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THE COMPLEX BORA FLOW IN THE LEE OF SOUTHERN VELEBIT

Ivana Stiperski1, Branka Ivančan-Picek1, Vanda Grubišić2

1 Meteorological and Hydrological Service, Zagreb, Croatia E-mail: [email protected] 2 University of Vienna, Vienna, Austria

Abstract: The complex flow structure in the lee of the Southern Velebit is investigated by means of very high-resolution numerical simulations carried out with the NRL COAMPS model. The focus is placed on a wintertime severe bora episode. The spatially complex and temporally highly variable 3D flow structure is characterized by a hydraulic jump over the lee slope and a pronounced wake over the Zadar area. The wake structure appears very sensitive to the upstream flow evolution and is highly non-stationary. The reversed flow along the wake centerline as well as flow separation and rotor formation are observed during different stages of the bora evolution. Sensitivity experiments show the height of Velebit exerting a strong influence on the flow structure, including the onset and strength of the bora flow and the structure of the wake. The terrain of the Zadar peninsula, although significantly lower than that of Velebit, is shown to influence the characteristics of the fully developed bora flow.

Keywords: bora, critical level, orography, rotors, upstream conditions, wake

1 INTRODUCTION

The strong bora winds are a common occurrence along the eastern mountainous Adriatic coast, especially in wintertime. The most severe Bora is found in the foothills of Southern Velebit, characterized by significant horizontal and vertical wind speed variability over a relatively small area (e.g. Grubišić, 2004; Belušić and Klaić, 2006). The highest bora windspeed ever recorded in Croatia (69 m/s) was measured in the lee of the southern tip of Velebit. Located only a short distance away, in the lee of one of the highest Velebit peaks (1757m), the area of the city of Zadar (ZD) (Fig. 1) is characterized climatologically by winds considerably weaker compared to its surroundings. In this study we investigate small-scale characteristics and spatial variability of the severe Bora flow in the wider Zadar area with the aim of identifying reason for the “Zadar calm”. A severe bora event of 20 December 2004 is examined during which a sodar system was operated at the Zadar Zemunik (ZZ) airport.

Figure1. Location of Southern Velebit with broader Zadar area (left) and the innermost model domain (right). Letter codes in the right panel indicate stations Zadar (ZD), Zadar airport (ZZ), Obrovac (OB), and Pag Bridge (MB). W1-W2 marks the baseline of the vertical cross section through the wake. Figure 2. Time series of the measured (grey) and modeled (orange) 10-minute wind speed (solid) and wind direction (dotted line) at Zadar (ZD) and Obrovac (OB).

2 BORA OBSERVATIONS AND MODEL SETUP

The examined bora event developed under the influence of a lee cyclone in the Gulf of Genoa and a high-pressure system over central Europe. The upstream flow impinging on the Velebit was characterized by a synoptically induced critical level at 3.2 km and several inversions with the bases at 0.6, 1.3 and 2.3 km. The bora flow showed significant spatio-temporal variability as seen by comparing the winds measured at the automatic weather stations Pag Bridge and Obrovac with those at Zadar, where the wind was distinctly weaker (Fig. 2). The simultaneous sodar measurements at ZZ reveal a weak low-level flow and the maximum bora speeds above 300 m MSL (Ivančan- Picek et al. 2007). The analysis of the spatial and temporal bora flow variability was facilitated by real-data three-dimensional numerical simulations, carried out with the nonhydrostatic NRL's COAMPS model with 6 nested domains. The horizontal resolution in the innermost domain was 333 m. In addition to the baseline run, two sensitivity experiments were conducted, one with the reduced Velebit height (rhV), and the other in which the topography of the Zadar peninsula was removed (nZ).

3 RESULTS

The numerical results shown in Fig. 3 show a hydraulic jump over the steep lee slopes of the Velebit, followed further downstream by a pronounced wake over the city of Zadar and beyond. The wake is spatially and temporally highly variable (Gohm et al. 2008). It is especially well developed in the initial stages of the bora development (Fig. 3 left and center), when a reversed flow is present along the wake centerline (Schär and Smith 1993). The reversed flow at the ground is also present towards the end of the examined period (from 21-23 UTC; Fig. 4), with trapped lee waves and weak and shallow lee-wave rotors over the Zadar peninsula (Hertenstein and Kuettner 2003), suggesting a possibility of the lee wave resonance (Grubišić and Stiperski 2009). The reduction of the Velebit height leads to the disappearance of the wake; instead the high-speed flow at low levels extends far downstream (Fig. 3 right). The removal of the Zadar peninsula affects most strongly the flow separation and the rotor zone (Fig. 4 right), with the reversed flow that is both stronger (> -5ms-1) and higher reaching.

Figure 3. Flow at 10 UTC 20 Dec 2004. Surface mountain-perpendicular windspeed (color) and streamlines (left). Vertical cross-sections of mountain-perpendicular windspeed (gray shade), potential temperature (lines) and horizontal wind direction (arrows) along W1-W2 for the baseline run (center) and the rhV run (right).

Figure 4. As in Fig.3 except at 23 UTC 20 Dec 2004 and the nZ run in the right panel.

4 CONCLUSIONS

The new observations and numerical simulations of the flow in the lee of Southern Velebit provide an insight into the nature of the “Zadar calm”, which lies within the wake of the highest terrain. The wake is strongly influenced by the upstream flow evolution and shows high spatio-temporal variability. Both the terrain of the Southern Velebit and the low-elevation Zadar peninsula were found to exert a strong control of the wake flow.

REFERENCES

Belušić, D., Z. B. Klaić 2006: Mesoscale dynamics, structure and predictability of a severe Adriatic bora case. Meteorol. Z., 15, 157–168. Gohm, A., G. J. Mayr, A. Fix, and A. Giez, 2008: On the onset of bora and formation of rotors and jumps near a mountain gap. Quart. J. Roy. Meteor. Soc.,134, 21-46. Grubišić, V., 2004: Bora-driven potential vorticity banners over the Adriatic. Quart. J. Roy. Meteor.Soc., 130, 2571-2603. Grubišić, V., and I. Stiperski, 2009: Lee wave resonances over double bell-shaped obstacles. J. Atmos. Sci., In press. Hertenstein, R.F., and J.P. Kuettner, 2003: Rotor types associated with steep lee topography: influence of the wind profile. Tellus, 57, 117-135. Schär, C., and R.B. Smith, 1993: Shallow-water flow past isolated topography. Part I: vorticity production and wake formation. J. Atmos. Sci. 50, 1373-1400. Ivančan-Picek , B., V. Grubišić, I. Stiperski, M. Xiao, and A. Bajić, 2007: "Zadar calm" during severe bora. Proc. 29th Intern. Conf. on Alpine Meteorology, Chambery, France, 261–264.