Quarterly Journal of the Royal Meteorological Society Q. J. R. Meteorol. Soc. 136: 1554–1571, July 2010 Part B

Sea-land breeze development during a summer bora event alongthe north-eastern Adriatic coast

Maja Telisman Prtenjak,*Mladen Viher and Jadran JurkovicAndrija Mohorovicic Geophysical Institute, Department of Geophysics, Faculty of Science, University of Zagreb, Croatia

*Correspondence to: Maja Telisman Prtenjak A.Mohorovicic Geophysical Institute, Dept. of Geophysics, Horvatovac 95,Zagreb, Croatia 10000. E-mail: telisman@irb.hr

The interaction of a summer frontal bora and the sea-land breeze along thenorth-eastern Adriatic coast was investigated by means of numerical simulationsand available observations. Available measurements (in situ, radiosonde, satelliteimages) provided model validation. The modelled wind field revealed several regionswhere the summer bora (weaker than 6 m s−1) allowed sea-breeze development:in the western parts of the Istrian peninsula and Rijeka Bay and along the north-western coast of the island of Rab. Along the western Istrian coast, the position ofthe narrow convergence zone that formed depended greatly on the balance betweenthe bora jets northward and southward of Istria. In the case of a strong northern(Trieste) bora jet, the westerly Istrian onshore flow presented the superposition ofthe dominant swirled bora flow and local weak thermal flow. It collided then withthe easterly bora flow within the zone. With weakening of the Trieste bora jet, theconvergence zone was a result of the pure westerly sea breeze and the easterly borawind. In general, during a bora event, sea breezes were somewhat later and shorter,with limited horizontal extent. The spatial position of the convergence zone causedby the bora and sea-breeze collision was strongly curved. The orientation of the head(of the thermally-induced flow) was more in the vertical causing larger horizontalpressure gradients and stronger daytime maximum wind speed than in undisturbedconditions. Except for the island of Rab, other lee-side islands in the area investigateddid not provide favourable conditions for the sea-breeze formation. Within a borawake near the island of Krk, onshore flow occurred as well, although not as asea-breeze flow, but as the bottom branch of the lee rotor that was associated withthe hydraulic jump-like feature in the lee of the Velika Kapela Mountain. Copyrightc© 2010 Royal Meteorological Society

Key Words: sea-breeze/bora interaction; sea-breeze front; convergence zone; lee rotor

Received 30 September 2008; Revised 11 February 2010; Accepted 23 April 2010; Published online in WileyOnline Library 16 August 2010

Citation: Prtenjak MT, Viher M, Jurkovic J. 2010. Sea-land breeze development during a summer bora eventalong the north-eastern Adriatic coast. Q. J. R. Meteorol. Soc. 136: 1554–1571. DOI:10.1002/qj.649

1. Introduction

Bora (‘bura’ in Croatian) is a strong and cold downslope,mainly north-easterly, wind with a higher occurrence in thecold part of the year. It has been analysed and documentedfor over a century (e.g. Mohorovicic, 1889; Defant, 1951;Yoshino, 1976; Makjanic, 1978; Jurcec, 1980; Klemp andDurran, 1987; Smith, 1987; Bajic, 1989; Poje, 1992; Orlic

et al., 1994; Enger and Grisogono, 1998; Heimann, 2001;Klaic et al., 2003; Grubisic, 2004; Belusic and Klaic, 2006;Kraljevic and Grisogono, 2006; Grubisic and Orlic, 2007;Pullen et al., 2007; Grisogono and Belusic, 2009). Figure 1depicts the north-eastern Adriatic region, representing thebora-affected area. This area covers the Istrian peninsula,Kvarner Bay and the mainland. The highest points in thearea are Cicarija (∼1100 m above mean sea level, MSL),

Copyright c© 2010 Royal Meteorological Society

Sea-Land Breeze Development During a Summer Bora Event 1555

(a)

(b)

Figure 1. (a) Configuration of nested model grids over the study area on the north-eastern Adriatic coast. Frames indicate the coarse-grid (A), mid-frame(B) and fine-grid (C) WRF model domains, respectively. (b) The fine-grid domain with the positions of measuring sites; hourly meteorologicalmeasurements (squares): 1 = Pula Airport, 2 = Rijeka and 3 = Senj, climatological measurements (full black circles numbered 4–24, see Table I) andair-quality monitoring stations (triangles): 25 = Opatija, 26 = Rijeka and 27 = Krasica. Krasica is the highest placed air-quality station. Lines A1B1,A2B2, A3B3 and A4B4 show the bases of the vertical cross-sections used in section 4.3. Topography contours are given for every 100 m between 0 and1700 m. Abbreviations in Figure 1(b) are GG = Grate Gate, SG = Senj Gate, GJ = Gornje Jelenje and VP = Vratnik Pass. The highest points in Istria arethe mountain massifs of Cicarija (∼1100 m MSL) and Ucka (∼1400 m MSL). Kvarner Bay encompasses, besides the smaller Rijeka Bay, the islands ofKrk (the biggest one), Cres, Losinj, Rab. East of Kvarner Bay, high mountains such as Risnjak (∼1500 m MSL) rise up, including Velika Kapela (∼1500 mMSL) and Velebit (∼1600 m MSL).

Ucka (∼1400 m MSL), Risnjak (∼1500 m MSL), VelikaKapela (∼1500 m MSL) and Velebit (∼1600 m MSL), andthe two main mountain passes are Gornje Jelenje (betweenRisnjak and Velika Kapela, 882 m MSL; GJ in Figure 1(b))and the Vratnik Pass (between Velika Kapela and Velebit,694 m MSL; VP in Figure 1(b)). The coastal slopes of VelebitMountain represent the regions with the highest probabilityof bora occurrence (e.g. Ivatek-Sahdan and Tudor, 2004;Horvath et al., 2007). The average annual probability ofbora occurrence in Senj (station 3 in Figure 1), for example,is greater than 35% (Luksic, 1975; Makjanic, 1978), withthe most probable occurrence in January and the least inJune. The average bora speed there is around 11 m s−1,with a maximum in December and a minimum in June

and July (Luksic, 1975). Generally, along the coast, bora hasmaximum speeds in the morning and minimum speeds inthe afternoon, with the average duration in the range of20–70 h (Poje, 1992).

Grisogono and Belusic (2009) pointed out that thereare over 25 places with bora-like phenomena worldwide(e.g. southern California, the Rocky Mountains, Japan, NewZealand). However, although the basic aspects of bora flowsare reasonably well known, the particulars are highly relatedto local topography (mountains, gaps, islands, etc.). Verysimplified, a bora may blow over approximately 1 km highmountains, so that the airflow is only partly blocked, whilelarge, steep waves appear above the mountain, overturn andeventually break. This process usually leads to a hydraulic

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1556 M. T. Prtenjak et al.

jump-like formation that is sometimes associated with lee-side eddies.

Several types of bora flow have been detected with regardto synoptic conditions based mainly on the position of thecyclone and anticyclone at the surface level (Jurcec, 1980,1988; Heimann, 2001; Pandzic and Likso, 2005). The firstbora type is commonly called a ‘cyclonic’ bora, when thebora blows along the northern Adriatic coast in combinationwith a strong sirocco wind along the southern Adriatic. Thisbora type develops when the Genoa cyclone moves south-eastwards along the Adriatic. Its duration is relatively short,usually no more than two days. A similar bora type is calleda ‘frontal’ bora, associated with cold air advection from thenorth-east. This type sometimes represents the most severebora events and can be recorded throughout the year. It ischaracterised locally by a sudden increase in the bora speedand a brief duration. Both of these types represent ‘dark’bora events, since they are usually connected with cloudinessand heavy precipitation. An ‘anticyclonic’ bora forms underthe prevailing influence of a continental high-pressure areaabove Croatia without a well-defined cyclone to the south.This type of bora without cloudiness occurs throughoutthe year as well, although it is weaker during the summer.‘Anticyclonic’ boras are usually deeper and weaker than the‘dark’ bora types (e.g. Gohm et al., 2008; Grisogono andBelusic, 2009).

