THE SEA BREEZE AT VENICE, AS RELATED TO DAILY

GLOBAL SOLAR RADIATION

DAR10 CAMUFFO

CNR-ICTR: Istituto di Chimica e Tecnologia dei Radioelementi, I-35100 Camin Padova, Italy and Universitd di Padova, Scuola di Per$eezionamento in Chimica Nucleare, Italy

(Received in final form 15 February, 1982)

Abstract. Although the sea breeze at Venice and on her hinterland is influenced by orography - mainly the Alps - to the north and the PO Valley to the west, the search for a correlation between the frequency of development of the sea breeze and the daily global solar radiation seems to be desirable, and may be useful for the management of emissions from the industrial area near Venice. Three different cases are examined: (i) the sea breeze occuring in the absence of any appreciable gradient wind; (ii) the sea breeze superimposed on a prevailing wind; (iii) the sea breeze not developing at all. The frequency distributions of these cases related to the global solar radiation at Venice are discussed.

1. Introduction

At Venice, due to the proximity of the coast and to the enhanced daytime thermal difference between land and sea, diurnal flow usually develops. The influence of the orography on a larger scale and in particular the differential heating between the Alps and PO Valley (see Figure 1) cause some differences from the usual coastal flows, dominated by an extended mountain-sea interaction (Camuffo et al., 1979). On the contrary, on the northern and western Venetian hinterland where the influence of the Alps and the PO Valley dominates, a diurnal circulation occurs but with no evidence of the arrival of marine air, so that an analysis of mixing ratio is not very useful. For each site in the hinterland, the departures depend on the distance of the site under examination from the Alps and from the sea, as well as on the intensity of the radiation and on the homogeneity of the cloud cover over the Alps and plain. Since the typical sea breeze and other interactions are caused by differences in incoming solar radiation, an analysis of the diurnal circulation compared with the solar global radiation R, seems to be desirable. The overall effect is due to the insolation at Venice, on her hinterland and on the Alps. Only a differential cloud cover over the Alps and over the plain during extended anticyclonic conditions may permit the occurrence of only one of the effects. However, this situation is rather rare and beyond the scope ofthis paper. Thus a statistical approach seems to be useful in order to determine whether an intermittent episode control strategy for management of emissions from the industrial area near Venice would be possible on the basis of local meteorological data and particularly of incoming radiation.

2. The Diurnal Circulation in the Venetian Hinterland

The diurnal circulation in the Venetian hinterland and PO Valley is driven by an interaction between orography (Alps and Apennines), plain and sea. In the morning

Boundary Layer Meteorology 23 (1982) 175-184. 0006-8314/82/0232-0175$01.80. Copyright 0 1982 by D. Reidel Publishing Co., Dordrecht, Holland, and Boston, U.S.A.

176

Ligu rian Sea

Fig. 1, Typical pressure pattern over northern Italy during fine weather favourable to the development of a tertiary circulation. The orography responsible for the flows at Venice (VE) are indicated on the map.

Surface analysis for 8 August, 1972 at 12.00 GMT by Borghi and Giuliacci (1979).

i \

L i3-- --. 3 -csIfl

Fig. 2. Three-hourly pressure tendencies in the summertime, between 06.00 and 09.00 GMT. Rising and faalling tendencies are indicated with sohd and broken lines, respectively. The numbers indicate the absolute

values in tenths of mbar (after Reiter, 1971).

