Low-Frequency Velocity and Temperature Fluctuations in Katabatic Valley Flows

Stone, G.L.; Hoard, Donald E.

doi:10.1175/1520-0450(1989)028<0477:lfvatf>2.0.co;2

Cited by 19 publications

(8 citation statements)

References 9 publications

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“…An additional mechanism was delineated in section 2c, where alongslope velocity oscillations due to internal waves impinging normal to the slope are a dominant flow mode. The presence of lowfrequency oscillations in katabatic flow has been noted in many studies (e.g., Tyson 1968;Stone and Hoard 1989), with time periods on the order of an hour. Note that these time scales are much larger than those of turbulent fluctuations (15 s based on rms velocity 0.13 m s Ϫ1 and integral length scale 2 m) and typical buoyancy periods (ϳ120 s) within katabatic flows.…”

Section: B Pulsations Of the Flowmentioning

confidence: 80%

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: B Pulsations Of the Flowmentioning

confidence: 80%

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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“…By tracking the position of the zero contours near the top of the figure we see that the waves originate over the centre of the strip and propagate toward the right and left across the slope. Observational studies have revealed the existence of lowfrequency oscillatory disturbances superimposed on the mean katabatic flow (Tyson, 1968;Doran and Horst, 1981;Gryning et al, 1985;Stone and Hoard, 1989;Helmis and Papadopoulos, 1996;Monti et al, 2002). Several of these studies suggest that these oscillations are internal gravity waves.…”

Section: Numerical Simulations Of 2d Katabatic Flow (Strip Flow)mentioning

confidence: 99%

Coriolis effects in homogeneous and inhomogeneous katabatic flows

Shapiro

Fedorovich

2008

Quart J Royal Meteoro Soc

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Katabatic flows along a planar slope in a viscous stably-stratified fluid are investigated analytically and numerically, with an emphasis on flow features arising from the action of the Coriolis force. Two idealized flow types are considered: turbulent flow along a uniformly cooled slope, and two-dimensional laminar flow induced by a cold strip of finite width running down the slope. In the case of turbulent flow along a uniformly cooled slope, the downslope velocity exhibits a boundary-layer structure, but the cross-slope velocity and buoyancy fields spread inexorably upward. This behaviour is qualitatively similar to that found in laminar flows along a uniformly cooled slope. In contrast, in the two-dimensional strip flow, a steady state is reached, in which the cross-slope wind and buoyancy fields vanish far above the slope, but the downslope and slope-normal velocity fields do not vanish. These latter two flow components comprise two purely horizontal along-isentrope counter-flowing currents: an upslope current entering the top of the boundary layer on one side of the strip, and a downslope current flowing out of the boundary layer into the environment on the other side of the strip. The slope-normal vorticity associated with these currents originates in the stretching of planetary vorticity in a broad zone of convergent flow over the cold strip.

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“…Doran and Horst, 1981). Stone and Hoard (1989) and Poulos et al (2007) demonstrate the modulation of drainage flows by wave motions. Often, drainage flows at a given point arrive as a microfront with a wind‐direction shift and sudden temperature decrease (Blumen et al, 1999).…”

Section: Introductionmentioning

confidence: 97%

Non-stationary drainage flows and motions in the cold pool

Mahrt

Richardson

Seaman

et al. 2010

Tellus A: Dynamic Meteorology and Oceanography

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The initial formation of drainage flows and subsequent interaction with the cold pool are examined by contrasting winds measured with 2‐sonic anemometers at three stations along a gentle slope for a 45‐d observational period in late summer and early fall. On clear nights with weak winds, the station at the bottom of the slope experiences downslope flow only at the beginning of the evening, which quickly yields to light winds of variable direction after formation of a cold pool on the valley floor. A second station, several hundred metres up the slope, experiences drainage flow in the first part of the evening, which diminishes and yields to light and variable winds in the middle of the night as the influence of the cold pool deepens and engulfs the station. Drainage flow continues throughout the night at the third station located still farther up the slope and above the cold pool throughout the night. The interplay between the drainage flow, large‐scale flow and submeso motions leads to frequent large shifts of wind direction. The character of the wind‐direction variability differs substantially between the three stations. Large remaining uncertainties are noted.

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Low-Frequency Velocity and Temperature Fluctuations in Katabatic Valley Flows

Cited by 19 publications

References 9 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

Coriolis effects in homogeneous and inhomogeneous katabatic flows

Non-stationary drainage flows and motions in the cold pool

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