Gap Winds and Wakes: SAR Observations and Numerical Simulations

125

This study investigates the onset phase of a strong Adriatic bora windstorm that occurred on 4 April 2002. The target area is a gap about 20 km wide embedded in the coastal mountain barrier of the Dinaric Alps that favours strong jet-like winds. Airborne-aerosol back-scatter lidar measurements on board the DLR Falcon research aircraft, together with surface and upper-air observations, are used to verify high-resolution numerical experiments conducted with the mesoscale atmospheric model RAMS and a single-layer shallow-water model (SWM). Especially during the breakthrough phase of the bora, the flow at the gap exit exhibits a complex spatial structure and temporal evolution. On a transect through the centre of the gap, a hydraulic jump forms; this is located close to the coast throughout the night, and starts to propagate downstream in the early morning. On a transect through the edge of the gap, a lee-wave-induced rotor becomes established, due to boundary-layer separation. It starts to propagate downstream about two hours after the jump. This flow evolution implies that the onset of strong winds at the coast occurs several hours earlier downstream of the centre of the gap than downwind of the edge of the gap. Consequently, the wind field in the vicinity of Rijeka airport, located downwind of the gap, is strongly inhomogeneous and transient, and represents a potential hazard to aviation. Measured bora winds at the surface exceed 20 ms −1 , and the simulated wind speed in the gap wind layer exceeds 30 ms −1 . The simulated turbulent kinetic energy exceeds 10 m 2 s −2 .RAMS indicates that wave-breaking near a critical level is the dominant mechanism for the generation of the windstorm. Gap jets can be identified downstream of several mountain passes. The simulated wave pattern above the Dinaric Alps, the wave decay with height due to directional wind shear and the strong flow descent on the leeward side of the barrier are supported by measured back-scatter intensities. Basic bora flow features, including gap jets and jumps, are remarkably well reproduced by SWM simulations. The RAMS reference run captures observed flow phenomena and the temporal flow evolution qualitatively well. A cold low-level bias, an overestimated bora inversion strength, and a slightly too-early bora onset are probably related to insufficient turbulent mixing in the boundary layer. The amplitude of trapped gravity waves, the time of the bora breakthrough and the inversion strength are found to be quite sensitive to the turbulence parametrization.

Section: The Shallow-water Modelmentioning

confidence: 72%

Section: Upstream Profilementioning

confidence: 99%

Section: The Swmmentioning

confidence: 99%

See 1 more Smart Citation

On the onset of bora and the formation of rotors and jumps near a mountain gap

Gohm

Mayr

Fix³

et al. 2008

125

“…In the pulsations case the bora extends over the sea and has pronounced jets associated with the mountain gaps and wakes associated with the mountain peaks (e.g. Pan and Smith, 1999;Gohm and Mayr, 2005a;Belušić and Klaić, 2006). During the no-pulsations period individual jets and wakes are not distinguishable, except for the jet emanating from the Vratnik Pass.…”

Section: Upstream Profilesmentioning

confidence: 99%

Numerical simulation of pulsations in the bora wind

Belušić

Žagar

Grisogono

2007

Numerical simulation of a long bora episode is presented and compared with measurements. The goal is to resolve the quasi-periodic oscillations of the bora gusts, i.e. the pulsations. The model reproduced well the approximately 7 min periodicity of pulsations and the upstream structure of the atmosphere, and can thus be used for the detailed dynamical considerations.The results of previous studies are confirmed, as well as the hypotheses on the mechanisms responsible for the disappearance of pulsations. Specifically, it is shown that the upper-tropospheric jet induces strong positive shear throughout the troposphere and consequently the local nonlinearity of the incoming flow weakens. Henceforth, the low-level wave breaking is diminished and so the pulsations cease. From the three previously proposed generating mechanisms of pulsations, the results obtained point to the Kelvin-Helmholtz instability as the primary mechanism in this case.Additionally, it seems that the situations with absence of pulsations may be related to the formation of the mountainwave-induced rotor.

“…But in the case of south föhn, advection on the synoptic scale will be shown to reverse the situation, to have colder air to the south. An overview of gap flow literature is given in Pan and Smith (1999) and Mayr et al (2007). As a brief introduction to the topographic situation for the föhn flows discussed in this paper, we show a crosssection along the crest of the central Alps looking north, in the flow direction of south föhn, in Figure 2.…”

Section: Introductionmentioning

confidence: 99%

Föhn as a response to changing upstream and downstream air masses

Mayr

Armi

2008

ABSTRACT:Observations of föhn from the field phase of the Mesoscale Alpine Programme (MAP) are used to study how differences between the air masses upstream and downstream of the central Alpine crest determine whether the flow can descend to the lee as either shallow föhn, when it passes through passes in the mountains, or deep föhn, when it overflows the Alpine crest. First, the föhn case of 30 October 1999 is examined using ECMWF analyses and radiosonde data at various upstream and downstream locations. Additional measurements from aircraft, dropsondes, an instrumented car and automatic weather stations are then used for a detailed study of the föhn flow across the Brenner Pass. Advection of cold air around the eastern edges of the Alps and warm air around the western edge of the Alps ahead of a synoptic ridge set up a reservoir of colder air on the south side of the Alps and a reservoir of warmer air to the north. The depth to where the air was colder on the southern side was sufficient for a shallow föhn to flow through the pass. After the passage of the ridge axis, synoptic cold air advection provided another source of colder air, this time from the southwest, growing deeper with time and having a synoptically imposed cross-barrier flow component. The maximum depth to where the air upstream was colder than downstream extended just above the peaks of the highest mountains. An analysis of the detailed measurements across the Brenner Pass showed that this depth was also the top of the layer that descended and accelerated down the lee slopes of the Wipp Valley. Upstream, air above the föhn layer had an even stronger cross-barrier component yet did not descend because it did not have lower potential temperatures than the downstream side at that level. Deep föhn never developed. An examination of other well-documented MAP föhn cases confirmed the conclusion from the 30 October event that shallow and deep föhns -at least for the central Alps -are mostly a response to differences in air masses between the upstream and downstream side. A cross-barrier component of the flow was only a modification but in itself not sufficient to cause the flow to both descend and accelerate down the lee slope, unless potential temperatures on the upstream side were lower in this layer than on the downstream side.