Athens, Greece - WIT Press€¦ · Environment, National Observatory of Athens, GR-11810 Athens, Greece Abstract According to past experience, the nearly stagnant conditions caused

Features of the Athens Basin wind flow in

view of recent experimental work

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

J.A. Kalogiros,* A.T. Soilemes," M. Petrakis

"Department of Applied Physics, University of Athens,

33 Ippokratous Street, GR-10680 Athens, Greece

Institute of Meteorology and Physics of the Atmospheric

Environment, National Observatory of Athens, GR-11810

Athens, Greece

Abstract

According to past experience, the nearly stagnant conditions caused by thepresumed equilibrium between the Saronikos Gulf sea breeze and an opposingsynoptic flow is identified as the principal mechanism leading to highpollution episodes in Athens during the summer. Previous experimental workcould not answer several questions related to air mass flow over the"transitional" areas of the Athens Basin such as the offshore, the coastalregions and the natural openings. In this context, recent experimental workfocuses on the inland propagation of the southerly sea breeze from the coastto the northern part of the basin mainly under moderate northerlybackground winds. An experimental campaign was designed to cover the basickey-locations revealing the wind flow over the Athens Basin. With the aid ofa network of meteorological stations, two tethered balloons and a high rangeacoustic sounder operated over a two months summer period it is attemptedto address some features of the wind flow over the Athens Basin. Resultsfrom this experimental campaign are presented and discussed.

1 Introduction

The Athens Metropolitan Area (AMA) is a region of well-known air pollutionproblems caused by the concentration of industrial, transport and serviceactivities in an area of 450 krn inhabited by nearly 4 million people. Themain residential and service activities area is found in the Athens Basin(Figure 1), a coastal valley washed by the Saronikos Gulf to the south. TheAthens Basin is surrounded by the Hymettos Mt to the east, the Penteli andParnitha Mts to the north and the Egaleo Mt to its west. The main pollutantsource in the city centre is automobile traffic, while the west part of thebasin is significantly affected by the industrial emissions.

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

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In such an area of complex topography, featuring sea to land, urban torural and mountain to plain transitions, the contribution of local flows to theobserved wind and thermal field is significant and actually dominates underweak synoptic pressure gradient The numerical and observational studiesconcerning air quality and atmospheric transport and diffusion mechanisms inAMA expectedly dealt with such periods of weak synoptic flow. Specifically,the pollutants' advection downtown Athens by the southerly Saronikos Gulfsea breeze and the transport from the Triassion Plain to the city centre aremain factors of the air quality problem. In order to understand the differentmechanisms of pollutant transport and advection, a number of fieldcampaigns were organised in the recent years in the AMA. The first campaignby Lalas et aL [3] dealt with the Saronikos Gulf sea breeze circulation andprovided strong evidence that air pollutants recirculate in the daily cycle ofsea and land breeze, enhancing the next day's pollution levels. The secondcampaign by Lalas et aL [4] addressed the issue of the horizontal and verticaldistribution of ozone over Athens and confirmed that ozone generated overthe sea, significantly enhancing the ground concentrations at the coastal areas,but slightly at the city centre. The third campaign by Asimakopoulos et aL[1] studied the transport mechanisms from the industrial area of ThriassionPlain to the city of Athens using tracer gas and tetroon tracking techniques.

160.0-

140.0

120.0-

100.0-

80.0-

60.0-

40.0-

20.0-

20.0 40.0 60.0 80.0 100.0 120.0 140.0 160.0 kill

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


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Two advection paths were identified, the straightforward one through thenorthern opening between Mts Egaleo and Parnitha and the second onethrough the Daphni opening of Egaleo Mt (see Figure 1).

