area). These three regions are physically defined by topography. … · 2014. 5. 12. · 116 Urban Pollution area). These three regions are physically defined by topography. The elongated

Air mass exchange between the Athens Basin and

the Messogia Plain

C.G. Helmis, D.N. Asimakopoulos, K.H. Papadopoulos,

J.A. Kalogiros, P. Kassomenos, P.O. Papageorgas, S. Blikas

Department of Applied Physics, University of Athens,

33 Ippokratous Street, GR-10680 Athens, Greece

Abstract

The new airport of Athens will be established at the Messogia Plain which islocated in the eastern part of the Attika Peninsula. The Messogia Plain is arural area separated from the Athens Basin by the 1000 m-high HymettosMountain. On the west side of Hymettos Mt lies the well known Athens Basinwhich combines industrial and human activities. The present work deals withthe issues of air mass exchange between the two areas and the local thermalflows coupling or separating them. In this respect, a four month experimentalcampaign was organised in an attempt to answer questions related to theabove issues. The methodology that was used included the following steps: (a)identification of the principal synoptic types recorded during theexperimental period that favour high air pollution episodes in the Athensarea, (b) determination of the recurrent surface wind flow patterns over theMessogia Plain and the Athens Basin in relation to the synoptic, background(large scale) conditions, and (c) analysis of the flows over the mountains andthrough the natural openings.

1 Introduction

The Athens Metropolitan Area (AMA) is inhabited by four million residentsin an area of 450 km̂ . The concentration of commercial, transport, publicservice and industrial activities in conjunction with the special topographicfeatures of the area has led to significant air pollution problems whichmotivated the scientific community towards both experimental and numericalmodelling studies with the aim of understanding the dominant atmosphericflow and pollutant transport patterns.

The AMA features a complex topography which includes the AthensBasin (mainly residential and public services activities), the Thriassion Field(mainly industrial activities) and the Messogia Plain (rural-suburban

Transactions on Ecology and the Environment vol 6, © 1995 WIT Press, www.witpress.com, ISSN 1743-3541

116 Urban Pollution

area). These three regions are physically defined by topography. Theelongated Hymettos Mt separates the Athens Basin from the Messogia Plainand the low-height Egaleo Mt lies between the Athens Basin and theThriassion Field. The extended mountain ranges of Penteli and Parnithaisolate the AMA from the Greek mainland, leaving a relatively narrowphysical passage to the north. These features are considered to be importantfor the ventilation of AMA.

Early studies addressed the possible influence of local thermally inducedcirculation on the air pollution dispersion. Such circulation mainly include thethree seaMand breeze cells acting in the region, namely the Saronikos Gulf seabreeze (blowing from SSW during the daytime and reversing to southwardduring the night), the Elefsis Gulf sea breeze (southerly in the daytime andnortherly in the night) and the Evoikos Gulf Sea breeze (easterly in thedaytime and westerly during the night). The first one has been the subject ofmost analyses since it is the one that affects the Athens Basin.

Specifically, the wind flow over the Athens Area and the possibletrajectories of air pollutants has been studied by Lalas et al [8,9],Asimakopoulos et al. [2,3] and Helmis et al. [5,6] analysed some features of theatmospheric boundary layer in the city centre and in the surrounding inlandor coastal areas. The openings of the Athens Basin to the northeast and west(emphasising on the nocturnal pollutant transport from the industrial area tothe Athens Basin over and around the Egaleo Mt) were the subject of thework of Asimakopoulos et aL [1] and Varvayianni et aL [12]. Additionally, theeffect of Hymettos Mt. as a natural barrier to the low-level atmospheric flowhas been studied by Deligiorgi et aL [4], Helmis et aL [6] and Varvayianni etaL [13]. Also, the analysis of a 10-year long record of high air pollutionepisodes in the Athens Basin by Kallos et aL [10] identified four distinctsynoptic types favouring them along with their seasonal preference.

