MAY 2002 1317B A R G A G L I E T A L .

q 2002 American Meteorological Society

An Integrated Forecast System over the Mediterranean Basin: Extreme SurgePrediction in the Northern Adriatic Sea

A. BARGAGLI, A. CARILLO, G. PISACANE, P. M. RUTI, M. V. STRUGLIA, AND N. TARTAGLIONE

Climate Section, ENEA, Rome, Italy

(Manuscript received 4 January 2001, in final form 24 September 2001)

ABSTRACT

A previsional system for the forecast of the state of the Mediterranean Sea and of surges in the northernAdriatic Sea has been designed and tested. The system consists of a limited area model (the Bologna LimitedArea Model, BOLAM), which computes high-resolution pressure and surface wind fields, to serve as the inputof a wave model (WAM) and of a shallow water model (two-dimensional Princeton Ocean Model, POM-2D).

Results of two case studies are presented, chosen to test the ability of the system to predict extreme surgeevents originating from cyclonic circulation over the Mediterranean basin. The two case studies cover the periods4–9 October 1998 and 11–22 November 1996. Comparison with observed buoy data shows that the results ofthe WAM simulations forced by the wind fields obtained from the high-resolution BOLAM model are significantlybetter than those obtained using a lower-resolution forcing from the ECMWF analyses.

Two domains corresponding to the entire Mediterranean basin and to the Adriatic–Ionian basin, respectively,have been used to run the POM-2D model. It is found that, in the Adriatic–Ionian run, the simulated sea elevationshows some discrepances in the amplitudes of the main peaks and in their phases, which are removed byextending the domain of integration to the entire Mediterranean. This better performance is due to the correctrepresentation of the principal barotropic modes and of the pressure forcing on the basin.

1. Introduction

The city of Venice often experiences flooding eventsinduced by abnormally high water levels at the threeentrances of the lagoon. Piling up of water in the north-ern part of the Adriatic basin is usually produced by adepression pattern, typical of extratropical cyclones,which causes the surface pressure gradient to align withthe major axis of the Adriatic Sea. The phenomenon isreinforced by the effect of local orography, forcing low-level winds to blow from the southeast along the sameaxis. As a consequence, water levels significantly higherthan the expected astronomical tide level are reportedseveral times a year, especially between October andJanuary.

Moreover, during recent years subsidence has devel-oped, partly as a by-product of urban and industrialdevelopment of the inner land: between 1900 and 1970,the whole area of the lagoon has ‘‘sunk’’ as much as23 cm. Future prospects are not reassuring, and allowneither optimistic reliance on a reduction in the fre-quency of flooding events, nor understatement of thedanger and costs implied by their intensity.

Due to the frequency of flood events in Venice, at-

Corresponding author address: Dr. A. Carillo, ENEA, Sc.P. 91,Via Anguillarese 301, 00060 Roma, Italy.E-mail: adriana.carillo@casaccia.enea.it

tention has been devoted to the development of a reliablewarning system. A statistical model based on 50 pre-dictors, represented by tidal levels and atmosphericpressure values at different stations, has been operativein Venice (Canestrelli and Pastore 1997) for more than10 yr. However, this model is useful only for short-rangeforecasts, which could possibly be extended using theforecast atmospheric pressures as predictors. However,as this kind of model is based on the hypothesis of theanalogs, it is hard to capture events not experiencedbefore by the system.

As mentioned by Cavaleri (1999, section 9.2), a pos-sible solution to overcome these problems is a forecastsystem based on the numerical integration of the dy-namical equations. Such an approach had already beenused by Finizio et al. (1972) to simulate the surge inthe northern Adriatic Sea with a one-dimensional oce-anic model. In recent years, more complex systems,based on the coupling among atmospheric and oceanicmodels, have been developed (Lionello 1995; Lionelloet al. 1998a,b).

These systems consist of three modules: a limited area(LAM) atmospheric circulation model, a wave model,and a shallow water model. The regional atmosphericcirculation model provides a high-resolution forcing forthe wave model (surface wind) and for the shallow watermodel (pressure field and surface wind).

High-resolution (of the order of 10 km) forcing, not

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FIG. 1. Finite-difference grids for the operative meteorologicalcodes (HR outer domain, VHR inner domain).

available from current general circulation models(GCMs), is needed mainly because the Mediterraneanbasin is surrounded by a complex orography that has astrong influence on the atmospheric flow. This influence,ranging from local to synoptic scales, can produce pre-cipitating clouds through orographic lifting, triggeringof convection, indirect effects of flow splitting or block-ing, induced waves, and channeling of the wind.

Surge prediction systems have also been developedin other key areas. A numerical model for the westerncoast of Norway was developed by Martinsen et al.(1979), based on the depth-integrated shallow waterequations. Their simulations show that the wind stressand the atmospheric pressure are of about equal im-portance in determining the largest storm surges.

The western coast of Alaska experiences coastalflooding events due to the storm surges associated withextratropical cyclones. A storm surge model forced bypredicted large-scale atmospheric fields has been usedby Blier et al. (1997) to simulate surge events, as apreliminary work for an operational implementation. Inthis case, the surge model is based on the depth-inte-grated quasi-linear shallow water equations, while theatmospheric forcing (mean sea level pressure and sur-face wind) is obtained from the National Centers forEnvironmental Prediction (NCEP) analysis for the hind-cast experiments.

An area where the prediction of storm surges is evenmore crucial, since coastal floods are one of the majorcauses of loss of life, is the Bay of Bengal. A numericalmodel based on the nonlinear shallow water equationshas been used for simulating storm surges in this area,on a grid covering both open sea and some estuarinechannels (Flather 1994). A high-resolution shallow wa-ter model has instead been used by As-Salek (1997) topredict negative surges that destroy coastal aquacultureinstallations in the area of the Meghna estuary, in thenorthern part of the Bengal Bay.

