LOCAL MESOSCALE CIRCULATION OVER VENICE AS

A RESULT OF THE MOUNTAIN-SEA INTERACTION

D. CAMUFFO*, F. TAMPIERlt, and G. ZAMBONt

* CNR, Laboratorio di Chemica e Tecnologia dei Radioelementi, Padova t CNR, Microfisica dell' Atmosfera, Bologna t CNR, Laboratorio per 10 Studio della Dinamica delle Grandi Masse, Venezia (Italy).

(Received in final form 7 October, 1978)

Abstract: The local sea-breeze circulation in Venice is found to have typical characteristics but the nocturnal flow is found to be quite complex .. This latter result is due to unequal cooling of the Alps and adjacent Iow-lying regions. The horizontal and vertical structure of the flow has been investigated using data obtained from surface anemometers and pilot balloon observations. Results show that the circulation is about 500 m in depth and achieves a maximum speed of 6 ms-I.

1. Introduction

The winds at Venice, located on the Tre Venezie Plain, are due to a combination of large-scale and local effects associated with the Alps, the plain, the lagoon, and the Adriatic Sea; see Figure 1. The most frequent large-scale flows in the area are the north-easterly Bora and the southerly Scirocco, both well known since Filiasi's study in 1794, while the most frequent daytime small-scale flow is the summer sea breeze, as documented by Montanari (1931) and Crestani (1933).

The large-scale flows are generally associated with high wind speeds, producing low pollutant concentrations in Venice; the small-scale flows are associated with low speed anticyclonic regimes, producing high pollutant concentrations. During these latter periods, the circulations in Venice are due to land-sea temperature differences and orographic effects.

The current paper concentrates on the characteristics of local-scale circulations, which are more common than the large-scale flows and sometimes are associated with high pollution episodes in Venice. The objective is to estimate the horizontal and vertical structure of the atmospheric boundary layer over the complicated topography near Venice.

2. Data

The study utilizes wind data collected on a routine basis at surface sites and data collected during special experimental test periods. The routine data were obtained from sites operated by the Meteorological Service of the Italian Air Force at St. Nicolo and Tessera, by the Hydrological Service at Padova, and by the National Research Council (CNR) Laboratory (Venice) at an offshore platform and in the center of the city (Figure 1). Standard cup anemometers were used at the sites

Boundary-Layer Meteorology 16 (1979) 83-92. 0006-8314/79/1601-0083$01.50 Copyright © 1979 by D. Reidel Publishing Company, Dordrecht, Holland, and Boston, U.s.A.

84 D . CAMUFFO ET AL.

N

* Platfo rm .

Fig. 1. Area of study showing observational sites and sea breeze axis of symmetry.

operated by the Meteorological and Hydrological Services, while special low-inertia aerovanes (Camufio, 1974) were used at the two CNR sites. The instrument height at all sites ranged from 15 to 25 m in order to reduce the influence of local obstacles. The minimum detectable speed for each of the instruments was 0.5 m s -1, and observations were averaged over lO-min intervals.

Experimental test periods were carried out in 1974 during 17-21 June, 22-26 July, and 14-18 October, and 1975 during 13-18 January. During these periods, both the vertical and horizontal structure of the wind field was investigated. The vertical structure was measured by use of double-theodolite tracked pilot balloons launched in Venice by the Center for Thermal and Nuclear Research of the National Electric Power Board and by the Cosmogeophysics Laboratory of CNR. Additional information concerning the vertical structure of the boundary layer was obtained using a monostatic acoustic sounder (Sodar) operated at a site in Venice by the Community Research Center of EURATOM and by slow-rate-of-rise radiosondes launched at various sites within the study area by CNR (Venice) and by EURATOM.

MESOSCALE CIRCULATION OVER VENICE 85

The horizontal structure was measured at five ground stations, aligned as shown in Figure 1. These are: (1) the CNR platform, 15 km offshore; (2) St. Nicolo, on the shoreline; (3) midway in the lagoon, at Venice; (4) Tessera, on the inner shore of the lagoon; (5) a station 15 km inland.

