Measurements of the bora wind at Senj (east Adriatic) with a three-axis anemometer showed that at the periods smaller than 1 min turbulent eddies propagating downstream prevail, in accordance with conventional wisdom. At the periods of 5-10 min bora pulsations were observed, with rotary spectral analysis pointing to hithertho unsuspected dynamics of these pulsations

Danijel Belušić

Mirko Orlić

Zoran Pasarić

English

Hrčak - Portal of scientific journals of Croatia

FIRST MAASUREMENTS OF BURA WIND AT SSNJ WITH A THREE-AXISANEMOMETERMirko OrliC 1, Danijel Belu5i6 t, ZoranPasarid It Andr;a Mohorovidii Geophysical Institute, Faculty of Science, Horvatovac bb, 10000 Zagreh,CroatiaE-mail: orlic@olimp.irb.hrAbstract: Measurements of the bora wind at Senj (east Adriatic) witb a three-axis anemometer showed that at theperiods smaller than I min turbulent eddies propagating downstream prevail, in accordance with conventionalwisdom. Ai lhe periods of 5-10 min bora pulsations were obsewed, wilh ro1ary spectral analysis pointing to hitherthounsuspected dynamics of tiese pulsations.Keywords - bora, pulsations, rotation, Adriaticl.INTRODUCTIONBora (bura i:r Croatian) is a cold, dry wind that blows above the Adriatic from the aortheast quadraat,most often during winter season. Its average speed is usually not high, but - due to the gustiness - thebora speed maxima may surpass 60 m/s. In a recent paper (BeluSie et al. 20Aq il has been shown that thebora pulsations appear at the periods of about 3-8 min, that they are highly variable in time and that theymay disappear and reappear even inside a single bora episode. All previous investigations of the borawind in the area were based on measurements of horizontal wind components only. ln this paper firstresults of measurements performed with a three-axis anemometer at Senj are considered.2. DATA DDSCRIPTION AND ANALYSKWe have mounted the WindMaster ulfasonic anemometer produced by Gill Instruments at Senj{44.99oN, 14.90"8,2 m above MSI,) at a height of 13 m above ground, on 18 March 2004.Ilrecordedthree wind components with a sampling *equency of 4112. Wi& the aim of illustrating the data collected,we consider here results obtained for the bora episode extending from 24 April (14:02 h LST) to 27 April{16:58 h LST) 2004. The episode was a moderate one, with the horizontal (vertical) wind speeds notsurpassing 30 m/s {+10 m/s).Timo Ih: after bo.a onsetFigure 1. Horizontal (up) and vertical (down) wind components: I s and l0 mil values.Eoo=308To simpliff the analysis, we have decomposed the wind vector in a coordinate sydtem having thehorizontal axis (B) aligned with the direction of maximum wind variance and the vertical axis (z) pointingupwards. Figure 1 shows a subset of B and z time series. Pulsations are clearly visible in both time series,although reduced in the second one.We have also calculated time-running power spect a (energy density of wind components plottedversus fiequency and time) for the episode considered. They were obtained by calculating a series ofpower spectra over shorter (3.5 h) sliding subin:ervals, progressing in time with a 30 min time step.Within each subinterval, spectrum was calculated and averaged over 3 Hanning windows (e.g. Papoulis1911) of 4096 s length. The spectrum thus obtained for B component of the wind is shown in Fig. 2. Itreveals that the strongest pulsations occurred between 481h and 60th hour after the onsel of the boraepisode, at the periods of 5-10 min.654o5+agoatEsuoEeoo.E zqF726660181260Jo-:\z'c GEa '-aoc1.5 g)c)ct!ol-ro)oJ0.50 1020 15 7 54m/s321.51.21Period lminjFigure 2. Time-running power spectrum of the horizontal wind component.ln order to document movement of the wind vector in the B-z plane, we have utilized rotary spectra.The main idea behind such an analysis is the decomposition of vector movemelt at a certain frequencyinto clockwise and anticlockwise rotary motions. The superposition of these two rotary motions gives theresultant vector whose tip follows a circular, elliptical or linear path - depending on the ratio of the twoconstituents. The method has been widely used in physical oceanography since its formulation at thebeginning of the seventies (Gonella 