Some recent papers (Hubbert et al. 1990; Tang et al.1997) have been concerned with the study of the effectof tropical cyclones on storm surges on the Australiancoasts, which are strongly related to the excitation ofcoastally trapped waves [a theoretical framework canbe found in Fandry et al. (1984)]. Tang et al. (1997)have shown that a depth-integrated barotropic numericalmodel is appropriate both for the simulation of the con-tinental shelf waves and of secondary effects, such ascoastally trapped waves. Hubbert et al. developed astorm surge model using a depth-integrated shallow wa-ter model forced by wind stress and pressure gradientscomputed by a regional atmospheric model. In a suc-cessive work (Hubbert et al. 1991) the model developedinto a real-time system for forecasting tropical cyclonestorm surges.

In this work, we describe an operational forecast sys-tem for the prediction of high water levels in the lagoonof Venice and of the state of the Mediterranean Sea,which has been recently developed at the National En-

vironmental and Energy Agency (ENEA). The operativeintegrated system consists of a limited area model (theBologna Limited Area Model, BOLAM), coupled witha wave model (WAM) and a high-resolution shallowwater model (the two-dimensional Princeton OceanModel, POM-2D) of the northern Adriatic Sea. BothBOLAM and WAM cover the whole Mediterranean ba-sin. The atmospheric model is run over a wider domainat a resolution of 0.38, using coarser boundary data fromthe European Centre for Medium-Range Weather Fore-casts (ECMWF), and on a smaller nested domain at aresolution of 0.18 (see Fig. 1). In the operative config-uration of the system, water levels at the entrances ofthe lagoon computed by POM are given as input to afinite-element model of the lagoon itself, to predict sur-face levels in Venice.

We concentrate, in particular, on the ability of thesystem to predict surge levels in the northern AdriaticSea and focus on the POM-2D tuning needed to improvethe prediction skill. A critical evaluation is given of thesystem performance for two relevant case studies.

This paper is organized as follows. In section 2 wedescribe the integrated forecast system and the modelsused. In section 3 there is a description of the two me-teorological cases studies and of the WAM performance,while section 4 contains the results of the surge simu-lations. Section 5 summarizes the main conclusions ofthe study.

2. The modeling chain

The quality of the modeling chain’s results dependson the quality of the atmospheric forcing (mean sea levelpressure and surface wind). As mentioned in the intro-duction, this is strongly affected by the representationof the complex orographic features present in the area.International scientific programs such as the Alpine Ex-periment (ALPEX; Speranza et al. 1986), the PyrennesExperiment (PYREX; Bougeault et al. 1990), and theMesoscale Alpine Programme (MAP; Bougeault et al.

MAY 2002 1319B A R G A G L I E T A L .

FIG. 2. Finite-difference grid for VHR model used in thesimulations.

2001; http://www.map.ethz.ch/map-doc/) have been or-ganized in the last 20 years in the European area withthe aim of improving the understanding and the mod-eling of the impact of mountainous massifs on the at-mospheric behavior. In the PYREX framework a directcomparison of analyses produced by the ECMWF GCMand by a mesoscale atmospheric model (PERIDOT)showed that the mesoscale model gives a more realisticrepresentation of the orographic pressure drag (Lott1995). The same mesoscale atmospheric model PERI-DOT, run with a 10-km grid mesh and 40 sigma levelsin the vertical, has been compared with observations ofthe PYREX campaign (Masson and Bougeault 1996),showing good skill by the model in reproducing themesoscale features of the wind field.

In this study, in order to achieve a satisfactory rep-resentation of the orography surrounding the Adriaticand the central Mediterranean Sea, the limited area mod-el BOLAM has been used. A nesting in two steps hasbeen set, which allows a smooth transition from thecoarser resolution of the boundary data (ECMWF anal-ysis with a spectral truncation of T213) to a very highresolution (VHR) grid in the region of interest. There-fore the VHR BOLAM (10-km resolution) is driven bya high-resolution (HR) BOLAM (30-km resolution),which is in turn driven by the ECMWF analysis. Bound-ary data for the HR model are given every 6 h, andevery 3 h for the VHR model. As we are using analysesas input for the LAM chain, the numerical experimentsperformed cannot be considered a forecast but a hindcastsimulation. Both the HR and VHR models run with 40sigma levels. Since we are mainly concerned with thePOM model setup for an accurate prediction of the seasurge in the northern Adriatic Sea, the VHR domain isconfined to the area between 29.58–468N and 98–278E,which is displayed in Fig. 2.

The HR and VHR model hindcasts start every day at0000 UTC. For each VHR hindcast we skipped the first6 h (spinup time) and we used the next 24 h to produce

a continuous forcing dataset for the WAM and POMmodels. WAM is forced by the surface wind, whilePOM-2D is driven by the surface wind stress field com-puted by WAM and by the sea level pressure field com-puted by BOLAM.

