59 W)(44.73 N; 4.08 E)

(33.63 N; 10.03 W)(39.23 N; 1.16 W)

(36.05 N; 7.53 W)(38.48 N; 2.50 W)

Vertical resolution 40 sigma level.Dynamics Primitive equation, nonhydrostatic.Radiation Dudhia (1989) scheme for shortwave radiation, Rapid radiative transfer model (RRTM) for longwave radiation.Surface processes 5 layer soil diffusion scheme (Dudhia, 1996).Boundary layer MRF with a bulk critical Richardson number of 0.25 and a coefficient of proportionality of 0.0.Cumulus A modified version of the Kain and Fritsch (1990, 1993).

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optimal resolution for the Atmospheric Boundary Layer (ABL)behaviour.

The longwave and shortwave radiation schemes werebased on the work of Mlawer et al. (1997) and Dudhia(1989), respectively. A modified version of the Kain andFritsch (1990, 1993) scheme was employed for the cumulusparameterisation, and a simple five-layer soil surface modelbased on Dudhia (1996) was also used. The Medium Rangeand Forecast (MRF) ABL parameterisation (Hong and Pan,1996) was used in the four domains with modifications in thevalues of the bulk critical Richardson number (Ribcr) and thecoefficient of proportionality (b) based on the meteorologicalinformation generated in the DOMINO (Diel Oxidant Mech-anisms in relation to Nitrogen oxides) campaign, which wasperformed in November–December 2008 at El Arenosillo(Van Stratum et al., 2012; Sinha et al., 2012). Using meteoro-logical soundings performed during this campaign, a value of0.25 was defined for Ribcr, and b was set to 0.0. This value ofRibcr was consistent with its use in Borge et al. (2008) for theentire Iberian Peninsula (Ribcr=0.3).

In addition and taking into account the DOMINO results,to gain a better approximation to the temporal evolution ofthe potential temperature and specific humidity in this areaa decrease in the value of the moisture availability (M) wasset in the WRF configuration. Soil moisture availability is themajor factor that controls the amount of surface evaporationthat takes place in the model; it is defined as a function ofland use and primarily represents the wetted-area fraction inthe absence of vegetation (Oncley and Dudhia, 1995). There-fore, changing the soil moisture availability is equivalent tochanging the latent heat flux in the surface energy budget,which affects the land–surface temperature (Carlson andBoland, 1978). The value of M ranges from 1.0 over water to aminimum of 0.05 over land. For most of the areas defined bythe U.S. Geological Survey (USGS) (Anderson et al., 1976), thesoil moisture availability value is variable. Two seasonal values(summer and winter) are defined, which do not change duringthe simulation and hence cannot reflect the impact of inter-mittent precipitation. We used the DOMINO observations todecrease the original M values of the main land-use categoriesdefined over this region by the USGS (Dryland Cropland andPasture (0.3), Cropland/Woodland Mosaic (0.35), Mixed shrubland/ Grassland (0.15) and deciduous Broadleaf Forest (0.5)) to0.1 (Oncley and Dudhia, 1995; Lam et al., 2006).

A comparison of the simulated and observed potentialtemperature and specific humidity profiles at El Arenosilloand Gibraltar was performed to demonstrate the reliability of

this configuration (Fig. 2). Three periods were selected asrepresentative of the cold season (22, 23 and 24 of November2008) and also of the warm season (24 May 2000, 29 June2006 and 15 June 2007). The simulated data were extractedfrom the inner domain (3 km of spatial resolution) (Fig. 1b).In the cold season, the forecasted convective layer presented acolder temperature profile with differences of nearly 1–2 K,while the humidity differences ranged between 1–1.5 g kg−1

at El Arenosillo and were wet or dry depending on the day(Fig. 2a). At Gibraltar, in the cold season, the forecasted resultswere similar to the observations, detecting different featuresin the temperature of the convective layer depending on theday and displaying more accuracy in the humidity profiles. Inthewarm season at El Arenosillo, themodel showed humiditydifferences in the range of 1–2 g kg−1 in both wet and dryconditions and obtained a colder temperature profile in twocases and a warmer profile in one, with differences rangingfrom 0.5 to 3 K and more accuracy in the first 400 m. How-ever, in Gibraltar, the model results were similar to the obser-vations in the first hundred metres, highlighting the greataccuracy observed on 15th May 2007.

These results obtained with the WRF-ARWmodel and useof the configuration defined above showed the tendency ofthe model to present results that were in adequate agree-ment with the ABL key features, successfully reproducing theABL development and behaviour in both seasons and in thetwo sites considered.

2.3. Sea–land breeze patterns in the coastal area

In Adame et al. (2010), two sea–land breeze patternswere identified based on the surface wind observations andassociated with different synoptic situations referred to aspure and non-pure. The pure breeze occurs under a weakisobaric gradient over the Iberian Peninsula and is associatedwith the northern location of the Azores anticyclone and thedevelopment of relatively low pressures over the north ofAfrica. This meteorological scenario can coincide with thedevelopment of a thermal low over the Iberian Peninsulaplateau. However, its influence on the establishment andevolution of this mesoscale process is minimal in the studyarea. The absence of synoptic flow allows the development ofthe mesoscale circulation, with a nocturnal flow from thenortheast (perpendicular to the shoreline) from midnightuntil midday. At this time, the winds begin to blow from thesouth or southwest, remaining until midnight, when the landflows are again detected.

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OBS_22/11/2008(11:00 UTC) OBS_23/11/2008(14:00 UTC) OBS_24/11/2008(16:00 UTC) WRF_22/11/2008(11:00 UTC) WRF_23/11/2008(14:00 UTC) WRF_24/11/2008(16:00 UTC)

Fig. 2. Examples of simulation, for the finer domain, and observed vertical potential temperature and specific humidity profiles at El Arenosillo and Gibraltar sitesin a) cold season on 22nd (11:00 UTC), 23rd (14:00 UTC) and 24th (16:00 UTC) November 2008 and a) warm season on 15th June 2007 (11:00 UTC), 29th June2006 (16:00 UTC) and 24th May 2000 (16:00 UTC).