In the course of the South European Cycles of Air Pollution (SECAP)program, an experimental campaign was designed to cover the basic key-locations determining the surface wind flow over the Athens Basin in relationto the crucial questions that have been appraised during past experimentaland theoretical studies. Previous experimental work, as outlined in thepreceding paragraphs, could not answer several questions related to air massflows over the transitional areas of the Athens Basin such as the offshore, thecoastal regions and the topographic openings of potential ventilation of thebasin. The experimental conditions and period were such as to allow theanalysis of the wind flow in the Athens Basin during the expected stagnantconditions caused by the presumed equilibrium between the Saronikos Gulfsea breeze and an opposing synoptic flow [2]. This mechanism is one of thetwo principal mechanisms leading to high pollution episodes in Athens andthe major one during the warm period. According to this process, theprevalence of the local summer northerly winds (etesians) would delay orinhibit the onset of the southerly Saronikos Gulf sea breeze. The latter israther insignificant since the domination of the moderate synoptic flowwould improve air quality over the Athens Basin. The main objective of thepresent study is to assess the features of the former case. In particular, thestudy focuses on the inland propagation of the southerly sea breeze against amoderate offshore wind. It is worth mentioning that vertical profiles of thesea breeze over the Saronikos Gulf are presented, the first ones ever obtainedin this area.

2 Experimental setup

Measurements span the period from 18/6/93 to 29/7/93. The surfacemeteorological stations installed are given in Table 1, and Figure 1 depictstheir exact locations. Data were stored as means and standard deviations over10 min intervals. In addition to the surface stations the following systemswere used:• Two tethered meteorological profiler systems each consisting of a 4.3 m^balloon and an instrumented measuring package for measuring profiles ofwind speed, direction, temperature and humidity in the lowest 800 m of theatmospheric boundary layer over selected periods [8]. The periods of interestwere those with a prevailing northerly, offshore wind (the warm periodetesians' wind regime).

Table 1: Experimental layoutStation

Code NameGARHOEEKT

OperationalPeriod

18/6 - 28/723/6 - 29/724/6 - 29/7

Recorded Parameters

U, DIR, TU, DIR, T

U, DIR, T, Humidity

• A monostatic acoustic sounder at the National Observatory of Athens(NOA) for continuously monitoring the thermal structure of the atmospheric


4 Urban Pollution

boundary layer and the vertical component of the wind covering heights upto 800 m above the ground.

The above data were supplemented by the hourly means of barometricpressure, air temperature, humidity, wind speed and direction measured bythe permanent surface meteorological station operated by the Institute ofMeteorology and Physics of the Atmospheric Environment of the NationalObservatory of Athens (NOA) on a small hill (110 m high) in the centre ofAthens, close to the acoustic sounder antenna. The two balloon systemswere flown on July, 27 at NOA and in the sea area northeasterly of Aeginaisland from a small sea vessel.

3 General features of the wind flow during the experimental period

The synoptic conditions were typical of the season studied with N-NE winds(the 'etesians') for more than half of the time period of the experiment. Themain points drawn from the frequency distribution of the wind direction forevery surface station are the following:• During the night there is a western shift of the northerly flow at GARstation, probably in response to the thermal contrast between the cool inlandregion and the air masses affected by the presence of the Southern EvoikosGulf to the east or due to adjustment of the surface flow to the synopticflow.• At HOE station, which is the most windy location, the wind directiondistribution is essentially the same between day and night. Moreover, the twomodes of the distribution lie along the major axis of the Athens Basin,confirming the well known fact that air flow in the interior of the AthensBasin is channeled.• At the coast (EKT), the high frequency of the S-SW directions reflects theoften occurrence of the Saronikos Gulf sea breeze. The persistence ofsoutherly directions during night reflects the observations of the sea breezeblowing until early night (sometimes even until around midnight).• In the centre of Athens (NOA), the high prevalence of northerly winds inconjunction with the reduced frequency of southerly flows in the daytimeshows the effect of an opposing flow on the inland propagation of the seabreeze developed at the shoreline. The nocturnal southerly directions areexplained as in the case of the coastal station.• The comparison of the peaks of the day-time southerly directions at EKTand NOA stations yielded the rough estimate that about less than half of thesea breezes reach the centre of Athens during the experimental period, whichis characterised by an offshore synoptic flow.

