QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY Q. J. R. Meteorol. Soc. 133: 1371–1388 (2007) Published online in Wiley InterScience (www.interscience.wiley.com) DOI: 10.1002/qj.129 Numerical simulation of pulsations in the bora wind Danijel Beluˇ si´ c, a * Mark ˇ Zagar b and Branko Grisogono a a Department of Geophysics, Faculty of Science, University of Zagreb, Zagreb, Croatia b Meteorological Office, Environmental Agency of Slovenia, Ljubljana, Slovenia ABSTRACT: Numerical simulation of a long bora episode is presented and compared with measurements. The goal is to resolve the quasi-periodic oscillations of the bora gusts, i.e. the pulsations. The model reproduced well the approximately 7 min periodicity of pulsations and the upstream structure of the atmosphere, and can thus be used for the detailed dynamical considerations. The results of previous studies are confirmed, as well as the hypotheses on the mechanisms responsible for the disappearance of pulsations. Specifically, it is shown that the upper-tropospheric jet induces strong positive shear throughout the troposphere and consequently the local nonlinearity of the incoming flow weakens. Henceforth, the low-level wave breaking is diminished and so the pulsations cease. From the three previously proposed generating mechanisms of pulsations, the results obtained point to the Kelvin–Helmholtz instability as the primary mechanism in this case. Additionally, it seems that the situations with absence of pulsations may be related to the formation of the mountain- wave-induced rotor. Copyright  2007 Royal Meteorological Society KEY WORDS downslope windstorm; wave breaking; jet; trapped lee waves; rotor; Kelvin–Helmholtz instability Received 6 October 2006; Revised 17 April 2007; Accepted 12 June 2007 1. Introduction Bora is a cold gusty downslope windstorm blowing at the eastern Adriatic coast. It is generated when the incom- ing north-easterly airflow from the land passes over the Dinaric Alps and descends at the coast. The bora’s sever- ity stems primarily from its well- known gustiness (e.g. Petkovˇ sek, 1982, 1987; Beluˇ si´ c et al., 2004 (BPO04 here- after); Beluˇ si´ c et al., 2006 (B06 hereafter)): maximum wind gusts are about twice the mean hourly values (e.g. Beluˇ si´ c and Klai´ c, 2004, 2006), and during severe bora cases may reach hurricane strength (Beluˇ si´ c and Klai´ c, 2006). It is, therefore, of great importance to learn about the behaviour of the bora wind gusts. Strong gustiness is also a property of other downslope windstorms around the world, e.g. the Boulder downslope windstorm (Neiman et al., 1988) and the downslope windstorms in Iceland ( ´ Ag´ ustsson and ´ Olafsson, 2007). Due to their dynamical similarity (e.g. Smith, 1987), certain results from the bora studies may be applied to other such phenomena, and vice versa. The bora may form under several synoptic conditions (e.g. Jurˇ cec, 1981; Heimann, 2001). This also implies dif- ferent aerological conditions which were usually studied by means of the upstream radiosonde measurements (e.g. Baji´ c, 1987). The most frequent or ‘standard’ bora set- up includes a well-defined upstream bora layer capped by a critical level (A critical level is defined as the level * Correspondence to: Danijel Beluˇ si´ c, Department of Geophysics, Fac- ulty of Science, University of Zagreb, Horvatovac bb, 10000 Zagreb, Croatia. E-mail: dbelusic@irb.hr where the phase speed of internal gravity waves equals the ambient wind speed in the direction of wave prop- agation. Since the mountain waves are stationary, the critical level is located at the height where the cross- mountain wind speed equals zero.) and/or a low-level inversion (Glasnovi´ c and Jurˇ cec, 1990), which basically prevent a more or less upward leakage of energy from the bora layer. There are, however, situations without the mean-flow critical level and the low-level inversion, thus leaving the upstream bora layer depth undefined. In these situations the bora shooting flow forms under the critical level induced by the wave breaking in the lee (e.g. Klemp and Durran, 1987; Enger and Grisogono, 1998; BPO04; Grisogono and Enger, 2004; Kraljevi´ c and Grisogono, 2006). The latter two studies address the boundary-layer variations some tens and hundreds of kilometres away from the mountain, while here we focus on details of lee-side instabilities. The fact that downslope windstorm gusts exhibit quasi- periodic behaviour was recognized some decades ago (Petkovˇ sek, 1976, 1982, 1987; Rakovec, 1987; Neiman et al., 1988). It has been shown that the bora pulsations appear at periods between approximately 3 and 11 min (BPO04) and are of non-local origin (B06), meaning that their appearance at the surface is unrelated to any local generating mechanism. There were several attempts to explain the dynamical origin of these pulsating features (This is the origin of the term ‘(quasi-periodic) pulsa- tions’ that is used throughout the study.), mostly by using idealized numerical simulations. Generally, three mecha- nisms were proposed: Copyright  2007 Royal Meteorological Society