Recently, research efforts have shifted to the developing Messogia Plainpartly in view of the construction of the new international airport of Athensin this area. The first campaign dealt with the Evoikos sea breeze's featuresand impact on the Messogia Plain, indicating the possibility of overriding themountain barrier of Hymettos Mt. A second campaign was aimed at theanalysis of the nocturnal wind flow regime in the east slope of Hymettos Mt.However, the questions of the ventilation of the Athens Basin through thenorth and south openings of the Athens Basin towards the Messogia Plain, thegeneral matter of air mass exchange between the urban basin and theneighbouring rural-suburban plain, the local thermal flows coupling orseparating them and the anticipated environmental impact of the plannedairport in the Messogia Plain called for an extensive observational andnumerical modelling approach, since most previous relevant experimentalstudies were rather limited in area coverage and time duration, mainly due tothe lack of financial resources. Besides, numerical simulations of the windflow and transport provided interesting results that could not be evaluatedagainst representative and high quality field data. These questions wereaddressed experimentally by the deployment of an extended network ofmeasuring stations which are described in section 2. Section 3 reviews theprincipal synoptic types favouring air pollution episodes in the AMA andconnects them with the surface flow patterns observed during theexperimental period. The recurrent flow patterns of surface winds are


Urban Pollution 117

analysed in section 4 in relation to the background atmospheric conditions,giving emphasis to the mass exchange through the openings between theMessogia Plain and the Athens Basin.

2 Experimental layout

A network of surface meteorological stations, acoustic sounders and tetheredballoons covered the Messogia Plain, the surrounding areas and the openingsnorth and south of the Hymettos Mt. The results and experience from paststudies and the main topographic features provided the basis for selecting thecrucial representative locations of the surface meteorological stations.

The surface meteorological network comprised 11 locations (Figure 1),where 10 min mean and fluctuating values of temperature, wind speed anddirection, and humidity were measured at a standard height of 10 m abovethe surface. Two high range monostatics acoustic sounders were operated inthe centre of Athens (NOA) and in the Messogia Plain at the location of thenew airport (SPA). In the same two locations tethered balloon systems and athree-axis Doppler sodar were operated in periods of particular interest.

Saronikos Gulf \ 2r, KOR

20.0-

20.0 40.0 60.0 120.0 140.0 160.0 klTI

Figure 1: The Athens Metropolitan Area and the locations of all surfacestations (height contours of 200 m).


118 Urban Pollution

In the following discussion, the surface meteorological stations areorganised in groups: the MAK, LOU and MAR stations are referred to as theeast coast stations; the SPA and INT stations are the inland stations; NOA isthe urban station; EKT is the west coast station, HYM is the mountain station;HOE, STA and KOR are located at the openings between Parnitha-PenteliMts (the north opening of Athens Basin), Penteli-Hymettos Mts (the westopening of Messogia Plain) and Hymettos-Panio Mts (the south opening ofMessogia Plain), respectively. The experimental period spans four months,from June to September 1994, when thermal local circulation systems developand complicate the observed flow patterns. Moreover, this is the period ofmaximum human activities in the Messogia Plain, as well as the rush periodof air traffic.

3 Classification of surface wind flow patterns in the Messogia Plainduring the summer period

The effect of complex terrain features on the background wind flow is due tomechanical (e.g. blocking, acceleration) and thermodynamical (e.g. sea-landthermal contrast, mountain-plain circulation, surface cover inhomogeinities)processes. Alternately, local flows such as sea breezes may be classified as'pure' (undisturbed) only in limited periods of weak background forcing, sincetheir interaction with the background field should be considered ascontinuous. Therefore, the categorisation of the dominant surface wind flowpatterns is done in the context of a more general classification based on thedominant synoptic types. When dealing with the issue of air quality and airmass transport it is further useful to identify those synoptic types thatpromote poor pollution dispersion. As a first step in the current methodology,the following six prevalent synoptic flow types are determined for theexperimental period:• Type I: A high pressure system slowly progresses eastward of Greeceadvecting warm air masses from the south, strongly stabilising the lowtroposphere, therefore suppressing vertical mixing. Low wind conditionsfurther limit horizontal transport. This type was observed for 23.8% of theexperimental days.• Type II The warm sector of a usually shallow depression causesstabilisation of the lower troposphere due to warm air advection from thesouth (2.5% of the experimental days)..* Type III: Cool air masses behind a cold front reside over Greece. Theassociated weak northwest flow and clear sky conditions allow the formationof surface temperature inversions, which limit the extent of vertical mixing.This type was observed for 3.3% of the experimental days.• Type IV: The combination of the thermal low pressure system coveringthe Anatolian Plateau with the anticyclonic system covering theMediterranean Sea and the Northeast Europe establishes a northerly flowover the Aegean Sea and consequently over the AMA. This flow is relativelyweak during periods when the anticyclonic system extends eastward orstrengthens, permitting local flows to develop. This type was observed for25.4% of the experimental days.• Type IVE: This type is similar to the previous one. However, thecombination of the two systems is such that the northerly flow is persistent


