ELSEVIER Atmospheric Research 36 ( 1995 ) 139-156

, \ ' F .X I ( )NPI tf£1~.1( ' I~ .E ,~E. \ I~ . ( ' f t

An environmental experiment over Athens urban area under sea breeze condit ions

H . D . K a m b e z i d i s a, A.A. P e p p e s a, D. Me las b

aAtmospheric Research Team, Institute of Meteorology and Physics of the Atmospheric Environment. National Observatory of Athens, P.O. Box 20048, GR-118 10 Athens, Greece.

OLaboratory of Atmospheric Physics, Aristotle University of Thessaloniki, GR- 540 06 Thessaloniki, Greece.

Received 6 August, 1993; revised and accepted 19 March, 1994

Abstract

Instrumental and technical details as well as results from ATHens Internal Boundary Layer EXperiment (ATHIBLEX) are presented in this paper. This experiment was per- formed during June 1989 and June-July 1990. The aim of ATHIBLEX was to study the IBL formation over Athens urban area under sea breeze conditions and its association with the city environmental problems. The results show a well established IBL over Athens basin. A slab model is adopted to estimate IBL heights at positions where no measure- ments were taken. The orograhic effect of Athens basin on the wind profiles is also dem- onstrated, u'-, v'- and w'-power spectral densities from a uvw propeller anemometer in- stalled 4500 m downwind of the shoreline on the top of a hill 107 m a.m.s.1, exhibit the - 5 / 3 law.

I. Introduction

The most frequent case that an Internal Boundary Layer ( IBL) is formed within the Atmospher ic Boundary Layer is when a wind blows over the interface be- tween two surfaces having different roughness lengths i.e. z0~ and z02. The case considered here is for z0, << Zo2. An IBL may be of mechanical (synoptic weather s i tuat ion) a n d / o r thermal origin ( tempera ture difference between the two sur- faces). In the second case the IBL is called TIBL (Thermal IBL) . Since summer sea breezes in Athens are generated because o f the sea / land tempera ture differ- ence, convect ive condit ions are supposed and the IBL's are TIBL's.

IBL studies have been carried out by many researchers to mainly investigate: ( i) the air pollutant t ransport through the IBL mechanism (Van der Hoven, 1967;

0169-8095/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0169-8095 ( 94 )00031-8

140 H.D. Kambezidis et al. / Atmospheric Research 36 (1995) 139-156

Peters, 1975; Lyons, 1977; Mishra, 1980; Kerman et al., 1982), ( i i ) the influence of IBL on the fatigue of a wind turbine installed near the roughness change ( Smedman and HiSgstrSm, 1983; Bergstr6m et al., 1988 ) and (iii) the wind mod- ification (Sundborg, 1950; Bornstein, 1968; Oke and East, 1971; H6gstr6m et al., 1978). For the above reasons modelling of the IBL characteristics has been of interest (Elliot, 1958; Van der Hoven, 1967; Plate, 1971; Raynor et al., 1975; Peters, 1975; Jackson, 1976; Weissman, 1976; Lyons, 1977; Venkatram, 1977; Anthes, 1978; Panofsky and Dutton, 1984; Papageorgiou, 1988; Melas and Kam- bezidis, 1992 ). A review paper about IBL has been presented by Garratt (1990).

Though IBL experiments over towns have been performed, it is unusual and difficult to attempt an experiment over a city of Athens size with sloping terrain downwind. Therefore this study is of interest for the non-uniform and non-fiat terrain implicated. Furthermore ATHIBLEX project sheds light on IBL associa- tion with the known environmental problems of Athens.

2. E x p e r i m e n t a l detai l s - data co l lect ion

Fig. 1 shows the basin in which ATHIBLEX was performed during June 1989 and June-July 1990. The aim of the project was to investigate the influence of the sea breeze circulation, which causes IBL stratification over Athens, on the transport and dispersion of air pollutants, primarily that of 03, from the Saroni- kos Gulf inland. The project was sponsored by the General Secretariat of Re- search and Technology in Greece which belongs to the Ministry of Industry, En- ergy and Technology.

