The amplitude of lee waves on the boundary‐layer inversion

Sachsperger, Johannes; Serafin, Stefano; Grubı̆sı́c, Vanda; Stiperski, Ivana; Paci, Alexandre

doi:10.1002/qj.2915

Cited by 15 publications

(26 citation statements)

References 32 publications

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“…This might allow some trapped waves to exist beyond the limiting line. The separation between trapped lee waves and hydraulic jumps fits qualitatively well to that from regime diagrams of both Knigge et al [32] and Vosper [35], but is best described by the limiting line of Sachperger et al [41]. We can therefore conclude that the differences in obstacle shapes and flow characteristics do not have important repercussions for our results, providing confidence in analyzing experimental results for double obstacles.…”

Section: Flow Over An Isolated Obstaclesupporting

confidence: 73%

“…Both of these phenomena (regime transition and valley flushing) are therefore a complex interaction between non-linearity introduced by combination of F r and H 1 /Z i [41] and complex topography through the relation between lee-wave wavelength and ridge separation distance. This non-linearity in flow response and differences in the flow character are also clear from the difference in interface height between single obstacle and double obstacle experiments in Figure 7.…”

Section: Flow Over Double Obstaclesmentioning

confidence: 99%

“…The exact amount of amplitude increase or decrease (constructive or destructive interference) is not as clearly delineated as in [29], but depends on both the inversion height and Froude number, since the combination of these parameters controls flow non-linearity (cf. Equation (11) in Sachsperger et al [41]). Still, there appears to be no systematic separation of results according to either F r or H 1 /Z i , or their combination, that would identify one of those parameters as the dominant influence on the interference.…”

Section: Flow Over Double Obstaclesmentioning

confidence: 99%

“…Still, a notable exception is obvious: trapped lee wave regime covers a larger portion of the regime space than for a single obstacle. Firstly, trapped lee waves occur even below the limiting line of Sachsperger et al [41] where hydraulic jumps are expected. Secondly, trapped lee waves over double obstacles persist at higher Froude numbers than downstream of a single obstacle, despite the fact that NH 1 /U (and therefore also the limiting line separating the two) is similar.…”

Section: Flow Over Double Obstaclesmentioning

confidence: 99%

“…line separates trapped lee waves from vertically propagating waves according to Vosper [35]. The short-dashed line separates hydraulic jumps from trapped lee waves according to Sachsperger et al [41] with NH1/U equal to 0.5. Crosses correspond to cases of valley flushing.…”

Section: Flow Over Double Obstaclesmentioning

confidence: 99%

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

et al. 2017

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Abstract:In this article, we present an overview of the HyIV-CNRS-SecORo (Hydralab IV-CNRS-Secondary Orography and Rotors Experiments) laboratory experiments carried out in the CNRM (Centre National de Recherches Météorologiques) large stratified water flume. The experiments were designed to systematically study the influence of double obstacles on stably stratified flow. The experimental set-up consists of a two-layer flow in the water tank, with a lower neutral and an upper stable layer separated by a sharp density discontinuity. This type of layering over terrain is known to be conducive to a variety of possible responses in the atmosphere, from hydraulic jumps to lee waves and highly turbulent rotors. In each experiment, obstacles were towed through the tank at a constant speed. The towing speed and the size of the tank allowed high Reynolds-number flow similar to the atmosphere. Here, we present the experimental design, together with an overview of laboratory experiments conducted and their results. We develop a regime diagram for flow over single and double obstacles and examine the parameter space where the secondary obstacle has the largest influence on the flow. Trapped lee waves, rotors, hydraulic jumps, lee-wave interference and flushing of the valley atmosphere are successfully reproduced in the stratified water tank. Obstacle height and ridge separation distance are shown to control lee-wave interference. Results, however, differ partially from previous findings on the flow over double ridges reported in the literature due to the presence of nonlinearities and possible differences in the boundary layer structure. The secondary obstacle also influences the transition between different flow regimes and makes trapped lee waves possible for higher Froude numbers than expected for an isolated obstacle.

