scholarly journals On the Diurnal Variation of Mountain Waves

2008 ◽  
Vol 65 (4) ◽  
pp. 1360-1377 ◽  
Author(s):  
Qingfang Jiang ◽  
James D. Doyle
Keyword(s):  
Diurnal Variation ◽  
Froude Number ◽  
Flow Pattern ◽  
Surface Wind ◽  
Wave Drag ◽  
Prediction System ◽  
Wind Maximum ◽  
Mountain Waves ◽  
Ridge Crest ◽  
Deep Well

Abstract The diurnal variation of mountain waves and wave drag associated with flow past mesoscale ridges has been examined using the Coupled Ocean–Atmosphere Mesoscale Prediction System (COAMPS) and an analytical boundary layer (BL) model. The wave drag exhibits substantial diurnal variation in response to the change in the atmospheric BL characteristics, such as the BL depth, shape factor, and stability. During daytime, a convective BL develops, characterized by a shallow shear layer near the surface and a deep well-mixed layer aloft, both of which tend to decrease the wave drag. As a result, the convective BL could significantly weaken mountain waves and reduce the momentum flux by up to 90%. Near the surface, the flow pattern resembles a potential flow with a surface wind maximum located near the ridge crest. During nighttime, a shallow stable BL develops, and the modulation of wave drag by the stable nocturnal BL is governed by the BL Froude number (Fr). If the BL flow is supercritical, the drag increases as Fr decreases toward unity and reaches the maximum around Fr = 1, where the drag could be several times larger than the corresponding free-slip hydrostatic wave drag. If the BL flow is subcritical because of excessive cooling, the drag decreases with decreasing Froude number and the flow pattern near the surface resembles a typical subcritical solution with the wind maximum located near the ridge crest.

scholarly journals Can Mountain Waves Contribute to Damaging Winds Far Away from the Lee Slope?

2019 ◽  
Vol 34 (6) ◽  
pp. 2045-2065 ◽  
Author(s):  
Jeffrey D. Kelley ◽  
David M. Schultz ◽  
Russ S. Schumacher ◽  
Dale R. Durran
Keyword(s):  
Great Plains ◽  
Surface Wind ◽  
Surface Friction ◽  
Conveyor Belt ◽  
The Arctic ◽  
Wind Maximum ◽  
Mountain Waves ◽  
Arctic Front ◽  
Strong Winds ◽  
Lapse Rates

Abstract On 25 December 2016, a 984-hPa cyclone departed Colorado and moved onto the northern plains, drawing a nearby Arctic front into the circulation and wrapping it cyclonically around the equatorward side of the cyclone. A 130-km-wide and 850-km-long swath of surface winds exceeding 25 m s−1 originated underneath the comma head of the lee cyclone and followed the track of the Arctic front from Colorado to Minnesota. These strong winds formed in association with a downslope windstorm and mountain wave over Colorado and Wyoming, producing an elevated jet of strong winds. Central to the distribution of winds in this case is the Arctic air mass, which both shielded the elevated winds from surface friction behind the front and facilitated the mixing of the elevated jet down to the surface just behind the Arctic front, due to steep lapse rates associated with cold-air advection. The intense circulation south of the cyclone center transported the Arctic front and the elevated jet away from the mountains and out across Great Plains. This case is compared to an otherwise similar cyclone that occurred on 28–29 February 2012 in which a downslope windstorm occurred, but no surface mesoscale wind maximum formed due to the absence of a well-defined Arctic front and postfrontal stable layer. Despite the superficial similarities of this surface wind maximum to a sting jet (e.g., origin in the midtroposphere within the comma head of the cyclone, descent evaporating the comma head, acceleration to the top of the boundary layer, and an existence separate from the cold conveyor belt), this swath of winds was not caused by a sting jet.

scholarly journals Mountain waves produced by a stratified shear flow with a boundary layer. Part II: Form drag, wave drag, and transition from downstream sheltering to upstream blocking

2020 ◽  
Author(s):  
François Lott ◽  
Bruno Deremble ◽  
Clément Soufflet
Keyword(s):  
Reynolds Stress ◽  
Large Scale ◽  
Wave Drag ◽  
Wave Crest ◽  
Form Drag ◽  
Mountain Waves ◽  
Reflected Waves ◽  
Stable Case ◽  
Large Scale Flow ◽  
Neutral Case

AbstractThe non-hydrostatic version of the mountain flow theory presented in Part I is detailed. In the near neutral case, the surface pressure decreases when the flow crosses the mountain to balance an increase in surface friction along the ground. This produces a form drag which can be predicted qualitatively. When stratification increases, internal waves start to control the dynamics and the drag is due to upward propagating mountain waves as in part I. The reflected waves nevertheless add complexity to the transition. First, when stability increases, upward propagating waves and reflected waves interact destructively and low drag states occur. When stability increases further, the interaction becomes constructive and high drag state are reached. In very stable cases the reflected waves do not affect the drag much. Although the drag gives a reasonable estimate of the Reynolds stress, its sign and vertical profile are profoundly affected by stability. In the near neutral case the Reynolds stress in the flow is positive, with maximum around the top of the inner layer, decelerating the large-scale flow in the inner layer and accelerating it above. In the more stable cases, on the contrary, the large-scale flow above the inner layer is decelerated as expected for dissipated mountain waves. The structure of the flow around the mountain is also strongly affected by stability: it is characterized by non separated sheltering in the near neutral cases, by upstream blocking in the very stable case, and at intermediate stability by the presence of a strong but isolated wave crest immediately downstream of the ridge.