THE SEA BREEZE AT VENICE 177

when the sun is still low, the warming of the Venetian hinterland is generally delayed because of frequent cloudiness and haze. At the same time the Alps tower over the haze layer and offer a surface perpendicular to the incoming solar radiation. Significant warming takes place there and produces a southerly horizontal pressure gradient extending to the northern Adriatic sea coast, which is parallel to the Alpine chain. This pressure gradient is evident in three-hourly pressure tendencies, especially between 06.00 and 09.00 GMT during summer, as reported by Reiter (197 1) and shown in Figure 2. This gradient is locally reinforced by the thermal discontinuity across the coast, so that the sea breeze generally begins earlier in the day and is reinforced. At mid-day, as already shown in Figure 1, the topography causes a deep depression over the orographic features and also causes two isolated high pressure areas, the principal one over the northern Adriatic sea and the second one over the northwestern PO Valley. As may be observed, the isobars converge west of Venice: therefore, the air flowing from the inner PO Valley along the lee of the Alps meets the marine air west of Venice. Since the pressure pattern in Figure 1 develops within a relatively short time and stays for some hours, the flow is due to: (i) the isoallobaric wind which at Venice blows from the sea towards the Alps and is related to the development of the sea breeze; (ii) the southwest gradient wind at Venice. The veering of the breeze is due to the combined effect of the Coriolis force and mesoscale interaction. This circulation has been studied by Gandino (1976), Borghi and Giuliacci (1979, 1980) and Tampieri et al. (1981).

Of course the diurnal tertiary circulation is best developed during summer days characterized by anticyclonic conditions. However, when the weather is not fine over the whole region, the sea breeze develops at Venice but the pressure pattern is reduced and the veering of the wind is due only to the Coriolis force and not to the pressure gradient over the PO Valley. Since Venice falls within the thermal discontinuity between land and sea, the influence of the orography and PO Valley is less effective at Venice than over the hinterland. Nevertheless in the following we will speak of ‘sea breeze’ at Venice even if the coast is overcast and the Alps-sea mesoscale interaction causes the usual diurnal circulation.

In contrast, when the Alps-sea interaction dominates, the wind direction changes simultaneously over the whole hinterland, or may also change near the Alps before it does so near the coast. In addition, the wind direction change is not associated with an increase of the mixing ratio: on the contrary the latter has been observed to be stationary or drop (Camuffo and Bernardi, 1982). Therefore, the diurnal circulation occurs as an isoallobaric wind generated by the Alps-sea pressure gradient, thereby transporting continental air before the arrival-when possible - of marine air. Sometimes the diurnal flow may be shifted to the east by the pressure pattern over the PO Valley, so that, on the basis of the coastal topography, the marine air cannot reach Padova, about 30 km west of Venice, in the region of isobaric convergence. On the contrary, two typical nocturnal flows, i.e., from NE and NW, arrive frontally as shown by a change in mixing ratio immediately after the change in wind direction.

178 DAKIO CAMUFFO

3. Relationships Between Daily Global Solar Radition and Diurnal Breeze

The measured values of the daily global solar radiation R, at Venice, relative to the 9-yr period under examination, were grouped into 50 ly classes. Also the diurnal evolution of the wind vector was analyzed and each day was classified according to the following cases: (i) the sea breeze occurs in the absence of any appreciable gradient wind; (ii) the sea breeze is superimposed on a prevailing wind; (iii) the sea breeze does not develop at all. In the first case (indicated by B), the wind vector describes an ellipse during the sea breeze, beginning from the SSE. In the second (D), the wind is represented by the sum of two vectors: the prevailing gradient wind and the breeze. However, the difference between the hourly vectors of the actual wind behaves as in the first case. In the third group (N) the wind vector is rather constant. Class R is typical of high pressure surface patterns during summertime, when the gradient wind is very light, so that the sea breeze begins late in the morning blowing from the SSE and afterward veers to W until about midnight, when it ceases and is replaced by a land breeze. The local ventilation is essentially due to the tertiary circulation driven by the pressure pattern caused by R, in the region between the Adriatic sea and the Alps. In the present classification the sea breeze must be clearly marked; in addition, during the period typical of the land breeze, the wind speed had to be less than 3 m s- ‘. Cases with light southerly wind during the night and showing the typical diurnal modulation were associated with class D, on the other hand. This is typical of sunny days but with a moderate or fresh wind due to the synoptic pressure pattern, when the mesoscale pressure gradient due to the incoming solar radiation causes a diurnal variation of the wind field. As a consequence, the hourly differences between the instantaneous values of the actual wind and the average wind vector (averaged over the whole day), describe a cycle similar to that in B and the major axis of the ellipse lies between 3 and 10 m s- ‘. Class N is typical of overcast days when the wind is quite constant in speed and direction; alternatively, hourly differences behave differently from case D.

