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INFLUENCE OF BACKGROUND FLOW ON EVOLUTION OF SARONIC GULF SEA BREEZE
C. G. H E L M I S , K. H. P A P A D O P O U L O S , J. A. K A L O G I R O S , A. T. S O I L E M E S and D. N. A S I M A K O P O U L O S
Department of Applied Physics, University of Athens, 33 Ippokratous Str., 10680 Athens, Greece
(First received 18 May 1993 and in final form 7 November 1994)
Abstract--Results from an experimental campaign at the coastline of the Saronic Gulf during the summer of 1992 are presented. The frontal intensity and the rotation of the wind hodograph at the shoreline during sea-breeze case,; are examined under different background flow conditions. The frontal intensity classifica- tion is based on the vertical velocities induced, as measured by a high resolution acoustic sounder. Three representative cases are presented. Conclusions are based on the analysis of all observed sea-breeze flows. Background oil-shore or shore-parallel flows are more probable to create a strong or weak front, respectively. The development of frontal characteristics under background on-shore flow is attributed to off-shore land features. The wind hodograph rotation is shown to be associated to the initial direction of the sea breeze, which is determined by the background flow direction. When the background flow possesses a westerly component the hodograph shows anticlockwise rotation, while an easterly background compon- ent causes the wind vector rotation to be clockwise.
Key word index: Sea breeze, front, hodograph rotation, Athens.
1. INTRODUCTION
The Greater Athens Area (GAA) includes the Athens Basin, the Mesogia Pllain and Thriassion Pedion and its northward limit reaches the Parnitha Mt. (Fig. 1). It is 450 km 2 area inhabited by nearly four million people and concentrates a large number of industrial activities often causing serious air pollution problems. The three different sea-breeze circulation cells which are formed at GAA (the Elefsis Gulf, the Mesogia Plain and the Saronic Gulf cells), have attracted the interest of researcher:~ for many years, due to their importance to the local climatic characteristics (see Fig. 1).
Both theoretical and experimental efforts have re- vealed aspects of the local sea-breeze circulation. Prezerakos (1986) studied the climatology of the Sa- ronic Gulf sea breeze under calm conditions giving information on the extent and rate of its inland pen- etration, based on a temporally coarse database from a network of surface meteorological stations. The contribution of the Saronic sea breeze circulation to increased levels of photochemical pollution and the possibility of pollutanl: re-circulation over the Athens basin due to the sea-land breeze diurnal cycle has been investigated by Lalas et al. (1983, 1987), Asimakopoulos et al. (1993), Kallos et al. (1993) and Moussiopoulos et al. (11993). Case studies of the struc- ture of the sea breeze front near the coastline have shown the influence of background flow on the frontal characteristics (Helmis et al., 1987). More recently,
a field campaign (Asimakopoulos et al., 1992) was also conducted to understand the mechanisms of pollutant transport and advection from the Thriassion Pedion to the city of Athens. Also, Steyn and Kallos (1992) have studied the dynamics of the rotation of the sea breeze direction over GAA for a typical initial wind flow.
It is also worth mentioning here, the recent experi- mental and theoretical studies of the Mesogia Plain sea breeze circulation system in relation to topogra- phy and synoptic conditions (Varvayianni et al., 1993a, b; Helmis et al., 1994a).
The above-mentioned studies have led to important conclusions concerning the influence of the Saronic Gulf sea breeze on the general flow pattern, but have concentrated either on typical case studies or on the examination of breezes developed under almost calm conditions.
There is an apparent lack of a database of sufficient duration and high temporal and spatial resolution concerning the development of the Saronic sea breeze flow under differing background flow conditions. A first step towards the construction of such a database was made in an experimental campaign held during the summer of 1992. The measurements were performed on the shoreline of the Saronic Gulf cover- ing a continuous period of two months (June and July), with the purpose to study the vertical structure and the systematic behaviour (if any) of the temporal variation of the direction of the Saronic Gulf sea breeze under different background flow conditions.
~ s ~ . : : /t ~ . I'#W....: 'x I ' " . . " " :" : ": ' - '" ,~ i
' :' PLA ,',~ " . . IN ~ : L A I N s P A T A ~ .~'-~sxaoN,c ~ut.~" " ~ ~ _ .: - . . . . %%5
?
INX ISL. < " /.r, . . / . :~'~ .i ¢
0 2 4 6 8 k i n , i 1 h i
Fig. 1. The Greater Athens area. The experimental site is indicated by a solid circle. The harbour of Pireas is inside the small Gulf seen just "above" the experimental site. Height contours of 100 m (dotted), 200 m
(solid thin) and 600 m (solid thick) above mean sea level are shown along with mountain-top heights.
