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Lee Waves (lee + wave)
Selected AbstractsSummary of the Sierra Rotors Project wave and rotor eventsATMOSPHERIC SCIENCE LETTERS, Issue 4 2008Vanda Grubi Abstract This study summarizes wave and rotor events observed during the Sierra Rotors Project (SRP). This summary is on the basis of numerical model simulations of SRP events. The events are classified according to the lee-side maxima of vertical and horizontal velocities. Generally, the above classification agrees with the classification on the basis of the obstacle-perpendicular ridge-top winds. The updraft maxima are found to preferentially lie within the leading-edge updraft of a lee wave in the immediate lee and near the crest height of the Sierra Nevada. Simulated waves and lower turbulence zones are illustrated with the wave and rotor structures in two of the strongest SRP events. Copyright © 2008 Royal Meteorological Society [source] Influence of lee waves on the near-surface flow downwind of the PenninesTHE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 627 2007P. F. Sheridan Abstract The results of a recent field experiment focusing on the near-surface pressure and flow fields downstream of the Pennines in northern England are presented. The main aim of the experiment is the improvement of wind forecasts downstream of orography. Trapped lee waves commonly occur in westerly flow in this region, and during the experiment there were numerous instances of apparent flow separation, indicating the formation of lee-wave rotors. The spatial variability of the near-surface flow in these circumstances is closely linked to the positions of lee-wave crests and troughs aloft, and appears to be a response to pressure gradients induced by the lee waves. For large-amplitude waves, it has been possible to demonstrate a correlation between the fractional change of the flow speed across the measurement array (which if large enough may lead to flow separation) and a normalized pressure-perturbation amplitude. For a group of lee-wave cases during which the cross-mountain flow is strong, a rapid decrease in the Scorer parameter within the lower portion of the troposphere appears to be a prerequisite for rotors to form. However, this does not guarantee their occurrence. For a fixed Scorer-parameter profile, idealized two-dimensional simulations indicate that the lee-wave-induced pressure-perturbation amplitude, and hence the occurrence of rotors, is controlled largely by the strength of the wind upstream close to the mountain-top level. It seems that the combination of a favourable Scorer-parameter profile and sufficiently strong low-level winds is required for rotors to develop. © Crown Copyright 2007. Reproduced with the permission of Her Majesty's Stationery Office. Published by John Wiley & Sons, Ltd. [source] Flow separation and rotor formation beneath two-dimensional trapped lee wavesTHE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 620 2006S. B. Vosper Abstract Numerical simulations of trapped lee waves generated in flow over a two-dimensional ridge are presented. It is shown that for sufficiently large amplitude waves flow separation occurs beneath the wave crests when a no-slip lower boundary condition is applied. The occurrence of separation corresponds to rotor motion, or recirculation, under the wave crests. The dependence of the wave-induced horizontal flow perturbations near the ground on the wave amplitude, wavelength and surface roughness is examined. It is shown that the normalized critical wave amplitude, above which rotors form, is a function of the ratio of the lee-wave horizontal wavelength to the surface roughness length. This normalized wave amplitude is defined as the ratio of the lee-wave pressure amplitude within the boundary layer, to the square of the friction velocity. Linearized turbulent equations for motion beneath the wave crests are considered and numerical solutions to the linear problem are compared with results from the simulations. When the waves are of sufficiently small amplitude that flow separation does not occur, the linear flow perturbations are shown to agree closely with the results from the simulations. It is also shown that linear theory provides a useful prediction of the occurrence of rotor formation. © Crown copyright, 2006. [source] Numerical simulations of rotors, hydraulic jumps and eddy shedding in the Falkland IslandsATMOSPHERIC SCIENCE LETTERS, Issue 4 2005P. F. Sheridan Abstract High-resolution three-dimensional simulations of flows over East Falkland, South Atlantic, are presented. With a temperature inversion upwind, lee waves, rotors and hydraulic jumps are found to occur, giving rise to highly unsteady phenomena such as wakes and eddy shedding. Such flows are known to represent a significant hazard to aviation. Copyright © 2006 Royal Meteorological Society [source] | ||||||||||