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Keywords:

  • sea-breeze/bora interaction;
  • sea-breeze front;
  • convergence zone;
  • lee rotor

Abstract

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. The case of 28–30 June 2004
  5. 3. Weather Research and Forecasting (WRF) model
  6. 4. Results and discussion
  7. 5. Conclusions
  8. Acknowledgments
  9. References

The interaction of a summer frontal bora and the sea-land breeze along the north-eastern Adriatic coast was investigated by means of numerical simulations and available observations. Available measurements (in situ, radiosonde, satellite images) provided model validation. The modelled wind field revealed several regions where 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 of the narrow convergence zone that formed depended greatly on the balance between the 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 of the dominant swirled bora flow and local weak thermal flow. It collided then with the easterly bora flow within the zone. With weakening of the Trieste bora jet, the convergence zone was a result of the pure westerly sea breeze and the easterly bora wind. In general, during a bora event, sea breezes were somewhat later and shorter, with limited horizontal extent. The spatial position of the convergence zone caused by 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 horizontal pressure gradients and stronger daytime maximum wind speed than in undisturbed conditions. Except for the island of Rab, other lee-side islands in the area investigated did not provide favourable conditions for the sea-breeze formation. Within a bora wake near the island of Krk, onshore flow occurred as well, although not as a sea-breeze flow, but as the bottom branch of the lee rotor that was associated with the hydraulic jump-like feature in the lee of the Velika Kapela Mountain. Copyright © 2010 Royal Meteorological Society


1. Introduction

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. The case of 28–30 June 2004
  5. 3. Weather Research and Forecasting (WRF) model
  6. 4. Results and discussion
  7. 5. Conclusions
  8. Acknowledgments
  9. References

Bora (‘bura’ in Croatian) is a strong and cold downslope, mainly north-easterly, wind with a higher occurrence in the cold part of the year. It has been analysed and documented for over a century (e.g. Mohorovičić, 1889; Defant, 1951; Yoshino, 1976; Makjanić, 1978; Jurčec, 1980; Klemp and Durran, 1987; Smith, 1987; Bajić, 1989; Poje, 1992; Orlić et al., 1994; Enger and Grisogono, 1998; Heimann, 2001; Klaić et al., 2003; Grubišić, 2004; Belušić and Klaić, 2006; Kraljević and Grisogono, 2006; Grubišić and Orlić, 2007; Pullen et al., 2007; Grisogono and Belušić, 2009). Figure 1 depicts the north-eastern Adriatic region, representing the bora-affected area. This area covers the Istrian peninsula, Kvarner Bay and the mainland. The highest points in the area are Ćićarija (∼1100 m above mean sea level, MSL), Učka (∼1400 m MSL), Risnjak (∼1500 m MSL), Velika Kapela (∼1500 m MSL) and Velebit (∼1600 m MSL), and the two main mountain passes are Gornje Jelenje (between Risnjak 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 Velebit Mountain represent the regions with the highest probability of bora occurrence (e.g. Ivatek-Šahdan and Tudor, 2004; Horvath et al., 2007). The average annual probability of bora occurrence in Senj (station 3 in Figure 1), for example, is greater than 35% (Lukšić, 1975; Makjanić, 1978), with the most probable occurrence in January and the least in June. The average bora speed there is around 11 m s−1, with a maximum in December and a minimum in June and July (Lukšić, 1975). Generally, along the coast, bora has maximum speeds in the morning and minimum speeds in the afternoon, with the average duration in the range of 20–70 h (Poje, 1992).

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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 meteorological measurements (squares): 1 = Pula Airport, 2 = Rijeka and 3 = Senj, climatological measurements (full black circles numbered 4–24, see Table I) and air-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 and 1700 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 are the mountain massifs of Ćićarija (∼1100 m MSL) and Učka (∼1400 m MSL). Kvarner Bay encompasses, besides the smaller Rijeka Bay, the islands of Krk (the biggest one), Cres, Lošinj, Rab. East of Kvarner Bay, high mountains such as Risnjak (∼1500 m MSL) rise up, including Velika Kapela (∼1500 m MSL) and Velebit (∼1600 m MSL).

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Grisogono and Belušić (2009) pointed out that there are over 25 places with bora-like phenomena worldwide (e.g. southern California, the Rocky Mountains, Japan, New Zealand). However, although the basic aspects of bora flows are reasonably well known, the particulars are highly related to local topography (mountains, gaps, islands, etc.). Very simplified, a bora may blow over approximately 1 km high mountains, so that the airflow is only partly blocked, while large, steep waves appear above the mountain, overturn and eventually break. This process usually leads to a hydraulic jump-like formation that is sometimes associated with lee-side eddies.

Several types of bora flow have been detected with regard to synoptic conditions based mainly on the position of the cyclone and anticyclone at the surface level (Jurčec, 1980, 1988; Heimann, 2001; Pandžić and Likso, 2005). The first bora type is commonly called a ‘cyclonic’ bora, when the bora blows along the northern Adriatic coast in combination with a strong sirocco wind along the southern Adriatic. This bora 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 called a ‘frontal’ bora, associated with cold air advection from the north-east. This type sometimes represents the most severe bora events and can be recorded throughout the year. It is characterised locally by a sudden increase in the bora speed and a brief duration. Both of these types represent ‘dark’ bora events, since they are usually connected with cloudiness and heavy precipitation. An ‘anticyclonic’ bora forms under the prevailing influence of a continental high-pressure area above Croatia without a well-defined cyclone to the south. This type of bora without cloudiness occurs throughout the 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 and Belušić, 2009).

Numerical bora simulations have investigated mostly severe bora episodes during the cold part of the year (e.g. Grubišić, 2004; Belušić and Klaić, 2006; Gohm et al., 2008). These studies reveal a spatial distribution of bora jets along the Adriatic coast as terrain-locked features with the main bora jet above the Vratnik Pass. An examination of weak winter boras showed a narrow jet attached to the mountain gap between Risnjak and Velika Kapela mountains that stretched above the northern part of the island of Cres and the tip of Istria (Gohm and Mayr, 2005). This jet and the primary bora jet emanating from Vratnik Pass over the sea near the island of Cres merge into a single, relatively broad, band of strong winds. Another significant bora jet is observed above the Šibenik and Split area (Grubišić, 2004; Gohm and Mayr, 2005; Gohm et al., 2008).

