A review of recent advances in understanding the meso- and microscale properties of the severe Bora wind

Abstract

A gusty downslope windstorm that blows at the eastern Adriatic coast is called bora. Similar winds exist at many other places on virtually all continents. Related hourly mean wind speeds surpassing 20 m s−1, with gusts reaching up to 50 or even 70 m s−1, in the coastal mountain lee areas are common (hurricane speeds). There has been substantial progress in bora observations and measurements, understanding, modelling and its more detailed prediction during the last 25 yr. It was generally thought before that bora was a falling, mostly thermodynamically driven wind; however, (severe) bora is primarily governed by mountain wave breaking. Understandings of bora interactions and influences on other processes have taken place as well, most notably in the air‐sea interaction, but are not completed yet. The overall progress mentioned would not be possible without airborne data, non‐linear theory and advances in computational techniques, most notably mesoscale numerical models. Some gaps in bora knowledge are also indicated, for example, dynamical transition from weak to moderate to strong to severe bora flows, where the latter are the main subject here, and vice versa. Moreover, the role of the boundary layer and waves on the upwind side of the bora evolution and the consequent lee side flow structures are inadequately understood; this is especially so for bora at the southern Adriatic coast. The focus here is on stronger bora flows at the NE Adriatic coast.

The role of advection and of work done by forces in the gustiness of the Bora flow

“…7). Gusts are developing quasi-periodically with an average period of 8.5 min which roughly corresponds with the 6 to 8 min period measured at Senj as also reported and simulated by Belušić et al (2009; for the same case with coAMPs and the same spatial resolution). The gusts over the slope precede those over the sea, but their dynamics is beyond pure translation down the slope -which may be seen from the much lower values between the maximum values of two areas of the strongest gusts (red slanted contours) in the Hovmøller diagram (Fig.…”

“…Belušić et al (2004Belušić et al ( , 2007. Their results suggest that Kelvin-Helmholtz instability (KHI) is the most likely pulsating mechanism (Grisogono and Belušić, 2009). This instability appears above the Bora shooting flow, which is essentially a jet with a maximum of about 300 -500 m AGL, and below the wave-breaking region.…”

“…Because turbulence and dissipation are typically parameterised in numerical models, their magnitudes in models also depend on the parameterisation used. Boundary layers and turbulence in complex terrain are poorly understood even today (Grisogono and Belušić, 2009;Baklanov and Grisogono, 2009;Rotach and zardi, 2007) and consequently any parameterisation is necessarily merely an approximation. The choice of parameterisation thus in particular affects the overall wave-turbulence interaction (e.g.…”

“…Some recent studies reveal that certain instances of Bora exhibit a downslope windstorm or hydraulic jump structure (AMs Glossary). A comprehensive review of Bora characteristics was recently published by Grisogono and Belušić (2009).…”

Possible observation of horizontal roll vortices over the Adriatic Sea during bora

“…The bora wind has been studied for decades by many researchers (for a recent review, see Grisogono and Belušić, 2009). It is a canonical example of a downslope windstorm that is generated by flow over an almost two-dimensional mountain.…”