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.