Numerical bora simulations have investigated mostlysevere bora episodes during the cold part of the year (e.g.Grubisic, 2004; Belusic and Klaic, 2006; Gohm et al., 2008).These studies reveal a spatial distribution of bora jets alongthe Adriatic coast as terrain-locked features with the mainbora jet above the Vratnik Pass. An examination of weakwinter boras showed a narrow jet attached to the mountaingap between Risnjak and Velika Kapela mountains thatstretched above the northern part of the island of Cres andthe tip of Istria (Gohm and Mayr, 2005). This jet and theprimary bora jet emanating from Vratnik Pass over thesea near the island of Cres merge into a single, relativelybroad, band of strong winds. Another significant bora jet isobserved above the Sibenik and Split area (Grubisic, 2004;Gohm and Mayr, 2005; Gohm et al., 2008).

During the last decade, more attention has also beendedicated to another frequent local coastal wind alongthe Adriatic coast, the sea-land breeze, SLB (Orlic et al.,1988; Nitis et al., 2005; Prtenjak et al., 2006; Trosic et al.,2006; Prtenjak and Grisogono, 2007; Prtenjak et al., 2008).Nitis et al. (2005) and Prtenjak et al. (2006) revealed theformation of several small-scale phenomena, e.g. mesoscaleeddies inside Rijeka Bay as well as convergence zones aboveIstria and the island of Krk. The mesoscale eddies developedinside Rijeka Bay over a 24-hour period. During the day,both the anabatic flow and the well-developed sea breeze(SB), which are caused by the coastal geometry and theterrain height, resulted in an afternoon anticyclonic vortexinside the shallow stable marine boundary layer. The night-time cyclonic eddy developed due to katabatic flow fromthe surrounding mountains. Above Istria, daytime-mergedSBs formed the convergence zone that moved eastward. Thesurface wind field is significantly channelled through VelebitChannel and the Great Gate. This hints at the goal of thisstudy, since the mesoscale wind characteristics were observedfor almost-undisturbed synoptic weather conditions. Thedetails of SLB development under considerable synopticforcing, e.g. during bora events, are still unknown.

Although Grisogono and Belusic (2009) have shownrecent progress and advances in research on meso- andmicroscale severe bora characteristics, they clearly pointedout some issues and questions that are not yet fully resolved.The authors suggested (among others) more extensiveanalyses of weak to moderate bora flows, which were stillnot sufficiently understood (and which are more frequentduring summer months), as well as the role of the lee-sideislands (e.g. islands within Kvarner Bay) during bora events.Furthermore, within the framework of the recent North-ern Adriatic Sea Current Mapping (NASCUM) project(http://poseidon.ogs.trieste.it/jungo/NASCUM/indexen.html), surface current structures in the northern

Adriatic Sea were monitored by high-frequency radars. Datafrom summer surface currents showed small-scale eddies(e.g. 5 km in radius) in front of the western Istrian coastduring relatively weak bora events (Cosoli et al., 2008). Thiscomplex flow pattern in the area has opened questions onits origin which have not yet been analysed in detail.

During the summer along the north-eastern Adriatic,mostly weak to moderate bora events (up to 20% of allsummer days) alternate with the sea breeze days (up to60% of all summer days), so the main goal here is toinvestigate moderate bora/SB interchange. Previous studiesexamined severe and/or winter bora cases along the Adriaticcoast when the interplay with thermal circulation was notpossible. Thus, they did not offer detailed insight into thefine-scale lower-tropospheric conditions responsible for thisparticular boundary-layer investigation, which, we believe,we have succeeded in doing in the present study. Air qualityissues can also be highly associated with the boundary-layerstructure. Bastin et al. (2006) investigated the combinationof the SB and the summer mistral (severe northerly wind)in Provence, southern France. They found that the presenceof the summer mistral prevents onshore SB penetration andweakens and delays the SB compared to cases of pure SB(Bastin and Drobinski, 2006). They observed that duringsuch a combined SB/mistral event, pollutants (e.g. ozone)stagnate close to the coastline and reach high concentrationsin the very densely inhabited coastal area. Still, since theseresults are connected with a particular geographic region,the open question is how far their results and conclusionscan be transferred to other, even more complex, topographicareas (e.g. the east Adriatic coast) and similar events.

Therefore, owing to the unknown boundary-layerbehaviour while the summer (moderate) bora and thesea breeze exchange, we here investigate their interplaywith specific regard to the north-eastern Croatian coast.During this interaction, we focus on the role of boundary-layer effects (especially on the western Istrian coast),on the role of the lee-side islands, and on the variousmechanisms that drive both the SLB and small-scaleform variability of the wind field in time and space (e.g.mesoscale eddies, convergence zones and bora rotors). Forthis purpose, a three-dimensional (3D) non-hydrostaticnumerical simulation of a real case by the WeatherResearch and Forecasting (WRF) model is used. The resultsare checked against available observational datasets andanalysed.

2. The case of 28–30 June 2004

To evaluate the interaction between bora events and the SLB,we selected one combined bora/SLB event that occurred on

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Figure 2. Surface diagnostic chart at 0000 UTC on 29 June 2004 for Europe (source: European Meteorological Bulletin).

28–30 June 2004. Figure 2 shows the synoptic-scale situationat the surface level on 29 June 2004. A surface anticycloneexisted over central and western Europe, with its centreover the Atlantic, near France. A shallow cold front crossedover the eastern Alps south-eastward, followed by a coldair outbreak, mostly in the lowest 2 km. The sudden boraonset occurred at 2200 UTC (which corresponds to 2400CEST, Central European Summer Time) on 28 June, lasting30 hours. On 29 June 2004, after the front passed, thebora speed suddenly increased at the coast, reaching itsmaximum before noon (at the foot of Velebit Mountain).On 30 June, during the early morning hours, the borawind stopped suddenly, and undisturbed local daytimecirculations developed. This chosen period represents a‘frontal’ shallow bora event and it is analysed furtherhere.

3. Weather Research and Forecasting (WRF) model

The meteorological fields used were obtained from theWRF model (version 2.2) developed at the National Centrefor Atmospheric Research. The WRF (http://www.wrf-model.org/index.php) is used in a variety of areas, includingstorm prediction and research, air-quality modelling, andpredictions of hurricanes, tropical storms, and regionalclimate and weather (e.g. Michalakes et al., 2004). TheWRF model consists of fully compressible non-hydrostaticequations on a staggered Arakawa C grid. Since an ArakawaC grid is used, the wind components u, v and w arerecorded at the respective cell interfaces and all othervariables as volumetric cells carry averages at the cell centres.The vertical coordinate is a terrain-influenced hydrostaticpressure coordinate. Here, the model uses the Runge–Kutta3rd-order time integration scheme, as well as 5th-orderadvection schemes in the horizontal and 3rd-order in thevertical directions. A time-split small step for acoustic and

gravity-wave modes is applied. The simulation uses a two-way nested configuration featuring a coarse domain with a9 km grid spacing (on the Lambert conformal projection)that covers the greater Adriatic area (Figure 1(a), frame A).The second grid is a nested domain with 3 km horizontalmesh size covering Croatia (Figure 1(a), frame B). Thefine-grid simulation covers an area of 124 × 130 points,with a 1 km horizontal grid spacing (Figure 1(a), frameC). The horizontal grid spacing of 1 km is coarse enoughfor the meaningful use of a turbulent kinetic energy (TKE)parametrization. Then the ratio of the energy-containingturbulence scale and the scale of the spatial filter usedon the equations of motion is small. It should mostlyprevent overlapping between the TKE parametrization andthe resolved boundary layer (e.g. Wyngaard, 2004). Sixty-five terrain-influenced coordinate levels were used, withthe lowest level at about 25 m. Spacing between levelsranged from 60 m at the bottom, and 300 m in themiddle and upper troposphere, to 400 m toward the topat 20 km. WRF dynamic and physical options used forall domains include the Advanced Research WRF (ARW)dynamic core; a Mellor–Yamada–Janjic scheme for theplanetary boundary layer; the Rapid Radiative TransferModel for the long-wave radiation and a Dudhia scheme forshort-wave radiation; a single-moment 3-class microphysicsscheme with ice and snow processes; the Eta surface layerscheme based on Monin–Obukhov (MO) theory and a five-layer thermal diffusion scheme for the soil temperature. Onthe coarse 9 km domain, the Betts–Miller–Janjic cumulusparametrization was used, but without parametrization inthe inner domains. Initialisation and boundary conditionsfor the mesoscale model were introduced with analysed datafrom the European Centre for Medium-Range WeatherForecasts (ECMWF). Data are available at a 0.25-degreeresolution at standard pressure levels every 6 h. Simulationsof 65 h were performed from 0600 UTC of 28 June 2004until midnight of 30 June 2004.

