A case study of a severe Adriatic bora on 28 December 1992

Ivančan-Picek, Branka; TUTIS, VLASTA

doi:10.1034/j.1600-0870.1996.t01-2-00001.x

Cited by 26 publications

(13 citation statements)

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“…In the case of weak upstream flow it was found that the factor contributing most to the development of pronounced lee slope winds is wave overturning which occurs beneath the inversion layer [ Jurčec and Glasnović , 1991]. Evidence has been found both against [ Ivančan‐Picek and Tutiš , 1996] and for simple hydraulic theory [e.g., Bajić , 1991; Vučetić , 1993]. Lazić and Tošić [1998] used real data simulation to investigate the influence of mountain height on bora trajectories.…”

Section: Introductionmentioning

confidence: 99%

February 2003 marine atmospheric conditions and the bora over the northern Adriatic

Dorman¹,

Carniel²,

Cavaleri³

et al. 2006

J. Geophys. Res.

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A winter oceanographic field experiment provided an opportunity to examine the atmospheric marine conditions over the northern Adriatic. Mean February winds are from a northeasterly direction over most of the Adriatic and a more northerly direction along the western coast. Wind speeds are fastest in jets over the NE coast during bora events and weakest in the mid‐northwestern Adriatic. Diurnal air temperature cycles are smallest on the NE coast and largest in the midwestern Adriatic. The maximum sea‐air difference is +10°C on the eastern coast and near zero on the midwestern Adriatic. Boras are northeasterly (from) wind events that sweep off Croatia and Slovenia, bringing slightly colder and drier air over the northern Adriatic. The main bora season is December to March. Winter 2002–2003 was normal for bora events. Synoptic‐scale temporal variations are correlated over the northern Adriatic. Fastest Bora winds and highest wind stress over the northern Adriatic is concentrated in four topographically controlled jets. The strongest is the Senj Jet, while the Trieste Jet extends across the entire northern Adriatic. Between each two jets is a weak wind zone. The greatest mean net heat loss is in bora jets in the NE Adriatic, where it was −438 W m−2 and is weakest in the midwestern northern Adriatic, where it was near zero. Wind stress is concentrated over the NE half of Adriatic in four bora jets, while wind stress is weak in the NW Adriatic. There is significant variation in wind stress mean and standard deviation structure over the northern Adriatic with each bora event.

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Section: Introductionmentioning

confidence: 99%

February 2003 marine atmospheric conditions and the bora over the northern Adriatic

Dorman¹,

Carniel²,

Cavaleri³

et al. 2006

J. Geophys. Res.

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“…This might also be a partial explanation for the spatial variability of the bora wind along the eastern Adriatic coast (e.g. Ivancan-Picek and Tutiš 1996;Brzovic 1999) in addition to the upstream flow preconditioning.…”

Section: (B) Backgroundmentioning

confidence: 99%

Boundary‐layer variations due to orographic‐wave breaking in the presence of rotation

Grisogono

Enger

2004

Quart J Royal Meteoro Soc

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SUMMARY A mesoscale numerical model is used to study the atmospheric boundary-layer (ABL) response to nonlinear orographic forcing with Coriolis effect, f , over a mountain with length (the cross-wind component) comparable to the Rossby radius of deformation, L R . The orographic-wave breaking occurring for Froude number Fr < 1, affected by f > 0, intensifies on the northern flank for westerly flows, as also found in other recent studies. A cumulative effect occurs as the Coriolis force lifts the northern ABL top and generates a stronger low-level jet (LLJ) than on the southern side. A differential layering also appears, since the specific humidity is higher in the lower southern ABL than in the related northern ABL, and vice versa. By contrast, there are higher values of the turbulent kinetic energy and humidity in the upper northern ABL.The breaking of flow symmetry around the orography due to f changes both the vertical vorticity and horizontal divergence field, (ζ, D), it modulates eddies and turbulence leading to the differential layering of the ABL. The stronger northern LLJ and its weaker southern counterpart, both meandering, together with the asymmetric wave breaking, induce strong lee-side fluctuations of the (ζ, D) field in the presence of f . The enhanced (ζ, D) production due to wave breaking over the distance ≈L R , the primary atmosphere-orography resonance occurs mainly in the vertical, while the 'f -enhancement' occurs in the horizontal plane. In this way, the initial mesoscale forcing may extend its effects over the synoptic scale.

