Numerical Simulation of Pulsating Bora Wind Gusts

2.1 NUMERICAL SIMULATION OF PULSATING BORA WIND GUSTS

Danijel Belušić1*, Mark Žagar2 and Branko Grisogono1 1University of Zagreb, Zagreb, Croatia 2Environmental Agency of Slovenia, Ljubljana, Slovenia

1. INTRODUCTION ridge towards the east (Fig. 1).

Bora wind is a relatively cold, predominantly northeasterly downslope windstorm blowing o 52 N down the western side of the Dinaric Alps and 30 plunging on the Adriatic Sea. Bora winds may 9360 -1 40 o reach mean wind speeds of over 20 m s . Many 48 N previous studies have examined the multi-scale 9060 bora nature using measurements, numerical o simulations or theoretical concepts. Lately, a 44 N strong emphasis has been given to the interaction between the bora and the Adriatic o 40 N using state-of-the-art coupled numerical models 8960 (e.g. Pullen et al., 2007). 30 o 9280 However, the bora main feature is its 36 N 40 0o o gustiness. The maximum bora gusts are almost 30 E o o 6 E o o 24 E twice the hourly mean wind speeds, which 12 E 18 E brings the bora wind to hurricane strengths when -1 gusts surpass 50 or even 60 m s . It has been o 52 N shown that the temporal variability of bora gusts 9360 stems from the superposition of two 8960 o components: 3–8 min quasi-periodic oscillations 48 N (i.e. pulsations) and high-frequency local 40 turbulence (e.g. Belušić et al., 2006). The o pulsating component has also been reported in 44 N other similar winds, e.g. the Boulder downslope 30 8980

o windstorm (e.g. Neiman et al., 1988). Previously 40 N proposed generating mechanisms of the pulsations include Kelvin-Helmholtz instability o 30 (KHI), vortex tilting in the wave-breaking region 36 N 9080 0o o 30 E and propagating lee waves (e.g. Belušić et al., o o 6 E o o 24 E 2007). Recently, the possibility of disappearance 12 E 18 E of the pulsations within a bora episode has been Figure 1. 300 hPa geopotential (contour interval 20 reported (Belušić et al., 2004). The peculiarity is gpm) and wind speed (shaded, interval 5 m s-1) at 06 that even when the pulsations are absent, the UTC 8 Dec (up) and 06 UTC 9 Dec (down) 2001. mean bora wind speed remains intact.

This study employs a high-resolution 3. NUMERICAL SIMULATION numerical simulation for examination of the bora wind properties in situations both with and The atmospheric component of the non- without pulsations. The period of interest is 8 – 9 hydrostatic, fully compressible mesoscale model December 2001. TM COAMPS (Hodur, 1997) has been used. Four

nested domains have been used, with grid 2. SYNOPTIC SITUATION spacing of 9, 3, 1 and 0.33 km and 51 x 51, 61 x

61, 91 x 91 and 91 x 91 grid points, respectively A cut-off low was present above Greece with (Fig. 2). There were 81 vertical levels reaching associated northeasterly winds at 300 hPa. The the height of 24 km, with higher vertical maximum wind speed above the northern resolution near the surface and lower near the Adriatic region increased to over 35 m s-1 on 9 model top. For details of the model setup see December 2001 due to shifting of the upper-level Belušić et al. (2007). The model output is given ______every 30 s at Senj and 50 s for 3D fields. The * Corresponding author address: Danijel Belušić, start of the simulation is at 00 UTC 8 December University of Zagreb, Faculty of Science, Dept. of 2001. The initial and boundary conditions are Geophysics, Zagreb, Croatia, email: [email protected]. obtained from the ECMWF operational analyses.

1 All model data presented here are taken from performed quite well except between hours 17 the 1-km domain, except for the analysis in and 23, where the model did not capture the Section 7. cessation of the bora. The fine-scale model variability seems to underestimate the recorded one. a) 35

46 N 25

45 N 10

3 u (m/s) 5

2 0 44 N Ad r −5 ia ti measured cSe −10 model a model 11.5 km −15 43 N 1 6 12 18 24 30 36 42 Time (h) 13 E 14 E 15 E 16 E 17 E

0 200 400 600 800 1000 1200 1400 30

80 25

60 Krk A 20

S u (m/s) V

Y (km) 40 15

20 10 0 10 20 30 40 50 60 Time (min)