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1558 M. T. Prtenjak et al.

(a)

(b)

(c)

Figure 3. Modelled (grey line) versus measured (black line) surface winds (10 m above the ground) from 0600 UTC (0800 CEST) 28 June, until midnight30 June 2004 for three stations: Pula Airport, Rijeka and Senj. The positions of the measuring sites are indicated in Figure 1(b). In order to show exactdiscrepancies between the measured and modelled directions, wind directions spanning a range of 0–90◦ are sometimes expanded by 360◦.

4. Results and discussion

4.1. Model versus measurements

In the study area, we first use in situ hourly windmeasurements (speed and direction) from three stations(Pula Airport (1), Rijeka (2) and Senj (3)) to validatethe simulation results in the fine-grid domain. Table IIshows statistics for the employed model, namely root meansquare error (RMSE), mean absolute error (MAE) and theindex of agreement, d-index, (e.g. Willmott, 1982; Mastura,2009), while Figure 3 illustrates a comparison between10 m measured and modelled wind (diagnosed by the MOsimilarity theory from model fields). Throughout the period,the wind at Pula Airport is simulated satisfactorily, while inRijeka and Senj, the modelled wind speed was overestimatedcompared to measured wind speed during the bora event.It is important to note that Rijeka and Senj are situatedin very complex topography, so the model overestimationis partly due to the smoothed topography used in oursimulation. However, similar wind speed overestimationshave occurred in bora wind simulations performed by othermesoscale models: MEMO6 (Klaic et al., 2003) at the same

1 km horizontal grid spacing and RAMS (Gohm and Mayr,2005; Gohm et al., 2008) at a higher horizontal grid spacingthan here. Klaic et al. (2003) reported that, at these sites,measured maximal values are questionable due to the lee-side positions of the two measuring sites. In Figure 3(c),apart from the standard anemometer measurements inSenj, additional special measurements are displayed forthe same town. They were performed by the WindMasterultrasonic Gill anemometer placed at a nearby location,approximately 300 m toward the coast (44.99◦N, 14.90◦E:Orlic et al., 2005). The additional measured wind speedis 22% higher, on average, than the standard measuredone during the examined bora event. For larger boraspeeds, deviations between the two measuring sites areeven higher – approximately 30–40% (Belusic and Klaic,2004; Klaic et al., 2009). Modelled wind speed is closerto the associated special measurements (marked by 2 inTable II) than to the standard measured data (d-index 1in Table II). The special measuring site is probably bettersuited for comparison with model results because of weakerlocal influences.

The wind direction is reproduced very well at each ofthese three stations (Figure 3). More variations in wind

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Table I. Stations in the north-eastern Adriatic used in the study. The table shows the type of station, where M = mainmeteorological station, C = ordinary meteorological station, AQ = air-quality monitoring and R = radiosonde station,and the geographic characteristics (latitude, longitude and height above sea level, ASL). The sites are also shown in Figure 1.

No. Station (type of station) lat long ASL (m) No. Station (type of station) lat long ASL (m)

1 Pula Airport (M) 44◦54′ 13◦55′ 63 16 Ucka (C) 45◦17′ 14◦12′ 13722 Rijeka (M) 45◦20′ 14◦27′ 120 17 Volosko (C) 45◦22′ 14◦19′ 463 Senj (M) 45◦0′ 14◦54′ 26 18 Kukuljanovo (C) 45◦20′ 14◦32′ 3554 Novigrad (C) 45◦20′ 13◦35′ 20 19 Crikvenica (C) 45◦10′ 14◦42′ 25 Porec (C) 45◦13′ 13◦36′ 15 20 Rijeka Airport (M) 45◦13′ 14◦35′ 856 Sveti Ivan na Pucini (C) 45◦3′ 13◦37′ 8 21 Malinska (C) 45◦07′ 14◦32′ 17 Rovinj (C) 45◦6′ 13◦38′ 20 22 Ponikve (C) 45◦04′ 14◦35′ 258 Pula (C) 44◦52′ 13◦51′ 43 23 Krk (C) 45◦02′ 14◦35′ 99 Pazin (M) 45◦14′ 13◦56′ 291 24 Rab (C) 44◦45′ 14◦46′ 2410 Abrami(C) 45◦26′ 13◦56′ 85 25 Opatija-Gorovo (AQ) 45◦20′ 14◦18′ 4011 Botonega (C) 45◦20′ 13◦55′ 50 26 Rijeka (AQ) 45◦19′ 14◦25′ 2012 Cres (C) 44◦57′ 14◦25′ 5 27 Krasica (AQ) 45◦18′ 14◦33′ 18613 Labin (C) 45◦11′ 14◦4′ 316 28 Senj additional (M) 44◦59′ 14◦54′ 214 Cepic (C) 45◦12′ 14◦9′ 30 28 Udine (R) 46◦3′ 13◦18′ 9415 Letaj brana (C) 45◦16′ 14◦8′ 120 29 Zagreb (R) 45◦49′ 16◦2′ 123

30 Zadar Airport (R) 44◦07′ 15◦23′ 88

direction can be noted for very low wind speeds when mostnumerical models fail to reproduce wind completely overthe very complex terrain (e.g. Baklanov and Grisogono,2007; Grisogono and Belusic, 2009). At Pula Airport on 30June, some wind direction discrepancies exist between themeasurements and the model, influenced by the convergencezone position above Istria. At all stations, the modelsuccessfully predicts the timing of the bora breakthroughnear the coast, as well as the SB timing. In contrast to thePula Airport and Rijeka stations, the bora wind started verysuddenly at Senj, reaching moderate bora speeds with thelongest duration under the same upstream conditions.

The radiosondes launched from Zadar Airport (Fig-ure 1(a)), the only one at the coast, are shown in Figure 4,allowing a vertical comparison of the model results withmeasurements. During the bora event, two layers can beobserved (visible in both the measurements and the model);the bora wind blew at about 4 m s−1 from the east-northeast(around 65◦) in the lowest 1600 m in the stable boundarylayer, with west-north-westerly wind above (Figure 4, upperrow). The maximum bora wind speed was 7.2 m s−1 at300 m height, observed only in measurements. Above thebora layer, the westerly wind, which the model reproducedsatisfactorily compared to the measurements, increases withheight. On the next day (Figure 4, lower row), the poten-tial temperature profile shows a convective boundary layerapproximately 2 km deep. Inside the boundary layer, in thelowermost 700 m, the SB developed as a weak north-westerlywind. The wind changes its direction with height (up to 90◦)in the next 1000 m. These SB characteristics agree withthe SB climatology (Prtenjak and Grisogono, 2007). Thewind is west-north-westerly above the boundary layer. Themodel reproduced the local circulation cell satisfactorily inthe somewhat lower boundary layer. The discrepancies incomparisons of the radiosondes from Udine and Zagreb(Figure 1(a)) were very similar to those shown here forZadar Airport and thus not shown.