The data relative to each quarter were separately analyzed to reveal seasonal effects if any; finally percentage, absolute and cumulative frequency distributions were com- puted.

3.1. PERCENTAGE FREQUENCY DISTRIBUTION

The percentage frequency distribution shows no seasonal variation except for the winter quarter NDJ (the capital letters stand for the months of November, December, and January, respectively). For this reason the quarters FMA, MJJ, and AS0 were combined in drawing Figure 3 and in computing the distribution function.

We will now consider case B for these three quarters of the year. The percentage frequency distribution Y,, ,, is linearly related to R,:

Y,,, ,. = O.l25R, - 3 (1)

The correlation coefficient is r = 0.97. The extrapolated threshold for the development of a breeze appears to be 24 ly. Since this value does not appear to be physically

THE SEA BREEZE AT VENICE 179

100

80

60

%

40

20

0

0 0 “ey

.’

0 3’ /- /’ 0

0

- ‘\

/ d 1 a

200 400 600 800

Rg( Ly/d) Fig. 3. Percentage frequency distributions of: (i) sea breeze events at Venice (circles); (ii) days with hourly departures of the actual wind according to the vector representing the sea breeze during unperturbed days (triangles) and (iii) days with no evidence of any breeze effect (dots), as a function of the daily global solar radiation R,. The best-fit curves for the three cases (indicated B, II, and N in the text) are also shown (Equations (1) (4) and (6) in the text). All months of the year are represented except November, December

and January.

meaningful, Equation (1) has been recomputed using only values of R, lower than 350 ly, which are more representative. However, the equation remains practically unchanged, namely:

Y B,y = 0.125R,- 3.48 (2)

and the threshold is 28 ly. Since this value does not furnish sufficient energy to develop a sea breeze, we are inclined to conclude that R, measured at Venice is not representative of incoming radiation over the whole hinterland. Unfortunately existing incoming radia- tion data are daily values. As a consequence, for a given R,, the situation is very different if the contribution to R, is due to strong radiation about noon or is due to weak radiation uniformly distributed throughout the whole day; and only on the basis of the hourly values of solar radiation can one predict the B, D, or N event.

During the NDJ quarter, different weather, characterized by fog in the morning or by windy cloudless days (particularly due to the ‘Bora Chiara’ which destroys any breeze) causes a different behaviour. The frequency of the sea breeze becomes:

Y B,NDJ = O.O6R,- 1.32 ;

this equation and the observed data are shown in Figure 4. The ‘threshold’ is 22.4 ly, very similar to the value quoted for Equation (1). The range of variability of R, in this quarter

180 DAK10 CAMUFFO

100

80

60

%

40

- I?‘\ - l \. a - - \‘.\ ,

l A

// -

\ d/ /

/

20

O- 0 100 200 300

Rg (Ly/d>

Fig. 4. As in Figure 3 but for the November to January quarter. The equations in the text for this quarter are (3), (5), and (7) for the events B, D, and N, respectively.

is between 25 and 250 ly with the exception of two outliers. Thus, throughout the whole year, the frequency of the sea breeze is linearly dependent on R, and is halved during NDJ.

With reference to case D, with low values of R,, the frequency distribution Y, is expected to be proportional to the incoming radiation as in the previous case. For high values of R,, on the other hand, Y, is expected to decrease since strong solar radiation supplies sufficient energy to develop a sea breeze. Therefore, the event D tend to be transformed into B except during the ‘Bora Chiara’ which mainly occurs in NDJ. Except for the winter quarter, the empirical formula of YD,>, is:

YD,>S = (-0.004R; + 36R, + 6300)"2- O.O25R,- 77 (4)

which is shown in Figure 3. The maximum is at R, = 375.7 ly at which point Y,,,.(375.7) = 32.8%; the frequency becomes zero for R, = 803.2 ly.