4 The Saronikos Gulf sea breeze

As noted in Section 1, the present experimental period is interesting in that itoffers the opportunity of observing sea breeze development under moderateot strong offshore wind. Table 2 summarizes all sea breeze days at EKT, theirduration, maximum intensity, maximum water vapour mixing ratio andadditionally gives information on the larger scale flow and the time of arrival(if existing) of the sea breeze flow at NOA. The HOE measurements in thetime period 1000-1400 LST are taken to be representative of the large scale


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flow. For reference, the 0000 and 1200 GMT radiosonde wind data at 700hPafrom the Hellenic Meteorological Service (HMS) station at Hellinikon nearEKT are included. The link of southerly winds at HOE with NW winds aloftdue to the pressure driven channeling is demonstrated. The followingconclusions are drawn:• On 24 out of 34 days the sea breeze managed to develop at the coast withmaximum intensity ranging between 2.1 and 6.1 ms~* and water vapour mixingratio 9-18 gkg"\ Under the conditions encountered during the experimentalcampaign, a typical sea breeze at the coast reaches a maximum surface windspeed of 4 ms", carrying 13 gkg~* of water vapour. The maximum intensity isattained at around 1700-1800 LST slightly later than for the pure (under weaksynoptic flow) sea breeze case.

Table 2: Characteristics of the Saronikos Sea Breeze flow (wind speedin ms * and wind direction in degrees)

EKT

Date

25/626/628/629/630/61/73/74/75/76/77/711/712/713/715/716/717/720/721/722/726/727/728/7

Duration(LST)

11-1214-1511-2211-219-2312-208-208-2014-1710-2210-1812-0111-2017-1911-2312-2314-1615-1710-2311-2016-1811-2212-23

STATIC

Max.windspeed

2.72.15.85.85.44.13.93.73.84.73.24.03.73.55.14.03.72.76.14.82.95.14.5

)N

Max.mixingratio(gkg"')

131010121516121613151813141191213141616131414

NOA STV

Duration(LST)

11-24

11-2310-2315-17

14-02

13-2015-2416-1817-1814-24

17-1911-2214-23

ITION

Max.windspeed

5.8

4.74.45.3

5.0

5.24.74.34.25.3

4.35.24.7

HOEwindspeed /direction

8/305/404/2005/2003/2207/507/207/203/1807/2109/306/2004/2206/507/309/2010/109/306/21012/308/309/30

Noctand

radiosoispee(

13/3156/27021/31518/270

15/33514/35517/35510/3158/33510/33520/2709/3156/3158/3554/3552/255/2704/458/3559/335

urnalnoonide data1/dir

7/31514/29522/29512/315

17/33514/35517/33514/27018/35512/27016/2709/3159/3558/3559/3555/35511/2701/35510/3559/335

• The sea breeze developed at the coast even at a 12 ms" surface offshorewind at HOE. Inspection of the days when no sea breeze was formed at thecoast has shown that the surface winds measured at HOE always exceeded 7ms~* and was from 340°-30°.• There is a clear trend of the flow onset at EKT to be delayed as theoffshore wind strengthens (Figure 2a). Figure 2b shows that when the seabreeze develops in the afternoon it endures for only 2-4 hours, otherwise itlasts for 12-14 hours irrespective of the offshore wind intensity. Also, the


Urban Pollution

surface flow at HOE seems to be more relevant to the determination of thepossibility of the sea breeze development than the synoptic flow taken at700 hPa.

14 —

_ 12-

0 10-

•o1

6-

(a)

0

0 0 0D D

0 0 0

' 1 ' 1 ' 1 ' 1 ' 1 'Time

of on

set a

t EKT

(LST) 16-

14-

12-

10-

8-

(b)

a

O D D

0 00 D

0

0

I I 1 I I I 1 I 1 I 1 I 1 I 110 12 14 16 18 10 12 14 16

time of onset at EKT (1ST) Duration at EKT (rrs)Figure 2: Time of sea breeze (a) onset at EKT as a function of the mean0600-1000 LST wind speed at HOE and (b) duration at EKT as a function ofthe onset time for the cases with background (synoptic) northerly surfaceflow at HOE.