We can conclude that despite both model limitations (e.g.model grid spacing or the MO scheme used) and sparse high-quality measurements, the model satisfactorily reproducedthe case analysed.

4.2. Near-surface wind field characteristics

The previous section introduced certain model capabilitiesof the simulation of bora/SB exchange. Here, we continueto analyse and verify model results for the near-surfaceflow pattern using wind measurements from the 27 stations(Figure 1(b)). Unfortunately, most of the wind data arecollected from ordinary meteorological stations (Table I).Since these stations provide only the strength of the wind inthree terms (according to the Beaufort scale), discrepanciesbetween measurements and the model may also be due towind strength conversions into wind speed (m s−1).

Figure 5 shows the 10 m wind field and potentialtemperature field on 28 June 2004 at 1300 UTC. Themeasurements and simulation results both show theprevailing relatively weak southerly winds over the largerpart of the Istrian peninsula and Kvarner Bay. The simulatedwind reached its highest speed within the Great Gate andthe Senj Gate due to channelling and downslope airflow inthe hinterland. In the study area, certain known mesoscalefeatures (Prtenjak et al., 2006) developed, e.g. convergencezones over the Istrian peninsula and the island of Krk,as well as a clockwise mesoscale eddy inside Rijeka Bay.The Istrian convergence zone is located about 35 kmeast from the western Istrian coast, where SB speeds werebelow 4.5 m s−1. Comparing measured and simulated wind(Table III) at 1300 UTC, the WRF model reproduced thewind direction satisfactorily and the wind speed somewhatless well. The surface air potential temperature rises towardthe centre of both the Istrian peninsula and the islandof Krk, showing SB formation at the coasts (Figure 5(c)).Although the model correctly simulates the coastal surface

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1560 M. T. Prtenjak et al.

(a) (b) (c)

(e) (f)

Figure 4. Modelled (solid line with circles) versus measured (grey line) vertical profiles of (a), (d) wind speed, (b), (e) wind direction and (c), (f) potentialtemperature at Zadar Airport from the radiosondes launched on 29 June 2004 at 1200 UTC (top) and 30 June 2004 at 1200 UTC (bottom). Position ofthe measuring site is indicated in Figure 1(a).

Table II. Some statistical indices – root mean square error (RMSE), mean absolute error (MAE) and the index of agreement(d-index) – for wind speed (WS; m s−1) and wind direction (WD; deg) between the model and measuring sites Rijeka,Pula Airport and Senj. In Senj, the comparison between the basic model simulation and the standard measuring site is

marked by 1 and that between the basic model simulation and a special measuring site is marked by 2.

Rijeka Pula Airport Senj Standard measuring site 1 Senj Special measuring site 2

WS WD WS WD WS WD WS WD

RMSE 1.6 31.9 2.5 65.6 4.2 39.3 3.2 57.1MAE 1.3 22.1 1.8 49.5 3.3 30.0 2.1 35.3d-index 0.87 0.93 0.71 0.95 0.84 0.95 0.91 0.95

air temperatures, in general, a slight underestimation of themeasured temperature values is noted in the middle of Istria.

Around 2200 UTC on the same day, the bora wind startedto blow along the north-eastern Adriatic coast (not shown),after the cold air outbreak began in the hinterland. However,due to the low-level cold-front transverse, the surface windfield during the bora event varies in time and space. In theearly morning on 29 June (Figure 6), the bora blew overnearly the entire area, except for two regions: within thevalley between Cicarija and Risnjak and within Rijeka Bay.Such a wind distribution means that the bora wind did notbecome fully established at the surface near the high coastal

mountains within Rijeka Bay. There, due to flow separationfrom the leeward slope of Risnjak Mountain, the bora layerwith north-easterly winds was lifted above 300 m off thesea. In the lowermost layer, a cold pool existed over RijekaBay which was blocked in front of the coastal mountainsof Istria (not shown). The whole wind distribution wasreproduced very well by the model (Table III), althoughwind speed was somewhat overestimated (Figure 6). Ingeneral, the bora wind varies spatially in strength, withthe formation of several bora jets through the mountainpasses, north and south of Istria. The northern jet formed(near the towns of Trieste and Koper), which is sited in

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(a) (b)

(c)

Figure 5. (a) 10 m wind vectors (m s−1) and surface air potential temperature in K (numbers near vectors) from meteorological and climatologicalstations (in Figure 1(b)); (b) modelled WRF wind field; and (c) modelled surface air potential temperature depicted every 0.5 K for 28 June 2004 at 1300UTC. The wind vectors are given at a horizontal resolution of 4 km, with reference vectors near the upper right-hand corner. The wind speed is depictedby filled areas (grey-scale legend on the right) with a 1 m s−1 interval.

Table III. Some statistical indices for the basic numerical simulation – root mean square error (RMSE), mean absoluteerror (MAE), the index of agreement (d-index) and the correlation coefficient (r) – between measured and modelled 10 m

wind speed (WS; m s−1) and direction (WD; deg) provided by measurements from the 27 stations in Figure 1(b).

Time 28 June 2004 1300 UTC 29 June 2004 0600 UTC 29 June 2004 1300 UTC 30 June 2004 1300 UTC

WS WD WS WD WS WD WS WD

RMSE 1.3 59.4 2.5 55.1 2.5 52.3 1.1 41.1MAE 1.1 44.4 2.2 40.2 2.3 34.0 0.9 31.3d-index 0.77 0.88 0.75 0.95 0.83 0.94 0.80 0.95r 0.53 0.81 0.72 0.91 0.76 0.90 0.62 0.86

a topographic incision between the Dinaric and the JulianAlps. The southern jets are associated with mountain gaps inthe Dinaric Alps range, the stronger one (around 14 m s−1)through Vratnik Pass and the weaker one through GornjeJelenje. Over the Istrian peninsula (according to both themeasurements and model), a moderate bora dominated,due to passage of the cold front. The maximum modelledbora wind speed of more than 16 m s−1 formed east of theIstrian peninsula, near the island of Cres. Above the island ofKrk, a moderate easterly bora veered toward another strong

narrow bora jet originating through Vratnik Pass. This jetachieved its maximum strength within the Senj Gate andthe eastern coast of the island of Cres. The event can beclassified as shallow, since the cross-mountain flow (i.e. thebora layer with north-easterly winds aloft) was restricted tothe lowermost 2.5 km of the atmosphere.

Figure 7 displays the surface wind field at 1000 UTC,around its maximum. The modelled wind at 1000 UTCis only somewhat similar to modelled wind at 0600 UTC.Figure 7 shows very well formed bora jets; the northern

Copyright c© 2010 Royal Meteorological Society Q. J. R. Meteorol. Soc. 136: 1554–1571 (2010)

1562 M. T. Prtenjak et al.

(a) (b)

Figure 6. (a) 10 m wind from meteorological and climatological stations and (b) from model simulations on 29 June 2004 at 0600 UTC. The windvectors are given at a horizontal resolution of 4 km, with reference vectors near the upper right-hand corner. The wind speed is depicted by filled areas(grey-scale legend on the right) with a 2 m s−1 interval.