During the last decade, more attention has also been dedicated to another frequent local coastal wind along the Adriatic coast, the sea-land breeze, SLB (Orlić et al., 1988; Nitis et al., 2005; Prtenjak et al., 2006; Trošić et al., 2006; Prtenjak and Grisogono, 2007; Prtenjak et al., 2008). Nitis et al.(2005) and Prtenjak et al.(2006) revealed the formation of several small-scale phenomena, e.g. mesoscale eddies inside Rijeka Bay as well as convergence zones above Istria and the island of Krk. The mesoscale eddies developed inside 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 the terrain height, resulted in an afternoon anticyclonic vortex inside the shallow stable marine boundary layer. The night-time cyclonic eddy developed due to katabatic flow from the surrounding mountains. Above Istria, daytime-merged SBs formed the convergence zone that moved eastward. The surface wind field is significantly channelled through Velebit Channel and the Great Gate. This hints at the goal of this study, since the mesoscale wind characteristics were observed for almost-undisturbed synoptic weather conditions. The details of SLB development under considerable synoptic forcing, e.g. during bora events, are still unknown.

Although Grisogono and Belušić (2009) have shown recent progress and advances in research on meso- and microscale severe bora characteristics, they clearly pointed out some issues and questions that are not yet fully resolved. The authors suggested (among others) more extensive analyses of weak to moderate bora flows, which were still not sufficiently understood (and which are more frequent during summer months), as well as the role of the lee-side islands (e.g. islands within Kvarner Bay) during bora events. Furthermore, within the framework of the recent Northern Adriatic Sea Current Mapping (NASCUM) project (http://poseidon.ogs.trieste.it/jungo/NASCUM/index_en.html), surface current structures in the northern Adriatic Sea were monitored by high-frequency radars. Data from summer surface currents showed small-scale eddies (e.g. 5 km in radius) in front of the western Istrian coast during relatively weak bora events (Cosoli et al., 2008). This complex flow pattern in the area has opened questions on its 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 all summer days) alternate with the sea breeze days (up to 60% of all summer days), so the main goal here is to investigate moderate bora/SB interchange. Previous studies examined severe and/or winter bora cases along the Adriatic coast when the interplay with thermal circulation was not possible. Thus, they did not offer detailed insight into the fine-scale lower-tropospheric conditions responsible for this particular boundary-layer investigation, which, we believe, we have succeeded in doing in the present study. Air quality issues can also be highly associated with the boundary-layer structure. Bastin et al.(2006) investigated the combination of the SB and the summer mistral (severe northerly wind) in Provence, southern France. They found that the presence of the summer mistral prevents onshore SB penetration and weakens and delays the SB compared to cases of pure SB (Bastin and Drobinski, 2006). They observed that during such a combined SB/mistral event, pollutants (e.g. ozone) stagnate close to the coastline and reach high concentrations in the very densely inhabited coastal area. Still, since these results are connected with a particular geographic region, the open question is how far their results and conclusions can be transferred to other, even more complex, topographic areas (e.g. the east Adriatic coast) and similar events.

Therefore, owing to the unknown boundary-layer behaviour while the summer (moderate) bora and the sea breeze exchange, we here investigate their interplay with 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 various mechanisms that drive both the SLB and small-scale form variability of the wind field in time and space (e.g. mesoscale eddies, convergence zones and bora rotors). For this purpose, a three-dimensional (3D) non-hydrostatic numerical simulation of a real case by the Weather Research and Forecasting (WRF) model is used. The results are checked against available observational datasets and analysed.

2. The case of 28–30 June 2004

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. The case of 28–30 June 2004
  5. 3. Weather Research and Forecasting (WRF) model
  6. 4. Results and discussion
  7. 5. Conclusions
  8. Acknowledgments
  9. References

To evaluate the interaction between bora events and the SLB, we selected one combined bora/SLB event that occurred on 28–30 June 2004. Figure 2 shows the synoptic-scale situation at the surface level on 29 June 2004. A surface anticyclone existed over central and western Europe, with its centre over the Atlantic, near France. A shallow cold front crossed over the eastern Alps south-eastward, followed by a cold air outbreak, mostly in the lowest 2 km. The sudden bora onset occurred at 2200 UTC (which corresponds to 2400 CEST, Central European Summer Time) on 28 June, lasting 30 hours. On 29 June 2004, after the front passed, the bora speed suddenly increased at the coast, reaching its maximum before noon (at the foot of Velebit Mountain). On 30 June, during the early morning hours, the bora wind stopped suddenly, and undisturbed local daytime circulations developed. This chosen period represents a ‘frontal’ shallow bora event and it is analysed further here.

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

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3. Weather Research and Forecasting (WRF) model

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. The case of 28–30 June 2004
  5. 3. Weather Research and Forecasting (WRF) model
  6. 4. Results and discussion
  7. 5. Conclusions
  8. Acknowledgments
  9. References

The meteorological fields used were obtained from the WRF model (version 2.2) developed at the National Centre for Atmospheric Research. The WRF (http://www.wrf-model.org/index.php) is used in a variety of areas, including storm prediction and research, air-quality modelling, and predictions of hurricanes, tropical storms, and regional climate and weather (e.g. Michalakes et al., 2004). The WRF model consists of fully compressible non-hydrostatic equations on a staggered Arakawa C grid. Since an Arakawa C grid is used, the wind components u, v and w are recorded at the respective cell interfaces and all other variables as volumetric cells carry averages at the cell centres. The vertical coordinate is a terrain-influenced hydrostatic pressure coordinate. Here, the model uses the Runge–Kutta 3rd-order time integration scheme, as well as 5th-order advection schemes in the horizontal and 3rd-order in the vertical 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 a 9 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 horizontal mesh size covering Croatia (Figure 1(a), frame B). The fine-grid simulation covers an area of 124 × 130 points, with a 1 km horizontal grid spacing (Figure 1(a), frame C). The horizontal grid spacing of 1 km is coarse enough for the meaningful use of a turbulent kinetic energy (TKE) parametrization. Then the ratio of the energy-containing turbulence scale and the scale of the spatial filter used on the equations of motion is small. It should mostly prevent overlapping between the TKE parametrization and the resolved boundary layer (e.g. Wyngaard, 2004). Sixty-five terrain-influenced coordinate levels were used, with the lowest level at about 25 m. Spacing between levels ranged from 60 m at the bottom, and 300 m in the middle and upper troposphere, to 400 m toward the top at 20 km. WRF dynamic and physical options used for all domains include the Advanced Research WRF (ARW) dynamic core; a Mellor–Yamada–Janjic scheme for the planetary boundary layer; the Rapid Radiative Transfer Model for the long-wave radiation and a Dudhia scheme for short-wave radiation; a single-moment 3-class microphysics scheme with ice and snow processes; the Eta surface layer scheme based on Monin–Obukhov (MO) theory and a five-layer thermal diffusion scheme for the soil temperature. On the coarse 9 km domain, the Betts–Miller–Janjic cumulus parametrization was used, but without parametrization in the inner domains. Initialisation and boundary conditions for the mesoscale model were introduced with analysed data from the European Centre for Medium-Range Weather Forecasts (ECMWF). Data are available at a 0.25-degree resolution at standard pressure levels every 6 h. Simulations of 65 h were performed from 0600 UTC of 28 June 2004 until midnight of 30 June 2004.