a. The atmospheric limited area model: BOLAM

The atmospheric model used in the system was de-veloped at Consiglio Nazionale delle Ricerche-Istitutoper lo Studio dei Fenomeni Fisici e Chimici della Bassaed Alta Atmosfera (CNR–FISBAT), Bologna, Italy(Buzzi et al. 1994). BOLAM is an explicit, primitiveequation, hydrostatic, three-dimensional gridpoint mod-el that uses pressure-like vertical coordinates (sigma co-ordinates). Horizontal discretization is performed on theArakawa C grid, with rotated latitude and longitude asindependent variables. The model integrates the equa-tions of momentum, mass continuity, and energy con-servation on a regular latitude–longitude grid. The prog-nostic variables are latitudinal and longitudinal windcomponents, potential temperature, specific humidity,and surface pressure. The time integration of the prog-nostic advection equations is performed using the for-ward–backward advection scheme (FBAS; Malguzziand Tartaglione 1999). A fourth-order horizontal hy-perdiffusion operator is applied to the prognostic vari-ables, and divergence damping of momentum is usedto reduce the growth of gravity waves. Both horizontaldiffusion and divergence damping are computed usingthe Euler scheme. An adjustment loop has been imple-mented, using a forward–backward scheme with re-duced time step, for those terms in the primitive equa-tions describing fast gravity modes. Model physics in-cludes vertical diffusion (Louis et al. 1981), soil waterand energy balance, a two-band radiation scheme (Ge-leyn and Hollingsworth 1979), dry-adiabatic adjust-ment, cumulus convection (Emanuel 1991), and large-scale precipitation. Relaxation to boundary conditionsis performed using the Davies scheme (Davies 1976),modified by Lehmann (1993), and is applied to all prog-nostic variables.

The model has been tested in a variety of differentatmospheric situations (Buzzi et al. 1994) and has scoredwell in a comparison with other state-of-the-art LAMsin the context of the PYREX experiment (Georgelin etal. 2000).

b. The WAM model

WAM is a third-generation wave model (WAMDIGroup 1988). It describes the evolution of the wavespectrum by solving the wave energy transfer equation.It has no constraints on the spectral shape, whose evo-lution depends on the spatial divergence of the energyflux and on the local sources of energy. The WAM in-tegration is forced by the atmospheric stress computedvia the relation ta 5 rCD , where U10 is the 10-m2U10

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FIG. 3. Finite-difference grid for (a) POM model, (b) Arc2 domain(displayed every two points in both the coordinate directions), and(c) entire Mediterranean basin (displayed every four points in boththe coordinate directions).

wind predicted by BOLAM, CD is the drag coefficient,and r is density. WAM then calculates the wave con-tribution to the total stress (taw), which, in turn, itera-tively corrects the total atmospheric stress (ta). The re-sulting stress is used to force the POM integration.WAM is integrated on sea points of the regular latitude–longitude grid of the VHR domain (Fig. 2), at a reso-lution of 0.18.

c. The POM model: Shallow water version

POM (Blumberg and Mellor 1987; Mellor 1991) is athree-dimensional primitive equation ocean model. Inthis work, it has been operated in a 2D configurationsolving the shallow water equations to compute surfaceelevation. We expect this approximation to be sufficientto capture the main features of the storm surges in theMediterranean, since they are essentially the barotropicresponse of the basin to the atmospheric forcing (Blieret al. 1997). The 2D configuration of the POM modelhas been used in other studies dealing with storm surgesin the northern Adriatic (Lionello 1995; Lionello et al.1998b) and with the Indonesian Seas circulation (Bur-nett et al. 2000a,b). The horizontal discretization is per-formed on the Arakawa C grid. An explicit Orlanskicondition (Orlanski 1976) for the elevation, with a re-storing to a prescribed value, has been selected betweenthe boundary conditions available in the model. How-ever, since the Orlanski scheme does not conserve thetotal water mass of the basin (Palma and Matano 1998),it is necessary to locate the open boundary where theelevation is close to zero. The amphidromic points, inwhich the tidal range vanishes, can be identified byexamining the barotropic modes of the Mediterraneanbasin. The first mode, computed by Schwab and Rao(1983) and by Beltrami Campagnani (2000), shows anoscillation of the western and eastern basins, with anamphidromic point in the Sicily channel (the Mediter-ranean mode). The second mode described by BeltramiCampagnani (2000) presents an amphidromic point inthe Sicily channel and another one in the Strait of Otran-to (the Adriatic mode). This mode divides the Medi-terranean into three parts, the western basin, the easternbasin, and the Adriatic sea; the three basins oscillatearound the amphidromic points.

Using a boundary condition located at Otranto theoscillation that involves both the Adriatic basin and theeastern basin cannot be entirely represented. For thisreason an integration domain including at least the Adri-atic and the eastern basin should be considered. Twodomains have been used in the studies reported here:one including the Adriatic and the central part of theMediterranean sea (Arc2 area; Fig. 3a), and the othercovering the entire Mediterranean (MED; Fig. 3b). Inthe Arc2 domain the Levantine Sea is not included, sinceit accounts for a small part of the total water mass ofthe eastern basin.

The boundary condition for the domain covering the

entire Mediterranean Sea has been imposed in the At-lantic region outside the Gibraltar Strait. The elevationto be imposed on this boundary has been derived, sim-ilarly to that done in Hubbert et al. (1990), by computingthe difference between the boundary pressure and a ref-erence Atlantic pressure, using an isostatic hypothesisfor the Atlantic. This boundary condition allows themass exchange between the Mediterranean Sea and theAtlantic Ocean. The initial elevation and current are setto zero for both the Arc2 and MED domains.

The effect of the initialization vanishes after differenttimes for the two domains. In fact, the Arc2 is char-acterized by an extensive open boundary on which out-flux of energy is allowed and the initial mean elevationis kept almost constant. Therefore, an initial perturbationof the elevation field with a low projection onto themain normal modes of the basin is rapidly radiated outof the boundary (in about 36 h for an experiment withoutforcing). On the other hand, the MED domain is almostclosed and the main mechanism responsible for the lossof energy of an initial perturbation is dissipation. In thiscase the initialization effects, in an unforced experiment,last for more than 2 days. In defining the grids for thetwo domains we had to take into account the need ofvery high resolution (;103 m) in the northern part ofthe Adriatic Sea, which derives from the fact that, inthe operative configuration, sea surface levels at thethree close locations of Chioggia (45.238N, 12.38E), Ma-

MAY 2002 1321B A R G A G L I E T A L .