It is established that whenever a L (ow) stagnates over south-east Asia Minor, the probability of sea breeze circulation is very high over Athens basin from the Saronikos Gulf. It is also known from local studies in the past (Lalas et al., 1983a, b) that sea breezes are responsible for air pollution episodes over Athens basin for two reasons: (i) they carry 03 from the Saronikos Gulf which is transported out there by the land breeze and (ii) they favour episodes formation because of the light wind associated with them (Catsoulis, 1983; Prezerakos, 1986 ) and the temperature inversions (Tselepidaki et al., 1983).

Athens climate is Mediterranean with hot summers and mild winters. The av- erage daily air temperature during the winter (December, January, February) is 9.9 °C and during the summer (June, July, August) 25.8 °C. The mean daily solar radiation flux on horizontal surface reaches 22 MJ m -2 in the summer months and 8 MJ m-2 in the winter months. A total number of 43 days has been registered at the National Observatory of Athens (NOA) in the period 1957- 1980 as sea breeze days blowing from early morning to late afternoon and only 12 days have been classified as interrupted sea breeze ones. In the first case, most of the sea breeze occurrences were in May, while in the second in August.

Fig. 2 shows the 3D topography of the area within which the experiments took place. It is seen that there is an average upslope of 15% from the shoreline. NOA is situated on the hill of Nymphs ( 107 m a.m.s.1. ) near the city centre. The loca-

H.D. Kambezidis et al. / Atmospheric Research 36 (1995) 139-156 141

N (

'%

-- *q53 . : -~ M

', '-~ l~!lll i,~,~...,'i: '~

. . . . . !i~v: ~ : l ~ - - - . d l l [ . 1! .t

. . . . . . . . . . . . . . . . . . . . . XELLIIflM ................... AIRPOILT-

I l ' I l 1

~; ~l l i l l ! ,

/ i . i ~',,', ,

t .i taykterTos' II J : . ~

, /~o~. / i / / ' /

: i " /

I (;'s / j

0 r /

i l l l l l l l '~ L-, '-~,' GULF 4 m r . , 0

Fig. 1. Map of Athens basin showing instrumental positions (I) AVEROF battleship, (1 ') lighthouse of Flisvos marina, (2) shoreline at Trokantero location and (3) NOA experimental site on the hill of Pnyx. 100 m (dashed line), 200 m (continuous line) and 600 m (dash-dotted line) contours are also shown.

142 H.D. Kambezidis et al. / Atmospheric Research 36 (1995) 139-156

k.

t~

e,

Fig. 2. A 3D topographical view of the experimental area shown in the rectangle of Fig. 1. Positions I, I', 2 and 3 are also indicated.

tion at NOA site where measurements were obtained is shown as (position) 3 in the Figure. This is the hill of Pnyx next to the hill of Nymphs with the same height; it lays 500 m away from the Acropolis. Tethered balloon flights and uvw

measurements were carried out at this location. At the shoreline an equipped meteorological telescopic mast was installed (position 2) for measuring mainly temperature and wind at four levels. Further into the sea there was an anemo- graph to measure the undisturbed conditions of the approaching flow. Table 1 gives all the instrumental details for ATHIBLEX. Note that this anemograph was installed at the prow of the First World War battleship AVEROF (now unarmed, used as a museum) standing 4 m out of it during 1989 experiments (position 1 in Figs. 1 and 2). For technical reasons, the same anemograph was placed at the top of a lighthouse 200 m away from AVEROF at the edge of a long quay during 1990 experiments; the instrument was again standing 4 m out of the lighthouse. In both cases the anemograph was clear of any obstructions in all possible sea breeze directions. All instruments used in both years were brand new except the

H.D. Kambezidis et al. / A tmospheric Research 36 (1995) 139-156 143

Table 1 Instrumental details during ATHIBLEX. All temperature and humidity sensors were in radiation shields

Position Distance ~ from Height m (m) Measurable Instrument Instrumental in Fig. 2 sea/land manufacture error

interface (m) and t y p e

m - + o + o 1 -300(1989) 7.0 Uh a, w d b MRI g 1082 _ 2 ~/o,_ I

l ' -500(1990) 10.0 ibid. ibid. ibid. 2 0 3.1, 5.8, 11.9,

19.5 (1989) /~h a VIS A100R _+2% 2.9, 7.4, 13.3,

22.1 (1990) ibid. ibid. ibid. 19.5 (1989) wd b VI k W200P _+ 1 ° 22.1 (1990) ibid. ibid. ibid.