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Section: Flow Over An Isolated Obstaclesupporting

confidence: 73%

Section: Flow Over Double Obstaclesmentioning

confidence: 99%

Section: Flow Over Double Obstaclesmentioning

confidence: 99%

Section: Flow Over Double Obstaclesmentioning

confidence: 99%

Section: Flow Over Double Obstaclesmentioning

confidence: 99%

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

et al. 2017

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Trapped mountain waves with a critical level just below the surface

Soufflet

Lott

Damiens

2019

Quart J Royal Meteoro Soc

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The trapped mountain waves produced when the incident wind near the surface is small compared with its value aloft are analyzed with a theory adapted from Long (1953) and compared with fully nonlinear simulations performed with the Weather Research and Forecasting model (WRF). Although small near‐surface incident winds occur naturally in fronts via a combination of the thermal wind balance and the boundary layer, they pose at least two problems in mountain meteorology: zero surface incident winds produce no wave in the fully linear case; they also correspond to places where mountain waves have a critical level. Despite these problems, theory and WRF show that, for small mountains, trapped lee waves (a) can occur and (b) are favored when the surface Richardson number J = N 2/(∂u/∂z)2 is small. This last result is related to the theoretical fact that the surface absorption of stationary gravity waves increases when J increases. The relation with flow stability is corroborated further by the fact that the trapped lee waves resemble the Kelvin–Helmholtz (KH) modes of instability that exist when J < 0.25. For medium mountains, some aspects of the theory still hold but need to be adapted, the more intense winds and foehn that occur along the lee side of the mountain having a tendency to increase the surface flow stability. For “initially” small J, this can limit the onset of trapped lee waves, again consistent with the fact that mountain‐wave surface absorption increases with surface flow stability. For large J, the dynamics produces wave breaking on the lee side, destabilizing the flow in the wake of the mountain. In the region where the Richardson number is small, trapped waves develop despite the fact that the surface Richardson number can be quite large, suggesting that the trapped lee waves now result from an absolute instability of the wake.

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Unravelling the March 1972 northwest Greenland windstorm with high‐resolution numerical simulations

Tollinger

Gohm

Jonassen

2019

Quart J Royal Meteoro Soc

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Thule Air Base in northwest Greenland experienced an extreme windstorm during the night of 8/9 March 1972. The event is not among official WMO records because the anemometer broke after recording the highest gust of 93 m s−1. A recent study re‐examined the event based on coarse‐resolution reanalyses and observations which, however, did not fully resolve the proposed storm processes. This is the first study that uses high‐resolution numerical simulations to investigate the processes associated with this windstorm. A cold‐frontal inversion and strong flow across the mountain ridge upstream of Thule Air Base (associated with a passing low pressure system) are shown to be key factors for the severe downslope windstorm in the lee of the ridge. It is shown that trapped lee waves occurred during the initial phase of the storm, but did not contribute to the highest wind speeds as proposed in the previous study. It is confirmed that rotor circulations occurred which possibly contributed to the large wind variability and, hence, large differences between individual observation sites. However, no rotors were present at the time of the highest simulated wind speed. Instead, wave breaking above the lee slope is found to indirectly contribute to the wind maxima by facilitating Kelvin–Helmholtz instability at the top of the shooting flow that caused intense wind speed pulsations near the surface. In agreement with the previous study, a corner jet was simulated, however it was not responsible for the strong winds in the vicinity of the air base. Sensitivity experiments showed that the flow field was considerably influenced by the high topography downstream of the air base and that simulations with very thin or no sea ice cover over Baffin Bay resulted in a weaker frontal inversion and up to about 30% lower maximum wind speeds.

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The amplitude of lee waves on the boundary‐layer inversion

Cited by 15 publications

References 32 publications

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

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

Trapped mountain waves with a critical level just below the surface

Unravelling the March 1972 northwest Greenland windstorm with high‐resolution numerical simulations

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