During the October campaign the synoptic situation was ideal, permitting the development of a sea-breeze circulation quite undisturbed by large-scale circula­tions.

3. Results

A. SURFACE WINDS: A STATISTICAL ANALYSIS.

A plot of the hourly distribution of wind direction at Venice during the months of high isolation (May to October) is shown in Figure 2. This distribution reveals that the most frequent daytime flow direction (SSE) is almost along the 'axis of symmetry' defined by bisecting the angle formed by the lines parallel to each coastal segment

,-.. ~ Cl) ...J

w ~ ~

DIRECTION

o 0-49

D 50-99

W:::::l@ lOO - 149

.. 150 -199

_ 200 -249

_ 250 -300

KEY

Fig. 2. Hourly distribution of wind direction in Venice during May to October. The numbers indicate the total cases that occurred over the 4-year period.

86 D. CAMUFFO ET AL.

(Figure 1). It also shows that the most frequent night-time flow direction (NE) is perpendicular to this axis. Thus the most frequent daytime flow direction is asso­ciated with a local sea breeze circulation, while the most frequent night-time direction is rotated by 90° in the clockwise direction. The complex explanation for this result is given in the following analysis.

In order to obtain insight into the dynamics of the local circulations, hourly wind distributions were analyzed during periods without significant synoptic-scale flow. During one such period (29 July, 1974 to 5 August, 1974), at about 1000 LST, the wind at the three sites shown in Figure 3 started to blow in a direction generally parallel to the 'axis of symmetry'. The sea-breeze circulation continued until about midnight, when the circulation at the two stations near Venice came under the influence of a north-easterly flow originating from the section of the Alps north and northeast of the Tre Venezie Plain. A similar situation is shown in Figure 4, representing the period 3-9 September, 1973.

The origin of the nocturnal flow is found in the effects on the pressure field induced by horizontal temperature gradients associated with unequal nocturnal cooling of the

OO~GMT

SURFACE ANALYSIS

DIRECTION

NNEEsESSWWNW o

.~ 12

,. 18

21

24-'--------:"-'------"

KEY CJ 1-2

DIRECTION

o

24 -'------'-.L.--.L~--,-_---' S NICOLO'

12

,. 18

21

DIReCTION

NNEEsESSWWNW

PADOVA

_ '-6 _ 1-8

Fig. 3. Hourly distribution of wind direction at three sites during the period from 291uly to 5 August, 1974. The numbers indicate the total cases occurring.

21

MESOSCALE CIRCULATION OVER VENICE

03 SEPTEMBER 1973 06' GM T

SURFACE ANALYSIS

DIRECTION

CJ 1-2

12

EO 3-4

DIRECTION

o

OFFSHORE PLATFORM

DIRECTION

_ 5-6

Fig. 4. As in Figure 3, for the period from 3-9 September, 1974.

87

Alps and adjacent low-lying regions (both land and sea). In fact, the high thermal capacity of sea water strongly reduces the diurnal amplitude of both air temperature and pressure over the sea. On the contrary, the daily thermal cycle in the mountain region is highly dependent on solar radiation. Thus whereas air density is continu­ously changing on the mainland, the lines of equal pressure tend to follow the coast. This effect is illustrated in the mean isallobaric pattern found during the early evening hours in summer when it reaches its maximum value. A complete documen­tation of the diurnal variation of this pattern is found in Reiter (1971) from which Figure 5 is reported. During periods of near calm synoptic circulation, the is alia baric wind originates in the Alps as a katabatic wind and initially constitutes the total regional flow; after being turned by the Coriolis force, however, the wind becomes parallel to the isallobars and thus to the coastline. In the Venetian area, this flow is substantially due to both (i) the very strong isallobaric gradient which always stays in the same direction, so that the surface isobaric pattern deepens and deepens, with the isopleth parallel to the coast; and (ii) the large distances between the Alps and the

88 D CAMUFFO ET AL

Fig. 5. Pressure change in mb during summer from 1800 to 2100 GMT from Reiter (1971).

sea, so that the flow may assume the observed northeasterly direction, consistent with the geostrophic approximation.

