engleski

Bora downslope windstorm

The eastern Adriatic coast, roughly N Mediterranean, is well known for Bora wind - a gusty downslope windstorm which typically blows from the NE quadrant with sustained wind speeds between 5-20 ms^-1, its gusts surpassing 50-70 ms^-1 in the lee of the coastal mountains. Similar bora-like flows occur elsewhere, at least at a couple of tens of other places, provided the presence of flow transcriticality. The bora has been studied at the NE coast more than at the central and SE Adriatic coast ; the latter bora cases appear as more complex but less frequent than their NE counterparts. Efforts to study bora have gradually moved toward progressively smaller spatio-temporal scales and turbulence nowadays. A tentative result, obtained by using hotwire anemometer with sampling frequency of 104 Hz, suggests that the inertial dissipation method provides an accurate estimate of the bora turbulent kinetic energy dissipation rate in the surface layer, ε ~ 1 m^2s^-3, at least for moderate wind speeds, O(10 ms^-1). Such and other bora turbulence details are needed for improving NWP and climate model turbulence parameterizations and to account for bora effects in traffic, engineering, air-pollution and coastal oceanography, to mention a few. Moreover, known occasional occurrence of pulsations and rotors is further enriched by secondary low-level jets (LLJs) that are up to a few tens of kilometers long and several kilometers apart (primary LLJs and wakes relate to the mountain main passes and peaks). Preliminary results on contrasting deep and shallow bora events reveal sub-mesoscale structures that have not previously been reported in bora research. For example, spatial structures of O(1 km) may appear further offshore in the convective marine atmospheric boundary layer more than 50 km in the lee of the coastal mountains.

dissipation rate, low-level jet, mountain wave, turbulence, dissipation rate

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