Urban Pollution 119

and strong. This is a typical feature of the warm period in the Aegean Seaand the characteristic northerly winds are called 'etesians' and may suppressthe development of thermal local flows. However, the ventilation of theAMA is usually very effective. Apparently, Type IV constitutes a weak'etesians' regime (39.3% of the experimental days).• Type V: It is characterised by strong northerly winds that do not fallinto the two previous types (5.7% of the experimental days).

According to the study by Kallos et al. [10], that was based on a one-decade long period observations covering all seasons, types I to IV arestrongly related to high pollution episodes in AMA. These types with theaddition of IVE and V were sufficient to describe the synoptic conditionsduring the experimental period.

In addition to the relation between the synoptic flow pattern and the airpollution episodes, two other factors are very crucial in this respect. The largescale, surface pressure gradient and the vertical thermal structure (affectedby the thermal advection) of the lowest 3 Km of the troposphere over thegreater area are important since they determine the efficiency of horizontal(mechanical) and vertical (thermal-convective) mixing, respectively. Thecategorisation of types I to V, in terms of these two parameters is given inTables 1 and 2. The results refer to the experimental period. It is noted thatII and III were observed in only a few days.

Table 1: Frequency distribution (%) of the experimental daysaccording to the large scale, surface pressure gradientpressuregradient

synoptictype

IIIIIIIVIVEV

>5 hPa/100km(strong)

31325.0

20.842.9

5hPa/100-550 km(moderate)

13.833.375.016.260.4429

5 hPa/550-1000 km

(weak)

37.933.3

51.618.814.3

4°C(strongwarm)31.033.3

16.1

3to4°C(moderatewarm)17.2

1296.2

lto2°C(weakwarm)27.633.325.038.720.828.5

-1 to 1 °C(very weak)

13.831325.022.637542.9

120 Urban Pollution

Regarding the thermal advection, it would be in principle desirable to knowthe detailed vertical structure in the lowest 3 km of the troposphere.However, the non-availability of such data dictated the use of the thermaladvection in the 850 and 700 hPa isobaric levels.

Types I and IV are both characterised by low pressure gradient-weakbackground winds, especially the former, while III, IVE and V feature strongpressure gradient, and type II possesses moderate background winds (Table 1).Types I and IV are characterised by warm advection in the lower troposphere(Table 2). It is worth-mentioning, here, that despite this apparent similarity oftypes I and IV, they are of a different synoptic origin. Usually, IV is a rathershort-lived type often shifting to IVE, and is related to generally cooler airmasses than for the case of I. Types III and V feature cold or insignificantadvection, while II has warm or negligible advection.

The above analysis, particularly shows that both I and IVE have thepotential of limited horizontal dispersion and vertical mixing at the sametime. However, their seasonal distribution is different, IVE being a summerfeature and type I being a feature of the transitional and winter season.