Figure 7. Same as in Figure 5(b) except for modelled 10 m wind (m s−1)on 29 June 2004 at 1000 UTC.

one (near Trieste, maximum wind speed ∼14 m s−1), theprimary southern stronger (∼15 m s−1) bora jet throughthe Vratnik Pass with a width of 25 km, and the secondsouthern, weaker (∼13 m s−1) bora jet through GornjeJelenje with a width of 15 km. Both narrow jets over KvarnerBay join together, forming one broad bora jet about 50 kmwide near the surface, several kilometres downstream of thecoast. This merged bora jet stretches from the middle of thepeninsula to the island of Losinj, with its centre above thetip of Istria. This wind distribution agrees very well with theresults of other observational and numerical winter severebora studies (e.g. Jurcec, 1980; Grubisic, 2004; Gohm et al.,2008). The modelled wind distribution reveals the westernpart of Rijeka Bay, especially around the town of Opatija(station 25 in Figure 1(b), at the foot of Uc ka Mountain) andat the north-western coast of the island of Krk around thetown of Malinska (station 21 in Figure 1(b)), as a shelteredarea. Since the bora typically weakens somewhat duringthe daytime due to the evolution of a convective boundary

layer over land (Grisogono and Belusic, 2009), the boradecreases its speed (over Istria) causing a bora wake regionover the flat western Istrian coast. This area is affected by theconvergence zone from the westerly onshore flow and theeasterly bora flow. The model shows that the bora wind atthe southern part of the jet north of Istria was considerablywhirled, supporting the onshore flow formation along thewestern Istrian coast. In section 4.3, we discuss this featureas the result of the unsteady bora/SB interaction.

Figure 8(a) shows the measured bora wind and theassociated potential temperature pattern at 1300 UTC,which agree satisfactorily with the modelled ones (Table III).Modelled wind in Figure 8(b) shows, apart from bora jets(the much weaker Trieste jet and the temporally almostunchanged southern jets), several simultaneous enlargedareas of bora minima (Figure 8(a) and (b)). They are thewestern Istrian coast, the sheltered areas in Rijeka Bay (thewestern sides of Rijeka Bay and the island of Krk) andsouthern part of the island of Rab. Depending on the borastrength, these areas of bora minima vary in space. Atthe western Istrian coast, the SB develops in the narrowarea despite the fact that the bora brings cold and drycontinental air and suppresses a daytime temperature rise(Figure 8(a) and (c)). There, the wind direction changesover time from south-west to north-west. Here, the strongerbora wind speed (>6 m s−1) south of Rovinj (station 7 inFigure 1(b)) did not allow daytime SB penetration over theland, maintaining the SB front over the sea (45.4◦N, 13.5◦E).In Rijeka Bay near Opatija, the low bora speed allows theformation of the weak thermally-induced perturbation, butwithin the sheltered areas near the town of Malinska andalong the narrow area on the western coast of the islandof Rab, only a redirection and weakening of the bora windoccurs.

A satellite image (Figure 8(d)) confirms the bora and SBinteraction. The satellite image was produced about twentyminutes earlier than ground observations (Figure 8(a)), themodelled WRF field (Figure 8(b)) and surface potentialtemperature (Figure 8(c)). A wide stratocumulus field thatspreads from the Croatian coast inland corresponds wellwith the lower surface potential temperatures in Figure 8(c).The convergence in the wind field results in air convection,

Copyright c© 2010 Royal Meteorological Society Q. J. R. Meteorol. Soc. 136: 1554–1571 (2010)

Sea-Land Breeze Development During a Summer Bora Event 1563

(a) (b)

(c) (d)

Figure 8. (a), (b), (c) Same as Figure 5 except on 29 June 2004 at 1300 UTC. (d) Satellite image taken on 29 June 2004 at 1238 UTC in the visible spectraby the NOAA16 satellite.

visible as convective cloud development above the westerncoast of the Istrian peninsula. Cumulus clouds are highenough to produce clearly visible shadows even at an imageresolution of 1 km. The cold-front passage (which is outsideof Figure 8(d), to the east) produced stronger convectivedevelopment, seen as dense stratocumulus over the highcoastal mountains (Risnjak, Velika Kapela and Velebit). Awide stratocumulus layer with sharp edges is an exampleof a typical orogenetic cloud induced by bora wind. Theplume of higher clouds moves in westerly winds, oppositeto the mostly north-eastern direction of the surface WRFwind modelled in Figure 8(b).

At 1600 UTC, in western Istria, the SB reaches a maximumspeed of about 6 m s−1 (Figure 9(a)). For the samewind field, the convergence and vorticity are displayedin Figure 9(b) and (c). The moderate grey-filled areas showwhere convergence (and consequently convection) occursduring the bora and SB interaction (Figure 9(b)). Comparingthe values of convergence during the study period aboveIstria, the highest values were on 29 June. These areas arealso associated with significant vorticity conditioned earlierby the bora jets (Figure 9(c)). The cyclonic vorticity (lightgrey) formed south-west of the Trieste bora jet maximum.

At the same time, south of Rovinj, moderate bora windsdetermined the formation of the anticyclonic vorticity(moderate grey). The penetration of the mature westernSB was approximately 20 km east (Figure 9(a) and (b)).Over the north-western coast of Rijeka Bay and the islandsof Krk and Rab, onshore flow developed. However, themoderate to strong bora (>6 m s−1) still obstructed SBformation above the rest of the north-eastern Adriatic coast.In the evening, on the western Istrian coast, the SB vanishedand a land breeze developed, blowing toward the sea. Itcoincides with the reinforced bora flow, especially above thetip of Istria (not shown).

During the night, the bora was stronger and still blewmostly in the form of jets, with the strongest one in Senj. Inthe morning on 30 June (Figure 10), the bora weakened andgradually stopped (until 1000 UTC), allowing simultaneousSB development on the north-eastern Adriatic coast. Amoderate bora blew only through Vratnik Pass towardthe tip of Istria and north of Trieste. In Rijeka Bay, thenorth-westerly SB formed in the presence of the weak boralimited to the very narrow coastal area. Above Istria, thewestern coast was under the SB (starting at 0700 UTC),which penetrated ∼10 km over the land. The SB flow

Copyright c© 2010 Royal Meteorological Society Q. J. R. Meteorol. Soc. 136: 1554–1571 (2010)

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(a)

(b) (c)

Figure 9. (a) Modelled 10 m wind (m s−1), (b) divergence of the near-surface wind field (s−1), and (c) vorticity of the near-surface wind field (s−1) for29 June 2004 at 1600 UTC. In the divergence field, divergence is depicted as light grey areas and convergence by medium grey colours. The positive(cyclonic) vorticity is light grey and the negative (anticyclonic) vorticity is medium grey.

diverged there, toward the bora jets, less northward andmore southward of Istria, forming an anticlockwise whirlin front of the coast between Novigrad and Rovinj (sites 4and 7 in Figure 1(b), respectively). This feature in the windcorresponds closely to the swirling in the surface currentsdetected during the NASCUM project (Cosoli et al., 2008).It seems that the wind distribution during the SB/borainterplay forced this small-scale phenomenon in the surfacecurrents. On the eastern Istrian coast, a south-easterly SBoverlapped with the weak bora wind, forming an onshoreflow that was twice as strong as the western SB. Over thenorthern part of the island of Krk, an SB developed as well.

Figure 11 shows the measured and modelled wind andtemperature distributions at 1300 UTC on 30 June, whichagree quite well (Table III). Still, the model somewhatunderestimates the maximum daily temperatures. MatureSBs developed at the coastlines of Istria as well as on the

coastlines of the islands. Thus, during the day (Figure 11(a)and (b)), the western and south-easterly Istrian SBs (witha maximum speed of 4.5 m s−1) merged, forming aconvergence zone along the peninsula. The convergencezone moved eastwards (with an average speed of 0.5 m s−1),although it was somewhat slower than in reality indicatedby measurements. The potential temperature distributionfollowed these merged SBs (Figure 11(c)). Over the islandof Krk, two convergence zones were generated: the weakconvergence zone due to the north-westerly SB aroundMalinska and another above the hilly area along the island(Figure 11(b)). In Rijeka Bay, a weak clockwise eddydeveloped that is in agreement with Prtenjak et al. (2006).The islands of Cres and Rab also generated weak thermally-induced circulations. Inside Velebit Channel, a weaksoutherly channelled flow met the weak northerly wind(near the town of Senj). The existence of a convergence

Copyright c© 2010 Royal Meteorological Society Q. J. R. Meteorol. Soc. 136: 1554–1571 (2010)

Sea-Land Breeze Development During a Summer Bora Event 1565

Figure 10. Same as in Figure 5(b) except for modelled 10 m wind (m s−1)on 30 June 2004 at 0900 UTC.

zone over the Istrian peninsula was confirmed by all threesources: ground observations (Figure 11(a)), the WRF windfield (Figure 11(b)) and satellite imagery (Figure 11(d)). Theimage shows convective clouds developed all over the area ofconvergence in the WRF model: over the central part of theIstrian peninsula and over the slopes of Cicarija and VelebitMountains.