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The other pattern identified, the non-pure breeze, isassociated with an isobaric configuration characterised bythe presence of the Azores anticyclone to the west of theIberian Peninsula and/or a low pressure systemover the BritishIsles in combination with the development of a low pressuresystem over the Iberian Peninsula plateau or in the westernMediterranean. Under these conditions, northwestern flowsare measured over the west of the Guadalquivir valley. Thesurface wind on the non-pure breeze days is determined bynight flows from the northwest, which remain from midnightuntil midday. At this time, the wind rotates in a cyclonic sense,positioned from the southwest until approximately 21:00 UTC,when it veers around to northwesterly flow once again.

3. Results and discussion

Several periods of both pure and non-pure breezes weresimulated from the large set of breeze days identified in thecoastal area (Adame et al., 2010). Although the breeze

mechanism can occur at any time of the year, the frequencyof breeze days in this area is greater than 30% between Mayand September. In addition, the intensity of the breezes ishigher in this period due to the prevalence of favourablemeteorological conditions. Hence, the warm season was con-sidered a reference for the study of the characteristics of thesea–land breezes.

Two periods that show similar features to other simulatedevents, were selected as representative of the pure and non-pure breeze patterns. For the pure breeze, the period from 11to 14 August 2003was selected, which occurred during one ofthe hottest summers in Spain (Díaz et al., 2006) and Europe(Zaitchik et al., 2006). For the non-pure breeze pattern, theperiod from 6 to 9 July 2001 was taken as a reference.

Each simulation was performed for 4 days (96 h), and thefirst 24 hwere treated as a spin-up period to avoid all possibleproblems related to the adjustment of the large-scale flowaccording to the local topography, land use, etc.; therefore, theremaining 72 h were considered for the analysis.

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3.1. Comparison of simulated variables

A comparison between the modelled and observed valueswas carried out using the meteorological observationscollected at the previously listed five stations (Fig 1a). Thewind direction and wind speed were selected as meteorolog-ical variables because determination of the wind fields wasone of the main aims of this work. The potential temperatureand specific humidity were also compared because the mainfeatures of breeze development, i.e., daily evolution, breezefront, inland penetration, etc., can be obtained from theirvariations (Kwon et al., 1998; Kolev et al., 1998, Ohashi andHidaji, 2002; Fovell, 2005). The meteorological observationsat each station were recorded at 10 m above ground level(hereafter agl) following the recommendations of the WorldMeteorological Organization (WMO WMO-No.8, 1996), andthe post-processing module of the WRF provided wind speedand direction information at 10 m agl and temperature andhumidity information at 2 m agl.

Figs. 3 and 4 show the hourly evolution of the modelledand observed wind data at each station during the pure andnon-pure wind patterns. At the El Arenosillo station, themodel was able to distinguish both wind patterns, although itregistered better results for the non-pure than the pure seabreeze. In this last example, the model values reproduced thewind change from northwest to southwest, showing sensibledifferences from 12:00 UTC, when the model predicted aprogressive change from southwest to northeast, and thewind observations remained in a westerly range until 00:00UTC.

At Puebla del Rio, the model was able to reproduce thetemporal variation of the pure sea breeze, from the northeastto northwest, showing slight time differences in the suddenwind changes registered. However, in the non-pure breezepattern, the model forecast showed a change from west tonorthwest while the observations provided evidence of amodification from the west to northeast. Taking into accountthe model configuration, this behaviour could be related tothe reduction applied in the moisture availability valuesdue to the closeness of this site to the coastal area and thepresence of wetlands in its surroundings.

At sites further inland, the model results reproduced thewind observations for each breeze pattern, registering betterresults in general than those from the coastal stations. For LaLuisiana, the forecast followed the wind variability from eastto south associated with the pure breeze case with only slighttime differences, while the dominance of the southwesternwinds related to the non-pure breeze was also faithfullyreproduced. An optimal correlation was also obtained atCordoba for both breeze patterns. These results were remark-able because this city is located in a site with complexorography, i.e., a small valley inside the Guadalquivir valleylocated close to the Sierra Morena mountains. At this station,the model values showed the variability of the pure breeze,registering the sudden changes during the day from thenortheast to west, especially at the end of the period whenthe model detected a variation of few hours in the obser-vations from the west to northeast. For the non-pure breezepattern, the model results were clearly similar to the observa-tions, showing the arrival of winds from the southwest. How-ever, at the end of the period, certain differenceswere detected,

observing the model's quick change to northerly winds whilethe observations were constant from the southwest.

At Linares, located in the upper valley, the simulationsshowed good agreementwith the observations in both patterns,with a prevalence of westerly winds in the non-pure case andeasterly flows in the pure case. However, in the pure breezeperiod, the model presented an opposite behaviour during thelast day because it indicated the arrival of easterly/northeasterlyflows while the observations noted southwesterly/westerlywinds.

Considering the set of results obtained for both breezepatterns, the model was able to reproduce the winds withbetter results for the non-pure than for the pure breezepattern. In addition, a small spatial development of both windcirculations was observed, especially in the non-pure pattern,which was clearly identified at the El Arenosillo coastal site,while the pure breeze was observed until Puebla del Rio,located a few tens of kilometres inland. In the case of thenon-pure breeze, although a diurnal cycle was noted in thewind speed, the wind direction was constant over three days.This observation is contrary to the concept of both sea andvalley breezes; these data will be analysed and discussed inthe following sections.

The wind speed observations along the valley wereoverestimated by the model (Figs. 3 and 4), with the differ-ences conditioned by the breeze pattern and the location ofthe site in the valley. Smaller differences were obtained forthe non-pure breeze, showing an optimal agreement in allsites and highlighting the results obtained at Puebla del Rioand Córdoba. However, the differences in the pure breezeswere smaller at the coastal area (El Arenosillo and Puebla delRío) than inland, where the model presented poorer results.In this case, the model found difficulties in reproducing boththe intensity and variability of wind speed, most likely asso-ciatedwith the higher variability of thewind registered underthis breeze pattern in contrast to the homogeneous windsobserved in the non-pure case.