Both these characteristics have not been systemati- cally assessed in previous studies, although they affect the dispersion characteristics during sea-breeze peri- ods. Moreover, the study of the sea breeze in the presence of a non-negligible background flow is quite interesting: it has been inferred by Kallos e t al. (1993) that pollution episodes in Athens are linked to condi- tions when the ambient flow is opposite to the Saronic sea-breeze direction, leading to stagnation.
2. T H E S U M M E R 1 9 9 2 C A M P A I G N
The experiment took place in a residential area at the shoreline of the Saronic Gulf, close to the harbour of Pireas (see Fig. 1). The instruments were placed 3 m from the water edge. The beach slope was very
small for a distance of 40 m where a small, relatively steep hill of 15 m height begins with a 10 m high building on the top of it, about 30 m further inland.
The instrumentation used consisted of the follow- ing:
(a) A tethered meteorological balloon facility ca- pable of measuring wind, temperature and humidity up to a height of 800 m (Soilemes e t al . , 1993).
(b) A high-resolution monostatic acoustic sounder operated at 4.3 kHz, measuring the vertical compon- ent of the wind up to 160 m with a sampling period of 10.9 s and the detailed thermal structure of the Atmospheric Boundary Layer (ABL) up to 400 m, (Asimakopoulos e t al. , 1987; Papageorgas e t al. , 1993). The strong temperature fluctuations due to vertical gradients of potential temperature and wind velocity
Influence of background flow on Gulf sea breeze 3691
\ o . - :" . . . . h 240 350 60 wind direct ion (°)
5500- (c,) 5000-
4500
4000
"~3500
. , ~ 3 0 0 0 -
. ~ 2 5 0 0
"~2000
1500-
1000-
I ~ I I
- - 0200 1400
500 -
0 240 3~0 ~o wind direct ion (o)
Fig. 2. The 0203 (solid) and 1400 (dashed) LST rawinsonde wind speed and direction profiles measured at HMS (see Fig. 1) for the 13/6 (a), 10/7 (b) and 17/7 (c) sea breeze cases.
correspond to echo layers appearing as black areas in the facsimile records.
(c) A 12 m high meteorological mast on the top of which were installed: a UVW propeller anemometer for the measurement of the three components of the wind; a fast temperature sensor; a cup anemometer
and a hygrometer. The data were stored at a rate of 1 Hz and the surface momentum and heat fluxes were directly calculated over 45-rain periods using the Eddy correlation method. Application of the effective transfer function of the propeller anemometers (Garratt, 1975) leads to the result that for a typical
3692 C.G. HELMIS et al. ~
frequency response of 2 Hz the measurement of Eddy fluxes are accurate within 10% (as far as high fre- quency loss is concerned) for heights exceeding 5 m over land and 10 m over sea for near-neutral and unstable conditions. The accuracy degrades as stabil- ity increases. The location of the above-mentioned instrumentat ion is also depicted in Fig. 1.
Section 3 presents and analyses the results of three typical sea-breeze cases under different background flow conditions with clear sky, Section 4 compares all the studied cases in relation to the front intensity and the rotation of the direction of the sea breeze, while the conclusions are outlined in Section 5.
