Nighttime Valley Waves

Porch, W.M.; Clements, W.E.; Coulter, R. L.

doi:10.1175/1520-0450(1991)030<0145:nvw>2.0.co;2

Cited by 14 publications

(5 citation statements)

References 7 publications

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“…For example, in Fleagle's (1950) model, the mechanism of pulsations is the adiabatic heating of fluid parcels as they flow along the slope, which leads to an adverse pressure gradient on the flow. Porch et al (1991) attributed these pulsations to the cross flows emanating FIG. 11.…”

Section: Discussionmentioning

confidence: 99%

“…Oscillations in katabatic flows have been predicted on theoretical grounds (Fleagle 1950;McNider 1982) and have been observed in field studies (Tyson 1968;Doran and Horst 1981;Porch et al 1991;Van Gorsel et al 2003). Their existence, however, has been attributed to different mechanisms.…”

Section: Velocity Oscillationsmentioning

confidence: 96%

“…If the friction can be written as F ϭ ϪkU, then the friction reduces as air decelerates, while the driving force continues to be enhanced by the radiational cooling, thus resulting in oscillatory flow behavior (also see McNider 1982). On the other hand, Porch et al (1991) argued that oscillations in katabatic flows can arise because of the presence of cross flows, which temporarily stop drainage currents from tributaries entering nearby valleys. As cold air builds up and buoyancy forcing increases in tributaries, there will be sudden releases of colder air onto the slopes periodically.…”

Section: Velocity Oscillationsmentioning

confidence: 99%

See 2 more Smart Citations

Quasi-Steady Katabatic Winds on Slopes in Wide Valleys: Hydraulic Theory and Observations

Princevac

Hunt

Fernando

2008

Journal of the Atmospheric Sciences

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Theoretical and field observational studies on mean velocity and temperature fields of quasi-steady nocturnal downslope (katabatic) flows on sloping surfaces are reported for the case of very wide valleys in the presence of weak synoptic winds. Because of the lateral constraints on the flow, Coriolis effects are considered negligible. The layer-averaged equations of Manins and Sawford were used for the analysis. It is shown that (i) in the absence of significant turbulent entrainment into the current (i.e., at large Richardson numbers Ri ϭ ⌬h cos␣ /U 2 ) the downslope flow velocity U is related to the slope length (L H ), slope angle (␣), and the buoyancy jump between the current and the background atmosphere (⌬) as U ϭ u (⌬L H sin␣) 1/2 , where u is a constant and h is the flow depth; (ii) on very long slopes h is proportional to L h (tan␣) 1/2 ; and (iii) under highly stable conditions (i.e., Ri Ͼ 1) the katabatic flow exhibits pulsations with period T 0 ϭ 2/N sin␣, where N is the buoyancy frequency of the background atmosphere. These predictions are verified principally using observations made during the Vertical Transport and Mixing Experiment (VTMX) conducted in Salt Lake City, Utah, in October 2000. By assuming the flow follows a straight line trajectory to the nearest ridgeline a good agreement was found between the predictions and observations over appropriate Richardson number ranges. For Ri Ͼ 1.5, u Ϸ 0.2, although u was a decreasing function of Ri at lesser stabilities. Oscillations with period T 0 are simply alongslope (critical) internal-wave oscillations with a slope-normal wavenumber, which are liable for degeneration into turbulence during their reflection. These critical internal waves may be responsible, at least partly, for weak sustained turbulence often observed in complex-terrain nocturnal boundary layer flows.

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Section: Discussionmentioning

confidence: 99%

Section: Velocity Oscillationsmentioning

confidence: 96%

Section: Velocity Oscillationsmentioning

confidence: 99%

See 1 more Smart Citation

Quasi-Steady Katabatic Winds on Slopes in Wide Valleys: Hydraulic Theory and Observations

Princevac

Hunt

Fernando

2008

Journal of the Atmospheric Sciences

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“…Using observations and numerical simulations, Nappo et al (2008), Viana et al (2010), and Udina et al (2013) found gravity waves following a density current, evident at the interfacial top of the cold air. Porch et al (1991) suspected that oscillations in a valley were initiated by the interaction between downvalley flow in the cold pool and tributary flow into the valley. The interaction of gravity waves and density currents was examined by Viana et al (2009) and Terradellas et al (2005).…”

Section: Potential Causes Of Wavesmentioning

confidence: 99%

Stably Stratified Atmospheric Boundary Layers

Mahrt

2014

Annu. Rev. Fluid Mech.

403

371

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Atmospheric boundary layers with weak stratification are relatively well described by similarity theory and numerical models for stationary horizontally homogeneous conditions. With common strong stratification, similarity theory becomes unreliable. The turbulence structure and interactions with the mean flow and small-scale nonturbulent motions assume a variety of scenarios. The turbulence is intermittent and may no longer fully satisfy the usual conditions for the definition of turbulence. Nonturbulent motions include wave-like motions and solitary modes, two-dimensional vortical modes, microfronts, intermittent drainage flows, and a host of more complex structures. The main source of turbulence may not be at the surface, but rather may result from shear above the surface inversion. The turbulence is typically not in equilibrium with the nonturbulent motions, sometimes preventing the formation of an inertial subrange. New observational and analysis techniques are expected to advance our understanding of the very stable boundary layer.

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“…IGWs have been reported on the top or leading edge of density currents [e.g., Sun et al , ; Viana et al , ]. IGWs generated by different density currents, for example, from the drainage flows in a valley and its tributaries [ Porch et al , ], may interact with each other. IGWs may at times appear to be above the SABL but are generated in the deep SABL upstream [e.g., Mahrt et al , ].…”

Section: Observations Of Waves and Wave‐turbulence Interactionsmentioning

confidence: 99%

Review of wave‐turbulence interactions in the stable atmospheric boundary layer

Sun

Nappo²,

Mahrt

et al. 2015

Reviews of Geophysics

137

124

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Flow in a stably stratified environment is characterized by anisotropic and intermittent turbulence and wavelike motions of varying amplitudes and periods. Understanding turbulence intermittency and wave-turbulence interactions in a stably stratified flow remains a challenging issue in geosciences including planetary atmospheres and oceans. The stable atmospheric boundary layer (SABL) commonly occurs when the ground surface is cooled by longwave radiation emission such as at night over land surfaces, or even daytime over snow and ice surfaces, and when warm air is advected over cold surfaces. Intermittent turbulence intensification in the SABL impacts human activities and weather variability, yet it cannot be generated in state-of-the-art numerical forecast models. This failure is mainly due to a lack of understanding of the physical mechanisms for seemingly random turbulence generation in a stably stratified flow, in which wave-turbulence interaction is a potential mechanism for turbulence intermittency. A workshop on wave-turbulence interactions in the SABL addressed the current understanding and challenges of wave-turbulence interactions and the role of wavelike motions in contributing to anisotropic and intermittent turbulence from the perspectives of theory, observations, and numerical parameterization. There have been a number of reviews on waves, and a few on turbulence in stably stratified flows, but not much on wave-turbulence interactions. This review focuses on the nocturnal SABL; however, the discussions here on intermittent turbulence and wave-turbulence interactions in stably stratified flows underscore important issues in stably stratified geophysical dynamics in general.

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Nighttime Valley Waves

Cited by 14 publications

References 7 publications

Quasi-Steady Katabatic Winds on Slopes in Wide Valleys: Hydraulic Theory and Observations

Quasi-Steady Katabatic Winds on Slopes in Wide Valleys: Hydraulic Theory and Observations

Stably Stratified Atmospheric Boundary Layers

Review of wave‐turbulence interactions in the stable atmospheric boundary layer

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