0 30 0 20406080 X (km)

Figure 2. Model domains; terrain contour interval 100 25 m. a) Domain 1 (9 km) with indicated domains 2 (3 km) and 3 (1 km). b) Domain 3 with indicated domain 4 (333 m). The line AB denotes the vertical cross- 20 sections shown in Fig. 6. S and V denote Senj and Vratnik Pass. 15 4. DATA u measured u model 10 The measurements were undertaken in Senj 0 10 20 30 40 50 60 (44.99°N, 14.90°E, 2 m MSL) at 13 m AGL with Time (min) a cup anemometer at 1 Hz sampling frequency Figure 3. Measured vs. modeled time series of 30-s u (for details see Belušić et al., 2004). The at Senj. Upper panel: Near-surface u with u at 11.5 coordinate system has been rotated so that the km AGL. Time is in hours after 00 UTC 8 Dec 2001. u component is in the along-wind direction. All Zoomed 1-h periods with (middle panel; 340th–400th subsequent analysis is performed on this rotated min of the simulation) and without (lower panel; th th u component. 1960 –2020 min of the simulation) pulsations near the surface. 5. MODEL PERFORMANCE AT SENJ The fine-scale variability in the model is seen Figure 3 depicts the time series comparison in the first part of the episode. Exactly this part of between the simulated and measured u at Senj. the episode is characterized by pulsations in the In terms of hourly averages, the model

2 measured data, which can be seen in the 1-h zoomed series in Fig. 3. It is seen that the model reproduced the pulsations well. The finer-scale 80 non-periodic variability that is superimposed on the pulsations in the measurements was not reproduced by the model. This is expected from 60 A this kind of models and is also of no importance for our study, which emphasizes the pulsations. S Figure 4 further corroborates the success of 40 the model in the range of frequencies of y (km) pulsations (i.e. periods around 7 min). B 20 25 7 min 20 0 15

10 80 5

60 A S 40 y (km) B 20

0 0 20406080 X (km) Figure 5. Horizontal cross-sections of u near the surface. Upper panel: Pulsations are present (10 LST 8 Dec). Lower panel: Pulsations are absent (13 LST 9 Dec). The line AB denotes the vertical cross-sections shown in Fig. 6. S denotes Senj.

Figure 6 shows the corresponding vertical cross-sections for the two situations. The low- Figure 4. Power spectral density of measured (solid level wave breaking is present in the situation violet) and modelled (dashed red) near-surface u at with pulsations. Also, there is a strong low-level Senj. Upper panel: Presence of pulsations in the first part of the episode (hours 3–15.5). Lower panel: shooting flow beneath the wave-breaking region Pulsations are absent in the second part of the and the offshore boundary layer jump (Fig. 6, episode (hours 31–45.2; see Fig. 3). upper panel). Therefore, as seen in both horizontal and vertical, this situation resembles the usual setup of the strong bora (e.g. Belušić 6. FLOW STRUCTURE and Klaić, 2006). To the contrary, when the pulsations are absent the low level setup is Figure 5 depicts the horizontal view of characterized by large-amplitude trapped lee situations with and without pulsations. There are waves (Fig. 6, lower panel). Hence, there is no the usual jets and wakes that extend over the significant low-level wave breaking. This implies sea when the pulsations are present. During the that the magnitude of the low-level negative absence of pulsations there is only the jet from shear between the bora jet and the usual the Vratnik Pass with lower wind speeds away stagnant, now absent, wave-breaking region is from the mountain than in the case with reduced and hence the Richardson number pulsations, while the flow reversal occurs which is a measure of dynamical stability is between the land and the Island of Krk. This increased. points to the occurrence of a mountain wave It is interesting to note that in the case induced rotor during the situation without without pulsations the wave breaking occurs pulsations, as discussed in Belušić et al. (2007). around the tropopause level instead. This is the

3 origin of the variability of u at 11.5 km seen in properties of pulsating bora wind gusts. The Fig. 3. chosen bora event is characterized by the presence of pulsations in the first part and their 426 416 406 absence in the second part. This provides an 15000 38 opportunity to study the detailed changes in the 376 6 u(m/s) 356 flow structure that lead to the disappearance of 45 pulsations, while high near-surface wind speeds 40 10000 remain. It is shown that when the pulsations are 35

306 30 present the flow is as in the usual strong-bora

25 setup. On the other hand, when the pulsations 5000 are absent the low-level wave breaking is replaced by large-amplitude lee waves that can sustain strong near-surface wind speeds close 0 S BAto the mountain. This is consistent with the results of Zängl and Hornsteiner (2007). The generating mechanism of the pulsations 428 420 appears to be KHI in this case. 414 15000 37 374 8 u(m/s) ACKNOWLEDGMENTS 362 45 10000 40 The work was supported by the Croatian 35 Ministry of Science, Education and Sports 304 30 (projects 119-1193086-1323 and 119-1193086- 25 5000 1311).