Fig. 5 shows the comparison between the potentialtemperature and specific humidity observed and modelledat the El Arenosillo and Cordoba stations for the pure (Fig. 5a)and non-pure (Fig. 5b) patterns. At El Arenosillo, the modeltended to simulate the observational variability with moreaccuracy. In both breeze patterns, the potential temperaturewas underestimated by the model. Nevertheless, the specifichumidity on the coast presented a slight underestimationfor both patterns, and inland, the model overestimated theobservations in both meteorological patterns with larger dif-ferences in the pure breeze case. During this period, althoughthe model tended to reproduce the temporal evolution ofthe observations, the differences were larger than in the non-pure case, reaching values up to 10–9 g kg−1. As mentionedpreviously, due to the special orography of Cordoba, themodel was unable to correctly simulate this parameter underthis meteorological scenario.

In both breeze patterns, the model was able to follow thedaily variability of the potential temperature at each site,although it had additional difficulties related to the dailymaximum values, especially in the pure circulation state. Inthis last pattern, the differences in the specific humidity werehigher, with special attention directed to the Cordoba results,which were completely in contrast to the observations. At

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12/08/2003 13/08/2003 14/08/2003 12/08/2003 13/08/2003 14/08/2003Time (UTC) Time (UTC)

Fig. 3. Simulated and observed wind direction and speed under pure sea breeze conditions from 12 (00:00 UTC) to 14 (23:00 UTC) August 2003, at El Arenosillo(ARE), Puebla del Río (PUE), La Luisiana (LLU), Córdoba (COR) and Linares (LIN). Local time=UTC+2.

7M.A. Hernández-Ceballos et al. / Atmospheric Research 124 (2013) 1–20

present, the causes of these results are unknown. However,under non-pure conditions, the simulation of the specifichumidity revealed the ability of the model to simulate the

variability in both sites, with better results at the coastal sitesthan inland, where the model tended to overestimate theobservations. In summary, the results obtained by the model

Non-Pure sea breeze

0:00 12:00 0:00 12:00 0:00 12:00 0:00 12:00 0:00 12:00 0:00 12:000

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Fig. 4. Simulated and observed wind direction and speed under nonpure sea breeze conditions from 7 (00:00 UTC) to 9 (23:00 UTC) July 2001, at El Arenosillo(ARE), Puebla del Río (PUE), La Luisiana (LLU), Córdoba (COR) and Linares (LIN). Local time=UTC+2.

8 M.A. Hernández-Ceballos et al. / Atmospheric Research 124 (2013) 1–20

for potential temperature and specific humidity were betterin the case of the non-pure breeze than in the case of the purebreeze.

Two statistical parameters were used to complete thecomparison between the observed and simulated variables(Gilliam et al., 2006). The classic approach of comparing

b) Non-pure sea breeze.

a) Pure sea breeze.

Tp_ARE_Obs Tp_ARE_WRF

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Time (UTC)7/07/2001 8/07/2001 9/07/2001

Time (UTC)

Tp_COR_Obs Tp_COR_WRF

Fig. 5. Simulated and observed potential temperature and specific humidity at El Arenosillo (ARE) and Córdoba (COR) sites a) from 12 (00:00 UTC) to 14 (23:00 UTC)August 2003 (pure sea–land breeze conditions) and b) from 7 (00:00 UTC) to 9 (23:00 UTC) July 2001 (nonpure sea–land breeze conditions). Local time=UTC+2.

9M.A. Hernández-Ceballos et al. / Atmospheric Research 124 (2013) 1–20

measurements is mostly surface-based, that may provide ageneral performancemeasure of the capability of themodel inreplicating the observed values (Emery et al., 2001). The rootmean square error (RMSE), which assesses the accuracy ofthe prediction, and the bias score (BIAS), which measures themodel's tendency to systematically overestimate or under-estimate a parameter, were chosen. Following the equationsshown in Jiménez-Guerrero et al. (2008), both parameterswere calculated from the hourly data for potential tempera-ture, specific humidity, wind speed and direction. For winddirection, it was necessary to consider the cyclic behaviour. Toobtain an estimate of the magnitude of the correctness of thewind vector, the absolute value of the difference between thesimulated and observed directions should be minimal.

Table 2 shows the statistical results for the 72 h treated asa reference in each simulation period. In general, the winddirection was underestimated (negative bias) for the purebreeze pattern and overestimated (positive bias) for the non-pure pattern. An exception was found for the pure breeze atEl Arenosillo, which presented a positive BIAS value (7°). Themaximum BIAS was found at Puebla del Río with −23° and45° for the pure and non-pure patterns, respectively, and thelowest values were obtained at Cordoba, with−12° and−2°,respectively. In the case of the RMSE, the results indicatedsmaller differences for the non-pure breeze at the coastal line(33°) than in the inland areas (73° at Cordoba and 63° atLinares); the approximation was better at Puebla del Río(91°) and La Luisiana (63°) for the pure pattern. These results

Table 2Bias score (BIAS) and root mean square error (RMSE) values obtained in two periods under pure and non-pure breeze situations at the five meteorologicalstations selected for wind speed (WS) and direction (WD), potential temperature (Tp) and specific humidity (q) data.

Sites Model integration time(72 h)

Statistic variable Pure breeze(12–14 August 2003)

Nonpure breeze(7–9 July 2001)

BIAS RMSE BIAS RMSE

El Arenosillo 24:00–96:00 WD 7.34 71.43 13.84 33.30WS −0.18 1.72 1.03 1.47Tp −2.36 3.11 −1.13 2.06q −1.24 2.46 −0.32 1.75

Puebla del Río 24:00–96:00 WD −23.33 91.63 45.24 115.33WS 1.10 1.52 0.27 0.93

La Luisiana 24:00–96:00 WD −13.68 63.86 22.23 83.61WS 1.78 2.12 0.35 1.16

Cordoba 24:00–96:00 WD −12.14 94.83 2.10 73.12WS 0.05 1.15 −0.38 1.13Tp −2.22 2.97 −0.77 1.28q −1.03 3.71 2.73 3.35

Linares 24:00–96:00 WD −15.18 95.57 11.36 62.99WS 1.04 1.80 −0.48 1.21

10 M.A. Hernández-Ceballos et al. / Atmospheric Research 124 (2013) 1–20

could be related to the meteorological conditions associatedwith each sea breeze. The non-pure breeze is associated witha synoptic forcing, which could favour better reproduction ofthe model in the case of a non-pure sea breeze versus a pureone.