Figures 8 and 11 allow us to compare the SB on thewestern Istrian coast during 29 and 30 June, respectively. On30 June, in the prevailing undisturbed synoptic conditions,the western SB was earlier and generally weaker than on29 June. The bora mostly limited the SB horizontal extentover the study area, like the mistral in Provence (Bastinet al., 2006). However, the overlap of the weak bora and SBdirections, as occurred on the eastern Istrian coast, resultedin a more time-persistent final south-easterly onshore flowthat slowed down the Istrian convergence zone displacementeastward.

4.3. Vertical structure

As we said in the previous section, on 29 June, at the westernIstrian coast, the onshore flow formed after 0800 UTC inthe lowermost 350 m. However, until noon, this flow didnot represent the westerly SB alone, and instead there wasthe superposition of the dominant swirled bora flow northof Istria and a very weak SB wind (see Figure 7). From1200 UTC, as the bora weakened in the Gulf of Trieste, theimpact of the northern bora jet on the onshore westerly(now thermally driven) wind became negligible. Here, thisunsteady SB/bora interaction is discussed in more detail.

Modelled vertical cross-sections (A1B1 in Figure 1(b))of wind, potential temperature and temperature over Istriaabove Porec (station number 5 in Figure 1(b)) are presentedin Figure 12. The arrows represent wind vectors with along-section horizontal (vh) and vertical (w) wind components.The potential temperature is shown by filled areas, andthe horizontal pressure gradient is shown by dashed lines.Along the cross-section A1B1 on 29 June 2004 at 1300 UTC(Figure 12(a)), the SB penetrated over the land only 6 kmfrom the western Istrian coast. The SB extended into the

400 m deep layer and collided with the bora at a well-marked SB front. At the front (that is ∼1.5 km high),the updraught was characterised by a maximum verticalvelocity of 0.8 m s−1 and turbulent kinetic energy (TKE) of0.5 m2 s−2. In the next 1400 m above the SB, the returncurrent and the north-easterly wind coincided, resulting ina higher wind speed than in the SB. Above the lowermost2 km, a westerly flow dominated in the study area. During theafternoon, at 1600 UTC, as the bora continuously weakened,the only slightly deeper SB (500 m) reached further aboveIstria, 10 km from the coast (Figure 12(b)). On the nextday at 1300 UTC (Figure 12(c)), the SB circulation existedand in a somewhat deeper layer (600 m), with weaker SBmaximum speeds than on 29 June, penetrating ∼16 km overthe land. Comparing Figure 12(a) and 12(c), the return aircurrent in the undisturbed synoptic conditions was weakeras well, squeezed into the 800 m deep layer between the SBbelow and the westerly winds above 1800 m.

In order to argue for the stronger afternoon SB duringthe bora, we evaluated the SB frontogenesis. The leadingedge of the SB (the SB front), which affects the wind speedbehind, can be described by the frontogenesis function,e.g. (d/dt)(∂�/∂x), as in Arritt (1993). The so-called‘convergence frontogenesis’ is very important for SB frontformation or intensification (defined as a product ofdivergence (div) and the potential temperature gradient(�-gradient) along a cross-section = −∂u/∂x × ∂�/∂x)).The �-gradient and divergence across the SB frontwere estimated (along the vertical cross-section A1B1(Figure 1(b)), and they are shown in Figure 12(d). Duringthe daytime on 29 June, both parameters varied in strength,reaching two maxima: the first one at 0900 UTC andthe second one in the afternoon (around 1400 UTC).In the morning, the narrow zone of interaction betweenthe westerly onshore and the easterly bora flows wascharacterised by a significant �-gradient, and it was followedby a strong low-level convergence. Generally, a strongconvergence can occur with strong opposing offshore windsand strong onshore winds (Miller et al., 2003). Later, thebora strength within jets redistributed in the area, weakeningnear the city of Trieste. The onshore western flow (now lessinfluenced by the bora) decreased over time. As the onshoreflow decreased, the low-level convergence became weakeras well. Around noon, the easterly bora pushed the onshoreflow backward over the sea. At the same time, the �-gradient across the front reached its minimum value, whichwas also smaller than for the pure SB case. After 1200UTC, with the overall bora weakening, the convection overthe land decreased the stable stratification of the air, butless than in undisturbed synoptic conditions (Figure 12).However, despite these temperature characteristics, theafternoon convection activities enhanced the �-gradientas well as the low-level convergence (Figure 12(d)). Becauseof moderate offshore bora flow, the locations of the strongest�-gradients and the maximum low-level convergenceoccurred in the same place, in the very narrow zone. Ingeneral, the reduced bora intensity (as the offshore synoptic-scale wind) decreased the width of the SB front, makingstronger temperature gradients across it than during anundisturbed SB event. Due to larger temperature gradients,the horizontal pressure gradients (which actually drive theSB) were higher as well. Figure 12(a) and (c) clarify howthe pressure gradients depend on the structure of the SBfront. In the presence of bora, the orientation of the gravity

Copyright c© 2010 Royal Meteorological Society Q. J. R. Meteorol. Soc. 136: 1554–1571 (2010)

1566 M. T. Prtenjak et al.

(a) (b)

(c) (d)

Figure 11. (a), (b), (c) Same as in Figure 5 except for 30 June 2004 at 1300 UTC. (d) The satellite image was taken at 1227 UTC in the visible spectra bythe NOAA16 satellite.

current head in respect to the x-axis (α) was more in thevertical (e.g. α in Figure 12(a) versus α in Figure 12(c)), andthe horizontal pressure gradients (along the SB front) weretwo times larger than in the pure SB case. The result was astronger maximum wind speed behind the SB front in themature SB during the bora. This result agrees with Arritt(1993), Miller et al. (2003) and Bastin et al. (2006). At thesame time, inland penetration of the SB front was slower(∼0.35 m s−1) than in undisturbed conditions (∼0.5 m s−1).In undisturbed synoptic conditions, the location of themaximum near-surface wind convergence differs somewhatfrom the location of the sharp �-gradient across the SBfront by as much as several km during the day. Therefore,the orientation of the gravity current head was less in thevertical; the horizontal pressure gradients, and consequently,the SB speeds were weaker on 30 June.

Figure 13 shows the A2B2 transect (in Figure 1(b)) abovethe western part of Rijeka Bay near Opatija, on 29 Juneat 1300 UTC and 1600 UTC and on 30 June 2004 at1300 UTC. The figure shows vertical cross-sections of wind,potential temperature and TKE. On 29 June, over the slopesof Risnjak Mountain at 1000 UTC, an event resembling ahydraulic jump occurred, with wake formation near Opatija.