3. ANALYSIS
3.1. The sea b r e e z e on 13/6/92 (on - shore b a c k o r o u n d
flow) The early morning conditions on the 13th of June
1992 (sunrise at 0507 LST and sunset at 1952 LST) were such that a low wind speed regime prevailed below 1000 m, while a moderate to low south-westerly flow (the background flow) was maintained aloft as was shown by the Athens (HMS in Fig. 1) rawinsonde at 0200 LST (Fig. 2a). It is quite often that several flow
regimes may be seen in a rawinsonde profile, reflect- ing influences from synoptic or mesoscale effects. Therefore, an issue of determining the ambient (background) flow conditions arises. In this work, the background flow is taken to be characterised by the wind speed and direction of the 0-2000 m layer, into which the sea breeze develops (see also the definition of the "ambient flow" as given in Banta e t al., 1993). The meteorological mast data show a WSW surface flow of about 3.5 m s - 1 during the night (Fig. 3a) and calms with variable wind direction from 0500 until 0815 LST. Later on, from about 0900 until 1400 LST a positive trend on temperature and a negative trend on humidity mixing ratio (Fig. 3b) was evident due to the increasing insolation (clear sky). The wind also increased significantly during the day, while the fac- simile recording of the acoustic sounder, which depic- ted the thermal structure of the ABL shows a low level (50 m) turbulent layer due to mechanical turbulence (see Fig. 4). This change is in accordance with the momentum and heat fluxes time-series estimates (Fig. 3c), where an increase in momentum flux and positive values of heat flux were evident at the same time period. After 1400 LST the intense influence of the background flow was interrupted by an intermittent decrease and backing of the wind. Since the air-flow
8 / , i , E , i , i , i , i , i , i , i , i , i , I 10~ 0 2 4 6 8 10 12 14 16 18 20 22 24
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7 _1.00 . . . . - 0 . 0 0 ,~ _ t \ , ,
- 1 . 5 0 J \/ ._.~ - q(~) ~ _ _ w,T, F -~ = - 2 . 0 0 | ~ , I , I ' I , I , I ' ~ , I , I , I , I - 0 . 10
0 2 4 6 8 10 12 14 16 10 20 22 24 T i m e (LST)
Fig. 3. Time-series of (a) the wind speed and wind direction, (b) temperature and humidity mixing ratio (w), and (e) vertical momentum (u'w') and heat (w'T') fluxes measured from the mast on 13/6/92. Fluxes are computed with the Eddy-correlation technique over 45-rain periods. All other quantifies correspond to I0
rain averages of the relevant parameters. The height of measurements is 12 m above ground.
Influence of background flow on Gulf sea breeze 3693
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14:51 16:06 17:22 18:38 19:54
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Fig. 4. Facsimile record of the acoustic sounder on 13/6/92. Weak fronts due to the sea breeze circulation appear at 1410, 1525, and 1620 LST and they are indicated by arrows•
was then blowing from the south and there was no air modification caused by the presence of the Salamis Island, as is the case tier a SW background flow (see Fig. 1), an increase of humidity mixing ratio and a decrease of temperature is expected. This also ex- plains the negative heat flux and the nearly zero value of the momentum flux. This change was also evident in the acoustic sounder facsimile record where inter- mittent activity regions corresponding to the passage of a weak front reaching 150 m in height appeared at 1410, 1525 and 1620 LST. These weak fronts were probably caused by tile combined effect of the back- ground flow and the sea breeze circulation established on the larger scale. The small intensity and the inter- rupted passage of the sea breeze front can be attributed to the WSW background flow that trans- ported relatively cold and moist marine air over the land decreasing the .,~a-land temperature contrast. The new regime was finally established after 1630 LST and was eleady indicated by the acoustic sounder facsimile which showed a more homogeneous and less turbulent record. The sea-breeze circulation died out at about 1900 LST when the surface wind shifted to the WSW background flow, the air temperature in- creased and the humi,rlity mixing ratio decreased.
3.2. The sea breeze on 10/7/92 (off-shore background flow)
During the early morning of the 10th of July 1992 (sunrise at 0511 LST and sunset at 1949 LST) a NNE background flow was observed in the lowest 1000 m of the atmosphere (Fig. 2b) with a 2.0 m s-1 surface wind decreasing to 0.5 m s- 1 by 0600 LST (Fig. 5a). After 0700 LST the wind strengthened mainly due to insolation, the temperature increased and the humid- ity mixing ratio decreased (Fig. 5b). At 1300 LST the wind speed increased and changed abruptly to south- edy directions, accompanied by a significant temper- ature depression and an increase of humidity mixing ratio• These were the results of the sea-breeze front, which crossed the coastline delayed by the opposing background flow. The front passage was associated with an elimination of the momentum flux and a downward heat flux as expected (Fig. 5c). The acoustic sounder facsimile record shown in Fig. 6 in- dicates the passage of a strong front extending up to 200 m. The thermal activity changed from thermal plumes to near-neutral conditions at low heights•
Figure 7 gives the time-series of the vertical com- ponent of the wind speed at nine different levels (25-159 m), which were estimated by the acoustic
3694 C.G. HELMIS et al.
,~ 360
= 270 o
lao
"~ 90
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- - wind direction X ~ - wind speed [ 1 0 ' ~
~ ~ ~ _ ' ~ ' -~.~..~-.._..~- ~
0 2 4 6 8 10 12 14 t6 18 30 22 24 "
. . . . . . temperature
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, 0 J ~ / ~ ~o~ = 5~ - - w ~ ~ . ~-1 ~
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0.00
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~ ,-o.Io 7
\ - ~ -. / - I \ r I ...'x r
v ~ - ~. \,. i \ I '~ --0-00"~
1(~, ' , , , , , , , , , , , , , , , , , , :_,'71:-o.,o; o ~ 4 8 a lO 12 ,4 18 la :~o 22 24
Time (LST)
Fig. 5. As Fig. 3, but for 10/7/92.
