The Complex Bora Flow in the Lee of Southern Velebit

The strong bora winds are a common occurrence along the eastern mountainous Adriatic coast, especially in winter. Bora is known to be particularly severe in the lee of the Velebit mountain range in the northern Adriatic, where leeward slopes are particularly steep and mountain passes scarce but prominent. The Velebit range is highest at its southern part, where it abruptly ends, resulting in steep terrain slopes. The highest bora windspeed ever recorded in Croatia (69 m/s) was measured in the lee of the southern tip of Velebit. This is also the area where bora winds are extremely spatially variable. Located only a short distance away from the locus of the bora maximum, the area of the city of Zadar, in the lee of one of the highest Velebit peaks (1757m), is characterized climatologically by weak winds, considerably weaker compared to its surroundings. The primary aim of this study is to investigate small-scale characteristics and spatial variability of the severe Bora flow in the wide Zadar area and to identify reason for the “Zadar calm”.

The complex flow structure in the lee of Southern Velebit is investigated with very high-resolution numerical simulations carried out with the NRL COAMPS model. The focus is placed on a wintertime severe bora episode occurring on 20 December 2004. A synoptically-induced critical level at the altitude ranging from 3 to 5 km depending on the time period and an inversion at the altitude ranging from 2.3-3.2 km, defines the upstream bora flow layer and leads to the formation of a hydraulic jump in the lee of the highest terrain. Downstream, the complex and temporally highly variable three-dimensional flow structure is characterized by a pronounced wake in the Zadar area surrounded by two jets, one emanating from a pass at the northern end of Southern Velebit and the second one originating as a southern tip jet. Shallow water theory calculated form the upstream soundings successfully predicts the occurrence of the wake in the lee of the highest terrain as well as the wake flow regime with the region of reversed flow in the wake center line observed at times between 10-12 UTC. Towards the end of the bora event amplifying undulations start to develop on top of the hydraulic jump that lead to flow separation and formation of shallow rotors underneath in the wake region. This reversed flow can be inferred also from the AWS data at Zadar.

Sensitivity experiments were conducted to examine the effects of the Zadar peninsula topography and the height of Southern Velebit on the structure of the bora flow. The influence of Velebit is particularly strong, governing the onset and strength of the bora flow within the domain as well as controlling the formation of the wake. The terrain of the Zadar peninsula, although significantly lower than Velebit, is also shown to influence the characteristics of the developed bora flow, especially the location of the point of flow separation. The rotor formation appears to be particularly sensitive to the small scale topography of the Zadar peninsula.