4. Results and discussion

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. The case of 28–30 June 2004
  5. 3. Weather Research and Forecasting (WRF) model
  6. 4. Results and discussion
  7. 5. Conclusions
  8. Acknowledgments
  9. References

4.1. Model versus measurements

In the study area, we first use in situ hourly wind measurements (speed and direction) from three stations (Pula Airport (1), Rijeka (2) and Senj (3)) to validate the simulation results in the fine-grid domain. Table II shows statistics for the employed model, namely root mean square error (RMSE), mean absolute error (MAE) and the index of agreement, d-index, (e.g. Willmott, 1982; Mastura, 2009), while Figure 3 illustrates a comparison between 10 m measured and modelled wind (diagnosed by the MO similarity theory from model fields). Throughout the period, the wind at Pula Airport is simulated satisfactorily, while in Rijeka and Senj, the modelled wind speed was overestimated compared to measured wind speed during the bora event. It is important to note that Rijeka and Senj are situated in very complex topography, so the model overestimation is partly due to the smoothed topography used in our simulation. However, similar wind speed overestimations have occurred in bora wind simulations performed by other mesoscale models: MEMO6 (Klaić 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 spacing than here. Klaić 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 in Senj, additional special measurements are displayed for the same town. They were performed by the WindMaster ultrasonic Gill anemometer placed at a nearby location, approximately 300 m toward the coast (44.99°N, 14.90°E: Orlić et al., 2005). The additional measured wind speed is 22% higher, on average, than the standard measured one during the examined bora event. For larger bora speeds, deviations between the two measuring sites are even higher—approximately 30–40% (Belušić and Klaić, 2004; Klaić et al., 2009). Modelled wind speed is closer to the associated special measurements (marked by 2 in Table II) than to the standard measured data (d-index_1 in Table II). The special measuring site is probably better suited for comparison with model results because of weaker local influences.

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Figure 3. Modelled (grey line) versus measured (black line) surface winds (10 m above the ground) from 0600 UTC (0800 CEST) 28 June, until midnight 30 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 exact discrepancies between the measured and modelled directions, wind directions spanning a range of 0–90° are sometimes expanded by 360°.

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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 = main meteorological 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)latlongASL (m)No.Station (type of station)latlongASL (m)
1Pula Airport (M)44°54′13°55′6316Učka (C)45°17′14°12′1372
2Rijeka (M)45°20′14°27′12017Volosko (C)45°22′14°19′46
3Senj (M)45°0′14°54′2618Kukuljanovo (C)45°20′14°32′355
4Novigrad (C)45°20′13°35′2019Crikvenica (C)45°10′14°42′2
5Poreč (C)45°13′13°36′1520Rijeka Airport (M)45°13′14°35′85
6Sveti Ivan na Pučini (C)45°3′13°37′821Malinska (C)45°07′14°32′1
7Rovinj (C)45°6′13°38′2022Ponikve (C)45°04′14°35′25
8Pula (C)44°52′13°51′4323Krk (C)45°02′14°35′9
9Pazin (M)45°14′13°56′29124Rab (C)44°45′14°46′24
10Abrami(C)45°26′13°56′8525Opatija-Gorovo (AQ)45°20′14°18′40
11Botonega (C)45°20′13°55′5026Rijeka (AQ)45°19′14°25′20
12Cres (C)44°57′14°25′527Krasica (AQ)45°18′14°33′186
13Labin (C)45°11′14°4′31628Senj_additional (M)44°59′14°54′2
14Čepić (C)45°12′14°9′3028Udine (R)46°3′13°18′94
15Letaj brana (C)45°16′14°8′12029Zagreb (R)45°49′16°2′123
     30Zadar Airport (R)44°07′15°23′88
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.
 RijekaPula AirportSenj Standard measuring site 1Senj Special measuring site 2
 WSWDWSWDWSWDWSWD
RMSE1.631.92.565.64.239.33.257.1
MAE1.322.11.849.53.330.02.135.3
d-index0.870.930.710.950.840.950.910.95

The wind direction is reproduced very well at each of these three stations (Figure 3). More variations in wind direction can be noted for very low wind speeds when most numerical models fail to reproduce wind completely over the very complex terrain (e.g. Baklanov and Grisogono, 2007; Grisogono and Belušić, 2009). At Pula Airport on 30 June, some wind direction discrepancies exist between the measurements and the model, influenced by the convergence zone position above Istria. At all stations, the model successfully predicts the timing of the bora breakthrough near the coast, as well as the SB timing. In contrast to the Pula Airport and Rijeka stations, the bora wind started very suddenly at Senj, reaching moderate bora speeds with the longest duration under the same upstream conditions.