2.9, 19.3 T c, R H d ROTRONIC _+0.1 °C (1989)

MP100 -+ 1% 2.6, 21.9 ibid. ibid. ibid.

(1990) 5.7, 11.7 7 ~c TChCE4 -+0.01 °C

(1989) 7.1, 13.0 ibid. ibid. ibid.

(1990) - 0 . 2 (1990) 7~w e CSLq07 -+0.2 °C

m a x

3 4500 variable up ~ AIR,Inc i -+ 0.25 ms- t o 1000 m wd b ibid. _+ 5.00 °

T ~ ibid. _+0.50 °C T 'e ibid. _+0.50 °C

4500 -0 .05 7~L CSL 107 _+0.2 °C m a x

4500 13.0 a' , v', w 'r YOUNG 27005 _+ 1%

acup anemometer, bpotentiometer wind vane, Cplatinum resistance thermometer, dhumicap sensor, ethermistor thermometer, fpropeller (Gill) uvw anemometer, gMeteorology Research, Inc., hZC Ltd., iCampbell Scientific Ltd., JAtmospheric Instrumentation Research, Inc.(Rx:TS-2A,Tx:TS-IA-I ), kVector Instruments. IAll distances refer t o t h e line joining positions 3 and 2. "All heights refer to m.s.l, except t h o s e f o r T L and u', v', w' which refer to ground level.

anemograph and the tethered balloon system. Nevertheless, they were calibrated before 1989 and 1990 campaigns against prototype instruments. Table 1 includes the errors related to each specific instrument which was also taken into account during the data analysis.

As shown in Fig. 2, the terrain has moderate complexity. The presence of the hill o f Pnyx at position 3 complicates the experiment because it introduces wind speedup (e.g. Taylor et al., 1987). At a distance of approximately 300 m from the shoreline up to 500 m upwind of NOA site there are blocks of buildings with estimated mean height of hb = 7 m. The average roof-top acts on the flow like a displaced surface; according to Panofsky and Dutton (1984) the displacement height is estimated to d-- 5 m.

144 H.D. Kambezidis et al. / Atmospheric Research 36 (1995) 139-156

To investigate the IBL t ime evolution, te thered bal loon flights were pe r fo rmed at N O A site. No other in te rmedia te measur ing locations were considered for purely technical reasons. Regular bal loon flights during selected sea breeze days were per formed. For simplicity, 1989 or 1990 is called "exper imenta l year" , each of the selected sea breeze days is called "exper imenta l day" and each of the bal- loon flights is n a m e d "exper imen ta l hour" . Table 2 gives the data sets obta ined during A T H I B L E X . The wind speed and direct ion were recorded by the anemo- graph on a chart recorder. All mas t measurables at shoreline were recorded on cassette via a Campbel l Scientific Ltd. (CSL) 21X data logger as minu te aver- ages. The u v w propel ler a n e m o m e t e r outputs were sampled at a rate o f 15 Hz by a CSL ESX 1 P R O M installed in a separate CSL 21X data logger. The sampled

Table 2 Data sets obtained during ATHIBLEX. The data complies with the limitation that the wind direction lies in the desired range. Runs with an asterisk indicate fast sampling periods only. Time is LST.

Run

I. Experimental year I: 1989 A.

Experimental hour:

B.