• The cases with southerly flow at HOE should be distinguished according towhether they result from a pressure driven channeling effect or from a trulysoutherly geostrophic flow. Only 30/6 and 6/7 fall into the second category.Both of them were early in developing sea breezes especially the 30/6 case,where the onshore flow was stronger than 6/7. The same did not hold true forthe "channeled cases", although a westerly geostrophic wind field would tendto enhance the developing mesoscale pressure gradient and the sea breezecirculation.• The change of air temperature and vapour content at the onset of the seabreeze at the coast is presented as a function of time of onset in Figure 3.When the sea breeze developed in early morning, the temperature drop wassmall, because the negative horizontal advection of heat was compensated bythe large warming rate due to insolation in comparison with the noon hours,where the temperature drop was significant. A southerly synoptic flow ("S"-marked points in the diagram) before the onset of the sea breeze had theeffect of advecting cool, moist air over land, thus minimizing thermal andhumidity changes when the sea breeze developed. When the sea breezedeveloped around 1100-1200 LST despite strong opposing synoptic wind (the"H"-marked points in Figure 3a), the warm land masses, that were presumablyadvected earlier over the sea, subsequently return with the sea breezecirculation leading to minor temperature changes, but significant watervapour increase.• Concerning the flow observed at NOA, Table 2 shows that in the presenceof an opposing flow only in 2 cases out of 15 did the sea breeze not reach thestation. However, on both these days the sea breeze lasted for only two hoursat EKT. The sea breeze arrival at NOA was marked by the directional shift, ashort interval of stabilized temperature and no humidity changes, at leastnear the surface. Anyway, with surface station data it is difficult to identify


Urban Pollution

a sea breeze event atwhen southerly windswind and temperature

•i

Temperature change at

onset (*C

)

en .i* c

o NJ -

o

-1

,1

,1

,1

,1

,1

, (a) ,H" S S,§o"D D D Dg

S D SD DD

least at NOAprevail The iwill be discus

s0D

0

-andnodifsed i

i0>0too>O)cCDO.02D>C'E

probably at other inland stationsication of the vertical profiles on the next section. The maximunQ

8-

7-

6-

5-

4-

3-

2-

1-

n

(b)0

^ DH

Sn D p

" I"S ° DDS S S' — • — i 1 B B ra i , , , , ,

10 12 16 18 12 14 16 18Time of onset at EKT (LST) Time of onset at EKT (1ST)

Figure 3: The change of air (a) temperature and (b) water vapour mixingratio after the onset of sea breeze at EKT. The points marked by "S" denotecases with southerly surface flow at HOE during 0600-1000 LST. The "H"cases refer to strong offshore flow at HOE station.

wind speeds measured at NOA are comparable if not greater than at thecoast, but this could be partly attributed to the speed-up effect of the hillwhere the NOA station operates. Also, there is usually a drop in the intensityof the opposing flow during the hour before the arrival of the sea breeze.

The next point that is important to be resolved is the possibility of theSaronikos Sea breeze to reach HOE or GAR. Prezerakos [6] studied pure seabreeze days (weak synoptic conditions) over the Athens Basin using surfacedata over a 25-year period. Among the meteorological surface stations heused was the one at Tatoi Airport, approximately 2-3 kms NW of HOE. ForJuly he found that when the Saronikos sea breeze had developed at the coast,it was observed at Tatoi between 1100-1700 LST and 1400-1700 LST withprobabilities 17% and 19%, respectively. In 55% of the cases it wasintermittent as concluded by 3-hours observations. Those frequencies ofoccurrence imply that we should be able to identify some sea breezes at HOEas well, although the period selected was windy with NE offshore wind.

The analysis excluded the days with southerly flow and concentrated onthe days when the sea breeze flow lasted for more than 3 hours. Three cases(28/6, 3/7 and 27/7) were found with some evidence of sea breeze arrival atHOE; on all of them the flow passed NOA at 1100 LST. On the 28/6, thesurface early morning flow was from NE blowing at 5 ms~* and on the 3/7 itwas from the same direction at 7 ms~*. As seen in Table 2, the sea breezedeveloped early at EKT, but the relatively strong opposing flow significantlydelayed (by 3 hours) its arrival at NOA as compared to 30/6 when it arrivedwithin 30 min. As a result, it reached HOE at 1300 LST on 28/6 and at 1800LST on 3/7. The time evolution on 28/6 is shown in Figure 4. The 27/7 caseis analysed in section 5. Figure 4 shows that prior to the arrival of theflow at HOE the wind was blowing from 60° at 5 ms~\ then the velocity