The radiosondes launched from Zadar Airport (Figure 1(a)), the only one at the coast, are shown in Figure 4, allowing a vertical comparison of the model results with measurements. During the bora event, two layers can be observed (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 boundary layer, with west-north-westerly wind above (Figure 4, upper row). The maximum bora wind speed was 7.2 m s−1 at 300 m height, observed only in measurements. Above the bora layer, the westerly wind, which the model reproduced satisfactorily compared to the measurements, increases with height. On the next day (Figure 4, lower row), the potential temperature profile shows a convective boundary layer approximately 2 km deep. Inside the boundary layer, in the lowermost 700 m, the SB developed as a weak north-westerly wind. The wind changes its direction with height (up to 90°) in the next 1000 m. These SB characteristics agree with the SB climatology (Prtenjak and Grisogono, 2007). The wind is west-north-westerly above the boundary layer. The model reproduced the local circulation cell satisfactorily in the somewhat lower boundary layer. The discrepancies in comparisons of the radiosondes from Udine and Zagreb (Figure 1(a)) were very similar to those shown here for Zadar Airport and thus not shown.

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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) potential temperature at Zadar Airport from the radiosondes launched on 29 June 2004 at 1200 UTC (top) and 30 June 2004 aSt 1200 UTC (bottom). Position of the measuring site is indicated in Figure 1(a).

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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 reproduced the case analysed.

4.2. Near-surface wind field characteristics

The previous section introduced certain model capabilities of the simulation of bora/SB exchange. Here, we continue to analyse and verify model results for the near-surface flow pattern using wind measurements from the 27 stations (Figure 1(b)). Unfortunately, most of the wind data are collected from ordinary meteorological stations (Table I). Since these stations provide only the strength of the wind in three terms (according to the Beaufort scale), discrepancies between measurements and the model may also be due to wind strength conversions into wind speed (m s−1).

Figure 5 shows the 10 m wind field and potential temperature field on 28 June 2004 at 1300 UTC. The measurements and simulation results both show the prevailing relatively weak southerly winds over the larger part of the Istrian peninsula and Kvarner Bay. The simulated wind reached its highest speed within the Great Gate and the Senj Gate due to channelling and downslope airflow in the hinterland. In the study area, certain known mesoscale features (Prtenjak et al., 2006) developed, e.g. convergence zones 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 km east from the western Istrian coast, where SB speeds were below 4.5 m s−1. Comparing measured and simulated wind (Table III) at 1300 UTC, the WRF model reproduced the wind direction satisfactorily and the wind speed somewhat less well. The surface air potential temperature rises toward the centre of both the Istrian peninsula and the island of Krk, showing SB formation at the coasts (Figure 5(c)). Although the model correctly simulates the coastal surface air temperatures, in general, a slight underestimation of the measured temperature values is noted in the middle of Istria.

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Figure 5. (a) 10 m wind vectors (m s−1) and surface air potential temperature in K (numbers near vectors) from meteorological and climatological stations (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 1300 UTC. 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 depicted by filled areas (grey-scale legend on the right) with a 1 m s−1 interval.

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Table III. Some statistical indices for the basic numerical simulation—root mean square error (RMSE), mean absolute error (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).
Time28 June 2004 1300 UTC29 June 2004 0600 UTC29 June 2004 1300 UTC30 June 2004 1300 UTC
 WSWDWSWDWSWDWSWD
RMSE1.359.42.555.12.552.31.141.1
MAE1.144.42.240.22.334.00.931.3
d-index0.770.880.750.950.830.940.800.95
r0.530.810.720.910.760.900.620.86

Around 2200 UTC on the same day, the bora wind started to 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 wind field during the bora event varies in time and space. In the early morning on 29 June (Figure 6), the bora blew over nearly the entire area, except for two regions: within the valley between Ćićarija and Risnjak and within Rijeka Bay. Such a wind distribution means that the bora wind did not become fully established at the surface near the high coastal mountains within Rijeka Bay. There, due to flow separation from the leeward slope of Risnjak Mountain, the bora layer with north-easterly winds was lifted above 300 m off the sea. In the lowermost layer, a cold pool existed over Rijeka Bay which was blocked in front of the coastal mountains of Istria (not shown). The whole wind distribution was reproduced very well by the model (Table III), although wind speed was somewhat overestimated (Figure 6). In general, the bora wind varies spatially in strength, with the formation of several bora jets through the mountain passes, north and south of Istria. The northern jet formed (near the towns of Trieste and Koper), which is sited in a topographic incision between the Dinaric and the Julian Alps. The southern jets are associated with mountain gaps in the Dinaric Alps range, the stronger one (around 14 m s−1) through Vratnik Pass and the weaker one through Gornje Jelenje. Over the Istrian peninsula (according to both the measurements and model), a moderate bora dominated, due to passage of the cold front. The maximum modelled bora wind speed of more than 16 m s−1 formed east of the Istrian peninsula, near the island of Cres. Above the island of Krk, a moderate easterly bora veered toward another strong narrow bora jet originating through Vratnik Pass. This jet achieved its maximum strength within the Senj Gate and the eastern coast of the island of Cres. The event can be classified as shallow, since the cross-mountain flow (i.e. the bora layer with north-easterly winds aloft) was restricted to the lowermost 2.5 km of the atmosphere.

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Figure 6. (a) 10 m wind from meteorological and climatological stations and (b) from model simulations on 29 June 2004 at 0600 UTC. 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 depicted by filled areas (grey-scale legend on the right) with a 2 m s−1 interval.

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Figure 7 displays the surface wind field at 1000 UTC, around its maximum. The modelled wind at 1000 UTC is only somewhat similar to modelled wind at 0600 UTC. Figure 7 shows very well formed bora jets; the northern one (near Trieste, maximum wind speed ∼14 m s−1), the primary southern stronger (∼15 m s−1) bora jet through the Vratnik Pass with a width of 25 km, and the second southern, weaker (∼13 m s−1) bora jet through Gornje Jelenje with a width of 15 km. Both narrow jets over Kvarner Bay join together, forming one broad bora jet about 50 km wide near the surface, several kilometres downstream of the coast. This merged bora jet stretches from the middle of the peninsula to the island of Lošinj, with its centre above the tip of Istria. This wind distribution agrees very well with the results of other observational and numerical winter severe bora studies (e.g. Jurčec, 1980; Grubišić, 2004; Gohm et al., 2008). The modelled wind distribution reveals the western part of Rijeka Bay, especially around the town of Opatija (station 25 in Figure 1(b), at the foot of Uč ka Mountain) and at the north-western coast of the island of Krk around the town of Malinska (station 21 in Figure 1(b)), as a sheltered area. Since the bora typically weakens somewhat during the daytime due to the evolution of a convective boundary layer over land (Grisogono and Belušić, 2009), the bora decreases its speed (over Istria) causing a bora wake region over the flat western Istrian coast. This area is affected by the convergence zone from the westerly onshore flow and the easterly bora flow. The model shows that the bora wind at the southern part of the jet north of Istria was considerably whirled, supporting the onshore flow formation along the western Istrian coast. In section 4.3, we discuss this feature as the result of the unsteady bora/SB interaction.