FIG. 4. ECMWF analysis of the mean sea level pressure (hPa) at(a) 1200 UTC 16 Nov, (b) 0000 UTC 18 Nov, (c) 0600 UTC 18 Nov,and (d) 1200 UTC 18 Nov 1996. Contouring every 2.5 hPa.

lamocco (45.338N, 12.328E), and Lido (45.448N,12.428E) have to be predicted.

To define the Arc2 domain we adopted a polar, non-uniform grid. The center of the reference frame is lo-cated at (Lat0, Lon0) 5 (48.738N, 12.438E). The dis-tance of a generic point P is obtained via the equation

2 2 2h 5 (Lat 2 Lat ) 1 (Lon 2 Lon ) ,p p 0 p 0

while the angle is defined as

j 5 arctan[(Lat 2 Lat )/(Lon 2 Lon )] 1 a ,p p 0 p 0 0

where a0 is defined by tan(a0) 5 (Latsw 2 Lat0)/(Lonsw

2 Lon0) and (Latsw, Lonsw) 5 (9.48N, 28.98E) (SWindicates the southwest extreme of the grid). The Med-iterranean domain is defined by a latitude–longitude gridwith nonuniform spacing. The grid spacing has a min-imum at (448N, 148E) where the interval is 0.0708 inlatitude and 0.0918 in longitude, respectively. Startingfrom this point the grid interval increases linearly by1% per grid point in both directions.

3. Synoptic description and results from WAM:The two test cases

a. The November 1996 event

The extreme surge events that took place during No-vember 1996 were due to a cyclonic circulation overthe Western Mediterranean region. The highest sea sur-face elevation in Venice was measured on 18 Novemberat 0600 UTC. Two series of hindcasts at the two dif-ferent resolutions were computed for November 1996,covering the period from the 11th to the 22rd. Figure4 shows a sequence of ECMWF analyses of the meansea level pressure field for the period starting at 1200UTC 16 November, while the corresponding fields pre-dicted by BOLAM-HR are shown in Fig. 5. A depres-sion, initially located to the south of the Baleari Islands(Figs. 4a and 5a), moves slowly toward the east, reach-ing the Gulf of Genoa after 36 h (Figs. 4b and 5b). Thepressure gradient aligns with the main axis of the Adri-atic Sea and gives rise to a southeasterly wind. In thenext 12 h the low covers the Alps while the stronggradient over the Adriatic Sea persists. Simulated andobserved patterns are in good agreement, but the sim-ulation reveals finer-resolution aspects, such as a splitof the low at 0000 UTC 18 November. Surface windduring this period is directed along the Adriatic axis,blowing from the southeast. Figure 6 shows the windfield simulated by BOLAM-VHR for the same period:here the influence of the orography surrounding theAdriatic basin is evident.

To obtain a further experimental validation of theLAM results, we cannot use observations included inthe assimilation cycle for the ECMWF analysis, suchas those of coastal synoptic stations, that record the windstrength and direction. On the other hand, observationaldata over the sea should be preferred, due to the absence

of the orographic influence. In the 1996 case, obser-vational data of wind strength from the oceanographictower located 15 km off the coast of Venice were avail-able. The tower data (every 3 h), the corresponding

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FIG. 5. Mean sea level pressure (hPa) simulated by BOLAM-HRat (a) 1200 UTC 16 Nov, (b) 0000 UTC 18 Nov, (c) 0600 UTC 18Nov, and (d) 1200 UTC 18 Nov 1996. Contouring every 2.5 hPa.

FIG. 6. The 10-m wind field (m s21) predicted by VHR at 0000UTC 18 Nov 1996.

FIG. 7. Comparison of wind magnitude (m s21) at Venice, mea-surement (solid line), VHR simulation (dashed line), and ECMWFanalysis (open circle).

simulated BOLAM-VHR wind (every 3 h) used to forcethe WAM and POM models, and the ECMWF analysiswind (every 6 h) are plotted in Fig. 7. Note that in theperiod from 1800 UTC November 14 to 0600 UTC

November 17 observational data have not been recordedby the instrument.

It can be noted that the ECMWF analysis stronglyunderestimates the observed data all over the period.The BOLAM results are consistently better, particularlyafter 17 November, even if they are not able to reproducesome sharp peaks, such as those at 0600 UTC 17 No-vember, 0600 UTC 20 November, and 1200 UTC 22November. The bias values for BOLAM-VHR andECMWF are, respectively, 21.96 and 26.20, while therms errors are 4.9 and 7.5.

An indirect validation of the BOLAM surface wind

MAY 2002 1323B A R G A G L I E T A L .

FIG. 8. Significant wave heights (m) simulated by WAM at 0000UTC 18 Nov 1996 (ECMWF wind data used as forcing fields).

FIG. 9. Significant wave heights (m) simulated by WAM at 0000UTC 18 Nov 1996 (VHR wind data used as forcing fields). Thenumbers show the position where wave measurements are available:1, Venice; 2, Pescara; 3, Monopoli; 4, Crotone; 5, Catania; 6, Mazaradel Vallo; and 7, Ponza.field can be achieved through the analysis of the WAM

performance in predicting significant wave height overthe Adriatic basin (Cavaleri 1999, section 10.5). Exper-imental data from a wave measurement network of di-rectional buoys managed by the National Department ofTechnical Services are available. Data from the ocean-ographic platform off the coast of Venice are also used.Two WAM runs were made over the VHR domain, atthe resolution of 0.18: one using as input the BOLAM-VHR wind fields, and another one using winds from theECMWF analyses. Figures 8 and 9 display the significantwave height fields HS at 0000 UTC 18 November for thetwo runs; in Fig. 9 the available buoy locations are alsoshown. It can be seen that the high resolution of the VHRfield allows the WAM model to represent finer-scale fea-tures. Moreover, the VHR run produces larger areaswhere the significant wave height exceeds 2 m.