1972), and it has been introduced to rneteorologists by O'Brien andPillsbury {1974). Rotary spectra and spectra of related parameters, computed for the whole bora episodeusing 263 windows of fi24 s length, are given in Fig. 3. They show markedly different behavior atperiods below and above I min. At high frequencies vector tips draw ellipses of high eccentricify, theirmain axes depart considerably fiom the horizontal plane and the vectors tum in the negative (clockwise)sense. Low frequencies are dominated by pulsations of the 5-10 min period, with the correspondingellipses having evea higher eccentricity, the main axes being almost horizontal and the vectors tuming inthe positive (anticlockwise) sense.Furthermore, we have calculated time-running rotary spectra, using the same approach as utilized in{he calculation of power spectra. Figure 4 illustrates the way bora pulsations develop over time and inparticular a prevalence of positive over negative rotation at the 5-10 min periods during the second halfofthe episode.Power spectrum309Pos.lryes.J:::::::::\::::::had6EEooo-IJ'b(gogc.9()>,E:(t€e5-'(Uo(r(t)ooco(6oOIas.oc)x(g.Eoop(!T(!E10'ron1oo1010'1A-'10"10- I 0-' 10- -110Frequency lcpsl Frequency lcpslFigure 3. Spectra of rotary components and related parameters for the whole bora episode considered,0 10 20 1.2 1.5 2 3 45715 15754 3 2 1.5 1.2mls Period [min]Figure 4. Time-running rotary spectra.Finally, in Fig. 5 temporal evolulion of 5.7 min variability is documented by the amplitude ratio andphase shift between B and z wind componenls and by the ellipse orientation and rotary coefficient. Thebora pulsations in the later part of the episode are characteizsdby low vertical-to-horizontal amplituderatio, by phase shift between horizontal and vertical wind components departing from 180o, by the mainellipse axes positioned in the hoizontal plane and by the positive turning of the wind vectors.ooo(goo(!oEi:aNa1.5 €-'6ciOIUct'o0.5 go,oJ0: l-:-:-:[ ps]Rotary spectra310_ 300O)(l)E;'2ooc.ttE 1oo(g-c(La o.4c(l):O a.2o8obJg -o.2oCE-0.45o)(!)p.oco(sc€) -1O.Coa-4.9E o.soE o,E o.''oo 1224{El I Io 122436486072 o 1224 36486A72Time [h] after bora onset Time [h] after bora onsetFigure 5. Results of cross-spectral analysis of the horizontal and vertical wind components (up) and ofrotary spectral analysis (down), for ihe 5.7 min period.3. CONCLUSIONWe have seen (Fig. 3) that the bora rotary spectra can be divided into two regions of differentbehavior divided by the period of I mia. In the high-frequency part there is prevailing negative tuming ofthe wind vectors, which agrees with the expected dynamical setup wherein turbulent eddies are advectedin the direction of the mean wind. On the other hand, in the low-&equency part vectors are turning in apositive sense. This is particularly pronounced when the pulsations are present (Figs. 4 and 5). At thosetimes horizontal amplitude becomes much larger than the vertical one, and the phase difference drops*om above 180" to around 100". This relation befween lhe pulsations and the positive turning, logetherwith the associated phase-shift decrease, is in contrast with the presen! theories of the pulsalion dynamics,which presume downstream propagation of the pulses. A possibility that the bora pulsations are related tohitherto unsuspected dynamical processes opens way to further research which should encompass alargemrmber of bora episodes.Acknowledgement: The work was supported by the Ministry of Science, Education and Sparts af theRepublic of Croatia (grant Na. 0119330).RAFERNNCESBelu3i6, D., M. Pasarii, M. Orli6, 2004: Quasi-psriodic bora gusts related to the structure of thetroposphere. Q. J. R. Meteorol. Soc., 130, I 103-l 121.Gonellg J., 1972: A rotary-component method for analysing meteorological and oceanographic vectortime series. Deep-Sea Res., 19, 833-846.O'Brien, J. J., R. D. Pillsbury, 1974: Rolary wind spectra in a sea breeze regime. J. Appl. Meteorol.,l3,820-825.Papoulis, A.,1977: Signal Analysrs. McGraw Hill, Auckland, 431 pp.604836't2oLo72311

FIRST MEASUREMENTS OF BURA WIND AT SENJ WITH A THREE-AXIS ANEMOMETER

Abstract

Similar works

Full text

Available Versions

Hrčak - Portal of scientific journals of Croatia