Experimental hour: II. Experimental year II: 1990 A. Experimental hour:

B. Experimental day B: 4 July Experimental hour:

Experimental day A: 26 June

1. 1005-1045 h (IA1) 2. 1150-1215 h (IA2) 3. 1340-1405 h (IA3)

Experimental day B: 28 June

1. 1640-1655 h (IBI)

Experimental day A: 8 June 1. 0813-0830 h (IIAI) 2. 1011-1028 h (IIA2)* 3. 1124-1158 h (IIA3) 4. 1228-1245 h (IIA4)* 5. 1415-1453 h (IIAS) 6. 1518-1535 h (IIA6)* 7. 1718-1752 h (IIAT)

1. 0916-1013 h (IIB1) 2.1136-1202 h (IIB2) 3. 1142-1159 h (IIB3)* 4. 1237-1324 h (IIB4) 5. 1451-1508 h (IIBS)* 6. 1452-1544 h (lIB6) 7. 1627-1715 h (IIB7) 8. 1748-1837 h (IIB8) 9. 1855-1932 h (IIB9)

10. 1858-1915 h (IIBI0)* 11. 2030-2047 h (IIBI 1 )* 12. 1956-2045 h (IIB12) 13. 2059-2147 H (IIB13)

H.D. Kambezidis et al. / Atmospheric Research 36 (I995) 139-156 145

data was stored in a CSL SM716 solid state memory. The data from the tethered balloon was printed on paper. Special software was developed to pre-process the data (for any corrections required) and then derive results. Apart from the above specific instruments, any meteorological and/or solar radiation data required for the study was obtained from the meteorological and actinometric stations that are run by the Institute of Meteorology and Physics of the Atmospheric Environ- ment (IMPAE) situated at NOA site.

3. R e s u l t s - D i s c u s s i o n

The determination of IBL height, h(x) , at any distance, x, downwind can be made in various ways. Gamo et al. ( 1982 ) showed that when comparing h deter- mined from temperature and turbulence, the latter gave higher values. Smedman and Hrgst r rm ( 1983 ) got a higher value from the temperature profile than from a sensible heat flux one. Ogawa and Ohara (1985) showed that h determined from the wind profile is lower than that resulting from the temperature one. This study investigates convective TIBL's i.e. the driving force is the land/sea temper- ature difference; therefore h is determined where a temperature inversion is found (Venkatram, 1977; Anthes, 1978 ) only.

In the past, few IBL experiments were performed on rather fiat terrain e.g. Pan- ofsky and Townsend (1964), Panofsky and Petersen (1972), Smedman and Hrgstr6m ( 1983 ), Ogawa and Ohara (1985). The differentiation in our case is the complexity and non-uniformity of the terrain. Because of lack of any relative measurements, z0~ and Zo2 were empirically estimated to be 0.004 m and 1 m respectively. [Nevertheless, applying Kondo and Yamazawa's method (1986) for z02 a value very close to unity was obtained. ] These values correspond to light sea undulation and city environment as is the case. The locations 1', 2 and 3 in Figs. 1 and 2 lie on a straight line more or less and they were chosen to be so. From position 2 the view is not clear southwards. Therefore sea breeze cells in the range 190-250 ° were taken into account in both experimental years.

According to some measurements performed with tethered balloon offshore (position 1' ) to a height of 120 m a.m.s.1, in the morning of June 18, 1990 showed neutral stability near sea level with a temperature inversion aloft. This implies the origin of the (daytime) TIBL being out in the sea according to fig. lb of Stunder and SethuRaman's paper (1985 ). This was confirmed by all tempera- ture profiles at the mast; soon after sea breeze starts, the whole 21-m mast is within the TIBL and shows near neutral profiles. After sunset an IBL still exists over Athens area since sea breeze cells continue to progress inland in the form of gravity or density currents (Stull, 1988 ). The stratification over Saronikos Gulf must be then stable; this implies IBL origin at the coast. Indeed, all evening runs show no temperature inversion at the mast which implies that the latter is com- pletely out of the IBL. Fig. 3 shows temperature profiles at positions 2 and 3 for the experimental day IIA.