The second step of the analysis is to link the observed surface wind flowpatterns to the synoptic types (i.e. the driving force) already described. Thevariations of large scale, surface pressure gradient and the thermal advectionwithin each class are taken into account to explain the possible variability ofthe surface wind field. A direct interpretation of the coupling of thermallyinduced flows to the background conditions is, thus, attempted. The sixrecurrent surface wind flow patterns A-E defined below were sufficient toclassify 97% of the experimental days. It is noted in advance that they expressa smoothly varying degree of interaction between the local flows and thesynoptic flow. The most interesting details of each pattern are analysed in thenext section.• Pattern A: Pure or undisturbed local flows in both the Messogia Plain(Evoikos seaUand breeze) and the Athens Basin (Saronikos Gulf seaMandbreeze).• Pattern B: Limited interaction of the local flows with the synoptic flow.• Pattern C Strong interaction of the local flows with the synoptic(usually northerly) flow.• Pattern Dl: Prevalence of the northerly winds all over the AM A for thewhole day.• Pattern D2: Modification of the background flow by sea-land thermalcontrast• Pattern E: Transitional background southerly or westerly winds interactpositively with the Saronikos Gulf sea breeze, but negatively with theEvoikos Gulf sea breeze. Local nocturnal flows develop.

In the course of this analysis, it became evident that there was not a one-to-one correspondence between synoptic and surface wind flow patterns;more than one surface wind field emerged within each of the classes. Asnoted above, the apparent variability are attributed to the difference in theintensity of the large scale, surface pressure gradient and the thermaladvection. Table 3 is a cross-tabulation of the flow patterns.


Urban Pollution 121

Table 3: Frequency of occurrence (%) of the surface flow patterns forevery synoptic flow type during the experimental period (number of

cases in brackets)

surfacpatter

synoptic typeIIIIIIIVIVEV

j.

3233

10

\

(8)(1)

(3)

12

33232

B

(3)

(1)(7)(1)

C

32(8)

27(8)9(4)25(1)

I

4

177850

)1

(1)

(5)(35)(2)

I

8

2011

)2

(2)

(6)(5)

1

1267673

25

E

(3)(2)(2)(1)

(1)

According to Table 3, type I mainly favours an A or C surface wind flowtype, while IV favours C, B and Dl, D2 as well. This implies that type IV hasthe potential of disturbing or even overriding local flows. It was previouslystated that, although of different origin, I and IV appear similar, yet theformer has slightly stronger background wind and more intense warmadvection than the latter. Therefore, it is concluded that the stability of thelow level air found in I is sufficient to prevent the downward mixing fromthe higher levels. Types IVE and V, which have strong background winds andno advection, favour the dominance of synoptic conditions over AMA. Thesmall sample of II and III types leads to an E type of surface wind field.

Table 4: Frequency of occurrence (%) of the large scale surfacepressure gradient scales for every surface flow pattern during the

experimental period (number of cases in brackets)

patter A B C Dl D2 Epressuregradient

strong - - - 21(9) 15(2) 22(2)moderate 8(1) 25(3) 14(3) 63(27) 23(3) 33(3)weak 25(3) 42(5) 62(13) 12(5) 47(6) 33(3)

very weak 67(8) 33(4) 24(5) 4(2) 15(2) 11(1)

Table 5: As in Table 4, but for the thermal advection scales

surfacpatter

thermaladvection

strongmoderateweak

very weakcold

A

42(5)25(3)25(38(1)

178171741

B

(2)(1)(2)(2)(5)

(

101057149

C

(2)(2)(12)(3)(2)

I

22194730

)1

(1)(1)(8)(20)(13)

I

3123388

)2

(4)(3)(5)(1)

1

J'J

222222

(3)

(2)(2)(2)


122 Urban Pollution

We further investigate separately the effect of the pressure gradient andthe thermal advection strength on the surface flow pattern in Tables 4 and 5,respectively. As expected, the distribution of both the pressure gradientforcing and the thermal advection is totally compatible with the degree ofdisturbance of thermal circulation from the background wind, with theexception of type E which shows no particular trend. Therefore, a weaksynoptic forcing and a warm advection, which may lead to air pollutant'saccumulation, permit the decoupling of the local flows from backgroundconditions.