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

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Figure 8(a) shows the measured bora wind and the associated 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 almost unchanged southern jets), several simultaneous enlarged areas of bora minima (Figure 8(a) and (b)). They are the western Istrian coast, the sheltered areas in Rijeka Bay (the western sides of Rijeka Bay and the island of Krk) and southern part of the island of Rab. Depending on the bora strength, these areas of bora minima vary in space. At the western Istrian coast, the SB develops in the narrow area despite the fact that the bora brings cold and dry continental air and suppresses a daytime temperature rise (Figure 8(a) and (c)). There, the wind direction changes over time from south-west to north-west. Here, the stronger bora wind speed (>6 m s−1) south of Rovinj (station 7 in Figure 1(b)) did not allow daytime SB penetration over the land, maintaining the SB front over the sea (45.4°N, 13.5°E). In Rijeka Bay near Opatija, the low bora speed allows the formation of the weak thermally-induced perturbation, but within the sheltered areas near the town of Malinska and along the narrow area on the western coast of the island of Rab, only a redirection and weakening of the bora wind occurs.

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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 spectra by the NOAA16 satellite.

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A satellite image (Figure 8(d)) confirms the bora and SB interaction. The satellite image was produced about twenty minutes earlier than ground observations (Figure 8(a)), the modelled WRF field (Figure 8(b)) and surface potential temperature (Figure 8(c)). A wide stratocumulus field that spreads from the Croatian coast inland corresponds well with the lower surface potential temperatures in Figure 8(c). The convergence in the wind field results in air convection, visible as convective cloud development above the western coast of the Istrian peninsula. Cumulus clouds are high enough to produce clearly visible shadows even at an image resolution of 1 km. The cold-front passage (which is outside of Figure 8(d), to the east) produced stronger convective development, seen as dense stratocumulus over the high coastal mountains (Risnjak, Velika Kapela and Velebit). A wide stratocumulus layer with sharp edges is an example of a typical orogenetic cloud induced by bora wind. The plume of higher clouds moves in westerly winds, opposite to the mostly north-eastern direction of the surface WRF wind modelled in Figure 8(b).

At 1600 UTC, in western Istria, the SB reaches a maximum speed of about 6 m s−1 (Figure 9(a)). For the same wind field, the convergence and vorticity are displayed in Figure 9(b) and (c). The moderate grey-filled areas show where convergence (and consequently convection) occurs during the bora and SB interaction (Figure 9(b)). Comparing the values of convergence during the study period above Istria, the highest values were on 29 June. These areas are also associated with significant vorticity conditioned earlier by the bora jets (Figure 9(c)). The cyclonic vorticity (light grey) formed south-west of the Trieste bora jet maximum. At the same time, south of Rovinj, moderate bora winds determined the formation of the anticyclonic vorticity (moderate grey). The penetration of the mature western SB was approximately 20 km east (Figure 9(a) and (b)). Over the north-western coast of Rijeka Bay and the islands of Krk and Rab, onshore flow developed. However, the moderate to strong bora (>6 m s−1) still obstructed SB formation above the rest of the north-eastern Adriatic coast. In the evening, on the western Istrian coast, the SB vanished and a land breeze developed, blowing toward the sea. It coincides with the reinforced bora flow, especially above the tip of Istria (not shown).

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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) for 29 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.

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During the night, the bora was stronger and still blew mostly in the form of jets, with the strongest one in Senj. In the morning on 30 June (Figure 10), the bora weakened and gradually stopped (until 1000 UTC), allowing simultaneous SB development on the north-eastern Adriatic coast. A moderate bora blew only through Vratnik Pass toward the tip of Istria and north of Trieste. In Rijeka Bay, the north-westerly SB formed in the presence of the weak bora limited to the very narrow coastal area. Above Istria, the western coast was under the SB (starting at 0700 UTC), which penetrated ∼10 km over the land. The SB flow diverged there, toward the bora jets, less northward and more southward of Istria, forming an anticlockwise whirl in front of the coast between Novigrad and Rovinj (sites 4 and 7 in Figure 1(b), respectively). This feature in the wind corresponds closely to the swirling in the surface currents detected during the NASCUM project (Cosoli et al., 2008). It seems that the wind distribution during the SB/bora interplay forced this small-scale phenomenon in the surface currents. On the eastern Istrian coast, a south-easterly SB overlapped with the weak bora wind, forming an onshore flow that was twice as strong as the western SB. Over the northern part of the island of Krk, an SB developed as well.

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

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Figure 11 shows the measured and modelled wind and temperature distributions at 1300 UTC on 30 June, which agree quite well (Table III). Still, the model somewhat underestimates the maximum daily temperatures. Mature SBs 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 (with a maximum speed of 4.5 m s−1) merged, forming a convergence zone along the peninsula. The convergence zone moved eastwards (with an average speed of 0.5 m s−1), although it was somewhat slower than in reality indicated by measurements. The potential temperature distribution followed these merged SBs (Figure 11(c)). Over the island of Krk, two convergence zones were generated: the weak convergence zone due to the north-westerly SB around Malinska and another above the hilly area along the island (Figure 11(b)). In Rijeka Bay, a weak clockwise eddy developed 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 weak southerly channelled flow met the weak northerly wind (near the town of Senj). The existence of a convergence zone over the Istrian peninsula was confirmed by all three sources: ground observations (Figure 11(a)), the WRF wind field (Figure 11(b)) and satellite imagery (Figure 11(d)). The image shows convective clouds developed all over the area of convergence in the WRF model: over the central part of the Istrian peninsula and over the slopes of Ćićarija and Velebit Mountains.

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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 by the NOAA16 satellite.