In general, the observations of wind speed were mainlyoverestimated along the valley during the pure pattern withthe exception of El Arenosillo (low BIAS value). For thispattern, the BIAS values obtained were at approximately1.0 m s−1, while the RMSE reached the maximum value atLa Luisiana of 2.12 m s−1 and the minimum at Cordoba of1.15 ms−1. The statistical values were generally smaller inthe non-pure pattern than in the pure pattern, reaching themaximum value at the coastal area (El Arenosillo). In thissense, during the non-pure breeze, the BIAS values along thevalley were between 1.03 m s−1 and −0.48 m s−1 and theRMSE was below 1.47 m s−1. These results could be associ-ated with the large homogeneity observed along the valleyduring the non-pure breeze, promoting the southwesternflows at every site in contrast with the additional variabilityidentified in the pure case.

Relative to the statistical values obtained for potentialtemperature and specific humidity, the model showed betterresults in the non-pure case than in the pure case for bothsites considered. The specific humiditywas underestimated inthe coastal area for both patterns while in Cordoba, thesevalues were under predicted for the pure breeze and overpredicted for the non-pure breeze. In addition, the RMSE waslarger in Cordoba, in the range of nearly 1.5 g kg−1. However,an opposite behaviour between the modelled and observedvalues was shown for the potential temperature because ad-ditional differenceswere obtained between themeteorologicalseries at El Arenosillo than at Cordoba, with underestimatedobservations by the model in both sites.

In summary, the WRF model was able to simulate thenon-pure breeze better than the pure breeze along the valley.This observation could be driven by the different weatherconditions associatedwith the development of each state: thesynoptic forcing in the case of non-pure breeze and the weakpressure gradient for the pure breeze. Moreover, the specialcharacteristics of the coastal area and the Guadalquivir valley

must be taken into account. The coastal area covers theDoñana National Park, a wetland/coastal reserve at the deltaof the Guadalquivir river with a great variety of ecosystems,i.e., marshes on clay soils, dunes and forests, and scrublandson sandy soil, among others, in addition to different soil con-ditions and humidity due to its unevenness.

Numerical simulations with weak synoptic forcing showedthat horizontal variations in soil moisture may produce strongmesoscale circulations (Avissar and Liu, 1996; Lynn et al., 1998;Baker et al., 2001). As a suggestion, this effect could influencethe WRF simulations of the pure breeze while remaining at aminimum for the non-pure pattern.

Nevertheless, the statistical results obtained in this work arein agreement with the results obtained in previous works thatfocus on the performance of theWRFmodel (Jiménez-Guerreroet al., 2008; Borge et al., 2008; Challa et al., 2009; Papanastasiouet al., 2010).

3.2. Horizontal extension of sea–land breezes

Once the pure and non-pure breezes in the coastal area(El Arenosillo) were identified, the horizontal wind vectors at10 m and the horizontal specific humidity at 2 m for theinner domain (D4) were used to study their spatial evolutionalong the Guadalquivir valley.

Fig. 6 shows the wind field and the specific humidityrepresentative of the pure breeze conditions on 12 August2005. In the map of the nocturnal period, at 5:00 UTC, thesurface winds along the Guadalquivir valley were mostlyobserved from the east/northeast direction, highlighting thechannelling carried out by the valley over the flows. Theinfluence of mountain breezes (which were only slightlyobserved in the north portion of the valley) on the windbehaviour was minimal. At the same time, strong easterlywinds governed the surface dynamics in the Mediterraneancoast of the southern Iberian Peninsula. At approximately8:00–9:00 UTC (not shown in Fig. 6), flows coming from theMediterranean coast began to converge with the winds fromthe Guadalquivir valley through the transverse valleys locatedin the Betic mountain chain, revealing the connection betweenboth areas.

d) 12-08-2003 23:00 UTC

Pure sea breeze

Specific Humidity (g kg -1) 10 m/s

c) 12-08-2003 18:00 UTC

b) 12-08-2003 12:00 UTCa) 12-08-2003 05:00 UTC

Fig. 6. Simulated wind field at 10 m and specific humidity at 2 m (finer domain) under pure breeze conditions on 12 August 2003 at a) 05:00, b) 12:00, c) 18:00and d) 23:00 UTC. The reference wind speed used in the domain is indicated by an arrow below the figures. Local time=UTC +2.

11M.A. Hernández-Ceballos et al. / Atmospheric Research 124 (2013) 1–20

At midday, as Fig. 6b shows, there was a transition periodat 12:00 UTC in the coastal area characterised by an undefinedbehaviour of the wind; inside the valley, the wind continuedto blow from the east/northeast. At this time, it was possibleto define two meteorological areas along the Guadalquivirvalley due to the arrival of the Mediterranean flows. The firstwas associated with the channelling of the Mediterraneanflows toward the mouth of the valley via an east/northeastcomponent with an elevated wind speed. The other waslocated in the upper valley in which the northeasterly windswere less intensive than in the coastal area of Guadalquivirvalley. However, this division was more remarkable consid-ering the horizontal distribution of the specific humidity(Fig. 6b). The Mediterranean flows increased the humidity ofthe lower half, with values greater than 13–14 g kg−1, whileon the east side, the humidity values were lower than12 g kg−1. These results could be responsible for the largestdifferences in the specific humidity obtained in Cordoba cityin the simulation because it is close to the entrance of theMediterranean flows over the valley.

The channelling of the Mediterranean flows to the mouthof the valley limited the spatial development of the pure seabreeze along this region (Fig. 6b and c). For this reason, theMediterranean flow towards the lower valley is consideredto be the major factor that characterises and determinesthe evolution of the pure sea breeze along the Guadalquivirvalley.