Datm 10/ 7/1992 400
v
100
0
~ " . . . . . . T,,~'i;~T~ . . . .
Fig. 6. Facsimile record of the acoustic sounder on 10/7/92. The sea breeze front passed the shoreline at 1300 LST (indicated by the arrow) and the sea breeze circulation lasted until 1730 LST.
sounder. At 1301 LST a downdraft of about 1.5 m s-1 is evident up to 92 m, followed by an updraft at all heights, induced by the ¢onvergen¢~ zone on the sea brcczc front, with a value of 2.5 m s - t at the higher levels (Simpson et al., 1977; Ogawa e t al., 1986; Helmis
et al., 1987; Prakash e t al., 1992; Chiba, 1993). The peak values of the updraft motion appeared earlier at the lower levels (below 75 m) in respect to the higher ones probably because of the amplification of the updraft motion at these heights due to the terrain
Influence of background flow on Gulf sea breeze 3695
slope described in Section 2. On the other hand, the 1-min averages of the vertical velocities reveal a transition from negative or near-zero values to posit- ive ones at 1302 LST ,'dmost at all measurement levels (also discernible in Fig. 7). The persistent positive values of vertical velocities at the lower heights after the passage of the sea-breeze front is also an indica- tion of the terrain influence. Thus, the time difference between the peak values of the vertical component of the wind speed in this small, relative to the sea breeze vertical extent, height range does not constitute a con- tradiction to the model of Simpson et al. (1977); the latter implies a shape of the sea-breeze front like a gravity current with a nose of intense turbulence (downdrafts followed by updrafts), the air near the ground being frictionally retarded.
At about 1730 LST the sea breeze circulation was destroyed by the synoptic flow and the plumes activ- ity re-established as depicted by the acoustic sounder facsimile record.
3.3. The sea breeze or117/7/92 (background f low paral- lel to the coastline)
On the 17th of July (sunrise at 0516 LST and sunset at 1946 LST) the early morning conditions support- ed weak N N W flow at all heights (Fig. 2c). Later on (Fig. 8a), a slow increase of the wind and temperature were evident. In particular, during the time period 0800-1200 LST the wind direction fluctuated between the nocturnal flow (land breeze) and the developing sea breeze flow until the latter was established. After 1200 LST the wind direction turned SW, the tem- perature decreased and the humidity mixing ratio increased as shown in Fig. 8b. The corresponding facsimile record of the acoustic sounder (Fig. 9) at the
same time showed that the sea-breeze front was cross- ing the shoreline, extending up to 200 m. In this ease, the structure recorded by the acoustic sounder was not so strong as was on the 10th of July and the associated updrafts did not exceed 2.0 m s - L The momentum and heat fluxes time-series shown in Fig. 8c did not exhibit any significant change with the passage of the sea-breeze front, because they were kept at nearly zero values before the front arrival. These pre-frontal small values of the fluxes are at- tributed to the southerly gusts which transported air of marine origin. The stable marine layer is character- ised by downward heat flux which on the average balances the upward heat flux that is found in the unstable land air. The southerly gusts may be part of what Banta et al. (1993) defined as a minor (precursor) sea breeze caused by the local thermal contrast be- tween the coast and the adjacent sea. The wind profile from the tethered-balloon system at that time showed an intermittent surface layer of southerly directions covering about 20 m. The sea breeze lasted until 1700 LST when the surface wind shifted slowly to 90 °. At that time the wind speed decreased from 3.5 to 1.5 m s - ' and the temperature increased significantly. The humidity mixing ratio, the downward mo- mentum flux and upward heat flux were restored to their initial values before the sea breeze. The thermal activity was also recovered to the expected pattern (thermal plumes) for this time of day.
The vertical structure of the sea breeze monitored by the successive ascents of the tethered balloon is given in Fig. 10. The contours in Figs 10b and c are the result of 16 profiles (balloon ascents and descents) spaced less than 1 h apart, while only five representa- tive profiles of potential temperature are shown in
v
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. I ~ , . . , , . ~ , , ~ _ . A...A.._ ^ . ^ ~ r , ~ T k 4 ~ , ~ , , ~ . , . . . . . . I i ~ ,,v,,-- ,.'-~ "v~v"v. ~ - v - - v - , ,iv- v -wv,,,,- " " "- Vl
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| ~ - - v ¥ - ~ Y - r v - ~ v V - . ~ - - , v - i -V" . ^ . 1 2 6 m - - , o . u
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"" 1 0 9 m 2 0
[ . . . . ~ . , ,,~ n. ^ . . . . ~.~_ ~ ^ ~ . . . . . ^,~,~/t.-, . . . . . . . "o'o "-" l ~ . . . . W ~ V ~ v ~ i ' d U V ~ ' ' v " i " " w - - ~ V ' - • - - . ~ v - • , ~
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0 2 4 6 8 10 12 14 16 18 20 22 24 ""
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"~ -0 .5 0 I I / \ - o . oo
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T i m e (LST)
Fig. 8. As Fig. 3, but for 17/7/92.