In Fig. 10 we compare significant wave height sim-ulated by WAM, forced by VHR fields (filled circle)and by the ECMWF analyses (open square) with thewave height measured by buoys (open circle). Com-parisons are shown for the following locations in theAdriatic Sea, indicated by numbers in Fig. 9: Venice(1), Pescara (2), Monopoli (3), and Crotone (4). Clearly,there is a very good simulation of the wave height atthe two southern locations (Monopoli and Crotone). Onthe other hand, in Pescara and Venice the significantwave height peak on 15 November is almost completelymissed. During these days a pressure gradient directedalong the Adriatic Sea gives rise to a wind directedtoward the NW. This situation is due to a low pressurearea that is present over the Alps on 14 November andthen slowly moves westward, covering a large area ofthe northern Tyrrenian Sea during the next day.

The comparison of wave heights at Venice is consis-tent with the results of the direct comparison of windmagnitudes (Fig. 7), which shows an underestimationin the simulated wind during the period 11–14 Novem-ber.

In Table 1 the bias and the rms are shown both forthe ECWMF and VHR simulations for all the availablebuoys. Data for Mazara (number 6 in Fig. 9) are notincluded because of the lack of measured data for mostof the period. The wave height bias is negative in al-most all the locations, but it is reduced when the inputwind fields come from BOLAM-VHR. The rms valuesshow an improvement using VHR forcing for the north-ern Adriatic buoys, while both the simulations displayhigh rms values for the buoy of Ponza. This makessense, since Ponza is close to the western boundary ofthe computational domain and the prevalent winds inthe Tyrrenian Sea during the period of maximum waveheight are from the west. This clearly leads to an un-derestimation of the swell part of the wave height.

b. The October 1998 event

The October 1998 surge events are also characterizedby a cyclonic circulation over the Western Mediterra-nean region. The cyclonic regime persists from 5 to 7October, determining a pressure gradient along the mainaxis of the Adriatic Sea. The large-scale circulationshows for this period the development of an omegablocking. The 1996 and the 1998 events are quite similarin the synoptic development, even if the pressure min-imum in the latter event is less pronounced. Anotherdifference is related to the contribution of the tide tothe elevation in Venice: in the October 1998 event thetide amplitude is close to its maximum, while in theNovember 1996 event it is less relevant. Therefore, inthe 1998 event the correct simulation of the phase ofthe surge has the greatest importance.

Two series of hindcasts at the two different resolu-tions were computed for the period 3–9 October 1998.In Figs. 11a–d show the ECMWF analyses of the mean

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FIG. 10. Wave height (m) measured (open circle) and simulated by WAM with VHR wind fields (filled circle) and withECMWF wind fields (open square) at (a) Venice, (b) Pescara, (c) Monopoli, and (d) Crotone.

sea level pressure at 0600 UTC 5 October, 1200 UTC5 October, 0600 UTC 7 October, and 1200 UTC 7 Oc-tober. It can be seen that the depression located overthe Gulf of Lion on 5 October moves to the east, cov-ering the Italian peninsula and broadening during thetwo following days. The corresponding pressure gra-

dient has a strong longitudinal component. These fea-tures are well reproduced in the results of the BOLAM-HR simulation shown in Figs. 12a–d. It should be noted,however, that on 7 October a stronger sea level pressuregradient is simulated in the Adriatic area. This discrep-ancy is particularly interesting, since the highest sea

MAY 2002 1325B A R G A G L I E T A L .

TABLE 1. Bias and rms error for wave height at buoys (see Fig. 9),using ECMWF and VHR results. For Nov 1996.

Bias

ECMWF VHR

Rms

ECMWF VHR

Venice (1)Pescara (2)Monopoli (3)Crotone (4)Catania (5)Ponza (7)

20.5420.5020.1820.0720.1620.37

20.2320.31

0.050.01

20.1320.53

0.550.500.210.200.180.51

0.430.330.160.210.180.53

FIG. 11. ECMWF analysis of the mean sea level pressure (hPa) at(a) 0600 UTC 5 Oct, (b) 1200 UTC 5 Oct, (c) 0600 UTC 7 Oct, and(d) 1200 UTC 7 Oct 1998. Contouring every 2.5 hPa.

surface elevation in Venice was measured at 0800 UTCof the same day.

In Fig. 13 we show significant wave heights simulatedby WAM, forced by VHR wind fields (filled circle) andby ECMWF analysis fields (empty square), and waveheights measured by buoys (empty circle), for four lo-cations: Pescara, Monopoli, Crotone, and Catania (num-ber 5 in Fig. 9). The Venice data are not available forthis event.

The results are discussed for each of the locations,starting from the northern buoy (Pescara). The peak inthe VHR simulation is correct both in amplitude and intime, but there is a clear underestimation during theprevious two days. During 6 October the mean energyof the measured waves is directed westward, with amean frequency of about 0.15 Hz, while wind wavesfrequently change direction. This suggests that the meanenergy is related to the swell component. Therefore, theobserved underestimation could be due to an insufficientquality of the simulated wind far from the station.

The VHR simulated height at Monopoli is larger thanthe observed one during 7 and 8 October. Analyzing thecomponents of the simulated significant wave height, itis found that the wind waves are predominant duringthis period. Then the strong positive bias could be con-nected to an overestimation of local wind by the model.For both the two southern locations (Catania and Cro-tone) the growing phase in the VHR simulation is ingood agreement with observed values, but stops tooearly (9 h before the observation).