Melas and Kambezidis ( 1992 ) used ATHIBLEX data to evaluate some empir-

146 H.D. Kambezidis et al. / Atmospheric Research 36 (1995) 139-156

30

25

(O 2 0 i

E

C

E 1 5 "

.E

~10- I

0 22

1000

900

; ; ;#¢ 0813-883@(LST) ==:== 1124-1 IB8(LST) =--=--= 1416-1453(LST) &&&== 1718-1762(LST)

i

I

I i I i t I I I I

2 3 24 25 2 6 27 P o L e n L l e t T e m p e r a L u r e ( d e g C)

a i

;;+~¢ 0813- @B38(LST) = : : e : 1124-115B(LST) ===== 1416-1463(LST) ~ - " 1718-1762(LST)

2B

800 "7 700-

E

© 6 g g -

E

5 0 0 - r

4@O- ZIZ

3ge"

2001 1001 ,

21 2 2 2 3 24 2B 2 6 27 2 8 2 9 30 P o L e n L l a l T e m p e r a L u r e ( d e g C)

Fig. 3. Potential temperature profiles for runs IIA 1, IIA3, IIA5 and IIA7 at positions 2 (a) and 3 (b).

H.D. Kambezidis et al. /Atmospheric Research 36 (1995) 139-156 147

ical formulae and also similarity and slab models. They showed that for such a complex terrain as Athens area, all of the empirical formulae and similarity models used failed to predict the TIBL height correctly. The slab models work better if both convective and mechanical turbulence are taken into account. Gryning and Batchvarova's (1990) slab model was adopted in their work with very good agreement with observations. Table 3 gives the comparison between observed (experiment) and (slab model) TIBL heights. Categorising these runs as morn- ing (before 1400 h LST), afternoon ( 1400-1800 h LST) and evening (after 1800 h LST), taking the mean of the observed heights at positions 2 and 3 and esti- mating the IBL height at intermediate locations through the above mentioned slab model, Fig. 4 is drawn. This Figure shows the TIBL evolution throughout a typical sea breeze day over Athens area. Lyons and Cole (1976), for instance, have pointed out the importance of IBL over urban areas situated on coastlines to the transport (by the IBL) and diffusion (fumigation point) of air pollutants. A simple example of a 150-m stack of a power plant belonging to the Public Power Corporation (PPC) located just on the coastline is shown in Fig. 4. The fumiga- tion point varies according to IBL height and therefore the diffusion of the air pollutants released by the stack affects a large residential area of Athens. Part of these pollutants can also be transported by the IBL mechanism some 10 km into the basin. For these reasons PPC decided to cease its operation in 1982.

Appendix I gives the analytical expressions for some empirical formulations and the slab model proposed by Melas and Kambezidis (1992). Applying all these expressions to a distance 4500 m downwind of position 2, i.e. at position 3, Table 4 is obtained. From the Table it is seen that the slab model performs better on the

Table 3 Comparison between observed and estimated (Melas and Kambezidis, 1992) mean TIBL heights in metres a,m.s.1, at position 3. Early morning runs IIA1 and IIB1 have not been considered since sea breeze had not reached position 3. Atmospheric stability is estimated at position 3, 13 m a.g.l.

TIBL height Run Observed Estimated z~ L

IAI 209 186 -0 .76 IA2 171 130 -0.51 IA3 302 222 - 0.34 IBI 209 311 -0 .17 IIA3 368 404 - 0.46 IIA5 279 392 -0 .24 I1A7 363 373 - 0.26 IIB2 199 169 -0 .92 lIB4 344 331 - 1.97 lIB6 391 316 -0 .23 IIB7 273 264 -0 .34 IIB8 302 247 -0 .37 IIB9 155 153 -0 .34 IIBI2 180 190 -0 .50 lIB13 189 136 -0 .33

148 H.D. Kambezidis et al. / Atmospheric Research 36 (1995) 139-156

3 5 0

u

:31313-

TIBL 2: e~ternoon (14@0-IE~0 LST) I: morning (=unPtee-140@ LST) 3 : even trig ( 1 8 0 0 - s u n r t se )

O)

E

© 2@@-

E

1613-

f-

031 @13- o.J

(]) -F

B13

PPC 8 r e c k

Po8 It ton 3

Poo ttton ~13 @ 1 0013 2131313 213130 413130 B131313 613130

D i s L o n c e d o w n w Lnd ( m )

Fig. 4. TIBL mean height daytime evolution downwind of the sea/land interface.

Table 4 Mean TIBL height in metres a.m.s.l, estimated by Eqs. ( 1 ) - ( 5 ) for x = 4500 m and compared with ATHIBLEX observations.