Date1 17/ 7 / 1 ~ 2 4 0 0
v
.. 200
100
0
Fig. 9. Facsimile record of the acoustic sounder on 17/7/92. The sea breeze front passed the shoreline at 1200 LST (indicated by the arrows) and the sea brcczc circulation lasted until 1700 LST. The tethered
balloon tracks are also evident.
17137
Fig. 10a for better presentation. The early morning flight revealed a stable layer in the first 500 m (Fig. 10a). Upper layer moderate northerly winds were de- coupled from the surface (Fig. 10c) through the inver-
sion layer. This layer was subsequently eroded from below and by 0900 LST convection developed up to 200 m, also observed in the acoustic sounder facsimile record in Fig. 9; the layer from 80 to 200 m with
Influence of background flow on Gulf sea breeze 3697
o :---- ~--~------w-""~'~. ~ , " ! ~ " - - - " - ~ o 7 a 9 ~o 1~ 12 ~a ~4 ~5 to
Time (LST)
Fig. 10. Tethered balloon data for the 17/7/92 sea breeze flow: (a) potential temperature profiles, (b) time-height plot of humidity mixing rwlio and (c) time-height plot of wind speed. The time-height plots are constructed using data from
16 profiles spaced less than I h apart.
flow. It is worth noting that the isotaches reflect the frontal shape (between 1100 and 1300 LST in Fig. 10c). The cooling effect reaches the 200 m level and is also evidenced by the inversion that remained after the decay of the sea breeze. The progressing warming of the upper layers (Fig. 10a) is due to heat redistribu- tion within the ABL rather than the effect of regional warm advection, as concluded from the comparison of the 0200 and 1400 LST rawinsonde temperature pro- files above 1000 m (not shown). The water vapour (Fig. 10b) is concentrated in a shallow layer of 50 m depth. The slope of the 10 g kg - 1 isoline reveals the upper limit for vertical advection of humidity.
4. T H E I N T E R A C T I O N B E T W E E N T H E S E A B R E E Z E A N D
T H E B A C K G R O U N D F L O W
4.1. The in tens i ty o f the sea-breeze f r o n t
The prevailing synoptic wind, the land-sea thermal contrast, the frictional retardation, the surface heat- ing, the geographical barriers are all factors that influ- ence the time of onset, the intensity and the nature of sea breeze circulation. The background flow is prob- ably the most important factor in the evolution of the sea breeze circulation (Ogawa et al., 1986; Helmis et
al., 1987; Prakash et al., 1992). Under calm conditions, the sea-breeze front is weak and progresses inland normal to the coastline from early morning till late evening. If the background flow is in the same direc- tion as the sea breeze (on-shore), the inland penetra- tion of the sea breeze can be even greater, but the front is also weaker in terms of vertical motion intensity. With opposing background flow (off-shore) the pas- sage of the sea breeze from the coastline is retarded and the front is quite strong. When the background flow is parallel to the coastline the sea-breeze front is diffuse and not well defined.
Table 1 shows the intensity of the sea-breeze front against the background flow for the sea-breeze cases of the experiment. Since only background flows from SW to NNE where observed, the remaining directions are not shown. The strength of the sea-breeze front was derived by the acoustic sounder data: a strong front appears as a steep structure in the facsimile record extending up to 200 m and a strong updraft
almost steady potential temperature is a statically unstable, mixed layer. The upper northerly winds de- scended to about 100 m (Fig. 10c) and aided in the erosion of the stable layer. The third temperature profile (Fig. 10a) shows a low-level cooling corres- ponding to the influence of the fluctuating sea breeze (Fig. 8a). The next profile marks the onset of the sea breeze, which is about 200 m deep with a jet below 100 m (Fig. 10c). The cooling is limited below the jet, destroying the mixed layer. The flow then rapidly
deepens and strengthens, while a jet (maximum strength 6.8 m s- 1) develops at 100 m in a 500 m-deep
Table 1. Classification of the sea-breeze cases (number of days) with respect to the intensity of the front and the rotation (CR: clockwise, ACR: anticlockwise) of the sea-breeze direction against the background flow (10/6/92-31/7/92). The SW sector
of wind directions is the on-shore one
Background Strong Weak No flow front front front CR ACR
over 2.0 m s-1. It can be seen that the strong front cases were limited to mainly opposing (off-shore) di- rections and to a lesser extent to directions parallel to the coastline. On the other hand, the weak fronts appear mainly with background flows parallel to the shore and less with off-shore or on-shore background flows. Sea breeze eases with no front can occur with almost any wind direction, the off-shore cases with no front corresponding to low background wind (almost calm weather). These conclusions are in fair agree- ment with the preceding discussion.