As the day progresses, favoured by the increase in thethermal contrast between sea and land surfaces as well as bythe channelling that the Guadalquivir valley exerts over thewinds that surround the western limit of the Betic mountainsystem, the pure sea breeze penetrated inland by a few tens ofkilometres, up to nearly 30–40 km as observed in the windfields at 18:00 UTC (black line of Fig. 6c). This value representsthe maximum spatial development observed for the pure seabreeze. At that time, the Mediterranean flows began to bechannelled to the north side of the valley, favouring theprogressive establishment of homogeneous humidity valuesalong the valley. At approximately 21:00 UTC (not shown inFig. 6), the sea breeze in the coastal area halted and the windbegan to blowwith low intensity in an east/northeast directionalong the valley, coinciding with the decrease in the intensityof the Mediterranean flows. These conditions favoured thepredominance of the easterly/northeasterly winds along thevalley during the night.

These results were consistent with the wind evolutionobserved at the fivemeteorological stations (Fig. 3). The typicalwind direction evolution under breeze conditions was ob-served in the two stations close to the coastline (extension ofthe marine breeze to 30–40 km inland), while at sites locatedinland, its variation could be explained by the connection be-tween flows from the Guadalquivir valley and MediterraneanSea. As noted above, similar results were found in the simula-tions performed during other pure breeze periods.

12 M.A. Hernández-Ceballos et al. / Atmospheric Research 124 (2013) 1–20

Fig. 7 shows the main characteristics for the non-puresea–land breeze period (8 July 2001). During the nocturnalperiod, the surface winds in the valley were dominated bysouthwestern/western flows with low intensity, as shown inFig. 7a (panel at 05:00 UTC). In this case, the model resultsshowed more intensive winds in the mountainous areassurrounding the valley, which could be related to the devel-opment of mountains breezes. Nevertheless, the synopticnorthwestern winds over the coastal area of the Gulf of Cadizwere dominant and were measured at the coastal station ofEl Arenosillo (Fig. 4). These flows could be attributed to thehigh-pressure system located in the Atlantic Ocean. Thesenorthwestern winds reached the coastal area favoured bythe lower heights of the orography at the northwest of theHuelva province because this area is the end of the SierraMorena mountain chain and the beginning of the Portugueseplateau. As a result, this synoptic flow isolated the inside ofthe Guadalquivir valley from the Atlantic influence (Fig. 7a).

At that time, southwestern flows predominated over theMediterranean sea, without any wind connection betweenthe Guadalquivir valley and Mediterranean sea such as thatoccurring in the pure breeze events. In addition, a cycloniccirculation over the Alboran Sea was observed in that meteo-rological scenario, coinciding with times during which therewere no strong flows from the east in the Alboran Sea. Thisvortex could be the result of the interaction between thegeneral flow from the southwest and the land breeze, a strongKatabatic flow that appears in southern Sierra Nevada. A

Nonpure

c) 08-07-2001 17:00 UTC

Specific Humi

a) 08-07-2001 05:00 UTC

Fig. 7. Simulated wind field at 10 m and specific humidity at 2 m (finer domain) unand d) 23:00 UTC. The reference wind speed used in the domain is indicated by an

similar case was observed in the eastern Mediterranean, offthe coast of Turkey (Alpert et al., 1999).

Under non-pure breeze conditions, at approximately9:00–10:00 UTC (Fig. 7b), the northwesterly flows detectedin the coastal area during the night began to change to thewest–southwest direction, while in the valley, the windcontinued to blow mainly from the southwest. At midday,the advection of southwesterly winds over the mouth ofthe Guadalquivir valley was clearly defined. From this timeforward, a progressive homogenisation of the southwesternwinds along the valley occurred, also increasing the flowintensity as reflected in Fig. 7c at 17:00 UTC. In this scenario,southwestern flows in the upper valley were measured whenthe synoptic northwestern flows were greater over the entiredomain. This behaviour could be associated with the oro-graphic characteristic in this portion of the valley, which isnarrower and thus increases the channelling effect of theGuadalquivir valley over themarine flows. Thiswindbehaviourfavoured the gradual homogenisation of moisture along thevalley. In addition, a progressive increment of specific humidityvalues was detected during the day in the inland areas, asshown in Fig 7c. This enhancement could be related to theadvance of the marine air masses because its movement wasmainly strengthened along the valley by the progressive arrivalof the southwestern flows.

During this diurnal period, a communication channelwas established between the Guadalquivir valley and theMediterranean Sea, similar to the one found in the pure

sea breeze

d) 08-07-2001 23:00 UTC

dity (g kg -1) 10 m/s

b) 08-07-2001 10:00 UTC

der nonpure breeze conditions on 8 July 2001 at a) 05:00, b) 10:00, c) 17:00arrow below the figures. Local time=UTC +2.

13M.A. Hernández-Ceballos et al. / Atmospheric Research 124 (2013) 1–20

breeze condition but in the opposite direction. Hence, theflows went from the valley to the Mediterranean Sea. Thiswind behaviour in the valley began to subside at approxi-mately 21:00 UTC when the northwesterly winds were pre-dominant again over the mouth of the valley, as observed at23:00 UTC (Fig. 7d). At that time, the high humidity valuesreached in the inland locations of the valley were remarkable.Again, in the nocturnal hours, the wind behaviour tended tobe undefined along the Guadalquivir valley while it decreasedin intensity.

This horizontal behaviour of the wind was in agreementwith the behaviour observed along the valley at the meteo-rological stations (Fig. 4). The daily change of the wind di-rection, from northwest to southwest, was observed only inthe coastal area while the rest of the valley remained in theadvection of southwestern flows during the day due to thevalley channelling.

According to these results, the non-pure breeze wasidentified in the coastal area at a range of 20–30 km due tothe influence of southwesterly winds in the valley duringthe day. As the horizontal distribution of specific humidityshowed, the marine air masses along Guadalquivir valleywere channelled to reach the inland areas. Therefore, it waspossible to find the transport of marine flows along the valleyassociated with the advection of southwesterly flows butwithout associating it to the development of sea breezes inthe valley.

