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2.1 NUMERICAL SIMULATION OF PULSATING BORA WIND GUSTS

Bora wind is a relatively cold, predominantly northeasterly downslope windstorm blowing down the western side of the Dinaric Alps and plunging on the Adriatic Sea. Bora winds may reach mean wind speeds of over 20 m s. Many previous studies have examined the multi-scale bora nature using measurements, numerical simulations or theoretical concepts. Lately, a strong emphasis has been given to the interaction between the bora and the Adriatic using state-of-the-art coupled numerical models (e.g. Pullen et al., 2007). However, the bora main feature is its gustiness. The maximum bora gusts are almost twice the hourly mean wind speeds, which brings the bora wind to hurricane strengths when gusts surpass 50 or even 60 m s. It has been shown that the temporal variability of bora gusts stems from the superposition of two components: 3–8 min quasi-periodic oscillations (i.e. pulsations) and high-frequency local turbulence (e.g. Belušić et al., 2006). The pulsating component has also been reported in other similar winds, e.g. the Boulder downslope windstorm (e.g. Neiman et al., 1988). Previously proposed generating mechanisms of the pulsations include Kelvin-Helmholtz instability (KHI), vortex tilting in the wave-breaking region and propagating lee waves (e.g. Belušić et al., 2007). Recently, the possibility of disappearance of the pulsations within a bora episode has been reported (Belušić et al., 2004). The peculiarity is that even when the pulsations are absent, the mean bora wind speed remains intact. This study employs a high-resolution numerical simulation for examination of the bora wind properties in situations both with and without pulsations. The period of interest is 8 – 9 December 2001.