4 Detailed description of the surface flow patterns

It was found methodologically effective to represent the daily course of thesurface wind flow at the stations' locations in the form of frequency contoursof the average hourly wind speed and direction distribution of all the daysfalling within a particular pattern. This was facilitated and justified by thesignificant degree of similarity between individual realisations of the surfaceflow pattern. Pattern A is thoroughly discussed to show the features of theundisturbed thermal circulation.

4.1 Pattern AThis pattern was observed on 10% of the experimental period, with thehighest frequency of occurrence during early summer (30% in June). It isnoted that its low frequency of occurrence in late summer (September)should not be considered as the typical case and is attributed to theexceptionally persistent 'etesians' regime this year. It develops under synopticflow types I (75%) or IV (25%) with weak large scale pressure gradient andmoderate to strong warm advection that permit the undisturbed developmentof local flows within the AMA (see Tables 3, 4 and 5). Figure 2 shows therange of the daily surface wind flow at the most interesting experimentallocations (LOU, STA, KOR and EKT), denoted in Figure 1, while the featuresof the flow at the rest stations are also described below.

During the daytime, the Saronikos Gulf Sea breeze develops, blowingfrom southwest to southeast directions (NOA, EKT). It is weak at the coast(

Urban Pollution 123

(a) LOU (b)STA

0 2 4 6 8 10 12 14 16 18 20 22Time (LSI)(c) KOR

0 2 4 6 8 10 12 14 16 18 20 22Time (1ST)

(d) EKT

0 _2 4_ 6 8 10 12 14 16 18 20 22

300-

2 4 6 8 10 12 14 16 18 20 22Time (1ST)

0 2 4 6 8 10 12 14 16 18 20 22"̂ \\h#Wg

300-

240-

180-

0-0 2 4 6 8 1 0 1 2 1 4 1 6 1 8 20 22

Time (1ST)Figure 2: Frequency of occurrence (%) per hour of the wind speed and thewind direction at the surface station (a) LOU, (b) STA, (c) KOR and (d) EKTfor the surface flow pattern A during the experimental period.

interval 1100-1800 LST, accompanied by a temperature drop.The Evoikos Gulf Sea breeze affects the Messogia Plain blowing from E

to SSE directions. This sea breeze propagates inland and affects both INT andSTA. However, these two locations, along with KOR, may be affected by boththe west and east coast sea breezes of Attika Peninsula (the Saronikos and theEvoikos Gulf ones, respectively), as seen from Figure 1 which clearly depictsthe difference in their relative distances from the two coastlines.

The obvious differences between the characteristics of the east and westcoast sea breezes are the following:• At the east coastal stations (LOU, MAK) the circulation starts one hourearlier and peaks much earlier at a lower wind speed, compared to the westcoast (EKT). The morning low winds are not observed at the east coast.• The vector rotation of the Evoikos breeze is clockwise (veering), leadingto enhancement of the wind speed when it blows from SE directions. Thiseffect, first noted by Varvayianni et aL [12], is due to the supply of fresh cool


124 Urban Pollution

marine air from the open sea of the South Evoikos (Figure 1) as opposed tothe early stages of the flow which involves the limited channelled waters ofthe Evoikos Gulf and is affected by the presence of Evia Island to thenortheast. This suggestion is reinforced by the fact that the sea breeze atMAR is stronger than at the two other east coast stations.• The west coast sea breeze endures inland until late evening or evenmidnight, a feature not present at the Messogia Plain.

The east branch of the Saronikos Gulf sea breeze passes through thesouth opening into the Messogia Plain, reaching KOR between 1100 and 1500LST. The air is accelerated in passing through, an effect verified by numericalsimulations. The dominance of the west coast breeze at KOR should beanticipated in view of its location (see Figure 1). The southwest flow at KORis maintained until late evening, a characteristic of the Saronikos Gulf seabreeze inside the Athens Basin. At the interior of the Messogia Plain (INT), aweak east flow blows from sunrise to 1100 LST responding to the warming ofthe east slope of Hymettos Mt. The east coast sea breeze arrival is observedaround 1200 LST, blowing at 3-4 m/s from SE. However, after 1700 LST, theflow is interrupted by SSW winds that denote the arrival of the west coastsea breeze. Indeed, locations MAR, SPA, INT are all affected, after 1600-1700LST, by such a flow that is weakening in its northward progress. Finally, thewest opening (STA) seems to be affected by SE winds, peaking at 1200-1500LST, a possible combination of local thermal flows with the east coast breeze.The inland propagation rate of this breeze is observed to increase withdistance from the coast.