3.3. Vertical structure

To analyse the vertical behaviour of the lower atmo-sphere, the horizontal wind fields were calculated at levels of300 m, 500 m, 1000 m, 1500 m and 2000 m asl. In addition,the cross-section of the horizontal (u) and vertical (w) windcomponents (isotachs) (Orgill et al., 1992; Rao et al., 1999;Challa et al., 2009) and specific humidity along the Guadal-quivir valley axis (Fig. 1a) (covering a distance ~320 kmfrom (36.88 N, −6.94 W) to (38.12 N, −3.66 W)) were alsostudied.

Fig. 8a shows the wind behaviour at different verticallevels at 18:00 UTC, which is representative of the hour inwhich the pure breeze reached its maximum spatial devel-opment on the surface, as shown in Fig. 6. At lower levels(300 and 500 m), the valley surroundings delimited the flowsinside, while in the upper levels, the wind circulation wasclearly conditioned by the large-scale flows from the east andwas influenced by the orographic effect. The presence ofonshore flowswas detected over the coastal area, which couldbe caused by the thermal contrast and the channelling ofsoutheasterly winds. However, the dominance at the upperlevels of an intense advection from east/southeast limited thepresence of southwestern flows to the lower levels, whichindicated the low vertical development of the pure breeze atlevels less than 500 m.

Thewind pattern at 2000 m also indicated the influence ofMediterranean flows over the valley in this breeze configu-ration. The Mediterranean flows over the centre of the valleyincreased their vertical coverage throughout the day, detect-able at 2 km in the afternoon (Fig. 8a). As shown in this figure,in the upper levels, the Mediterranean stream divided thevalley during the day, without showing any change in its

movement to the mouth of the valley, in contrast to thesurface levels where the sea breeze was established (Fig. 6).For this reason, the winds in the upper heights were alsodifferent along the valley. Constant eastern winds wereobserved in the lower half and coastal areas, but the windwas variable in the north half.

Following this breeze pattern, thewest–east cross sectionsof the u and w-wind isotach and specific humidity confirmedthe limited horizontal and vertical extension of these breezepatterns along the valley and its division due to the arrivalof Mediterranean flows. Fig. 9a shows the simulated cross-sections at 15:00 UTC coinciding with the intense flows fromthe Mediterranean to the Guadalquivir valley as well as at18:00 UTC because this time showed the maximum spatialdevelopment of the pure breeze. At 15:00 UTC, a moist airmass was detected over the coastline (CL), with a verticaldevelopment lower than 500 m and wrapped by the zeroisotach. This pattern indicated the development of the puresea breeze along the valley. The maximum vertical motionassociated with the sea breeze front was approximately0.4 m s−1, with vertical winds up to a height of 1 km. Thisdevelopment was confirmed by comparing both cross-sectionsat 15:00 UTC and 18:00 UTC. In this time interval, the air masscreated a horizontal displacement along the valley coveringapproximately 30–40 km, with a vertical development of thehead of the sea breeze close to 500 m. This displacement alongthe valley was also detected in the vertical motion, reachingvalues between 0.2–0.4 m s−1. These facts confirmed the smallhorizontal development of the pure sea breeze along the valley,indicated in Figs. 3 and 6, and the shallow vertical developmentof this pattern.

Additionally, at 15:00 UTC, the Mediterranean flow overthe valley was detected near the centre, with a spatial limitbetween 75 and 150 km as measured from the coast line, anda vertical development close to 2 km. This area was charac-terised by a large easterly component and small humidityvalues compared with the closest regions. This characteristiccould be logical due to its origin from the Mediterranean Sea.This difference allowed us to detect the edge of this wind.These limits were well identified at 18:00 UTC by the estab-lishment of a westerly component representativeness of thewind channelling towards the inland portion of the valley(Fig. 9c), which had a vertical penetration of nearly 2 km andan upward motion at approximately 0.6–0.8 m s−1 (Fig. 9d).In addition, the ascending and descending motions between150 and 200 km could be evidence of the existence of Bénardcells in the Guadalquivir valley, which are a particular type ofconvection characterised by the heating of horizontal airfrom below (Fig. 9b). In this case, the topography character-istics of the Guadalquivir valley, together with the positivetemperature gradient along the valley and increased warmthassociated with the month of August (the hottest month inthis area), favour the definition of this type of thermalconductivity structure.

Analysing thewind pattern at different levels correspondingto the non-pure breeze, the entrance of the north/northwestwinds along the coastal area up 1 km was shown at night,while in the upper levels, we observed the dominance of west/southwestern flows over the entire region, as is shown in Fig. 8bat 08:00 UTC. At approximately 9:00–10:00 UTC, a progressivechannelling of thewesterlywinds along the valleywas detected,

a) Pure sea breeze at 12-08-2003 18:00 UTC b) Non-Pure sea breeze at 08-07-2001 08:00 UTC

300 m

10 m/s

2000 m

500 m

1000 m

1500 m

300 m

2000 m

500 m

1000 m

1500 m

Fig. 8. Simulated wind fields (finer domain) at heights of 300, 500, 1000, 1500 and 2000 m under a) pure breeze conditions on 12 August 2003 at 18:00 UTC andb) non pure breeze conditions on 8 July 2001 at 8:00 UTC. Local time=UTC +2.

14 M.A. Hernández-Ceballos et al. / Atmospheric Research 124 (2013) 1–20

Pure sea breeze at 12-08-2003 15:00 UTC Pure sea breeze at 12-08-2003 18:00 UTC

CL 30 90 150 210 270 km CL 30 90 150 210 270 km

CL 30 90 150 210 270 km CL 30 90 150 210 270 km

Specific Humidity (g kg-1)

w-wind component (m s-1)

a) c)

b) d)

Fig. 9. Cross section of specific humidity and u-wind isotach and vertical motion on 12 August 2003 at a,b) 15:00 UTC and c,d) 18:00 UTC along the Guadalquivirvalley corresponding to pure breeze. The bold line is the zero u-wind isotach which is used to determine the position and depth of the sea breeze. Wind at altitudesbelow the zero isotach has a westerly (onshore) component, while those at higher altitudes have an easterly (offshore) component. Local time=UTC +2.