4.2. The rotation of the sea-breeze direction
Previous studies concerning the temporal rotation of the Saronic Gulf sea breeze concentrated on the analysis of flows developing under calm background conditions (pure sea-breeze flows). The rotation of the sea-breeze wind vector has been studied theoretically by Neumann (1977), Burk and Staley (1979) and Kusuda and Alpert (1983). It is now accepted that it is determined by the Coriolis acceleration (causing a constant veering throughout the breeze cycle for the Northern Hemisphere); the interaction of the flow with the large scale and mesoscale pressure gradient, which are temporally variable and may cause clock- wise or anticiockwise rotation; friction and adveetive forcing. The general conclusion is that antidockwise rotation of the wind hodograph is only possible in the presence of complex topography near the shoreline (Kusuda and Alpert, 1983), presumably due to the development of mesoseale pressure gradients which may induce either clockwise or anticlockwise rota- tion.
In this work, the case of background forcing is studied by presenting the wind hodographs for the three experimental days analysed in the previous sec- tions and considering sea-breeze days in which rota- tion was clearly observed.
The rotation of the wind vector for 13/6/92 can be seen in Fig. 11a, which possesses an antielockwise rotation after the onset of the sea breeze. This elliptic hodograph is representative of all sea-breeze cases (observed during the experimental period) which de- veloped under SW background flow. The main axis of the ellipses were roughly parallel to each other for all SW eases, with a clear tendency to become parallel to the v-axis as the background forcing (0200 LST rawin- sonde, Fig. 2a) becomes weaker. There was also one day with SW-SSW background flow that initially exhibited a clockwise rotation, then shifting to anti- clockwise after 1500 LST.
Before commenting on the cause of the antielock- wise rotation, the hodographs of the two other case studies are presented. For 10/7/92 (NNE background wind), after an initial clustering of points (off-shore wind) a clockwise rotation develops (Fig. l lb). No other NNE cases were observed during the campaign.
The 17/7/92 case (NW background wind) shows near calm conditions at the surface, which is friction- ally decoupled from the flow aloft. This is also verified
a ) - 4
- 3
- 2
1
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1 3 / 0 6 / 1 9 9 2
' l ' I ' I 2 3 4
u ( m s -t)
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(b)
i I ' I ' I ' -3 -2
lS
v (m s-*) -4
3 10/07 /1992
- 2
- 1 - 9 20 ~ l I r I
3 4
- u (m ,-') -2
----3
--4
(o) =.=..~.3 s u r f a c e . . . . 300-400m layer
/ '
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- 4 - 3 - 2 -.4 I t I t I I , ' l i
/
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v (ms -1)
- 4
-- / / 9
-a , 17/0~/1992 - / /
~ 2
1 ] ~ 2 3 4
= ~ i l I I i I • 1 8 "__/I H U (m s-Z) i
l
" I 17
2 ~ 3
- 4
'114
Fig. 11. Wind hodographs on: (a) 13/6/92, (b) 10/7/92, and (c) 17/7/92. Mean hourly winds (squares) have been decom- posed in their u, v components along the East-West and North-South axis, respectively, with the following conven- tion: north wind ties on the positive v-axis, while an east wind lies along the positive u-axis. Numbers denote the corres- ponding time (LST) of the hourly wind. The sign of rotation
is measured around the centre of the hodograph curve.