Urban Pollution

28/6/9330

4:00 8:00 12:00 16:00 20:00 0:00 4:00

I6 -

"8 . ,<D 4 -Q.T3

I 2^

(b)

4:00 8:00 12:00 16:00 20:00 0:00 4:00

4:00 8:00 0:00 4:0012:00 16:00 20:00Time (LSI)

Figure 4: Time series of (a) temperature and water vapour mixing ratio (onlyat EKT), (b) wind speed and (c) direction at the surface stations for 28/6/93.

dropped to 3 ms~*, the direction gradually shifted to 220° and thetemperature dropped by 1°C. It is not clear if the wind regime after 1700 LSTwas still affected by the sea breeze. An interesting feature of the figure is thebacking at EKT and veering at HOE. Similar patterns are seen in the 3/7 case(not shown).

On the other hand, the GAR station is not affected by the Saronikos seabreeze, except for indication for an early afternoon arrival on 27/7, which isdiscussed in section 5. Therefore, under the experimental conditions, it seemsthat the opening between the Hymettos and Penteli Mts is not affected bythe N-S thermal contrast during daytime.


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5 Analysis of a case study (27/7/93)

On the 27th of July 1993 the synoptic conditions over Athens Basin induced aweak northwest flow according to the nocturnal radionsonde at Hellinikon(not shown here) that was also apparent in the early morning surface winddirection at EKT, GAR and HOE (Figures 5b and 5c). The coastal windbacked slowly to south directions, reaching 210° at 1200 LST indicating theonset of the Saronikos sea breeze flow. According to the noon radiosondeprofiles (also not shown here) the sea breeze flow was confined below 1000 mabove which the synoptic northwest flow persisted.

27/7/93

~~i i—i—i—i—i—i—i—i—i—|—i—rkOO 8:00 12:00 16:00 20:00 4:00

i

1

-o

~i 1 1 1 1 1 1 1 1 1 1 1 1 1 1 r4:00 8:00 12:00 16:00 20:00 0:00 4:00

;360 -

270 -

180 -

90 -

0 -l I I | I I I | I I I | I I | | |

4:00 8:00 12:00 16:00 20:00Time (LST)

0:00 4:00

Figure 5: As in Figure 4 for 27/7.793.


10 Urban Pollution

Due to the sea breeze cool air advection, the EKT temperature did notrise in early morning (in contrast to the inland GAR and HOE stations) andthe water vapour mixing ratio was as high as 14 gkg (Figure 5a). The winddirection at the rest surface stations changed to south directions with a timedelay relative to the coast. Thus, it seems that the sea breeze arrived at NOA,GAR, and HOE at about 1130, 1430, 1900 LST, lasting until 2100, 2000 and2100 LST, respectively. Actually, the GAR wind direction data snowed thatthe sea breeze endured for only half an hour (1430-1500 LST: south tosouthwest wind directions) and after that time period it apparently interactedwith the Evoikos Gulf sea breeze (southeast wind directions). The arrival ofthe Saronikos sea breeze at the surface stations was combined with anincrease of wind speed up to 6 ms~* at GAR, 5 ms~* at NOA, and a delay ofthe fall off of wind speed at the afternoon (4 ms") at HOE, a stabilization ofthe temperature at NOA, and a temperature decrease at GAR (from 32 to 30°C) and HOE (rapid fall). After 1500 LST the intensity of the sea breeze flowat the coast increased (5 ms~* at 1800 LST). Later, the wind there shifted toeast directions and declined after 2200 LST, indicating the destruction of theflow.

(a) wind speed (m/s) (b) wind direction (deg)900

10 12 14 16 18Time (LST)

(c) potential temperature (*K)

20 10 12 14 16 18 20Time (LST)

(d) water vapour mixing ratio (g/kg)

8 10 12 14 16Time (LST)

12 14 16Time (LST)

18 20

Figure 6: Time-height plot of (a) wind speed, (b) wind direction, (c) potentialtemperature and (d) water vapour mixing ratio obtained by the balloonflights at NOA on 27/7/93.


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Figure 7: The facsimile record by the acoustic sounder operating at NOA on27/7/93 during (a) 0900-1221 LST and (b) 1544-1905 LST.