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Figures 8 and 11 allow us to compare the SB on the western Istrian coast during 29 and 30 June, respectively. On 30 June, in the prevailing undisturbed synoptic conditions, the western SB was earlier and generally weaker than on 29 June. The bora mostly limited the SB horizontal extent over the study area, like the mistral in Provence (Bastin et al., 2006). However, the overlap of the weak bora and SB directions, as occurred on the eastern Istrian coast, resulted in a more time-persistent final south-easterly onshore flow that slowed down the Istrian convergence zone displacement eastward.

4.3. Vertical structure

As we said in the previous section, on 29 June, at the western Istrian coast, the onshore flow formed after 0800 UTC in the lowermost 350 m. However, until noon, this flow did not represent the westerly SB alone, and instead there was the superposition of the dominant swirled bora flow north of Istria and a very weak SB wind (see Figure 7). From 1200 UTC, as the bora weakened in the Gulf of Trieste, the impact of the northern bora jet on the onshore westerly (now thermally driven) wind became negligible. Here, this unsteady SB/bora interaction is discussed in more detail.

Modelled vertical cross-sections (A1B1 in Figure 1(b)) of wind, potential temperature and temperature over Istria above Poreč (station number 5 in Figure 1(b)) are presented in 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, and the 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 km from 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 vertical velocity of 0.8 m s−1 and turbulent kinetic energy (TKE) of 0.5 m2 s−2. In the next 1400 m above the SB, the return current and the north-easterly wind coincided, resulting in a higher wind speed than in the SB. Above the lowermost 2 km, a westerly flow dominated in the study area. During the afternoon, at 1600 UTC, as the bora continuously weakened, the only slightly deeper SB (500 m) reached further above Istria, 10 km from the coast (Figure 12(b)). On the next day at 1300 UTC (Figure 12(c)), the SB circulation existed and in a somewhat deeper layer (600 m), with weaker SB maximum speeds than on 29 June, penetrating ∼16 km over the land. Comparing Figure 12(a) and 12(c), the return air current in the undisturbed synoptic conditions was weaker as well, squeezed into the 800 m deep layer between the SB below and the westerly winds above 1800 m.

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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 reference vectors near the lower right-hand corner. The potential temperature is depicted by the filled areas (legend on the right) and the horizontal pressure gradient is shown by dashed lines (every 0.000015 hPa m−1). The position of Poreč station (dot 5 in Figure 1(b)) is shown by an arrow. (d) Changes with time 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 front along A1B1 vertical cross-section.

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In order to argue for the stronger afternoon SB during the bora, we evaluated the SB frontogenesis. The leading edge of the SB (the SB front), which affects the wind speed behind, can be described by the frontogenesis function, e.g. (d/dt)(∂Θ/∂x), as in Arritt (1993). The so-called ‘convergence frontogenesis’ is very important for SB front formation or intensification (defined as a product of divergence (div) and the potential temperature gradient (Θ-gradient) along a cross-section = − ∂u/∂x × ∂Θ/∂x)). The Θ-gradient and divergence across the SB front were estimated (along the vertical cross-section A1B1 (Figure 1(b)), and they are shown in Figure 12(d). During the daytime on 29 June, both parameters varied in strength, reaching two maxima: the first one at 0900 UTC and the second one in the afternoon (around 1400 UTC). In the morning, the narrow zone of interaction between the westerly onshore and the easterly bora flows was characterised by a significant Θ-gradient, and it was followed by a strong low-level convergence. Generally, a strong convergence can occur with strong opposing offshore winds and strong onshore winds (Miller et al., 2003). Later, the bora strength within jets redistributed in the area, weakening near the city of Trieste. The onshore western flow (now less influenced by the bora) decreased over time. As the onshore flow decreased, the low-level convergence became weaker as well. Around noon, the easterly bora pushed the onshore flow backward over the sea. At the same time, the Θ-gradient across the front reached its minimum value, which was also smaller than for the pure SB case. After 1200 UTC, with the overall bora weakening, the convection over the land decreased the stable stratification of the air, but less than in undisturbed synoptic conditions (Figure 12). However, despite these temperature characteristics, the afternoon convection activities enhanced the Θ-gradient as well as the low-level convergence (Figure 12(d)). Because of moderate offshore bora flow, the locations of the strongest Θ-gradients and the maximum low-level convergence occurred in the same place, in the very narrow zone. In general, the reduced bora intensity (as the offshore synoptic-scale wind) decreased the width of the SB front, making stronger temperature gradients across it than during an undisturbed SB event. Due to larger temperature gradients, the horizontal pressure gradients (which actually drive the SB) were higher as well. Figure 12(a) and (c) clarify how the pressure gradients depend on the structure of the SB front. In the presence of bora, the orientation of the gravity current head in respect to the x-axis (α) was more in the vertical (e.g. α in Figure 12(a) versus α in Figure 12(c)), and the horizontal pressure gradients (along the SB front) were two times larger than in the pure SB case. The result was a stronger maximum wind speed behind the SB front in the mature SB during the bora. This result agrees with Arritt (1993), Miller et al.(2003) and Bastin et al.(2006). At the same 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 the maximum near-surface wind convergence differs somewhat from the location of the sharp Θ-gradient across the SB front by as much as several km during the day. Therefore, the orientation of the gravity current head was less in the vertical; the horizontal pressure gradients, and consequently, the SB speeds were weaker on 30 June.

Figure 13 shows the A2B2 transect (in Figure 1(b)) above the western part of Rijeka Bay near Opatija, on 29 June at 1300 UTC and 1600 UTC and on 30 June 2004 at 1300 UTC. The figure shows vertical cross-sections of wind, potential temperature and TKE. On 29 June, over the slopes of Risnjak Mountain at 1000 UTC, an event resembling a hydraulic jump occurred, with wake formation near Opatija. Within the wake, a thermally-induced perturbation began to develop (not shown). In the lowermost zone, 200 m deep and 9 km wide, the onshore southerly flow formed as a superposition of upslope wind (due to the mountainous coast) and the weak SB marking the front. The bora wind redirected the onshore flow westward slightly, compared to the climatological wind hodograph for Opatija (Prtenjak and Grisogono, 2007). The bora wind existed in the 700 m above the land, retarding the inland penetration of the opposing thermally-induced wind (Figure 13(a)). The onshore flow (up to ∼6 m s−1) barely penetrated 4 km inland, occupying 13 km horizontally and the first 400 m vertically. Behind the front, that occupies the lowermost 1.5 km, the head of the thermally-induced flow formed with strong upward speed (>1 m s−1). The north-easterly wind lifted up above the southerly onshore flow, suppressing its vertical extent (compare Figure 13(a) and 13(c)). The area of the SB/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 southerly wind (around ∼3 m s−1) weakened as well, extending over 16 km horizontally, only 2 km further inland than at 1300 UTC. The front was still visible in the meteorological fields and was associated with high TKE values (the highest at 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 wind above. At 1800 UTC, the thermally–induced onshore wind vanished 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 m deep layer and the vertical thermal circulation was closed at about 2 km. The weaker onshore flow (speeds ∼3 m s−1) penetrated over 30 km further inland than at the same time on the previous day (Figure 13(a) and (c)).