Period of ATHIBLEX Eq. (1) Eq. (2) Eq. (3) Eq. (4) Eq. (5) day

Morning 260 114 257 425 304 255 Afternoon 300 46 265 421 319 290 Evening 180 61 78 96 180 180

average. This is so because all empirical formulations include the sea/land tem- perature difference i.e. they take into account the turbulence due to the thermal effect. In cases with complex terrain, the mechanical turbulence generated should be included for more accurate results. This is done by the slab model.

During experimental day A of year II, some synoptic wind from W or NW occurred. This did not prevent sea breeze to fully develop because Mt. Aegaleo (see Fig. 2 ) "shaded" the basin. The wind direction profiles at position 3 showed a marked shift from southerly directions to westerly or north-westerly ones above 400-450 m where the average height of the mountain is. The sea breeze cell starts from a direction almost perpendicular to the coastline, but then instead of veer-

H.D. Kambezidis et al. /Atmospheric Research 36 (1995) 139-156 149

(a) lo

1 { } -~

10 -2~

10 -4

I

N

~]~ 10-6

1{} - r

o~ =- 1 (3 -°-i

E 10+%

1 0 " ' " !

(J~ 10 -11 x RUN IIB 5 o RUN IIB11

,. - 5 / 3

0

I o

\ %

- 5 / 3 0

u

I ~ " 12 i i i i i i i i I i i i i i i i i I l l l i l l l I l l l i l l l l I i i i i i i i I l l l l l l l I l l l n l l l I I i i i i I I ~ I I I I I I | l l l l l l q i i I I I I I ~ l l l l l l q i i i i i i i ~

1 0 - " 1 0 - 7 1 0 - ' 1 0 - + 1 0 " 4 1 0 " : 0 1 0 " 2 1 0 -~ 1 10 10 2 10 = 10 4 10 +

F r e q u e n c y ( H z ) (b) 1

1(9 -~

1 ( 9 - 2 I

N I

oJ 1(9 - s I

0)

(kJ E

1(9 - " :

>

CO

1(9 -~ 1(9 -2

RUN I IBB

• , ~ -5/3

I I l l l l l I I I I I I I I I I I i i i i i i i I

1(9 -1 1 1(9

E r e q u e n c y Fig. 5.

I I I ~ l l l l I I f I I ; l l l I

1 0 2 1(9 s

( H z )

150 H.D. Kambezidis et al. / Atmospheric Research 36 (1995) 139-156

(C ) 1 0 - '

1 0 "=

1 8 -:-

l e "'"

I~ -m

= 1 Q -~ N

ZE 10 -7.

I 1 e -a. CO

c~ E 1 e -'--

~--~ 10 "~=~

(,0 l e -" ~.

10 "'='

**xNX X - 5 / 3 I *

= RUN II85 o RUN II811 % \ -5/3

o I.

i llllllq i iiii.~ i llllrlq i l lllnq i immj i Hl.n I I llll,q ; llllllq i llHnq t ~Ingl I I ;IHIIIj

18 -G 1 0 " 1~ "~ 1 0 "= 1~ "~ 1 1 0 10 = 1~ z l e " l e = 10 +

F r e q u e n cy ( H z ) Fig. 5. (continued) S= (a) , S~ (b) and Sw (c) power spectral densities of u'-, v'- and w'- components for runs IIB5 and liB 11. The straight line with slope - 5/3 indicates local isotropic conditions.

ing as a result of the coriolis effect (Koschmieder, 1935; Sutcliffe, 1937; Haur- witz, 1947; Estoque, 1961, 1962; Pearce, 1962), it shifts continuously until late in the evening when it becomes south-east and finally later at night it results in an offshore wind, i.e. land breeze. It is Prezerakos' (1986) opinion that when the synoptic pressure gradient is very weak, the regional air circulation in the Athens basin during sea breezes is cyclonic. This is attributed to surface friction, the pres- ence of the surrounding mountains and the Saronikos Gulf.