Within the wake, a thermally-induced perturbation beganto develop (not shown). In the lowermost zone, 200 m deepand 9 km wide, the onshore southerly flow formed as asuperposition of upslope wind (due to the mountainouscoast) and the weak SB marking the front. The bora windredirected the onshore flow westward slightly, compared tothe climatological wind hodograph for Opatija (Prtenjak andGrisogono, 2007). The bora wind existed in the 700 m abovethe land, retarding the inland penetration of the opposingthermally-induced wind (Figure 13(a)). The onshore flow(up to ∼6 m s−1) barely penetrated 4 km inland, occupying13 km horizontally and the first 400 m vertically. Behindthe front, that occupies the lowermost 1.5 km, the head ofthe thermally-induced flow formed with strong upwardspeed (>1 m s−1). The north-easterly wind lifted upabove the southerly onshore flow, suppressing its verticalextent (compare Figure 13(a) and 13(c)). The area of theSB/bora interaction was characterised by large TKE values,especially near the ground (Figure 13(a)). At 1600 UTC(Figure 13(b)), as the bora became weaker, the southerlywind (around ∼3 m s−1) weakened as well, extendingover 16 km horizontally, only 2 km further inland than at1300 UTC. The front was still visible in the meteorological

Copyright c© 2010 Royal Meteorological Society Q. J. R. Meteorol. Soc. 136: 1554–1571 (2010)

Sea-Land Breeze Development During a Summer Bora Event 1567

(a) (b)

(c)

(d)

Figure 12. Vertical cross-sections (A1B1 in Figure 1(b)) of the modelled wind (m s−1), the potential temperature (K) and horizontal pressure gradient(hPa m−1) on 29 June 2004 at (a) 1300 UTC and (b) 1600 UTC, and (c) on 30 June 2004 at 1300 UTC. The arrows represent the wind vectors (vh, w)with the along-section horizontal (vh) and vertical (w) wind components. The wind vectors are given at a horizontal grid spacing of 1 km, with referencevectors near the lower right-hand corner. The potential temperature is depicted by the filled areas (legend on the right) and the horizontal pressuregradient is shown by dashed lines (every 0.000015 hPa m−1). The position of Porec station (dot 5 in Figure 1(b)) is shown by an arrow. (d) Changes withtime during 29 June 2004 (grey) and 30 June 2004 (black) of the potential temperature gradient (K m−1) and divergence (s−1) across the sea-breeze frontalong A1B1 vertical cross-section.

fields and was associated with high TKE values (the highestat the ground), although lower than those at 1300 UTC.The onshore flow also occurred in a slightly deeper layer,100 m thicker than at 1300 UTC, with a north-easterly windabove. At 1800 UTC, the thermally–induced onshore windvanished and the bora started to be reinforced. The next day,the onshore flow started to develop earlier, at 0800 UTC.The well-developed onshore flow occurred in the 900 mdeep layer and the vertical thermal circulation was closed atabout 2 km. The weaker onshore flow (speeds ∼3 m s−1)penetrated over 30 km further inland than at the same timeon the previous day (Figure 13(a) and (c)).

The hourly concentrations of surface O3, shownin Figure 14 due to absence of high-quality windmeasurements, support the hypothesis of the developmentof thermally-induced flow above the western part of RijekaBay. The concentrations were measured hourly in the greaterRijeka area, at three chosen air-quality monitoring stations(stations 25–27 in Figure 1(b) and Table I). For the pureSLB case on 28 and 30 June (see wind for Rijeka in Figure 3),O3 concentrations mostly followed a diurnal cycle of the

local circulation, with winds having maxima in the earlyafternoon. The bora, on the other hand, is associated withlow pollution levels, since it advects pollutants over RijekaBay. A considerable decrease in daytime O3 concentrationswas observed on 29 June: only 45 µg m−3 of O3 wasmeasured in Rijeka, which was half that in Opatija at thesame time. Still, in Opatija, a daily cycle of O3 concentrationsexists, in contrast to Rijeka and Krasica, which have amajor bora influence. This significant deviation in O3

concentrations, between Rijeka and Krasica on one handand Opatija on the other, support the conclusion of theevolution of a shallow local circulation above Opatija, whichis suppressed by the north-easterly wind.

Figure 15, similarly to Figure 13, shows vertical cross-sections at 1300 UTC and 1600 UTC on 29 June of themeteorological fields over the next two observed bora wakes,explaining the certain role of the lee-side islands (e.g. Krkand Rab). The first row in Figure 14 corresponds to theA3B3 (see Figure 1(b)) transect above the eastern part ofRijeka Bay near Malinska. The second row belongs to A4B4(in Figure 1(b)), which transects the island of Rab.

Copyright c© 2010 Royal Meteorological Society Q. J. R. Meteorol. Soc. 136: 1554–1571 (2010)

1568 M. T. Prtenjak et al.

(a) (b)

(c)

Figure 13. Vertical cross-sections of the modelled wind (m s−1), potential temperature (K) and turbulent kinetic energy (m2 s−2) on 29 June 2004 at(a) 1300 UTC and (b) 1600 UTC, and (c) on 30 June 2004 at 1300 UTC along A2B2 (see Figure 1(b)) transecting the western part of Rijeka Bay nearOpatija. The arrows represent the wind vectors (vh, w) with the along-section horizontal (vh) and vertical (w) wind components. The wind vectors aregiven at a horizontal grid spacing of 1 km, with reference vectors near the lower right-hand corner. The potential temperature is depicted by the filledareas (legend on the right) with a 1 K interval and the turbulent kinetic energy is shown by the dashed lines every 0.25 m2 s−2.

Figure 14. Hourly averaged air-pollutant O3 concentrations (µg m−3)at three air-pollutant monitoring stations (see Figure 1(b) for locations):Opatija (triangles), Rijeka (solid line) and Krasica (circles) during 28 to 30June 2004.

On 29 June at 1300 UTC, along A3B3 (Figure 15(a)),the bora wind blew along the lee slopes of Velika KapelaMountain, accelerating toward the island of Krk, where ahydraulic jump occurred. The downslope wind increased its

speed to near 15 m s−1 in the foothills of Velika Kapela, and,associated with the hydraulic jump-like feature, the low-levelwind speed then decreased to 8 m s−1. The greatest TKEvalues (up to 3 m2 s−2) in the narrow, vertically-aligned bandwere associated with the hydraulic jump as well. The borawind continued to blow in the lowermost 2 km toward theisland of Cres. In Figure 15(b), at 1600 UTC, the hydraulicjump in the lee of Velika Kapela led to the formation of ashallow lee-wave rotor vertically, near the western coast ofthe island of Krk. Such an eddy has a certain resemblanceto the type-2 rotor in Hertenstein and Kuettner (2005)and explains the origin of the surface onshore flow towardthe island of Krk in Figure 9(a). Still, there are significantdifferences compared to the type-2 rotors obtained fromtheir idealised 2D study, e.g. an absence of the near-surfacejet below the reversed flow and small rotor-associatedturbulence (more details in Prtenjak and Belusic (2009)). Inthe afternoon, the rotor forms at 1500 UTC and lasts roughly3 hours. Since our model overestimates the bora strength,the modelled rotor formation is probably delayed relativeto reality (see Figure 8(a)). Unfortunately, measurements inthis narrow zone are highly limited. However, the existenceand location of this kind of hydraulic-jump rotor is alsosomewhat in agreement with the position of rotor clouds

Copyright c© 2010 Royal Meteorological Society Q. J. R. Meteorol. Soc. 136: 1554–1571 (2010)

Sea-Land Breeze Development During a Summer Bora Event 1569

(a) (b)

(c) (d)

Figure 15. Same as in Figure 13, except on 29 June 2004 at 1300 UTC (left) and 1600 UTC (right) along (a), (b) A3B3 and (c), (d) A4B4 sections inFigure 1(b). The A3B3 section transects the eastern part of Rijeka Bay near Malinska and the A4B4 transect is over the island of Rab.

observed by Mohorovicic during the nineteenth century(Mohorovicic, 1889; Grubisic and Orlic, 2007). It seems thatthe presence of the island of Krk favours the formation oflee-wave rotors: both the lee-side mountain wave-inducedrotors within Velebit Channel during winter bora events(Belusic et al., 2007; Gohm et al., 2008) and the hydraulicjump-like rotor in the lee side of the island of Krk duringsummer bora episodes. In contrast to the island of Krk, thelee side of the nearby island of Cres was without significantinfluence on bora flow (Figure 15(a) and (b)).