In NDJ the frequency distribution is concave in contrast with the preceding case, due to the completely different climatological situation. The observed data can be fitted by the empirical formula shown in Figure 4:

Y *, NDJ = 121.32 - (-R;,’ - 16R, + 11 396)“2 - O.O6R,. (5)

The percentage frequency distribution of events with no evidence of any breeze effect (case N) is:

THE SEA BREEZE AT VENICE 181

Y N.y = 100 - vI3.y + Y&J =

= 180 -(O.O4R, + 36R, + 6300)"*- 0. lR, (6)

for all months except the NDJ quarter. YN, y decreases and tends to vanish at the highest values of R,, when strong solar radiation can develop the breeze (B) or perturb the gradient wind (0). Equation (6) does not vanish asymptotically, but has its minimum at R, = 7 18.3 ly when YN, y (7 18.3) = 0.84%. At the upper limit of R, at Venice, Y,,(750) = 1.08”/ O, so that the frequency remains more or less constant at about 1% for the highest intensities.

For NDJ we obtain:

Y ,,,, NDJ = (-0.04R; - 16R, + 11 396)“2 - 20 . (7)

These equations are shown, together with the observed data, in Figures Tand 4.

3.2. ABSOLUTE AND CUMULATIVE FREQUENCY DISTRIBUTIONS

The above distributions Y,, Y,, Y, furnish, respectively, the percentages of events B,

k- 95 E t-

90 1

i

I 70 I--

l-

% 50 I-

c

30 1

10

5

I

I

‘4, ‘M \

‘.

I

0 200 400

Rg( Ly/d) Fig. 5. Cumulative frequency distributions ofthe events B (circles and dashed-dotted line), D (triangles and dashed line) and N (dots and solid line) at Venice, versus daily global solar radiation R,. Each quarter is

indicated with the initial letter of the middle months, i.e., D = NDJ, M = FMA, S = ASO.

182 DAR10 CAMUFFO

D, N, for a given R,. Therefore, the absolute frequency distributions of the three cases depend for each period on the frequency distribution of R,. The latter has been extensively described for Venice in a previous paper (Camuffo, 1978). In order to provide a quick method to evaluate the absolute frequency distributions, the data have been submitted to a graphical scrutiny for testing if a gaussian distribution can be used as a practical first-order approximation. Although the use of probability paper is not a rigorous test of normality, it nevertheless enables one to judge whether the population may be approximated by a normal distribution.

By plotting the cumulative frequency distribution on arithmetic probability paper, the graphical estimate ofthe theoretical straight line and therefore the mean values pe, pLo, and pN and the standard deviations o,, o,, and o, for each quarter have been computed, except for Y, and Y, in MJJ, when gaussian distribution is not a good approximation. In particular, cases B and D show a different character at R, greater than 650 ly, due to meteorological conditions for the highest intensities, generally occurring after showers. In this quarter also the distribution of R, is not gaussian, which causes the cumulative frequency distribution to be not normal in character. The values of p and 0 so obtained are reported in Table I. The cumulative frequency distributions are shown

TABLE I

Mean values p and standard deviations o of the frequency distributions of the B. D, and N events related to the R, in the quarters NDJ, FMA, MJJ, AS0 as

obtained from arithmetic probability paper

quarter

NDJ 182 62 165 57 132 62 FMA 425 134 372 137 228 142 MJJ - - - - 362 182 AS0 469 142 390 129 296 136

in Figure 5 for NDJ, FMA, and AS0 and in Figure 6 for MJJ. The straight lines are parallel and their spacing depends on the mean value of R, during each quarter. The winter quarter NDJ shows a different slope due to different climatological conditions.