Table 2 presents values of bias and root-mean squarefor each buoy, both for the simulation using BOLAM-VHR input and for that using ECMWF input. In general,the finer wind resolution corresponds to a better result;the ECMWF run always provides an underestimation ofwave heights. The only exception is represented byMonopoli where the rms error is higher for the VHRrun, due to the overestimation explained before.

4. The surge events

a. Surge simulation: 1996 event

The period we are studying was characterized by sev-eral flooding events with a maximum surge of 85 cmon 18 November. Including the effects of the astronom-ical tide and the 23 cm of subsidence in the lagoon,

gives a total elevation of 128 cm. Experimental data ofsea elevation are available from the oceanographic tow-er near Venice and from a network of altimeters man-aged by the National Department of Technical Services.

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FIG. 12. Mean sea level pressure (hPa) simulated by BOLAM-HRat (a) 0600 UTC 5 Oct, (b) 1200 UTC 5 Oct, (c) 0600 UTC 7 Oct,and (d) 1200 UTC 7 Oct 1998. Contouring every 2.5 hPa.

The elevations of sea level at Venice and Bari re-sulting from the shallow water model integration areplotted in Figs. 14a and 14b. The integration is on theArc2 area and covers the period 11–22 November 1996.

The model has been initialized with prognostic fieldsset to zero. Tide values have been subtracted from el-evation data to obtain pure surge. It can be seen fromFig. 14a that the main peaks for Venice are well sim-ulated in amplitude but not in phase, with a lag rangingfrom 2 to 4 h. The discrepancy in the period 13–16November, as already observed in section 3a, can beexplained by a deficiency in the simulated wind field.On the other hand, Fig. 14b shows that, with the ex-ception of the two major peaks, the simulation gives anunderestimation of the elevation for Bari.

In Fig. 15, we present the elevation at Venice obtainedfrom analogous experiments in which the POM inte-gration is extended over the entire Mediterranean do-main. Both the results of a run with BOLAM-HR forcingat the resolution of 0.38 and of a run using ECMWFanalyses forcing are presented. The results obtained us-ing the BOLAM-HR forcing show very good agree-ment, both in amplitude and in phase, with the observedelevation, while the general underestimation for the runforced by ECMWF analysis is evident. The lag problemobserved at Venice in the experiment with the Arc2domain (Fig. 14a) has been overcome; in particular, thephase error at 0700 UTC 18 November has disappearedand a similar improvement has been obtained for thesecondary peaks. The strong overestimation of the min-imum absolute values, present in the Arc2 run, is sig-nificantly reduced in the new simulation.

To obtain a quantitative assessment of the model skill,an error analysis of the runs was performed. Table 3gives a summary of the bias, root-mean-square error,and correlation coefficient for the sea surface elevationat the Venice station. The analysis of the observed andsimulated data was performed separately for high andlow frequencies, choosing a threshold period of 18 h.This period has been selected to separate the first Adri-atic mode, characterized by a period of 22 h, from modesof shorter periods. For low and high frequencies anamplitude ratio coefficient has been computed, indicat-ing the relative importance of the two range of fre-quencies in producing the total amplitude. If and2hLF

denote the mean quadratic values of the low- and2hHF

high-frequency components of the sea elevation, re-spectively, the amplitude ratios are defined as

2ÏhHFA 5 .HF

2 2Ï Ïh 1 hLF HF

The values presented provide a comparison of the resultsobtained for the Arc2, the MED, and the ECMWF ex-periments.

The amplitude ratio analysis shows a predominanceof the low-frequency component over the high-fre-quency component: the low-frequency amplitude ratiosrange from 0.84 to 0.87, which has to be compared witha value of 0.9 for the measured data. It can be notedthat the low-frequency component in the simulated dataalways has a negative bias with respect to the observed

MAY 2002 1327B A R G A G L I E T A L .

FIG. 13. Wave height (m) measured (open circle) and simulated by WAM with VHR wind fields (filled circle) andwith ECMWF wind fields (open square) at (a) Pescara, (b) Monopoli, (c) Crotone, and (d) Catania.

values. Looking at the correlation coefficients, twothings can be observed. There is an evident improve-ment going from the Arc2 to the MED experiment, thatis, extending the integration domain over the whole ba-sin. Bias and rms errors are strongly reduced and the

correlation coefficient grows up to 71%. On the otherhand, it should be noted that the ECMWF experimentshows the highest values of bias and the worst corre-lation coefficient.

The better results observed in the MED simulation

1328 VOLUME 130M O N T H L Y W E A T H E R R E V I E W

TABLE 2. Bias and rms error for wave height at buoys (see Fig. 9),using ECMWF and VHR results. For Oct 1998.

Bias

ECMWF VHR

Rms

ECMWF VHR

Pescara (2)Monopoli (3)Crotone (4)Catania (5)Mazara (6)Ponza (7)

20.4120.2020.2820.3520.3220.39

20.110.06

20.1420.0920.1520.18

0.410.230.290.350.320.39

0.230.370.230.170.200.26

FIG. 14. Sea elevation (cm) measured (open circle) and modeled(filled circle) using POM Arc2 with VHR input at (a) Venice, and(b) Bari. The astronomical tide has been subtracted. For Nov 1996.

FIG. 15. Sea elevation (cm) at Venice measured (open circle) andmodeled using POM on the entire Mediterranean basin, with BOLAMinput (filled circle) and ECMWF input (triangle). The astronomicaltide has been subtracted. For Nov 1996.

could be due to a better representation of the barotropicoscillation of the Mediterranean Sea caused by the pres-sure forcing. To test this hypothesis two experimentsusing only pressure forcing for Arc2 and MED havebeen performed (Fig. 16). The Arc2 experiment showstwo strong minima at the same times of the experimentwith total forcing (Fig. 14a). In the MED experimentthese minima are drastically reduced. This strong im-provement can be explained as follows. If a normalmode decomposition is applied, the forcing for the Nthnormal mode coefficient equation is computed by thespatial integration of the forcing times the Nth normalmode. Since the first gravitational modes have non-neg-ligible amplitudes in the area outside the Arc2 domain(Beltrami Campagnani 2000), including this area is im-portant for a precise determination of the coefficients.