On the western boundary of the plain, the wind field is affected by the channeling of the orographic extensions, so that at Padova a north-western katabatic flow predominates (see Figures 3 and 4).

B. A CASE STUDY: FROM 17 TO 18 OCTOBER. 1974

B 1. Vertical Structure

To determine the depth to which the nocturnal mountain breeze penetrates, pilot balloon observations were used to construct time sections of the airflow over Venice. Figure 6 shows the components of the flow along the principal direction of the nocturnal flow and along the normal one.

Sodar data were used to estimate the height of the top of the mountain circulation during periods with no piballaunches. Isotachs constructed during these periods are shown as dashed lines.

The nocturnal local mountain circulation is first evident at a height of about 150 m at about 1800 LST and is accompanied by a 'calm' area in the lowest 100 m; this lasts for about 5 h. During the next 9 h, the depth of this flow increases upward and downward (reaching the surface at 2300 LST) so that at its time of maximum vertical penetration, it reaches a height of about 500 m. The maximum intensity measured for this flow is about 6 m s -1 at a height of about 200 m. The negative flow centered at 600 m during the. hours after midnight is not a return flow, but is part of the large-scale circulation.

The dissipation of this mountain breeze is associated with convective activity due to surface heating and to the initiation of the sea-breeze circulation. The transition

MESOSCALE CIRCULATION OVER VENICE 89

,2 '5 '8 2' 00 03 06 09 '2 LST

18 OCTOBER 1974

(a)

12 15 18 21 00 3 6 9 12 lST

18 OCTOBER 197.

(b)

Fig. 6. Vertical time cross-section of (a) the northeast component and (b) the northwest component of the wind (in m S-I) in the Venetian area for 17-18 October, 1974. Light shading indicates negative components of the wind; heavy shading refers to wind speeds below 0.5 ms-I. The arrows indicate the top

of each sounding.

period between mountain and sea breeze on the 18th is somewhat complex, but it is interesting to note that the surface 'calm' periods associated with this process on this day and the preceding one are centered at almost identical times. Again a component of large-scale circulation is evident at upper levels throughout the entire period.

B2. Horizontal Structure

During the October test period, the horizontal time cross-section of the surface wind field is shown in Figure 7, where the wind components, parallel and normal to the symmetry axis in Figure 1, are shown. The grid time scale is 30 min and the space scale is as indicated in Section 2.

The wind component parallel to the northeastern coastline appears to behave quite regularly with increased intensity over the inner beach during the day, and over

90 D. CAMUFFO ET AL.

18 OCTOBER 1074

(a)

®

12 15 la 21 00 3 6 9 12 lST

IS OCTOBER 1974

(b)

Fig. 7. Horizontal time cross-section of (a) the northeast component and (b) the northwest component of the wind (in m S-I) in the Venetian area for 17-18 October. 1974. The cieled mimbers refer to the stations

indicated in Section 2. Shadings as in Figure 6.

both sea and lagoon at night. During the day, the maximum is due to the develop­ment of the sea breeze across the thermal discontinuity. At night the explanation is somewhat different. Due to radiative nocturnal cooling, the ground is shielded by a stable air layer over which the air flows, whereas the sea and lagoon can sense the nocturnal flow. The abrupt change in roughness between water and land causes deflection of the surface wind, which is to be found in the isopleth diagram relative to the normal component.

Finally in Figure 8 we give an example of the horizontal gradient that may exist between land and sea.

4. Conclusions

The local sea breeze in Venice has typical characteristics, whereas the nocturnal flow is quite complex. It is never a land breeze. and only sometimes does it arise in one of

MESOSCALE CIRCULATION OVER VENICE 91

_1151-1(1) ___ II5I-1Il1 00_000 T! 11 - Tl31

-I

12 1. .11 21 11 11 LSI

11 ICTIIEI 1174

Fig. 8. Temperature differences between some of the stations (numbers refer to Section 2) for 17-18 October, 1974.

the various forms of the classical Bora, which represents some type of barrier­modified flow.