2.00.0-2.0

2.00.0-2.0

42 2.0& 0.02> -2-0 -I

2.00.0-2.0

2.00.0-2.0

27/7/93 323m

- 2.00.0

- -2.0

187 m

85 m

51m

downdraft ^ updraft

10:30 10:42 10:54 11:06 11:18

2.00.0-2.0

2.00.0-2.0

2.00.0-2.0

11:30Time (LST)

Figure 8: The vertical velocity data by the acoustic sounder operating at NOAon 27/7/93 for the time period 1030 to 1130 LST (the sea breeze front passedby the station at 1100 LST).


12 Urban Pollution

The evolution of the sea breeze was also depicted in the rawinsonde dataobtained from the balloon flights at NOA station (Figures 6a-6d). The earlymorning surface temperature inversion was slowly destroyed from below andthe wind aloft was northerly. The sea breeze arrival at about 1100 LST wasevident in the wind direction (180-220°) along with a low level jet of 6 ms~*below 100 m (it is noted that the NOA station is on the top of an 110 m highhill). The inflow of cool air caused the formation of a stable layer above aconvective 50 m deep layer. The sea breeze cell extended up to 650 m atmidday (Figures 6a-6b). The water vapour mixing ratio (Figure 6c) did notseem to increase significantly with the arrival of the sea breeze (probablybecause of the 6 km inland penetration of the sea air and the correspondingdrying-out) until late afternoon (1900 LST), when the thermal structure ofthe atmosphere became almost neutral.

The sea breeze front arrival (exactly at 1100 LST) was evident in thefacsimile record of the acoustic sounder as a thermal structure extending upto 400 m (indicated by an arrow in Figure 7a). The vertical velocity

(a)600-

500-

400-

300-

200-

100-

(

700 -T

600-

500-

400-

300-

200-

100-

C

offshore"'•--.._

D 1 2 3 4 Swind speed (m/s)

" /

90 180 270 360wind direction (deg)

600-

500-

400-

300-

200-

100-

?nn -T

600-

500-

400-

300-

200-

100-

n

city centre

(_..£ \

3 1 2 3 4 5 6 7wind speed (m/s)

nonn nnnn , OTUouu-ujuu LO i city centre1000-1 100 LST /

-- 1300-1400 LST

90 180 270 360wind direction (deg)

Figure 9: The rawinsonde data: (a) wind speed and (b) wind direction obtainedby the balloon flights at the open sea (offshore), near Aegina island, and thecorresponding inland (city centre) ones on 27/7/93 during the time periods0800-0900 LST, 1000-1100 LST and 1300-1400 LST.


Urban Pollution 13

measurements by the acoustic sounder showed a downdraft (1.5 ms~*) justbefore the arrival of the sea breeze front and an extended updraft(2.5 ms") just after (Figure 8), like a gravity current as described bySimpson et aL [7]. The plumes' activity changed to a surface shear layerafter 1500 LST (the convective surface layer was suppressed) accordingto the facsimile record in Figure 7b. As vertical mixing presumably ceasedafter sunset, an elevated echo layer remained distinguishing the upperboundary of the formerly inflow region from the large scale structure.

Figures 9a-9d includes vertical profiles over the city centre (NOA) andoffshore (see Figure 1) for about the same time period. In early morning(0800-0900 LST), a 200 m deep, stable surface layer of NW winds was foundover the city, featuring a low-level jet, while northeast winds prevailed aloftin a less stable layer. On the other hand, over the sea, the wind profile wasuniform from northerly directions. The air temperatures over the two areasapproached each other at around 600 m. Later, in mid-morning (1000-1100LST), the near-surface stability over the city was eroded from the thermalactivity and the layer of NW winds became deeper

700

600-

500-

g 400-

f 300-

200-

100-

0

offshore (c) 700

600-

500-

g 400-g>® 300-

200-

100-

0-

city centre

297

700

299 301 303 305potential temperature (°K)

299

600-

500-

g 400-JC.o>j! 300-

200-

100-

offshore (d) 700

300 301 302 303potential temperature (°K)

304

—i—i—i—i—r4 6 8 10 12 14water vapour mixing ratio (g/kg)

600-

500-

g 400o>J 300

200-

100-

0

city centre

6 7 8 9 10water vapour mixing ratio (g/kg)

Figure 9 (continue): The rawinsonde data: (c) potential temperature, and (d)water vapour mixing ratio obtained by the balloon flights at the open sea andthe city centre.