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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 near Opatija. 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 reference vectors near the lower right-hand corner. The potential temperature is depicted by the filled areas (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.

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The hourly concentrations of surface O3, shown in Figure 14 due to absence of high-quality wind measurements, support the hypothesis of the development of thermally-induced flow above the western part of Rijeka Bay. The concentrations were measured hourly in the greater Rijeka area, at three chosen air-quality monitoring stations (stations 25–27 in Figure 1(b) and Table I). For the pure SLB 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 early afternoon. The bora, on the other hand, is associated with low pollution levels, since it advects pollutants over Rijeka Bay. A considerable decrease in daytime O3 concentrations was observed on 29 June: only 45 µg m−3 of O3 was measured in Rijeka, which was half that in Opatija at the same time. Still, in Opatija, a daily cycle of O3 concentrations exists, in contrast to Rijeka and Krasica, which have a major bora influence. This significant deviation in O3 concentrations, between Rijeka and Krasica on one hand and Opatija on the other, support the conclusion of the evolution of a shallow local circulation above Opatija, which is suppressed by the north-easterly wind.

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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 30 June 2004.

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Figure 15, similarly to Figure 13, shows vertical cross-sections at 1300 UTC and 1600 UTC on 29 June of the meteorological fields over the next two observed bora wakes, explaining the certain role of the lee-side islands (e.g. Krk and Rab). The first row in Figure 14 corresponds to the A3B3 (see Figure 1(b)) transect above the eastern part of Rijeka Bay near Malinska. The second row belongs to A4B4 (in Figure 1(b)), which transects the island of Rab.

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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 in Figure 1(b). The A3B3 section transects the eastern part of Rijeka Bay near Malinska and the A4B4 transect is over the island of Rab.

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On 29 June at 1300 UTC, along A3B3 (Figure 15(a)), the bora wind blew along the lee slopes of Velika Kapela Mountain, accelerating toward the island of Krk, where a hydraulic 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-level wind speed then decreased to 8 m s−1. The greatest TKE values (up to 3 m2 s−2) in the narrow, vertically-aligned band were associated with the hydraulic jump as well. The bora wind continued to blow in the lowermost 2 km toward the island of Cres. In Figure 15(b), at 1600 UTC, the hydraulic jump in the lee of Velika Kapela led to the formation of a shallow lee-wave rotor vertically, near the western coast of the island of Krk. Such an eddy has a certain resemblance to the type-2 rotor in Hertenstein and Kuettner (2005) and explains the origin of the surface onshore flow toward the island of Krk in Figure 9(a). Still, there are significant differences compared to the type-2 rotors obtained from their idealised 2D study, e.g. an absence of the near-surface jet below the reversed flow and small rotor-associated turbulence (more details in Prtenjak and Belušić (2009)). In the afternoon, the rotor forms at 1500 UTC and lasts roughly 3 hours. Since our model overestimates the bora strength, the modelled rotor formation is probably delayed relative to reality (see Figure 8(a)). Unfortunately, measurements in this narrow zone are highly limited. However, the existence and location of this kind of hydraulic-jump rotor is also somewhat in agreement with the position of rotor clouds observed by Mohorovičić during the nineteenth century (Mohorovičić, 1889; Grubišić and Orlić, 2007). It seems that the presence of the island of Krk favours the formation of lee-wave rotors: both the lee-side mountain wave-induced rotors within Velebit Channel during winter bora events (Belušić et al., 2007; Gohm et al., 2008) and the hydraulic jump-like rotor in the lee side of the island of Krk during summer bora episodes. In contrast to the island of Krk, the lee side of the nearby island of Cres was without significant influence on bora flow (Figure 15(a) and (b)).

On 29 June at 1300 UTC, along A4B4 further to the south in the study area, the north-easterly airflow was only slightly disrupted over the island of Rab, which is characterised by low topography (Figure 15(c)). The reason for this minimal disruption was the convection over land due to the daytime air temperature rise and roughness, which is higher than over the sea. The island acts to slightly increase the bora wind over the upwind half of the island, leading to a weak convergence and upward motion over the island. When the bora blows from the island toward the sea, a weak horizontal divergence and downward motion occur over the edge. This is consistent with the results of studies that examined the influence of surface roughness on wind (e.g. Yu and Wagner, 1975; Yoshikado, 1992; Prtenjak and Grisogono, 2002). At 1500 UTC, the weaker bora finally allowed a delayed SB onset above 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 bora flow above the centre of the island. The SB almost faded completely at 1900 UTC, when the bora was reinforced. On 30 June near Malinska and the island of Rab, a pure SB developed simultaneously, similar to that obtained in Prtenjak et al.(2006).

5. Conclusions

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. The case of 28–30 June 2004
  5. 3. Weather Research and Forecasting (WRF) model
  6. 4. Results and discussion
  7. 5. Conclusions
  8. Acknowledgments
  9. References

We used a 3D non-hydrostatic mesoscale meteorological model to study specific features in a very complex summertime wind regime along the north-eastern Adriatic coast. The main aim of this study was to examine the dynamic processes caused by the interaction of the most common wind regimes there: the moderate bora and the sea-land breeze (SLB), which have not been investigated before. The overall modelled results compared satisfactorily with available observational measurements, despite model and observational limitations.