During all runs of ATHIBLEL an overspeed was observed above the hill of Pnyx (position 3 ) in the region of 25-40 m a.g.1, due to the hill effect. The wind velocity profiles were therefore corrected for the speed-up following Taylor et al. (1987 ). It is very difficult, however, to estimate the effect of sloping terrain on the TIBL growth. In this study, the assumption was made that TIBL evolves as if there were no sloping terrain present (hill at position 3 ). There is some evidence (Hunt and Simpson, 1982 ) that when the height of the hill is much smaller than h, hill effects on TIBL growth can be neglected. In almost all cases of the experi- mental runs during ATHIBLEX the wind speed at low level ( 150-300 m a.g.l., position 3) showed a maximum in late afternoon together with an associated shift in wind direction towards west. This is so because sea breeze is influenced by coriolis force and synoptic flow; this has been the subject of various numerical

H.D. Kambezidis et al. / Atmospheric Research 36 (1995) 139-156 151

studies e.g. Estoque (1962), Pearson (1973), Estoque et al. (1976), Physick (1976) and Estoque and Gross ( 1981 ) or combined observational-numerical experiments e.g. Estoque ( 1981 ). A review on this subject has been given by At- kinson ( 1981 ).

Fig. 5 shows power spectral densities of u'-, v'- and w'- wind components ob- tained during runs IIB5 and liB 11 corresponding to highly and slightly unstable atmospheric conditions in the surface layer at position 3. In both runs all of the three wind components exhibit the - 5/3 law which implies local isotropy. From such a Kolmogorov spectrum an (inner) mixing scale l is estimated at the begin- ning of the inertial sub region. From Fig. 5a lu= 33 m, from Fig. 5b 1v=42 m and from Fig. 5c lw=30 m for run IIB5, for example. Therefore the air pollutants "feel" eddies of dimensions luX lvx lw i.e. 41,580 m 3. Another interesting feature from Fig. 5 is that in the high frequency region the power spectral densities show higher values as the atmospheric conditions become more unstable. This has been described by Kaimal et al. ( 1972 ) and Busch ( 1973 ).

4. Conclusions

ATHIBLEX was a 2D experiment performed over the Athens urban area. Its aim was to establish the convective TIBL height daytime evolution under sea breeze conditions. An IBL is well associated with air pollution transport and dif- fusion as is known from the international literature (e.g. Lyons, 1977 ). The TIBL was studied over a complex and non-homogeneous terrain within ATHIBLEX frame. Similar projects of this kind over coastal urban areas are very few world- wide. It was found that under sea breeze conditions the atmospheric flow just above the Saronikos Gulf has a neutral stability thus making the origin of the IBL to be out in the sea. It is only in the early morning and during the evening that the origin approaches or exists at the coastline.

Because of lack of intermediate observation points between the coastline and position 3, some empirical formulations (Peters, 1975; Weissman, 1976; Bergs- tr/Sm et al., 1988; Walmsley, 1989) and a slab model (Melas and Kambezidis, 1992) were tried to estimate TIBL height at points between the sea/land inter- face and NOA site. The slab model gave the best approximations to the observed heights in agreement with Melas and Kambezidis' (1992) investigation. These authors suggest that both mechanical and convective turbulence must be taken into account. Their model is a modification to that proposed by Gryning and Batchvarova (1990).

As far as the effect of the prevailing synoptic flow is concerned, it was shown that westerly winds do not affect the IBL development; Mr. Aegaleo "shades" Athens basin to a height of approximately 400 m a.m.s.1. Under such synoptic conditions the air pollutants continue to be influenced by the IBL presence. Moreover, air pollutants from the industrial area of Thriassion Pedion (Pe- dion = Field) west of Mt. Aegaleo may overleap the mountain and contribute to the pollution levels in the Athens basin (Asimakopoulos et al., 1992).

152 H.D. Kambezidis et al. / Atmospheric Research 36 (1995) 139-156

To give an idea about the wind speed turbulence levels at position 3, power spectral density functions of u'-, v'- and w'- wind components were derived dur- ing ATHIBLEX, they showed local isotropic conditions under unstable atmos- pheric conditions.

Appendix I

The empirical formulations as well as the slab model used in this work are described here analytically. The application is made for position 3.