Influence of background flow on Gulf sea breeze 3699
by the isotaches of Fig. 10c. The surface hodograph (solid line of Fig. 1 l c) possesses anticlockwise rota- tion. Its main axis is perpendicular to that of the SW case (Fig. l l a ) in correspondence to the perpendicu- larity of the respective background flows. This hodo- graph is also typical of the NW cases. Figure l lc includes the mean 3~ -400 m layer hodograph con- structed by the tethersonde data. The hodograph shows anticlockwise rotation, but its shape is con- siderably less elongated than the surface hodograph. The deviations of the two hodographs is a clear evid- ence of the topographic restriction that influences the surface wind (note the vertical directional shear be- tween surface and the upper layer at 1400 LST). Moreover, Zambakas (1973) argued that the elliptic form of the hodograph is attributed to frictional ef- fects. Concerning the SE flow aloft after 1400 LST, it is worth noting that the 1400 LST rawinsonde re- vealed a region of easterly winds at 1000 m (Fig. 2c).
Table 1 classifies the rotation of the direction of sea breeze, for the cases that were clear, in terms of the background flow direction. All SW (except one which was rather SSW), W and NW cases show anticlock- wise rotation. The only NNW case shows clockwise rotation, the N are divided in clockwise and anti- clockwise rotation cases and finally the only NNE case exhibits clockwise rotation. Both N hodographs are rather symmetric: with respect to the v-axis.
Thus, it is inferred that the critical cross-over back- ground direction for shifting from anticlockwise to clockwise rotation is N-NNE, while there is probably a second critical point at SSW. Since no more NE cases were found to enforce the present conclusion, two consecutive sea breeze days developing against moderate northerly vcinds from a recent experiment (summer of 1993, coastline of Saronic Gulf--unpub- lished results) were analysed. The first day had a NNW background wind and the hodograph showed anticlockwise rotation. The next one had a NE back- ground wind and possessed a clockwise rotation. Helmis et al. (1987) also studied two consecutive sea breeze days in relation to the frontal characteristics. Although not commented on in that study, the wind direction traces given in their Fig. 2 show that when there was an easterly flow in the morning, the sea breeze featured a clockwise rotation. The following day had weak background flow from the N and pos- sessed anticlockwise rotation.
Finally, it is noted that sea breezes developed under calm conditions had an erratic hodograph hindering the rotational direction distinction.
If the background wind forcing is accepted as the dominant factor, the anticlockwise-ciockwise rota- tion "transition" can be easily explained by assuming that the pure sea breeze would flow from the SSW. Under a background flow with a westerly component, the sea breeze is forced (deflected) to start as a SW flow (Fig. l la). As the day progresses, the sea-land thermal contrast grows and the SSW flow dominates. Following the maximum intensity of the circulation
(in the early noon), the wind shifts to more westerly directions, since the thermal forcing (mesoscale gradi- ent) is overriden by the background one (note the 1600 LST point in Fig. 1 la). This process would cause an anticlockwise rotation, as also observed.
Similarly, a background flow with an easterly (with respect to the SSW) component forces the sea breeze to start from SE directions and consequently veer, with the aid of the Coriolis acceleration (clockwise rotation), to S, then SW and back to NE. Certainly, it should be anticipated that other factors controlling the flow would obscure the NNE to SSW transition axis.
The present conclusions should be integrated in the context of previous studies concerning the Saronic Gulf sea breeze. Steyn and Kallos (1992) undertook a numerical simulation for a 5 m s - t N N W (330 °) synoptic flow and found anticlockwise rotation over the region of influence of the Saronic sea breeze. This is in agreement with the present experimental cases for NW directions.
On the other hand, Zambakas (1973) and Prezerakos (1986) found anticlockwise rotation in their extended climatological study. In order to ex- plain the anticlockwise rotation of pure sea breeze flows in an inland station, Zambakas speculated that the Saronic Gulf sea breeze is overridden by the in- tense breeze circulation of the open sea, the differing direction of which causes the apparent anticlockwise rotation. The same behaviour of the hodograph at three coastal stations was noticed by Prezerakos, who argued that the regional air circulation is controlled by the pressure gradient between the Aegean Sea and the Greek mainland.
It is worth noting that Bechtold et al. (1991) numer- ically simulated surface wind hodographs at a coast- line in relation to the variation of the synoptic wind direction (at a constant intensity). They found clock- wise rotation for all cases studied and concluded that the major effects on the hodograph shape are imposed by the wind component normal to the shoreline. These results contradict the present observational evidence, but it should be stated that they chose a synoptic wind with an intensity equal to the speed of advance of the sea breeze front.
5. CONCLUSIONS
The influence of the background flow on Saronic Gulf sea breeze has been studied in terms of frontal characteristics and time evolution of the wind vector on the shoreline for a period of two months. A classi- fication of sea breezes based on the background flow direction showed that it is, indeed, the main factor affecting the intensity of the circulation and the preva- lence of the sea breeze current over the background flow.