The nocturnal atmospheric circulation regime is characterised by weaksurface winds (1-2 m/s), blowing from north in the Athens Basin and fromwest-northwest at the Messogia Plain.

4.2 Pattern BThis pattern was observed on 10% of the experimental period, with noparticular month preference, except for its absence during June. It developsunder synoptic flow types I (25%) or IV (60%)-i.e. a reversed percentagecompared to A-with weak to moderate synoptic pressure gradient and limitedtropospheric warm advection that permit a slight interaction of local flowswith synoptic conditions imposing large scale northerly winds over AMA. Inthe following, only the important modifications of the characteristicsdiscussed in Pattern A are described.

The Saronikos Gulf sea breeze in the Athens Basin is delayed (but notweakened) by 1-2 hours at both the coastal and urban stations, exhibitingdistinct frontal characteristics [5,7]. Before the onset of the sea breeze, a NEwind prevails. The breeze's duration is still prolonged, except for a couple ofsituations when it was interrupted in the afternoon.

At the east coast the sea breeze starts as a NE flow, due to thebackground wind, and in the afternoon it shifts to SE and weakens (the opensea circulation opposes the northerly component of the background wind).The strongest surface winds are observed at the NE phase and are observedearlier than for pattern A. In the declining phase of the sea-land thermalcontrast the breeze shifts back to NE directions and then diminishes.

The most significant effect of the increased northerly flow is on theintrusion of the west coast sea breeze in the Messogia Plain. At the south


Urban Pollution 125

opening an ENE flow, which is the combined effect of the east coast seabreeze with the northerly background flow, usually prevails. However, thereare often shifts between NE and SW flow, during brief periods (usually, 1400-1700 LST) when the west coast breeze dominates. During late afternoon, thelonger lived Saronikos Gulf sea breeze dominates at the south opening andpropagates inland (at MAR, SPA and INT), although at a limited extent,compared with pattern A. The west opening (STA) is still characterised by aSE flow between 1100 and 1900 LST, delayed compared to pattern A. Also, thenocturnal features of pattern A are generally preserved.

4.3 Pattern CThis pattern was observed on 20% of the experimental period, mostly duringAugust and September. It develops under synoptic flow types I (40%), IV(40%) and IVE (20%). The development of C pattern under IVE synoptic typewas observed exclusively when the pressure gradient was weak. The moderatesynoptic pressure gradient and small low-level stability permit the significantdisturbance of local flows from large scale northerly winds over AMA. Figure3 shows the range of the daily surface wind flow at the most interestingexperimental locations. Pattern C is generally characterised by larger surfacewind speeds over the Messogia Plain than over the Athens Basin, which is areversal of the usual situation encountered in the previous two patterns. Theimportant modifications of the characteristics discussed in Pattern A aredescribed below.

The inland advance of the Saronikos Gulf sea breeze covers the distancefrom the west coast (EKT) to the urban station (NOA) in 3 hours as opposedto less than an hour for the undisturbed pattern A. The east component of thebackground wind seems more effective than the north component in delayingor cancelling the sea breeze. This is probably due to the fact that the largescale pressure gradient imposing an easterly wind component is opposite tothe mesoscale pressure gradient responsible for the development of the seabreeze. The wind vector rotation at the west coast is no longer observed. Thedistinction of the sea breeze of the east coast is obscured by the backgroundnortheast winds, but a shift to east directions is observed in the earlyafternoon.