The improvement in phase can also be explained bylooking at the normal modes. In fact, in a recent study(Briganti 2000) the normal mode frequencies have beencalculated for the Arc2 basin both for a closed boundaryand for a zero elevation boundary. In the case of a zeroelevation boundary, an anomalous principal gravita-tional mode, characterized by only one amphidromicpoint near the boundary, with an oscillation period of23.3 h is observed. This spurious mode can interact withthe first Adriatic gravitational mode, characterized bytwo amphidroms and a period of 22 h, producing anerror in the phase of the simulated oscillation.

b. Surge simulation: 1998 event

The flooding events of October 1998 were relevantin the recent history of Venice surges, because the max-imum elevation of 57 cm at 0800 UTC 7 October wasin phase with a high value of the astronomical tide.Including also the lagoon subsidence, the sea levelreached 120 cm.

Due to these characteristics it is crucial to predictboth the correct amplitude and phase of the maximumpeaks. In Fig. 17a the simulated sea elevation values atVenice obtained using the Arc2 domain are comparedwith the observed values. The principal peak at 1100UTC 7 October is underestimated and presents a phaselag of about 3 h. The peaks at 2000 UTC 5 October andat 1800 UTC 6 October are well simulated, while thepeak observed at 1200 UTC 8 October is considerably

underestimated. The comparison for the Bari station(Fig. 17b) shows a quite good agreement in the secondpart of the simulation, while in the first part the under-estimation is principally related to the 36-h spinup time.

Similarly to the 1996 event, better results are givenby the simulation over the entire Mediterranean domain

MAY 2002 1329B A R G A G L I E T A L .

TABLE 3. Bias, rms error, correlation coefficient, and amplitude ratio for sea elevation at Venice, for the Nov 1996 case. The values arecomputed separately for high and low frequencies (threshold value is 18 h). Experiments shown for Arc2, MED, and ECMWF domains.

High

Bias RmsCorrelationcoefficient

Amplituderatio

Low

Bias RmsCorrelationcoefficient

Amplituderatio

Arc2MED (VHR)MED (ECMWF)

20.0120.01

0.01

5.425.904.26

0.130.310.20

0.130.160.14

29.1725.86

214.54

19.6113.3619.08

0.570.710.49

0.870.840.86

FIG. 16. Sea elevation (cm) at Venice modeled by POM using onlythe pressure component on the entire Mediterranean basin (filled cir-cle) and on ARC2 domain (open circle). The astronomical tide hasbeen subtracted. For Nov 1996.

FIG. 17. Sea elevation (cm) measured (open circle) and modeledusing POM Arc2 with VHR input at (a) Venice and (b) Bari. Theastronomical tide has been subtracted. For Oct 1998.

(Fig. 18). Both the amplitude and the phase improvewith respect to the Arc2 experiment (Fig. 17a); in par-ticular, the 1100 UTC 7 October and 1200 UTC 8 Oc-tober peaks are now well reproduced. The ECMWF ex-periment shows a general underestimation and a phaselag for the peaks at 2000 UTC 5 October and at 1800UTC 6 October.

Table 4 gives a summary of the bias, root-mean-square error, and correlation coefficient for the sea sur-face elevation at the Venice station for the 1998 case.As in the 1996 event, the amplitude ratio comparisonshows a predominance of the low-frequency component,and all the experiments show a tendency to underesti-mate the observed values. Again, a significant improve-ment is observed going from the Arc2 to the MED con-figuration. The bias is similar for the two experiments,but the rms error for MED is reduced and the correlationcoefficient goes from 61% to 91%. Instead, the ECMWFexperiment shows higher bias and rms error values, evenif in this case the correlation coefficient is quite good,with a value of about 80%.

The improvement observed using the Mediterraneandomain suggests that the pressure could play an im-portant role in the surge phenomenon. Two experimentsto evaluate the relevance of the pressure forcing on theelevation in Venice have been performed, using thestress and pressure forcing separately. In Fig. 19 weshow the elevation in Venice for the run with stress only

(open squares), for the run with pressure forcing only(filled circles), and for the observations (open circles).It can be seen looking at the main peaks that the con-tribution of the pressure forcing to the surge on thenorthern Adriatic Sea is comparable with that comingfrom the stress.

Finally, since forcing at different resolutions havebeen used for the Mediterranean experiments (0.38) andthe Arc2 simulations (0.18), we want to verify if thishas had an impact on the results. In order to do that,we perform an experiment over the Arc2 domain, usingthe lower forcing resolution, for the 1998 case, wherethe differences between Arc2 and MED were more pro-nounced. In Fig. 20 results for the 0.38 resolution forcingapplied to the Arc2 area are shown, for the October

1330 VOLUME 130M O N T H L Y W E A T H E R R E V I E W

FIG. 18. Sea elevation (cm) measured (open circle) and modeledusing POM on the entire Mediterranean basin at Venice, with BOLAMinput (filled circle) and ECMWF input (triangle). The astronomicaltide has been subtracted. For Oct 1998.

FIG. 19. Sea elevation (cm) at Venice. Measured (open circle),modeled using POM on the entire Mediterranean basin with pressureforcing only (filled circle) and with stress forcing only (open square).The astronomical tide has been subtracted. For Oct 1998.