4. Conclusions

Since the sea breeze is dependent on both sea-land thermal difference and the pressure pattern determined by the warming of the Alps, the development of the sea breeze can be reinforced or begin earlier due to interactions. Some differences from the classical sea breeze can be noted inland, where the pressure gradient between the Alps, plain and sea dominates and where the beginning of the diurnal breeze is not associated with the frontal arrival of marine humid air. However, even when the sky is overcast at Venice, the Alps-sea interaction may generate a weak diurnal breeze blowing in the usual direction. This work was developed in order to improve our knowledge of the complex Alps-plain-

THE SEA BREEZE AT VENICE 183

%

70 f-

i d

L-l-1 i 0 200 400 600 800

R&/d)

Fig. 6. As in Figure 5 except for the MJJ quarter.

sea interaction as a necessary basis for a prediction of events B, D, N. Of course the study was conducted with available data, i.e., with daily totals of R,.

We have seen that at intermediate values of R,, the three distributions Y,, Y,, Y, are of the same order, so that, except for the extreme values of R, where only one of the three events prevails, the above relationships seem to be of little use for prediction. In a practical application, however, a prediction in the morning would consider only hourly values of the incoming radiation. Of course, the prediction would be improved with knowledge of radiation at two or possibly three stations inland and obviously, of the wind force. The latter is an additional useful tool for local forecasting. As an example, a fresh wind at Venice excludes case B but is not able to decide between cases D and N. Moreover, a southern wind crossing the Alpine chain may cause a Dark Bora on the northern Adriatic, which necessarily leads to case N; similarly when a high pressure area approaches the Alps from western Europe, some cold-air outbreaks from the NW and NE invade the Venetian hinterland after crossing the Alps through the Brenner and Trieste gates; the cold pools may generate instability and showers. The class may be D or N depending on the intensity of the event.

184 DAR10 CAMUFFO

It appears that a unique criterion cannot be applied for forecasting the winds at Venice. The best predictions are not obtained automatically with the help of some categorization, but only through a knowledge of the local dynamical climatology supported by both synoptic analysis and some objective criteria based on local data.

References

Borghi, S. and Giughacci, M.: 1979, ‘I Rilievi Appenninici come Fattore Condizionante della Circolazione Atmosferica della Valpadana’, Proc. of fhe Meeting on Apennine Meteorology, Reggio Emma, Italy, 7-10 April, 1979.

Borghi, S. and Giugliacci, M.: 1980, ‘Circulation Features Driven by Diurnal Heating in the Lower Atmospheric Layers of the PO Valley’, II Nuovo Cimento 3C, 1 - 16.

Cam&o, D.: 1978, ‘Cumulated Frequency Distribution of Daily Global Solar Radiation at Venice, Italy’, Arch. Met. Geoph. Biokl. Ser B 26, 45-50.

Camuffo, D., Tampieri, F., and Zambon, G.: 1979, ‘Local Mesoscale Circulation over Venice as a Result of the Mountain-Sea Interaction’, Boundary-Layer Mefeorol. 16, 83-92.

Camuffo, D. and Bernardi, A.: 1982,‘The DiurnalTrend ofthe Mixing Ratio with Reference to the Convective Activity of the Planetary Boundary Layer’, Boundary-Layer Meteorol. 22, 273-282.

Gandino, C.: 1976, ‘The Influence ofthe Alps on the Diurnal Winds’, Riv. Italiana di Geojsica e Science A@i 3, 150- 152.

Reiter, E.: 1971, ‘Digest of Selected Weather Problems of the Mediterranean’, Navy Research Foci/if!,, Technical Paper No. 9 - 7 1.

Tampieri, F., Scarani, C., and Trombetti, F.: 1981, ‘Summer Daily Circulation in the PO Valley, Italy’, Geophys. and Astrophys. Fluid Dynamics II, 97- 112.