The surface wind flow at the south opening is from the ENE and isparticularly enhanced. No intrusion of the west coast sea breeze is observed,although a weak southerly flow is occasionally apparent at both KOR andINT after 2000 LST. The most interesting feature of this pattern is the windflow through the west opening (STA). At the previous two patterns it wasseen that the east coast sea breeze manages in arriving there, blowing fromsoutheast direction. In this pattern, the sea breeze arrival is significantlydelayed by more than two hours and is characterised by varying winddirections mostly from S to SSE, while in 30% of the cases the NEbackground flow dominated. This frequent prevalence of northeasterly windsat the west opening could be due to a blockage of the penetration of the eastcoast sea breeze into the Athens Basin by the moderate background northerlywinds. Finally, the nocturnal regime is occasionally disrupted in the MessogiaPlain, but not in the Athens Basin.


126 Urban Pollution

(a) LOU

^̂ > "̂

(b) STA

0 2 4 6 8 10 12 14 16 18 20 22

300-̂

0 2 4 6 8 10 12 14 16 18 20 22

300-

0 2 4 6 8 10 12 14 16 18 20 22Time (1ST)(c) KOR

0 2 4 6 8 10 12 14 16 18 20 22Time (1ST)

(d) EKT

0 2 4 6 8 10 12 14 16 18 20 22Time (1ST)

0 2 4 6 8 10 12 14 16 18 20 22Time (1ST)

Figure 3: Frequency of occurrence (%) per hour of the wind speed and thewind direction at the surface station (a) LOU, (b) STA, (c) KOR and (d) EKTfor the surface flow pattern C during the experimental period.

4.4 Pattern DlThis pattern was observed on 40% of the experimental period, mostly duringJuly and August. It develops under synoptic flow types IVE (80%) and IV(10%). The strong northerly synoptic pressure gradient permit is sufficient tototally depress the local flows. Northeast moderate winds blow over theAMA, with a maximum intensity in the afternoon. The wind is reinforced bytopographic channelling in south opening of Messogia Plain, but not at thewest opening (possibly due to the mountain range blocking to the north ofSTA). The decline of the 'etesians' in the evening is not sufficient for localnocturnal flows to develop.

4.5 Pattern D2This pattern was observed on 10% of the experimental period, mostly duringAugust It develops under synoptic flow types IV (40%) and IVE (40% ). The


Urban Pollution 127

development of D2 pattern under IVE synoptic type was observed mainlywhen the pressure gradient was relatively weak. The moderate synopticpressure gradient and small low-level stability help in the domination of thebackground NNE winds, which are modified by mesoscale thermal gradients.Pattern D2 may be regarded as an intermediate condition between C and Dland usually consecutive days (during the etesians' wind regime IVE) of theexperimental period alternate between those three patterns. The most obviousdifference between C and D2 is the nocturnal flows regime, which issuppressed in D2.

Even though, a sea breeze over the Messogia Plain is not clearly depicted,the modification of the background flow is apparent around noon. Yet, thewest coast sea breeze is easily discerned in the Athens Basin when it managesto develop (in about 50% of the cases) against the strongly opposingbackground wind. In these cases its duration is short (1200-1600 LST), whileno vector rotation is observed. Its influence on the resultant surface windflow is a reduction of wind speed relative to the period prior to its initiationand strong frontal characteristics. This point is emphasised in view of therecognition of these conditions (see description of Type IV) as favourable forpoor dispersion over the Athens Basin. The situation at the east coastalstations is rather similar to pattern C, but at MAR and at the inland stationsSPA and KOR the surface flow is similar to Dl due to the fact that thethermal gradient weakens as the distance from the coast increases.

The south opening features strong NE winds. The flow at the westopening features the frequent prevalence of north winds-also observed forpattern C-for 20% of the cases. From cross-analysis of the flow evolution atSPA, INT and SPA it is concluded that the usual SSE flow at the west openingis closely linked to the flows at the two other stations, rather than being alocal circulation in the vicinity of STA.

4.6 Pattern EThis pattern was observed on less than 10% of the experimental period, mostlyduring June and September. It is connected with conditions of strong warmadvection from south and west directions. The limited number of days fallingin this category precludes a detailed analysis.