TABLE 4. Same as in Table 3 for the Oct 1998 case.

High

Bias RmsCorrelationcoefficient

Amplituderatio

Low

Bias RmsCorrelationcoefficient

Amplituderatio

Arc2MED (VHR)MED (ECMWF)

20.070.350.15

7.327.834.53

20.1220.16

0.24

0.190.200.18

210.51210.37217.26

16.4912.1219.17

0.610.910.81

0.810.800.82

1998 period. The comparison with the results obtainedfor the same domain with higher-resolution forcing doesnot show substantial differences.

5. Summary and conclusions

Two storm surge events in the northern Adriatic Seahave been simulated using an integrated forecast systemdeveloped for the prediction of the Mediterranean Seastate and of the surge in the lagoon of Venice.

The forecasting system consists of a limited area mod-el (BOLAM) for the simulation of the atmospheric con-ditions, of a wave model (WAM) for the simulation ofthe stress field, and of a shallow water model (POM)for the simulation of the sea elevation. WAM is forcedby the surface wind computed by BOLAM, while POM-2D is driven by the surface wind stress computed byWAM and by the sea level pressure field computed byBOLAM.

The November 1996 and October 1998 surge eventswere associated with a persistent omega blocking on thelarge scale and with a cyclonic circulation over the West-ern Mediterranean region, determining a pressure gra-dient along the main axis of the Adriatic Sea. The twoevents differ in the tide contribution: while in November1996 it is less relevant, in October 1998 the tide am-plitude is close to its maximum, making crucial a correctphase prediction. The comparison with wind strengthsmeasured at the CNR tower off the coast of Veniceshows good performance of the simulation with BO-

LAM at higher resolution; the same comparison showsa general underestimation for the ECMWF wind anal-ysis. Similar conclusions are reached evaluating theWAM performance in predicting significant wave heightover several locations in the Adriatic basin.

Two domains corresponding to the entire Mediter-ranean basin and to the Adriatic–Ionian basin (Arc2),respectively, have been used in the POM-2D simula-tions. Concerning the Arc2 experiments, it is found,after comparison with measurements of elevation in theAdriatic Sea at Venice and Bari, that the amplitudes arereasonably reproduced, with an underestimation of someof the main peaks and some discrepancies in the cor-responding phases. The same comparison for the MEDexperiment shows clear improvements, with a good rep-resentation of the maximum and minimum values ofelevation and an almost-perfect phase simulation. Thecomputation of the normal modes on the Arc2 domainshows a spurious mode (Briganti 2000) that can causethe phase lag problem found in running POM over thisdomain. This is overcome in the experiment using theMediterranean basin, in which the main barotropicmodes are correctly represented.

Another advantage in using the entire Mediterraneanbasin as a computational domain is a correct accountingof the total effect of pressure forcing. The experimentperformed to check the importance of pressure forcingon the Mediterranean basin has shown that the pressureforcing and the stress forcing are of about equal im-portance in determining the sea elevation amplitude at

MAY 2002 1331B A R G A G L I E T A L .

FIG. 20. Sea elevation (cm) measured (empty circle) and modelledusing POM Arc2 with HR input (filled circle) at Venice. The astro-nomical tide has been subtracted. For Oct 1998.

Venice. Similar results were found by Martinsen andGjevik (1979), in a study on storm surge along the Nor-way coast. An evaluation of the relative importance ofthe two forcings in the Mediterranean context, based onscaling arguments, is given in the appendix.

Acknowledgments. The authors wish to acknowledgeP. Malguzzi (CNR–FISBAT), P. Lionello (UN–PADO-VA), R. Iacono (ENEA), and two anonymous refereesfor their helpful suggestions and comments. We wishto thank L. Cavaleri and A. Tomasin for the CNR plat-form data of Venice, and the National Department ofTechnical Services (DSTN) for the oceanographic data.This work has been supported by the DSTN.

APPENDIX

Scale Analysis

A scale analysis can be applied to discern the relativeimportance of the pressure and stress forcing compo-nents. We consider a forced shallow water equation gov-erning a small sea level disturbances z in an ocean ofconstant depth h, as in Fandry et al. (1984):

2] ]z ]z2 2 21 f 2 c ¹

21 2]t ]t ]t

x y x y]F ]F ] ]F ]F5 f 2 2 1 .1 2 1 2]y ]x ]t ]x ]y

where c 5 and f is the Coriolis parameter. TheÏghforcing term includes the surface wind stress (t) and theatmospheric pressure (pa):

]P ]Pa ax y 21 x y(F , F ) 5 r t 2 h , t 2 h ,w 1 2]x ]y

where rw is the water density.Using a geostrophic approximation, the first term in

the forcing depends only on the wind stress while the

second term depends only on the pressure gradient. Us-ing the stress formula as in Gill (1982, p. 328), we obtain

2] ]z ]z r r ]Aa a2 2 21 f 2 c ¹ 5 f C \V \A 2 h f ,D21 2]t ]t ]t r r ]tw w

where ra is the air density, \V\ is the module of thewind vector, CD is the drag coefficient, A is defined as(21/raf ) ¹2pa, and a term involving the derivative of\V\ has been neglected. The importance of pressureversus stress can be assessed by computing the ratiobetween the two forcing terms:

C \V \AD .]A

h]t

Scaling ]A/]t as A/Dt, with Dt ù 10 f 21, we obtain

10C \V \D .f h

If we assign the values f ù 1024 s21, \V\ ù 10 ms21, CD ù 1023, the ratio of wind stress forcing to pres-sure forcing is of order of 1 if h ù 103 m (mean Med-iterranean depth), or of order of 10 if h ù 102 m (meannorthern Adriatic depth). This is consistent with the nu-merical findings.

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