Aleksandar D. Prodanov

Danijel Belušić

Zvjezdana Bencetić Klaić

English

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Wind measurements in Senj – underestimationof true bora flows1. IntroductionIt is well known that the strongest and most frequent bora over the Cro-atian coast of the Adriatic Sea is found in the region of Senj (Figure 1) (e.g.,Yoshino, 1976; Makjani}, 1978; Vu~eti}, 1991; Orli} et al., 1994; Penzar et al.,2001; Grubi{i}, 2004:, Dorman et al., 2006). According to field measurements(Yoshino, 1976), the strongest winds appear in the mountainous areas north-east of Senj, between the Alan and the Vratnik Pass (Figure 2). An investiga-tion of the wind-shaped trees in the area (Figure 3) was performed to estimatethe speed and direction of the prevailing winter winds (Yoshino, 1976). It re-GEOFIZIKA VOL. 26 No. 2 2009Figure 1. During the bora wind, the sea surface has a typical appearance. Bora tears up wavecrests which bubble up like a boiling water, spraying small particles of foam and water into air.Thus, visibility above the sea is reduced. This photo was taken in Senj during severe bora on 17November 2007. According to MHS (2007), during this episode wind gusts were occasionally ofhurricane strength (wind speeds over 40 ms–1). (Photo by Damir Sen~ar, copyright by Hina.)vealed that the strongest ENE winds, which blow out of the pass, hit thesouthern part of the Krk Island, then turn gradually into NNE winds, and fi-nally reach the Lo{inj Island. They thus define the strongest bora region asthat at the Adriatic coast, extending as far as 50–60 km inland. Consequently,a sharp contrast between the windward (bare) and leeward slopes (with vege-tation) of islands is seen (Figures 3 and 4).Since bora affects human activities (particularly traffic), induces rathercomplex responses on the Adriatic Sea (e.g., Orli} et al., 1994; Beg Paklar etal., 2001; Pasari} and Orli}, 2004) and occasionally causes some specific phe-nomena (Figures 4 and 5), it is the primary area of study for a number of sci-entists. (For a comprehensive review of recent findings, see Grisogono andBelu{i} (2009)).Thus, wind measurements in the Senj region are of the utmost impor-tance. Nevertheless, several modeling studies point to the underestimation ofwind speeds measured operationally at the Senj measuring site (44.99°N,14.90°E, 2 m above the mean sea level (ASL)) during bora flow (e.g., Klai} etal. 2003; Belu{i} and Klai}, 2004; Gohm et al., 2008). These motivated us tocompare operationally measured winds with special measurements data that246 Z. BENCETI] KLAI] ET AL.: WIND MEASUREMENTS IN SENJ ...Figure 2. Old postcard shows the northern portion of the Vratnik Pass (698 m) looking towardthe town of Senj, and further away, the Island of Krk. Due to the downwind position, the climateof Senj is strongly affected by bora.were available for two winter months. Here we show that true bora winds inSenj are stronger than those reported from operational measurements.2. Measuring sites and wind measurementsFigure 6 shows the positions of measuring sites. Site 1 belongs to the net-work of main meteorological stations operated by Meteorological and Hydro-logical Service of Croatia (MHS) (http://vrijeme.hr/aktpod_e.php?id=karta&param=GMP). It routinely provides hourly meteorological data. A cup ane-mometer is placed on the 10 m high mast located in the flat grassy backyard ofthe MHS premises. As seen from the lowermost two photos, site is surroundedby nearby houses and trees.Special wind measurements were performed during the two-month winterperiod (1 December 2001–31 January 2002), i.e., during the season generallyGEOFIZIKA, VOL. 26, NO. 2, 2009, 245–252 247Figure 3. Due to frequent bora winds, trees at the leeward slopes in the greater Senj area arewind-shaped. The windward side of the Krk Island (left) is bare. The photo was taken on a calmday in August 2009, a few kilometers north of Senj. (Photo by ZBK.)favorable for the strongest bora occurrence (e.g., Vu~eti}, 1991). A Micro--m-asta cup anemometer was placed on the roof of a city tower, at the height of15 m ASL (13 m above ground; Belu{i} et al., 2006), next to the sea (upper-most photo in Figure 6). Wind measurements were performed at the resolu-tion of 1 s.3. Results and conclusionFigure 7 shows that the differences between hourly mean wind speeds re-corded at two measuring sites (v2–v1) generally increase with the increase ofthe wind speed. Thus, for weak winds (speeds at site 2 between 0 and 5 m s–1),wind speeds measured at the operational site were on average 0.7 m s–1weaker than the speeds measured at site 2, while for strong winds (speeds atsite 2 over 15 m s–1), wind speeds at site 2 were on the average 6.8 m s–1 highercompared to site 1. Minimum absolute differences ¡v2–v1¡ varied between 0.0248 Z. BENCETI] KLAI] ET AL.: WIND MEASUREMENTS IN SENJ ...Figure 4. On 5 September 2009, bora over the northern Adriatic induced the onset of Langmuircells (e.g., Evans and Taylor, 1980), tubes of rotating water with the axes aligned roughly with thewind. Adjacent tubes rotate in opposite directions. At the water surface, parallel lines of foam, sea-weed or other debris are seen. Photo is taken a few km north of Senj. Ze~evo islet and furtheraway a bare, windward side of the Island of Krk is seen (photo by ZBK).m s–1 (weak winds) and 2.4 m s–1 (strong winds), while corresponding figuresfor maximum differences (v2–v1) were 4.1 and 15.0 m s–1, respectively.Figure 8 shows average differences between the hourly mean wind speedsmeasured at the two sites (v2–v1) with respect to the wind direction recordedat site 2. It clearly illustrates that discrepancies between the two sites are thelargest for the northeastern quadrant, i.e., for the bora directions (note thatdue to the geometry of Vratnik Pass, direction of bora in Senj is predominatelyeast-northeastern). During the investigated period, which according to Belu{i}et al. (2006) was characterized by a number of strong bora episodes, windspeeds at site 2 were on average 2.4, 2.4 and 0.9 m s–1 higher than speeds atsite 1 for the northeastern, eastern and northern winds, respectively. Simulta-neously, the smallest discrepancies are found for southwestern and westernwinds (around 0.2 m s–1), while for sirocco directions they are on the averageabout 0.5 m s–1.To some extent differences between wind speeds at sites 1 and 2, withspeeds at site 2 on the average higher than at site 1 for all eight main direc-tions and for all wind speed classes, may be attributed to the difference in ane-mometers’ altitudes. Thus, somewhat higher speeds are expected at 13 m (SiteGEOFIZIKA, VOL. 26, NO. 2, 2009, 245–252 249Figure 5. A large stationary cumulus cloud (rotor) (e.g., Belu{i} et al., 2007; Gohm et al., 2007;Grubi{i} and Orli}, 2007; Orli}, 2007) observed from Vrataru{a (approximately 5 km north-north-east of Senj) on 6 August 2008. Picture shows a view in the bora’s direction. The rotor is locatedbehind a 600 m high hill top, above the slope that is adjacent to the sea. Upwind of the rotor,smaller fragments of cumulus clouds are found. (Photo by ZBK.)250 Z. BENCETI] KLAI] ET AL.: WIND MEASUREMENTS IN SENJ ...Figure 6. The measuring sites in Senj. The operational site is denoted by (1). Two lowermost pho-tos show views approximately from the northwest (second from below) and north (lowermostphoto), respectively. Site with special wind measurements is denoted by (2). Dashed arrow showsapproximate direction of bora flow (i.e., orientation of the Vratnik Pass). (Photos by ZBK.)2) than at 10 m above ground (Site 1). Nevertheless, Figures 7 and 8 togetherwith the modeled winds for Senj (Klai} et al. 2003; Belu{i} and Klai}, 2004;Gohm et al., 2008) suggest that the true bora flows, and particularly strongand severe boras, are substantially stronger than operationally measured winds.This, together with the fact that the location of the operational site (Site 1) issheltered with respect to bora flow, indicates that the bora winds at Senj aresystematically underrepresented. This implication should be taken into ac-GEOFIZIKA, VOL. 26, NO. 2, 2009, 245–252 251Figure 7. Differences between hourly mean wind speeds obtained by special (v2) and routine (v1)measurements (m s–1) for the period 1 December 2001–31 January 2002. Left, center and rightpanel correspond to mean, minimum absolute and maximum difference between v2 and v1 withrespect to the wind speed recorded at site 2.Figure 8. Mean differences between hourly mean wind speeds obtained by special (v2) and rou-tine (v1) measurements (v2 – v1) (m s–1) for the period 1 December 2001–31 January 2002. Differ-ences are shown with respect to the wind direction measured at site 2.count when dealing with the Senj data. A more comprehensive study based ona longer set of additional simultaneous measurements could offer speed anddirection-dependent corrections to the operationally measured winds.Acknowledgements – We are indebted to MHS for providing the site 1 data. We alsothank Professor Mirko Orli} for the site 2 data and for suggesting the interpretation of pat-terns shown in Fig. 4 in terms of Langmuir cells.ReferencesBeg Paklar, G., Isakov, V., Kora~in, D., Kourafalou, V. and Orli}, M. (2001): A case study ofbora-driven flow and density changes on the Adriatic Shelf (January 1987). Cont. Shelf Res.,21, 1751–1783.Belu{i}, D. and Klai}, Z.B. 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Wind measurements in Senj – underestimation of true bora flows

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