Concerning the frontal intensity, the results confirm the earlier findings of Helmis et al. (1987) which were
3700 C.G. HELMIS et al.
based on a limited dataset: cases with background off-shore flow or flow parallel to the coastline are more probable to create a strong or weak sea-breeze front, respectively. When the ambient flow is parallel to the coastline and geostrophic balance indicates an on-shore imposed pressure gradient force, the sea breeze velocity and inland propagation may be en- hanced, however the vertical motions induced by the front are weaker compared to the case of an off-shore background flow. The off-shore case studied showed a sharply defined front reaching 200 m in height with vertical velocities up to 2.5-3.0 m s-1.
It is interesting to observe that the acoustic sounder facsimile record revealed front-like activity even for on-shore background flow, although past experience indicates absence of such characteristics. In the case presented in Section 3.1, the initial flow was blowing from the sea, but its upstream path passed over the Salamis Island (Fig. 1) at about 5-km offshore. The presence of this island is considered to significantly modify the lower part of the background flow causing warming and drying of the air. When the sea breeze from open sea sets up, its passage over the shoreline is evidenced as a front transporting unmodified marine air. The concept of taking into account off-shore topo- graphy has been also demonstrated in the recent study of Varvayianni et al. (1993a).
An interesting feature revealed is that the mo- mentum flux is significantly reduced and the heat flux reverses its sign becoming negative (downward) after the sea-breeze front passage. A downward heat flux is in agreement with the temperature profile in the sur- face inversion layer formed by the advection of cool marine air below a relatively warm land air mass. On the other hand, the wind speed increase after the onset of the sea breeze should imply increased downward momentum transport at least near the surface where the wind shear is increased due to the low-level wind maximum. The issue of a possible effect of local topo- graphy around the meteorological mast on the fluxes estimation is not considered responsible, because the acoustic sounder-derived turbulent fluxes showed the same behaviour (Helmis et al., 1994b). It should be stated that the acoustic sounder performs spatially averaged measurements which "filter out" local small-scale surface inhomogeneities. Evidence of a similar behaviour may be found in Chiba (1993). The largest contribution to the turbulent fluxes is made by the energy containing eddies which cover the low part of the frequency spectrum, which then falls sharply in the inertial subrange. In case of flow over a surface discontinuity (sea-land transition in the present case), the large (energy containing) eddies ad- just to the new surface conditions much slower than the smaller scales of motion (Panofsky et al., 1982). Therefore, for measurements performed at the shore- line air coming from the sea maintains its upstream characteristics: the small roughness length above sea should imply low values of momentum and heat fluxes as observed. It would be interesting to perform
the same analysis in an inland station after the growth of the thermal internal boundary layer.
In an experimental study, Prezerakos (1986) com- mented on the temporal variation of the sea-breeze direction on the coast and in inland stations where he mostly found anticlockwise rotation. Steyn and Kallos (1993) using a typical day with NW back- ground flow plotted a surface map of preferred anti- clockwise and clockwise rotation regions over GAA, attributing its basic features to balance between the pressure-gradient and terrain-gradient forcing. In the present study, the rotation of the wind hodograph is analysed in terms of the background flow direction. Two summer months have been covered with intense sea-breeze activity mainly under conditions of non- negligible background flow. It is shown that the direc- tion from which the sea breeze starts to blow is deter- mined by the direction of the background flow: it is therefore assumed that initially the sea breeze does not blow along the sea-land thermal gradient. Its initial direction is the resultant of the latter forcing and the background forcing. The consequent en- hancement of the sea-land thermal contrast as the day progresses, forces the sea breeze to align with the corresponding pressure gradient. This adjusting rota- tion is seen to be anticlockwise if the background flow possesses a western component and clockwise rota- tion if it possesses an easterly component. The major- ity of the sea breezes observed featured anticlockwise rotation, in accordance with previous findings. How- ever, cases with NNE and SSW background flow showed the clockwise rotation as expected by applica- tion of the simple principle presented in this study. Finally, days with almost calm conditions were hard to classify in terms of rotation, possibly due to the development of local circulations, which are elimi- nated under moderate background forcing.
It is suggested that collection of more experimental data are required to provide a climatological signifi- cance to the present conclusions. More specifically, data from several locations within G A A are desired (work is in progress). Subsequently, theoretical efforts for a variety of background conditions could assist in identifying the dominant factors affecting the tem- poral and spatial evolution of the Saronic sea breeze.
Acknowledgements--The present work belongs to the MED- POL framework programme of studies with the financial support of the Greek Ministry of Environment, City Plann- ing and Public Works and UNEP.
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