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increasing its effectiveness from 1 km to lower levels. Thebeginning of a gradual change in wind direction, from anorthwest to a west component over the coastline, began at12:00–13:00 UTC at levels under 1 km, while the westerlyinfluence remained in the upper layers. This fact revealed thevertical development of the pattern. The westerly wind wasobserved to end at a 500-m level at approximately 18:00–19:00UTC, increasing the vertical influence of the synoptic wind fromthe northwest over the coastal area hour by hour while theinfluence of thewest/southwest flows remained over the valley.

The cross-section revealed the air mass transport from thecoast to the inland areas and clearly showed the verticaldevelopment of the marine air mass. Fig. 10a shows thedominance of westerly winds at the surface levels, favouringthe increase of the humidity values over the coastal area andallowing the progressive penetration of marine air massestowards inland areas. This fact was observed by the presenceof air masses with higher specific humidity values along theGuadalquivir valley at 12:00 UTC (Fig. 10a). These air masseswere characterised by an upward vertical motion up to0.8 m s−1 in combination with downward areas reaching avertical development of 1–1.5 km (Fig. 10b). In addition, dueto the continuous channelling of westerly winds along thevalley, whichwere identified in Fig. 10c by the intensity of thewesterly surface winds, the marine air masses were able to

cover long distances, reaching sites located far away from thecoast and always depending on the intensity of the westerlywinds. In this case, at 18:00 UTC, the maritime air masseswere identified by the specific humidity (Fig. 10c) and theupward wind (Fig. 10d) at 150 km and 180 km inland, with amaximum ascent of 0.4 m s−1. Similar results were observedin other simulations, indicating the observation the so-callednon-pure sea breeze pattern is centred in the coastal area andis shown as an air mass displacement along the Guadalquivirvalley.

3.4. Mixing height in the Guadalquivir valley under breezeconditions

To investigate the behaviour of the mixing height in thestudy area under the two different sea–land breezes patterns,the spatial distribution of the mixing height fields obtainedfrom the finer domain were analysed as well as the potentialtemperature and specific humidity profiles at the previouslyselected five meteorological stations (Figs. 11 and 12).

In the case of a pure breeze pattern, during the morninghours, a stratification of the temperature and humidity wasobserved in the lower atmospheric region over the coastaland inland areas lasting until 13:00 UTC (not shown). Fromthis time until 20:00 UTC, the boundary layer structure was

Non-pure sea breeze at 08-07-2001 12:00 UTC Non-pure sea breeze at 08-07-2001 18:00 UTC

CL 30 90 150 210 270 km CL 30 90 150 210 270 km

CL 30 90 150 210 270 km CL 30 90 150 210 270 km

Specific Humidity (g kg-1)

w-wind component (m s-1)

a) c)

b) d)

Fig. 10. Cross section of specific humidity and u-wind isotach and vertical motion on 8 July 2001 at a,b) 12:00 UTC and c,d) 18:00 UTC along the Guadalquivir valleycorresponding to non-pure breeze. The bold line is the zero u-wind isotach which is used to determine the position and depth of the sea breeze. Wind at altitudesbelow the zero isotach has a westerly (onshore) component, while those at higher altitudes have an easterly (offshore) component. Local time=UTC +2.

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clearly determined by the arrival of the Mediterranean windsover the valley, dividing it into the two zones mentioned inthe previous sections. As shown in Fig. 11, at 15:00 UTC, theABL height distribution showed a decline in the centre of thevalley, while it reached greater heights on both sides. Puebladel Rio and La Luisiana presented boundary layer structuresthat were well defined with a mixing height of approximately1500 m agl. However, the boundary layer structure in theCordoba site was less marked and reached a shallower ABLheight than in the coastal area with a mixed layer up to1200 m agl. At the end of the valley, Linares reached a mixedlayer height of nearly 1700 m agl, driven in part by itsmountainous emplacement. This valley division, according tothe ABL behaviour, remained present until the entire valleywas under a similar wind regimen.

At El Arenosillo, Fig. 11 showed that coincident with thearrival of marine air masses, the vertical profile displayed astrong inversion layer and a sudden change of the specifichumidity. In this figure, the arrival of a wet air mass over thecoastal area at surface levels was also observed, confirmingthe development of a shallow boundary layer near the coastthat could be identified as the internal boundary layer regionformed under sea breeze advection. In this coastal area, thepotential temperature and specific humidity profiles at 18:00UTC were observable as the TIBL extends horizontally in arange of 30–40 km inland with a progressive decrease of itsvertical development and therefore was slightly detected atPuebla del Rio.

For the non-pure sea breeze pattern (Fig. 12), due to thehomogeneous entrance of the southwesterly flows along thevalley, a similar ABL height was detected during the day,reaching greater than 1600 m from 9:00–10:00 UTC to 19:00–20:00 UTC. Fig. 12 shows the ABL characteristics at 15:00coinciding with the highest ABL height observed in this period.However, differences between El Arenosillo and the rest of thesites were found that were associated with the features of thiswind pattern and only detected at the coastal site.

In inland sites, the potential temperature showed anincrease in the convective layer and a decrease in the specifichumidity moving away to the coastline. It is necessary toremark that the wettest convective layer was obtained atPuebla del Rio, which reached values higher than those shownat the coastal site. This result could be attributed to thedisplacement of the marine air mass over this area, as shownin Fig. 10. In addition, the isolation of the coastal area, whichreached a horizontal evolution of 30–40 km, could indicatethe merger between the TBIL associated with the sea breezemovement inland, as shown in Fig. 10, and the generic inlandABL much closer to the coast.

4. Conclusions

The aim of this study was to investigate the characteristicsof two sea–land breeze patterns, defined as pure and non-purebreezes, which were previously identified over the southwestcoast of the Iberian Peninsula. This area is characterised by the

12-08-2003 15:00 UTC

300 304 308 312 3160

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ght (

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0 4 8 120

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2400

2800

3200

3600

4000H

eigh

t (m

)

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ARE PUE LLU COR LIN

ABL height

ABL height

Fig. 11. Simulated boundary layer height field in the finer domain (D4) and vertical potential temperature and specific humidity profiles at El Arenosillo (ARE),Puebla del Río (PUE), La Luisiana (LLU), Córdoba (COR), Linares (LIN) sites along the Guadalquivir valley corresponding to pure breeze on 12 August 2003 at 15:00UTC. Local time=UTC +2.

