Complex bora flow in the lee of Southern Velebit

• Published in: Quarterly journal of the Royal Meteorological Society Vol. 138; no. 667; pp. 1490 - 1506
• Main Authors: Stiperski, Ivana, Ivančan-Picek, Branka, Grubišić, Vanda, Bajić, Alica
• Format: Journal Article
• Language: English
• Published: Chichester, UK John Wiley & Sons, Ltd 01.07.2012; Wiley
• Subjects: atmospheric rotors; boundary layer separation; critical level; Earth, ocean, space; Exact sciences and technology; External geophysics; Meteorology; secondary orography; undular bore; waek; wake vortex; wave breaking; Winds and their effects; Southern Europe; Europe; Croatia; Coupled model; critical level; wake vortex; Wind velocity; atmospheric rotors; digital simulation; urban areas; Forecast model; Hydraulic jump; undular bore; wave breaking; Boundary layer separation; Mesoscale; wake; winds; Observation data; spatial variations; Sodar; secondary orography; high resolution
• ISSN: 0035-9009; 1477-870X
• DOI: 10.1002/qj.1901

The complexity of bora flow in the lee of Southern Velebit, Croatia, is investigated by means of high‐resolution numerical simulations carried out with the US Naval Research Laboratory's (NRL) Coupled Ocean–Atmosphere Mesoscale Prediction System (COAMPS) model. The aim of this study is to identify reasons for the strong spatial variability of bora in the wider Zadar region and the uncharacteristically weak bora in the city of Zadar. The primary focus is put on the severe bora episode of 20 December 2004 during which a sodar (sonic detection and ranging) system was operated at Zemunik airport. Numerical results are verified against the available surface and upper‐air observations. Upstream conditions governing dynamics of this particular bora event are characterized by the presence of a mean‐state critical level at 3–4 km above mean sea‐level (AMSL) defining the upstream bora layer. Low‐level airflow over Zadar peninsula during this event displays marked temporal variability. During the morning, a highly three‐dimensional and unsteady wake (i.e. region of reduced wind speed) is established over Zadar peninsula, downstream of a hydraulic jump that forms in the lee of, but not collocated with, highest portions of the Southern Velebit terrain. During its most extensive phase the wake scales the entire length of Zadar peninsula in the horizontal and extends up to the critical level in the vertical. A wake vortex develops as the flow reverses along the wake centreline. In the evening hours, an undular bore forms in the same location, with large‐amplitude waves on top of it inducing boundary layer separation and rotors. Wake flow is affected both by changes in the upstream atmospheric structure and diurnal boundary‐layer evolution. Sensitivity experiments show that the influence of Velebit's height is particularly strong, governing the onset and strength of bora. The terrain of Zadar peninsula exerts a strong influence on the characteristics of the developed bora flow, particularly when non‐hydrostatic effects are significant. Boundary layer separation is particularly sensitive to downstream orography. Copyright © 2012 Royal Meteorological Society