Complex bora flow in the lee of Southern Velebit

Abstract

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.

Micro-Scale Properties of Different Bora Types

“…A similar threshold (5 m·s −1 ) was used by Lepri et al [35]. Furthermore, the detection algorithm required that these conditions must be satisfied for at least 6 h, with an allowed discontinuity of 1 h. This is to allow for possible weaker periods or even wind reversals within the bora episodes caused by lee rotor formation or low-level flow separation [22,38,39]. Bora episodes detected in this way were also visually checked (not shown).…”

Impact of mesoscale meteorological processes on anomalous radar propagation conditions over the northern Adriatic area

“…The bora wind blew for 30 h with stronger wind during the nighttime and weaker wind during the daytime, mainly because of the effect of the convective ABL in which turbulence generally decreases (and eventually dissipates) the mean wind speed. A main characteristic of the bora wind is its spatial variation in wind speed [e.g., Grisogono and Belušić , ; Prtenjak et al ., ; Stiperski et al ., ]. It is attributed to the formation of several bora jets that are associated with the mountain passes to the north and south of the Istria (Figure b).…”

Water Tank Experiments on Stratified Flow over Double Mountain-Shaped Obstacles at High-Reynolds Number

“…This was shown to be a consequence of surface friction, causing attenuation of wave amplitude downstream of the obstacles [9,10]. Secondary terrain, even substantially smaller than primary, was found to significantly modify the flow response in a realistic setting [34].…”