The nocturnal flow in the Venetian region does in fact arise due to effects on the pressure field induced by horizontal temperature gradients associated with unequal nocturnal cooling of the Alps and adjacent lowland regions. This flow turns out to be from the northeast in the Venetian area and from the northwest in the area of Padova. In one well documented case, the flow was initiated first at 150 m and at about 1800 LST, growing upward and downward (to the surface), reaching a maximum height of 500 m and a maximum speed of 6 m s -1 at a height of 200 m. The breeze was preceded and followed by near surface calm periods.

It is to be noted that during both 17th and 18th, the sea breeze appeared at about 1230 LST, with an inversion of the wind in the vertical, whereas the nocturnal circulation appeared frontally. Particularly, during the 18th, the development of the front followed the equation: h = 200 .ft, where h is the height (in m) of the internal boundary of nocturnal circulation and t is the time (in h) after the passage of the front.

This behviour was verified four times during summer and autumn field tests but can be only tentatively proposed. It is to be noted, however, that even if evidence is limited only to few cases, the Sodar facsimile confirmed the results in every case. These and other case studies show that thermally- and dynamically-induced layers behave quite independently. Such a situation causes a very complicated system of blackenings on the Sodar facsimile, each one related to the development of a special

92 D. CAMUFFO ET AL.

event, which can be correctly interpreted only by comparison with other vertical soundings relative to the temperature, humidity and wind fields.

Both the horizontal and vertical time cross-sections of the wind field show a complex structure. This result turns out to be a very general one for the Venetian lagoon, so that steady-state diffusion models cannot be expected to apply. Instead, pollution models must account for the complex flow.

Another result is that the vertical wind field can not be inferred on the basis of ground observations. A statistical analysis of the flow patterns carried out by Montanari (1931) agrees with the current results.

Finally, the nocturnal orographic influences in the Tre Venezie Plain are some­what different from their effects in the adjacent Po Valley, as reported by Gandino (1976). The nocturnal mountain winds in Venice have been shown to be parallel to the Alps, while in the Po Valley they were found to be perpendicular to the slopes. This difference exists because the Po Valley is bounded both to the north and south by mountains, while the Tre Venezie Plain is bounded by mountains to the north and by the Adriatic Sea to the south.

Acknowledgements

We are indebted to ENEL-CRTN (Trino Vercellese), to CNR-Cosmogeofisica Laboratory (Torino) to Aeronautica Militare Italiana, to Magistrato alle Acque, to Regione Veneto and to the Comune di Venezia for their support and co-operation.

In addition we would wish to thank Profs. G. Fea, S. Palmieri, O. Vittori, D. Anfossi for their stimulating discussions. Finally, we would like to thank Prof. R. D. Bornstein for review of the manuscript.

References

Camuffo, D.: 'A Multipurpose Integrating Anemometer', European Symposium on Offshore Data Acquisition Systems, 199-207.

Crestani, G.: 1933, 'La laguna di Venezia' (ed. Ferrari Co., Venezia), 305 pp. Filiasi, G.: 1794, 'Memoria delle procelle che annualmente sogliono regnare nelle maremme Veneziane',

Venezia 64 pp. Filiasi, G.: 1828, 'Osservazioni sopra le vicende annuali atmosferiche di Venezia e paesi circumvicini',

Venezia,36-37 Gandino, c.: 1976, 'The Influence of the Alps on the Diurnal Winds" Rivista Italiana di Geofisica e

Scienze Affini 3,150-152. Montanari, D.: 1931, 'Contributo allo studio sui regime anemologico a Venezia', L'aerotecnica 11,

741-759. Reiter, E.: 1971, 'Digest of Selected Weather Problems of the Mediterranean', Navy Research Facility,

Technical Paper no. 9-71, 2199 pp. Runca, E., and Zanetti, P.: 1973, 'Un'indagine preliminare sui problema dell'inquinamento atmosferico

nell'area Veneziana', IBM Venice Scientific Center, Report no. 7, 85 pp.