On 29 June at 1300 UTC, along A4B4 further to the southin the study area, the north-easterly airflow was only slightlydisrupted over the island of Rab, which is characterised bylow topography (Figure 15(c)). The reason for this minimaldisruption was the convection over land due to the daytimeair temperature rise and roughness, which is higher thanover the sea. The island acts to slightly increase the borawind over the upwind half of the island, leading to a weakconvergence and upward motion over the island. When thebora blows from the island toward the sea, a weak horizontaldivergence and downward motion occur over the edge. Thisis consistent with the results of studies that examined theinfluence of surface roughness on wind (e.g. Yu and Wagner,1975; Yoshikado, 1992; Prtenjak and Grisogono, 2002). At1500 UTC, the weaker bora finally allowed a delayed SB onsetabove the western coast of the island. In the following hours(1600 UTC in Figure 15(d)), a weak SB developed (from

10 to 20 km in the first 500 m), and collided with the boraflow above the centre of the island. The SB almost fadedcompletely at 1900 UTC, when the bora was reinforced.On 30 June near Malinska and the island of Rab, a pureSB developed simultaneously, similar to that obtained inPrtenjak et al. (2006).

5. Conclusions

We used a 3D non-hydrostatic mesoscale meteorologicalmodel to study specific features in a very complexsummertime wind regime along the north-eastern Adriaticcoast. The main aim of this study was to examine thedynamic processes caused by the interaction of the mostcommon wind regimes there: the moderate bora and thesea-land breeze (SLB), which have not been investigatedbefore. The overall modelled results compared satisfactorilywith available observational measurements, despite modeland observational limitations.

The measurements indicate and the modelled resultsconfirm many peculiarities in the wind field during aparticular moderate summer frontal bora event (28–30June 2004). The model simulated bora jets which are similarto those during strong winter bora events (e.g. Belusic andKlaic, 2006; Gohm et al., 2008) with diurnal variations. Atnight, the bora is stronger when its shooting flow prevails,and during the daytime the bora weakens, partly due to the

Copyright c© 2010 Royal Meteorological Society Q. J. R. Meteorol. Soc. 136: 1554–1571 (2010)

1570 M. T. Prtenjak et al.

development of a convective boundary layer over the land.The results revealed areas of bora wakes in regions with weakwinds where an unsteady bora/SLB interaction occurred.They form only for the weaker bora episodes during thewarm part of the year. These areas are the western partof the Istrian peninsula, the western coasts of Rijeka Bayand the island of Rab. Since the SB and the bora presentmostly opposite winds in the lowest atmospheric layer, boraflow (exceeding a certain strength) tends to suppress thedevelopment of the SB. Highly influenced by topography,the north-easterly wind is mostly enhanced and extended byreturn flow in SB circulation aloft (e.g. above the westerncoasts of Istria and Rijeka Bay). The bora is enhanced bythe land breeze at the tip of Istria, and by the SB on theeastern Istrian coast. The more detailed characteristics ofthe SB/bora interaction are as follows:

• Along the western Istrian coast, a narrow convergencezone formed. Its position and the strength of low-level convergence are highly dependent on the balancebetween the bora jets northward and southward ofIstria. In the case of a strong northern jet, the westerlyIstrian onshore flow presented the superposition ofthe dominant swirled bora flow (that follows thecoast toward the south) and the local weak thermalflow. In the case of a weak Trieste bora jet, theconvergence zone is a result of the westerly SB andthe easterly bora wind. Nevertheless, the western SBcannot extend far inland, since its horizontal extentis considerably limited by the bora. This agrees withthe almost similar mistral/SB interaction reported forsouthern France (Bastin et al., 2006) despite different(i.e. less complex) topographic characteristics. Still,the bora as an opposite wind (bora speed lower than6 m s−1) enhances frontogenesis at the SB front,since thermodynamic and kinematic characteristicsof the SB front coincided (contrary to the widerdistance between them for the SB in undisturbedsynoptic conditions). Then, the orientation of thegravity current head was more in the vertical andconsequently, the horizontal pressure gradients werelarger. The result was a stronger upward motion andlarger SB speeds behind (similarly to the theoreticalstudy made by Arritt (1993)) than in the pure SB event.Unlike Bastin et al. (2006), who observed that the SBwas significantly limited vertically as well, we find onlyslight variations in the SB vertical extent there. Abovethe SB, the return current coincides with the north-easterly synoptic wind, increasing it. The SB startsslightly later than under other conditions and lastsfor a somewhat shorter period during the day (dueto reinforcing of the bora wind). The spatial positionof the convergence zone caused by the meeting ofthe SB and the weak bora wind was highly curved,much more than in the pure SB case (e.g. Prtenjaket al., 2006). Since surface circulation patterns in thenorthern Adriatic are mostly driven by wind forcing,the wind distributions reported here (especially the SBformation) presumably caused the formation of thesmall-scale (5 km radius) eddy in surface currents infront of the western Istrian coast observed by Cosoliet al. (2008).

• Over the western part of Rijeka Bay, near Opatija, asmall area of local landward flow developed. The bora

slightly redirects the onshore flow westward, formingthe south-westerly onshore wind that propagatesonly 4 km inland. During the bora/SB interaction,the landward flow, which is substantially limitedvertically, is shorter than in the pure SB case andstronger concerning the maximum wind speed behindthe front. Under the influence of the north-easterlywind aloft, the return current over the SB mergeswith the north-easterly synoptic wind; it is difficultto distinguish them. The air-quality measurements ofozone indirectly supported the hypothesis of the weakthermal onshore flow near Opatija.

• The lee sides of the islands within Kvarner Bay (e.g.Rab, Krk and Cres) revealed different effects on boraflow. On the north-western coast of the island ofRab, convection over the island obstructs the borastrength. In the afternoon, with the bora weakening,an SB develops and collides with the bora over thecentre of the island. Soon afterwards, the SB ceasesdue to reinforcing of the bora wind. Over the northernpart of the island of Krk, in the bora wake, onshoreflow occurs, appearing earlier in measurements thanin the model. Nevertheless, a more detailed modelanalysis revealed that this onshore wind is not the SBflow but presumably the bottom branch of the leerotor that is associated with the hydraulic jump inthe lee of Velika Kapela Mountain. However, theexistence and location of this kind of hydraulic-jump rotor is also somewhat in agreement with thehistorical observation of the position of rotor clouds(e.g. Grubisic and Orlic, 2007). Comparing the overalleffects on the lee side of the islands, it seems that thepresence of the island of Krk favours the lee-wavesrotor formation, although at different locations: thelee-side mountain wave-induced rotors within Velebitchannel during winter bora events (Belusic et al., 2007;Gohm et al., 2008) and the hydraulic jump-like rotorin the lee-side of the island of Krk during summer boraepisodes (Prtenjak and Belusic, 2009). In contrast tothe island of Krk, the lee side of the nearby island ofCres did not produce specific small-scale phenomena.

The simulation presented here showed small-scaleformations like hydraulic-jump rotors and the developmentof thermal-induced flow in a very narrow zone (e.g.near Opatija) that are unfortunately impossible to verifycompletely without a dense grid of measurements in thedomain (which is not the case now). However, we believethat examination of the combined SB/bora event improvesour knowledge of the low-level wind field along the north-east Adriatic and will help in carrying out further researchand measurements.

Acknowledgements

Two anonymous referees are acknowledged for theiruseful suggestions. This work has been supported by theMinistry of Science, Education and Sport (BORA projectNo. 119-1193086-1311). The authors are indebted to theCroatian Weather Service for providing this study with themeteorological data and to Teaching Institute for PublicHealth, Rijeka, Croatia for O3 data and Ana Alebic Jureticfor available comments. We also thank Mirko Orlic forthe additional special wind measurements in Senj. The first

Copyright c© 2010 Royal Meteorological Society Q. J. R. Meteorol. Soc. 136: 1554–1571 (2010)

Sea-Land Breeze Development During a Summer Bora Event 1571

author is grateful to Josip Juras, Branko Grisogono, DanijelBelusic and Zeljko Vecenaj for constructive remarks andtechnical support from Zagreb, Croatia.

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