5 Concluding remarks

On the basis of the analysis of the preceding sections, some interestingconclusions can be drawn concerning the mass exchange between the AthensBasin and the Messogia Plain from the two natural openings betweenHymettos-Penteli Mts and Hymettos-Panio Mts during the warm period ofthe year, since a crucial issue of the wind flow over AMA is the degree ofcoupling or topographic isolation of the Athens Basin from its surroundingregions.

The surface flow at the west opening of Messogia Plain betweenHymettos and Penteli Mts shows a transport from the Messogia Plain towardsthe Athens Basin for all synoptic flow types I to IV (A, B, C and D2 surfaceflow patterns). The persistence of this transport, that is associated with theinland penetration of the east coast sea breeze, is larger for synoptic flowtypes I and IV (A and B surface flow patterns). For the cases when the


128 Urban Pollution

background northerly flow is moderate to strong (patterns C and D2), theflow through the west opening usually comes from the SSE while at the sametime the surface flow at the interior of the Athens Basin is northerly whilethe observed frequent (20-30%) prevalence of northerly winds at the westopening could be due to a blockage of the penetration of the east coast seabreeze into the Athens Basin. Under weaker background winds, the westopening flow is SE, which is the typical case of the east coast sea breeze. Forthe IVE and V synoptic types, the transport from Messogia Plain towardsAthens Basin is absent.

The direction of surface advection at the south opening of Messogia Plainbetween Hymettos and Panio Mts for synoptic flow types I and IV isdetermined by the development of an A, B or C surface flow pattern. Underpattern A, the east branch of the Saronikos Gulf sea breeze intrudes theMessogia Plain and reaches the central and west parts of the Messogia Plainin the late afternoon. Under pattern B, the usually northerly background flowlimits the propagation of the east part of the sea breeze of Saronikos Gulf inthe central part of the Messogia Plain. A reverse transport (northerly flow) atthe south opening is observed for pattern C, because the stronger opposing,northerly background flow does not permit the east part of the sea breeze ofSaronikos Gulf to penetrate into the Messogia Plain. The northerly, moderateto strong wind dominates for the IVE and V synoptic types (Dl, D2 and Esurface flow patterns) and the possible advection of air masses over theSaronikos Gulf could be realised from suppression of the west coast breeze atEKT. Generally, for these flow types, the wind is north at both the MessogiaPlain and the Athens Basin, therefore there is no communication between thetwo areas.

Acknowledgements

The authors wish to thank the "Athens Airport S.A." for financial support.

References

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2. Asimakopoulos, D.N., Helmis, C.G., Petrakis, M. & Tombrou M.Atmospheric Boundary Layer field measurements over coastal areas,Environmental Software, 1993, 8, 19-27.

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4. Deligiorgi, D.G., Helmis, C.G., Asimakopoulos, D.N. & Lalas, D.P.Quantitative acoustic sounding from the top of a steep mountain, Int.Journal of Remote Sensing, 1989, 9, 211-226.


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5. Helmis, C.G., Asimakopoulos, D.N., Deligiorgi, D.G. & Lalas, D.P.Observations of the sea-breeze front structure near the shoreline,Boundary Layer Meteorology, 1987, 38, 395-410.

6. Helmis, C.G., Deligiorgi, D.G., Asimakopoulos, D.N. & Lalas, D.P. Evidenceof the sea breeze flow at the top of Hymettos mountain, Int. Journal ofRemote Sensing, 1994,15, 479-487.

7. Helmis, C.G., Papadopoulos, K.H., Kalogiros, J.A., Soilemes, A.T. &Asimakopoulos, D.N. The influence of the background flow on theevolution of the Saronic Gulf sea breeze, Atmospheric Environment, 1995,accepted for publication.

8. Lalas, D.P., Asimakopoulos, D.N., Deligiorgi, D.G. & Helmis C.G. Sea breezecirculation and photochemical pollution in Athens, Greece, AtmosphericEnvironment, 1983,17,1621-1631.

9. Lalas, D.P., Tombrou-Tsella, M., Petrakis, M., Asimakopoulos, D.N. &Helmis, C.G. An experimental study of horizontal and vertical distributionof ozone over Athens, Atmospheric Environment, 1987,12, 2681-2693.

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