14 Urban Pollution

and well-mixed. A shallow sea breeze flow (below 100 m) had developedoffshore. In the early afternoon (1300-1400 LST), the sea breeze circulationcovered 600 m and at least 500 m offshore and over the city, respectively. Aninteresting feature was the drying and warming aloft over the sea relativelyto the city. This could be the effect of subsidence over the sea, caused by theremoval of cool air by the sea breeze.

According to the above, the balloon flights at the open sea reveal adifferent sea breeze cell which seems to dominate over synoptic flow after1300 LST with a wind shift to south directions up to 600 m and a low leveljet (below 100 m). Thus, the onset of the sea breeze flow observed earlier atthe EKT station seems to correspond to a small local sea breeze cell (like theones observed by Physick & Byron-Scott [5] in south Australia), while theregional (Saronikos Gulf) cell is delayed and possibly correspond to anincrease of the wind speed at EKT and a change in the thermal structure atNOA after 1500 LST (see Figures 6c and 8b, respectively).

6 Conclusions

The main objective of the oresent study was to identify the features of thedevelopment of the Saronikos Gulf sea breeze against an opposing (offshore)synoptic flow as well as important topographic and thermal effects on theevolution of sea breeze circulation.

It is seen that for 70% of the experimental days the sea breeze developedat the coast and it is significantly delayed by the strongest offshore windcases. In such cases, the frontal characteristics are enhanced. A typical seabreeze peaks at around 1700-1800 LST at the coast When the sea breezedevelopes in early morning, the temperature drop is small, because thenegative horizontal advection of heat is compensated by the large warmingrate due to insolation in comparison with the noon hours, where thetemperature drop is significant. A southerly synoptic flow before the onset ofthe sea breeze has the effect of advecting cool, moist air over land, thusminimizing thermal and humidity changes when the sea breeze developes.When the sea breeze developes around 1100-1200 LST despite strong opposingsynoptic wind, the warm land masses, that were advected earlier over the sea,subsequently return with the sea breeze circulation leading to minortemperature changes, but significant water vapour increase. Also, when thesea breeze develops in the afternoon it endures for only 2-4 hours, otherwiseit lasts for 12-14 hours irrespective of the offshore wind intensity.

Roughly, half of the sea breezes reach the centre of Athens city and theyusually persist until midnight. The onset of the sea breeze is marked by adirectional shift, a short interval of constant air temperature and no humiditychanges, at least near the surface. The sea breeze direction at the coast isobserved to continuously back in the course of the day, whereas in the centreof Athens it blows from the south-southwest directions. Ten percent of theobserved cases at the coast reach the northern part of the Athens Basin in thelate afternoon. Also, it seems that the opening between the Hymettos andPenteli Mts is not affected by the N-S thermal contrast during daytime underthe certain experimental conditions (etesians with N-NE wind direction).Finally, the mechanism of pressure-driven channeling is proposed to beimportant in producing low level southerly flows inside the basin in the


Urban Pollution 15

presence of west-northwest synoptic flow. This situation complicates thedepiction of sea breeze flows in the city centre.

The balloon flights at the open sea reveal a regional sea breeze cell whichseems to dominate over synoptic flow after noon, while the onset of the seabreeze flow observed earlier at the coast corresponds to a small local seabreeze cell

References

1. Asimakopoulos, D.N., Deligiorgi, D.G., Drakopoulos, C, Helmis, C.G.,Kokkori, K., Lalas, DP., Sikiotis, D. & Varotsos, C An experimental studyof nighttime air-pollutant transport over complex terrain in Athens,Atmospheric Environment 1992, 26B, No 1, 59-71.

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4. 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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Documents

Athens, Greece - WIT Press€¦ · Environment, National Observatory of Athens, GR-11810 Athens, Greece Abstract According to past experience, the nearly stagnant conditions caused