The measurements indicate and the modelled results confirm many peculiarities in the wind field during a particular moderate summer frontal bora event (28–30 June 2004). The model simulated bora jets which are similar to those during strong winter bora events (e.g. Belušić and Klaić, 2006; Gohm et al., 2008) with diurnal variations. At night, the bora is stronger when its shooting flow prevails, and during the daytime the bora weakens, partly due to the development of a convective boundary layer over the land. The results revealed areas of bora wakes in regions with weak winds where an unsteady bora/SLB interaction occurred. They form only for the weaker bora episodes during the warm part of the year. These areas are the western part of the Istrian peninsula, the western coasts of Rijeka Bay and the island of Rab. Since the SB and the bora present mostly opposite winds in the lowest atmospheric layer, bora flow (exceeding a certain strength) tends to suppress the development of the SB. Highly influenced by topography, the north-easterly wind is mostly enhanced and extended by return flow in SB circulation aloft (e.g. above the western coasts of Istria and Rijeka Bay). The bora is enhanced by the land breeze at the tip of Istria, and by the SB on the eastern Istrian coast. The more detailed characteristics of the SB/bora interaction are as follows:

  • Along the western Istrian coast, a narrow convergence zone formed. Its position and the strength of low-level convergence are highly dependent on the balance between the bora jets northward and southward of Istria. In the case of a strong northern jet, the westerly Istrian onshore flow presented the superposition of the dominant swirled bora flow (that follows the coast toward the south) and the local weak thermal flow. In the case of a weak Trieste bora jet, the convergence zone is a result of the westerly SB and the easterly bora wind. Nevertheless, the western SB cannot extend far inland, since its horizontal extent is considerably limited by the bora. This agrees with the almost similar mistral/SB interaction reported for southern France (Bastin et al., 2006) despite different (i.e. less complex) topographic characteristics. Still, the bora as an opposite wind (bora speed lower than 6 m s−1) enhances frontogenesis at the SB front, since thermodynamic and kinematic characteristics of the SB front coincided (contrary to the wider distance between them for the SB in undisturbed synoptic conditions). Then, the orientation of the gravity current head was more in the vertical and consequently, the horizontal pressure gradients were larger. The result was a stronger upward motion and larger SB speeds behind (similarly to the theoretical study made by Arritt (1993)) than in the pure SB event. Unlike Bastin et al.(2006), who observed that the SB was significantly limited vertically as well, we find only slight variations in the SB vertical extent there. Above the SB, the return current coincides with the north-easterly synoptic wind, increasing it. The SB starts slightly later than under other conditions and lasts for a somewhat shorter period during the day (due to reinforcing of the bora wind). The spatial position of the convergence zone caused by the meeting of the SB and the weak bora wind was highly curved, much more than in the pure SB case (e.g. Prtenjak et al., 2006). Since surface circulation patterns in the northern Adriatic are mostly driven by wind forcing, the wind distributions reported here (especially the SB formation) presumably caused the formation of the small-scale (5 km radius) eddy in surface currents in front of the western Istrian coast observed by Cosoli et al.(2008).

  • Over the western part of Rijeka Bay, near Opatija, a small area of local landward flow developed. The bora slightly redirects the onshore flow westward, forming the south-westerly onshore wind that propagates only 4 km inland. During the bora/SB interaction, the landward flow, which is substantially limited vertically, is shorter than in the pure SB case and stronger concerning the maximum wind speed behind the front. Under the influence of the north-easterly wind aloft, the return current over the SB merges with the north-easterly synoptic wind; it is difficult to distinguish them. The air-quality measurements of ozone indirectly supported the hypothesis of the weak thermal onshore flow near Opatija.

  • The lee sides of the islands within Kvarner Bay (e.g. Rab, Krk and Cres) revealed different effects on bora flow. On the north-western coast of the island of Rab, convection over the island obstructs the bora strength. In the afternoon, with the bora weakening, an SB develops and collides with the bora over the centre of the island. Soon afterwards, the SB ceases due to reinforcing of the bora wind. Over the northern part of the island of Krk, in the bora wake, onshore flow occurs, appearing earlier in measurements than in the model. Nevertheless, a more detailed model analysis revealed that this onshore wind is not the SB flow but presumably the bottom branch of the lee rotor that is associated with the hydraulic jump in the lee of Velika Kapela Mountain. However, the existence and location of this kind of hydraulic-jump rotor is also somewhat in agreement with the historical observation of the position of rotor clouds (e.g. Grubišić and Orlić, 2007). Comparing the overall effects on the lee side of the islands, it seems that the presence of the island of Krk favours the lee-waves rotor formation, although at different locations: the lee-side mountain wave-induced rotors within Velebit channel during winter bora events (Belušić et al., 2007; Gohm et al., 2008) and the hydraulic jump-like rotor in the lee-side of the island of Krk during summer bora episodes (Prtenjak and Belušić, 2009). In contrast to the island of Krk, the lee side of the nearby island of Cres did not produce specific small-scale phenomena.

The simulation presented here showed small-scale formations like hydraulic-jump rotors and the development of thermal-induced flow in a very narrow zone (e.g. near Opatija) that are unfortunately impossible to verify completely without a dense grid of measurements in the domain (which is not the case now). However, we believe that examination of the combined SB/bora event improves our knowledge of the low-level wind field along the north-east Adriatic and will help in carrying out further research and measurements.

Acknowledgments

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. The case of 28–30 June 2004
  5. 3. Weather Research and Forecasting (WRF) model
  6. 4. Results and discussion
  7. 5. Conclusions
  8. Acknowledgments
  9. References

Two anonymous referees are acknowledged for their useful suggestions. This work has been supported by the Ministry of Science, Education and Sport (BORA project No. 119-1193086-1311). The authors are indebted to the Croatian Weather Service for providing this study with the meteorological data and to Teaching Institute for Public Health, Rijeka, Croatia for O3 data and Ana Alebić Juretić for available comments. We also thank Mirko Orlić for the additional special wind measurements in Senj. The first author is grateful to Josip Juras, Branko Grisogono, Danijel Belušić and Željko Večenaj for constructive remarks and technical support from Zagreb, Croatia.

References

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. The case of 28–30 June 2004
  5. 3. Weather Research and Forecasting (WRF) model
  6. 4. Results and discussion
  7. 5. Conclusions
  8. Acknowledgments
  9. References
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