A. Empirical formulae: ( 1 ) Peters ( 1975 ) proposed a simple formula for the determination of the TIBL

height. The derivation of the formula is based on a constant heat flux method leading to a linear relationship between the TIBL depth, h, and fetch, x:

h(x) =2Hox/pCeu( TL -- Tw) ( 1 )

where: Ho (W m -2) = 0.39S according to Mishra and Onlock ( 1982 ) S net solar radiation measured at NOA, in W m - 2 p = 1.225 kg m -3 (mean air density) x = 4500 m (distance downwind of the sea/land interface) a(ms -1) mean wind speed at 10 m a.g.l., derived from corresponding mea-

surements at 13 m and being corrected for hill overspeed TL (K) measured at a depth of 5 cm below ground surface Tw (K) measured at a depth of 20 cm below sea surface Ce= 1004 m 2 s - 2 K-~ (specific heat of air under constant pressure). (2) Weissman ( 1976 ) suggested a parabolic extension to Peters' ( 1975 ) Eq.

(1) i.e.:

h(x) = (2Ho ×/pCpflU) 1/2 (2)

where: fl was calculated at position 2 from the mast potential temperature profiles as the only available temperature measurements most representative for the ov- erwater conditions.

(3) Bergstr/Sm et al. (1988) based on a work by Smedman and H/Sgstr/Sm ( 1978 ) analysed some good quality data and obtained:

h(x) -.~-0.2x 0"78-0"33z/L ( 3 )

where: z /L is the atmospheric stability and L is the Monin-Obukhov length. (4) Walmsley (1989):

x ' = {h' (x) [lnh' (x) - 1 ] + 1}/Bk (4)

where: x = 4500 m h' (4500) =h(4500) - 5, in metres B= 1.25 after Businger ( 1982 ). B. Slab model

H.D. Kambezidis et al. / Atmospheric Research 36 (1995) 139-156 153

Melas and Kambezidis, based on a work by Gryning and Batchvarova (1990), presented the following simplified expression for the slab model:

h ( x ) = ( 1 - 0.002La) [2CoCr( 1 +2A) ]1/2[ (TL

- T w ) x / f l ] l / 2 + l l l n ( - L a ) (5)

where: A = 0.2 after Driedonks ( 1981 ) g /T=9 .81 /303=O.032 , in m s -2 K -~ k=0.4 La = - ( CD ah ) 2 / ( g / T) k [ Cx ( TL - Tw ) ] , in metres.

N o m e n c l a t u r e

Latin symbols: B Co C~ C~ d h hb g g / T Ho h' k

L R H T T' TL To Tw s U

Uh

U,

W

wd U', U p, W ~

U, V, W

x, y, z Zo X'

= ~ / u , drag coefficient specific heat of air under constant pressure (m 2 s -2 K-1 ) bulk heat transfer coefficient displacement height (m) IBL height (m) height of obstacles (m) gravitational acceleration 9.81 (m s -2) buoyancy parameter (m s -2 K - 1 ) surface heat flux (W m - 2 ) = ( h - d ) / z o 2 =von K~irm~in constant = 0.4 inner scaling length of inertial subrange of variable s (m) Monin-Obukhov length (m) relative humidity (%) dry-bulb temperature (K) wet-bulb temperature (K) ground surface temperature (K) reference air temperature = 293 K sea surface temperature (K) net solar radiation (W m -2) longitudinal wind component (m s- 1 ) horizontal wind speed (m s- ~ ) friction velocity (m s- 1 ) vertical wind component (m s- ~ ) wind direction (deg) turbulent values of wind speed components (m s-~ ) the three wind components Cartesian co-ordinate system according to the u, v, w notion roughness length (m) =X/Zo2 (Zo2 land roughness length )

154 H.D. Kambezidis et al. / Atmospheric Research 36 (1995) 139-156

zlL atmospheric stability

Greek symbols: fl potential temperature lapse rate over water (K m - 1 ) p mean air density (kg m - 3 ) trw standard deviat ion of w-wind component (m s - 1 ).

An overbar over any measurable denotes average value.

Acknowledgement

The ATHIBLEX organiser, Dr. H.D. Kambezidis, expresses his thanks to NOA/ IMPAE personnel for helping him and his team to carry out successfully this campaign.

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