G

Baines, Anthony

doi:10.1007/978-3-476-04423-5_7

Cited by 5 publications

(5 citation statements)

References 0 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…F i is the ratio of the flow speed to the speed of long-wavelength interfacial waves on the inversion when the stratification above is neutral (Baines 1995). In the light of Vosper's results, we have attempted to examine the dependence of the observed flow on similar parameters.…”

Section: A Flow Regime Diagrammentioning

confidence: 99%

“…where g = g θ/θ 0 , g is the gravitational acceleration, θ is the magnitude of the potential temperature difference across the inversion and θ 0 is a reference potential temperature. F i is the ratio of the flow speed to the speed of long-wavelength interfacial waves on the inversion when the stratification above is neutral (Baines 1995). In the light of Vosper's results, we have attempted to examine the dependence of the observed flow on similar parameters.…”

Section: A Flow Regime Diagrammentioning

confidence: 99%

See 1 more Smart Citation

Observations of downslope winds and rotors in the Falkland Islands

Mobbs

Vosper

Sheridan

et al. 2005

Q. J. R. Meteorol. Soc.

View full text Add to dashboard Cite

SUMMARYA field campaign aimed at observing the near-surface flow field across and downwind of a mountain range on the Falkland Islands, South Atlantic, is described. The objective was to understand and eventually predict orographically generated turbulence. The instrumentation was based primarily on an array of automatic weather stations (AWSs), which recorded 30 s mean surface pressure, wind speed and direction (at 2 m), temperature and relative humidity for approximately one year. These measurements were supported by twice-daily radiosonde releases. The densest part of the AWS array was located to the south of the Wickham mountain range, across Mount Pleasant Airfield (MPA). In northerly flow the array provides a detailed study of the flow downwind of the mountain range. The dataset contains several episodes in which the flow downwind of the mountains is accelerated relative to the upwind flow. During some of these episodes short-lived (typically ∼1 hour) periods of unsteady flow separation are observed and these are associated with the formation of rotors aloft. Such events present a significant hazard to aviation at MPA. Examination of radiosonde profiles suggests that the presence of a strong temperature inversion at a height similar to the mountain height is a necessary condition for both downwind acceleration and the formation of rotors. The data are used to show that the downwind fractional speed-up is proportional to the non-dimensional mountain height (based on upstream near-surface winds and a depth-averaged Brunt-Väisälä frequency diagnosed from radiosonde data). Similarly, a relationship is established between a quantity that describes the spatial variability of the flow downwind of the mountains and the upstream wind and depth-averaged Brunt-Väisälä frequency. The dependence of the flow behaviour on the Froude number (defined in the usual way for two-layer shallow-water flow) and ratio of mountain height to inversion height is presented in terms of a flow regime diagram.

show abstract

Section: A Flow Regime Diagrammentioning

confidence: 99%

Section: A Flow Regime Diagrammentioning

confidence: 99%

Observations of downslope winds and rotors in the Falkland Islands

Mobbs

Vosper

Sheridan

et al. 2005

Q. J. R. Meteorol. Soc.

View full text Add to dashboard Cite

show abstract

“…The simpli ed problem of a single-layer uid with a free surface owing through a gap is related to the problem of a single-layer shallow-water ow over a ridge (see the reviews of its theory e.g. by Henderson (1966), Baines (1995) and Durran (1990)). The continuous acceleration of the ow across the ridge is related to the transition from a subcritical into a supercritical state at the summit, where the local Froude number F D U= p g ¤ H becomes unity (with g ¤ being the reduced-gravity and H the uid layer depth), i.e.…”