REFERENCES 0 S BABelušić, D., M. Pasarić, and M. Orlić, 2004: Quasi-periodic bora gusts related to the Figure 6. Vertical cross-sections corresponding to Fig. structure of the troposphere. Q. J. R. 5 along the line AB. Shown are potential temperature Meteorol. Soc., 130, 1103–1121. contours (2 K interval), wind speed along the cross- Belušić, D., and Z. B. Klaić, 2006: Mesoscale section (color) and TKE (red dashed, starts at 2 J kg-1 dynamics, structure and predictability of a with 2 J kg-1 (upper panel) and 6 J kg-1 (lower panel) severe Adriatic bora case. Meteorol. Z., 15, interval). 157–168. Belušić, D., M. Pasarić, Z. Pasarić, M. Orlić, and 7. DETAILED STRUCTURE OF PULSATIONS B. Grisogono, 2006: A note on local and non- local properties of turbulence in the bora flow. Figure 7 shows the horizontal view of a Meteorol. Z., 15, 301–306. pulsation development, while Fig. 8 depicts its Belušić, D., M. Žagar, and B. Grisogono, 2007: vertical structure. The four panels span an Numerical simulation of pulsations in the bora interval of 5 min, which is approximately the wind, Q. J. R. Meteorol. Soc., in press. period of pulsations. In the horizontal, pulsations Hodur, R. M., 1997: The Naval Research have almost elliptical shape. They propagate Laboratory's Coupled Ocean/Atmosphere downstream and eventually fade away. Mesoscale Prediction System (COAMPS). Vertical cross-sections (Fig. 8) show that the Mon. Weather Rev., 125, 1414–1430. pulsation is generated when the shear Neiman, P. J., R. M. Hardesty, M. A. Shapiro, strengthens. The creation of the pulsation and R. E. Cupp, 1988: Doppler lidar relaxes the strong shear below the wave observations of a downslope windstorm. breaking and hence reduces the dynamic Mon. Weather Rev., 116, 2265–2275. instability, i.e. the Richardson number increases Pullen, J., J. D. Doyle, T. Haack, C. Dorman, R. (not shown). Afterwards, the shear strengthens P. Signell, and C. M. Lee, 2007: Bora event again and the rhythmical process is repeated. variability and the role of air-sea feedback, J. This is in accordance with the KHI mechanism Geophys. Res., 112, C03S18, doi: (see further discussion in Belušić et al., 2007). 10.1029/2006JC003726. Zängl, G., and M. Hornsteiner, 2007: Can 8. CONCLUSIONS trapped gravity waves be relevant for severe foehn windstorms? A case study. Meteorol. A fine-scale numerical simulation of a bora Z., 16, 203–212. event is presented with the goal to examine the

4 (a) (c)

20 A A B

15 Y(km)

(b) (d)

20 A B A B

15 Y(km)

52510 15 20 52510 15 20 X(km) X(km) V(m/s): 22 24 26 28 30 32 34 36 38 40 Figure 7. Horizontal view of a pulsation development over the period of 5 min (∆t between consecutive panels is 100 s). Shown is the wind speed magnitude. A and B denote individual pulsations.

4000 (a) (c)

2 91 291 1 3000 29

288

285 2000 2 282 82 288 5 8 2 2 79

Z(m) 85 9 2 27 276

1000

273

0 A A B 4000 (b) (d)

1 9 2 3000 291

1 9 2

2 282 8 2000 8

8 Z(m)

276 8 276 2 8 3 3 2 27 27 2 8 9 5 9 27 1000 27

0 0 1020 30 0 1020 30 A B A B X(km) X(km) V(m/s): 29 33 37 Figure 8. Vertical cross-sections corresponding to Fig. 7 along the horizontal line indicated in Fig. 7. Shown are the wind speed magnitude (color) and potential temperature contours (interval 1 K).