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presence of the Guadalquivir valley, which acts as a naturalcommunication channel between the coastal and inland areas.TheWRF-ARWmodel was applied to analyse themain featuresof these mesoscale mechanisms. The pure breeze was investi-gated using the period from11 to 14 August 2003, and the non-pure typewas investigated from6 to 9 July 2001. Both episodeswere selected during the warm season because the frequencyof occurrence is elevated in these months.

The modelling results showed a short spatial develop-ment along the Guadalquivir valley for both breeze typesidentified in the coastal area. In the case of the pure pattern,northeasterly winds were predominant along the valleyduring the night periods, early morning, and until midday,identifying a sea breeze development from the southwestwith a maximum horizontal extension to approximately 20–

40 km inland. Unexpectedly, the arrival of Mediterraneanflows through transversal valleys was detected, limiting thehorizontal extension of the marine breeze along the valley.Under these meteorological conditions, the valley was clearlydivided into two meteorological areas.

For the non-pure breeze, during the nocturnal period, thenorthwestern winds prevailed in the mouth of the valley,while southwest flows with low intensity were predominantalong the valley. At sunset, there was a change in the coastalwind regime, which blew from the southwest and remainedin the same direction along the valley, increasing its intensity.In the diurnal period, communication between the Guadal-quivir valley and the Mediterranean through transversalvalleys was detected but in a manner opposite to that of thepure breeze. Hence, under this breeze pattern, it was possible

08-07-2001 15:00 UTC

292 296 300 304 308 312 3160

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2400

2800

3200

3600

4000H

eigh

t (m

) ARE PUE LLU COR LIN

ABL height

ABL height

Specific humidity (g kg-1)

Fig. 12. Simulated boundary layer height field in the finer domain (D4) and vertical potential temperature and specific humidity profiles at El Arenosillo (ARE),Puebla del Río (PUE), La Luisiana (LLU), Córdoba (COR), and Linares (LIN) sites along the Guadalquivir valley corresponding to nonpure breeze on 8 July 2001 at15:00 UTC. Local time=UTC +2.

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to consider air mass transport along the valley but unrelatedto the development of the non-pure sea breeze. The dailywindvariation of the breeze,with changes in thewinddirectionand speed according to the solar cycle, was only observed in anarea of 20–30 km inland because the wind blew along thevalley from the southwest during the day.

The analysis of the vertical structure of both meteorolog-ical scenarios showed the scarce vertical development of bothbreeze patterns. In addition, the simulation of the mixinglayer displayed the formation of a thermal internal boundarylayer (TIBL) for both types. Under pure breeze conditions,according to the mixing layer, the valley could be divided intofour areas during the diurnal time period. The first, from thecoast to tens of km inland, was influenced by the TIBL. Thesecond consisted of a zone that could present a mixing layerwith a height of approximately 1600 m. Next, a zone with

major atmospheric stability was influenced byMediterraneanflows. Finally, in themedium and upper valley, it was possibleto find mixing layer heights up to 1600 m.

The TIBL during the non-pure breeze state showed awider influence than the pure patterns. At the coastal site, anatmospheric stability or a lower mixing layer height no morethan 500–600 m was detected. From a distance of 30–40 kmto the inland areas, the vertical atmospheric structure wascharacterised by homogeneous mixing layer heights withtops at approximately 1500–1600 m.

The results obtained in this work will aid in interpretationof the air pollutants measured in this area. For instance, thecommunication between the Guadalquivir valley and theMediterranean Sea has been detected using these modelresults and could explain the pathways for transport ofpollutants between different zones. In addition, the vertical

19M.A. Hernández-Ceballos et al. / Atmospheric Research 124 (2013) 1–20

structure of the atmosphere under different meteorologicalscenarios will provide data on the pollutant dynamics at thesurface levels. Future work linked with meteorological simu-lations will be carried out in this area to improve the resultsobtained with the pure breeze. Moreover, other typical mete-orological scenarios governed by the synoptic scale will besimulated as well.

Acknowledgments

The authors want to thank the Spanish National Meteo-rology Service (the current “State Agency of Meteorology”)and the Network of Climate Information of Andalusia for thesupply of surface meteorological information and especially toDr. Jordi Vila-Guerau de Arellano fromWageningen University(The Netherlands) for his collaboration and support in thisstudy.

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Dr. Miguel Ángel Hernández Ceballos has a Degree in Physics, fromSeptember 2005, at the University of Salamanca. He is a doctor from 2011 atthe University of Huelva. He has participated in 7 international paperspublished in journals included in SCI and in 22 conference contributions. Hisresearch line is focused on the field of meteorology, using experimentalobservations and modelling tools.

Dr. José Antonio Adame Carnero has a Degree in Physics at the Universityof Seville in 1998. He is a doctor from 2005 at the University of Huelva. SinceDecember 2007 he is a senior researcher in the National Institute forAerospace Technology (Spain). He has participated in 9 research projects, in10 papers published in journals included in SCI and in 70 conferencecontributions. He has been the co-Director of one PhD. The research lines aremainly focused on air quality and meteorology.

Dr. Juan Pedro Bolívar Raya is a professor of the Applied Physics in thePhysics Department at the University of Huelva since 2004, and the directorof the Research Group for Radiation Physics and Environment since 1995. Hehas directed more than 10 national/autonomic competitive projects, andmore than 15 projects/contracts with companies and/or institutions. Hehas published over 60 papers in international journals in the field ofenvironment.

Dr. Benito A. of Morena Carretero has a Degree in Physics at theUniversidad Complutense of Madrid in 1975 and he is a doctor from theUniversity of Granada in 1995. Since 1975 he is a senior researcher in theNational Institute for Aerospace Technology (Spain), working on researchinglines related to total content of ozone, solar radiation, aerosols, air qualityand meteorology. He has been the main researcher of 35 coordinatedprojects and subprojects. Currently, he has published 8 books, more than 80publications in SCI journals and more than 200 conference contributions.During his career he has directed 8 PhDs.