Section: (A) Topographic Environmentmentioning

confidence: 99%

Hydraulic aspects of föhn winds in an Alpine valley

Gohm

Mayr

2004

Quart J Royal Meteoro Soc

View full text Add to dashboard Cite

SUMMARYThis study examines the applicability of single-layer hydraulic theory to Alpine föhn winds in the example of the ow in the Brenner Pass region (Austria/Italy). The south föhn is described as the gap ow over the Pass and along the associated Wipp Valley. Numerical shallow-water simulations for a wide range of initial conditions, including shallow-and deep-föhn cases, are discussed and compared with selected measurements collected within the Mesoscale Alpine Programme (MAP). The observational analyses are based on Doppler and aerosol-backscatter lidar data for a speci c föhn case, and on surface observations and radiosoundings for all föhn cases observed within the 70-day MAP Special Observing Period (SOP).Radiosoundings at a location upstream (south) and downstream (north) of the Brenner Pass reveal that the average MAP SOP föhn case had subcritical ow south and nearly critical ow north of the Brenner Pass. The hydraulic model indicates ow transition to a supercritical state near the Pass, a hydraulic jump 2 km further north, mainly subcritical ow in the upper Wipp Valley except near the Brenner and, especially for shallow föhn, a tendency to becoming supercritical in the lower Wipp Valley. The model results suggest that the vertical topographic contraction exerts stronger control for the ow at the Brenner gap than the lateral contraction. In accordance with Doppler lidar measurements, the hydraulic model captures the decrease of the wind speed in the across-valley direction from east to west in the northern part of the valley. This asymmetric ow pattern is a result of the complex valley geometry rather than of the in uence of upper-level synoptic winds, as suggested in a previous study.For some observed cases the strength of the temperature inversion at the top of the föhn layer decreased across the Brenner Pass, presumably due to entrainment processes. Consequently, the reduced-gravity decreased as well. There are two major reasons for the only moderate agreement between the hydraulic parameters retrieved from soundings and those provided by the model: the model assumes a constant value of reduced-gravity, and the calculation of hydraulic parameters for föhn cases without a strong and sharp temperature inversion is ambiguous. Essential ow patterns in the simulated elds, such as the location of the pressure minimum and wind speed maxima, compare well with the surface observations along a valley transect. Indications for a steeply ampli ed, or breaking, gravity wave resembling a hydraulic jump to the lee of the two mountain ridges are found in lidar observations as well as in the simulations.

show abstract

“…As an illustration of the effects of trapping on the gravity waves, consider a simple linear calculation. As shown by Baines (1995), the amplitude of two-dimensional linear disturbances are governed by the equation…”

Section: (A) Wave Reflection and Resonancementioning

confidence: 99%

Gravity waves generated by the dry convective boundary layer: Two-dimensional scale selection and boundary-layer feedback

Lane¹,

Clark²

2002

Q. J. R. Meteorol. Soc.

View full text Add to dashboard Cite

SUMMARYThis study examines the generation of gravity waves by the dry convective boundary layer in two dimensions. In particular, a number of cases are considered which range from zero background-flow conditions to conditions with strong wind shear. These experiments agree with previous studies that identified a distinct horizontal-scale selection of the gravity waves due to changes in wind speed. Detailed linear analysis is completed which attributes the horizontal-scale selection to two processes. The first process is the dissipation of downstream-propagating gravity waves by critical levels, and the second process is the trapping of upstream-propagating waves. The result is a narrow region of the generated spectrum of gravity waves that are able to propagate vertically. The experiments also agree with previous studies that have observed a resonant interaction between the boundary-layer thermals and the gravity waves. This resonance is described as a result of the trapped gravity waves, which form a resonant cavity near the top of the mixed layer. Finally, the experiments are used to evaluate the role of shear in the gravitywave generation. In cases with little resonance, shear is shown to play a minor role in the wave generation. In these cases, changes in the wave's amplitude spectrum are mostly due to linear wave refraction and the aforementioned selection mechanism.

show abstract

G

Cited by 5 publications

References 0 publications

Observations of downslope winds and rotors in the Falkland Islands

Observations of downslope winds and rotors in the Falkland Islands

Hydraulic aspects of föhn winds in an Alpine valley

Gravity waves generated by the dry convective boundary layer: Two-dimensional scale selection and boundary-layer feedback

Contact Info

Product

Resources

About