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“…Several authors have stressed the need to consider a three-dimensional view of the bora in order to explain the local differences of its structure (e.g. Glasnović and Jurčec 1990;Jurčec and Glasnović 1991;Ivančan-Picek and Tutiš 1996). Three-dimensional numerical case-studies with horizontal model mesh sizes larger than 5 km (Lazić and Tošić 1998;Tošić and Lazić 1998;Qian and Giraud 2000;Morelli and Berni 2003) revealed, for example, preferred regions with severe bora winds.…”

Section: Introductionmentioning

confidence: 99%

Numerical and observational case-study of a deep Adriatic bora

Gohm

Mayr

2005

Q. J. R. Meteorol. Soc.

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SUMMARYA detailed analysis of bora winds is presented that were observed on 28 March 2002 in the north-eastern part of the Adriatic Sea. Very high-resolution numerical simulations are compared with airborne, surface, and balloon observations. The key instrument for the verification of the numerical results is an aerosol backscatter lidar on board the DLR Falcon research aircraft. The high spatial resolution of the model and of the observational dataset allows several small-scale aspects of the bora winds to be explored. The study emphasises the great impact of boundary-layer effects including convective mixing and surface friction on the structure of the bora flow.The numerical model reveals that the diurnal cycle of the planetary boundary layer causes a diurnal variation of the gravity-wave amplitude and consequently a variation in the bora strength. The downslope flow is strongest during the night-time when the impinging air mass is stably stratified and exhibits a low-level jet. The bora strength weakens during the daytime due to the evolution of a neutrally stratified convective boundary layer. Lidar observations and simulations both suggest that the observed bora winds are driven by the dynamics of orographic gravity waves. In the deep north-easterly cross-mountain flow steeply amplified but non-breaking gravity waves as well as trapped lee waves are found throughout the troposphere. Vertical separation of the boundary layer over the steep leeward terrain slope prevents the bora flow from reaching the coast. As a result, the flow is kept aloft in a series of at least three unsteady trapped lee waves before it reattaches to the surface over the sea downstream of the mountains. No clear evidence is found for the existence of low-level rotors underneath these lee waves. The ability of the model to capture the trapped waves is found to depend on the horizontal model resolution. Bora winds are identified as jets emanating from several mountain gaps. The physical mechanism for the gap flow formation is boundary-layer separation controlled by surface friction. Flow separation is more effective over the highest terrain where a broad wake forms downstream with weak winds compared to the mountain gaps where the strong bora flow eventually reattaches to the surface. The strongest gap jet belongs to the Vratnik Pass upstream of the town Senj and forms the primary shear line in the northern Adriatic with a corresponding potential-vorticity (PV) banner. The numerical model reveals several secondary orographic PV banners and embedded PV filaments. The vertical PV distribution exhibits a two-layer structure with PV anomalies at lower (higher) elevations originating from mountain gaps (isolated mountain peaks). Surface friction has a minor direct effect on PV generation but has a major indirect effect in controlling the mechanism for the generation of PV. In a simulation without surface friction the primary PV banner forms as a result of gravity-wave breaking. In the more realistic simulation, however, the primary mechanism...

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A case study of a severe Adriatic bora on 28 December 1992

Cited by 26 publications

References 0 publications

February 2003 marine atmospheric conditions and the bora over the northern Adriatic

February 2003 marine atmospheric conditions and the bora over the northern Adriatic

Boundary‐layer variations due to orographic‐wave breaking in the presence of rotation

Numerical and observational case-study of a deep Adriatic bora

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