scholarly journals Fennec dust forecast intercomparison over the Sahara in June 2011

2016 ◽  
Vol 16 (11) ◽  
pp. 6977-6995 ◽  
Author(s):  
Jean-Pierre Chaboureau ◽  
Cyrille Flamant ◽  
Thibaut Dauhut ◽  
Cécile Kocha ◽  
Jean-Philippe Lafore ◽  
...  
Keyword(s):  
High Resolution ◽  
Surface Wind ◽  
Dust Emission ◽  
Airborne Lidar ◽  
Data Sets ◽  
Density Currents ◽  
Data Set ◽  
Near Surface ◽  
Western Sahara ◽  
Deep Well

Abstract. In the framework of the Fennec international programme, a field campaign was conducted in June 2011 over the western Sahara. It led to the first observational data set ever obtained that documents the dynamics, thermodynamics and composition of the Saharan atmospheric boundary layer (SABL) under the influence of the heat low. In support to the aircraft operation, four dust forecasts were run daily at low and high resolutions with convection-parameterizing and convection-permitting models, respectively. The unique airborne and ground-based data sets allowed the first ever intercomparison of dust forecasts over the western Sahara. At monthly scale, large aerosol optical depths (AODs) were forecast over the Sahara, a feature observed by satellite retrievals but with different magnitudes. The AOD intensity was correctly predicted by the high-resolution models, while it was underestimated by the low-resolution models. This was partly because of the generation of strong near-surface wind associated with thunderstorm-related density currents that could only be reproduced by models representing convection explicitly. Such models yield emissions mainly in the afternoon that dominate the total emission over the western fringes of the Adrar des Iforas and the Aïr Mountains in the high-resolution forecasts. Over the western Sahara, where the harmattan contributes up to 80 % of dust emission, all the models were successful in forecasting the deep well-mixed SABL. Some of them, however, missed the large near-surface dust concentration generated by density currents and low-level winds. This feature, observed repeatedly by the airborne lidar, was partly forecast by one high-resolution model only.

Flow past a sphere moving vertically in a stratified diffusive fluid

2000 ◽  
Vol 417 ◽  
pp. 211-236 ◽  
Author(s):  
C. R. TORRES ◽  
H. HANAZAKI ◽  
J. OCHOA ◽  
J. CASTILLO ◽  
M. VAN WOERT
Keyword(s):  
Froude Number ◽  
Rear Surface ◽  
Wave Drag ◽  
Thin Layers ◽  
Original Position ◽  
Linear Wave ◽  
Frictional Drag ◽  
Pressure Drag ◽  
Lee Wave ◽  
Vertical Jet

Numerical studies are described of the flows generated by a sphere moving vertically in a uniformly stratified fluid. It is found that the axisymmetric standing vortex usually found in homogeneous fluids at moderate Reynolds numbers (25 [les ] Re [les ] 200) is completely collapsed by stable stratification, generating a strong vertical jet. This is consistent with our experimental visualizations. For Re = 200 the complete collapse of the vortex occurs at Froude number F ≃ 19, and the critical Froude number decreases slowly as Re increases. The Froude number and the Reynolds number are here defined by F = W/Na and Re = 2Wa/v, with W being the descent velocity of the sphere, N the Brunt–Väisälä frequency, a the radius of the sphere and v the kinematic viscosity coefficient. The inviscid processes, including the generation of the vertical jet, have been investigated by Eames & Hunt (1997) in the context of weak stratification without buoyancy effects. They showed the existence of a singularity of vorticity and density gradient on the rear axis of the flow and also the impossibility of realizing a steady state. When there is no density diffusion, all the isopycnal surfaces which existed initially in front of the sphere accumulate very near the front surface because of density conservation and the fluid in those thin layers generates a rear jet when returning to its original position. In the present study, however, the fluid has diffusivity and the buoyancy effects also exist. The density diffusion prevents the extreme piling up of the isopycnal surfaces and allows the existence of a steady solution, preventing the generation of a singularity or a jet. On the other hand, the buoyancy effect works to increase the vertical velocity to the rear of the sphere by converting the potential energy to vertical kinetic energy, leading to the formation of a strong jet. We found that the collapse of the vortex and the generation of the jet occurs at much weaker stratifications than those necessary for the generation of strong lee waves, showing that jet formation is independent of the internal waves. At low Froude numbers (F [les ] 2) the lee wave patterns showed good agreement with the linear wave theory and the previous experiments by Mowbray & Rarity (1967). At very low Froude numbers (F [les ] 1) the drag on a sphere increases rapidly, partly due to the lee wave drag but mainly due to the large velocity of the jet. The jet causes a reduction of the pressure on the rear surface of the sphere, which leads to the increase of pressure drag. High velocity is induced also just outside the boundary layer of the sphere so that the frictional drag increases even more significantly than the pressure drag.

scholarly journals Drag Characteristics of Competitive Swimming Children and Adults

2008 ◽  
Vol 24 (1) ◽  
pp. 35-42 ◽  
Author(s):  
Per-Ludvik Kjendlie ◽  
Robert Keig Stallman
Keyword(s):  
Perturbation Method ◽  
Froude Number ◽  
Wave Drag ◽  
Maximal Velocity ◽  
Evaluation Tool ◽  
Competitive Swimming ◽  
Velocity Decay ◽  
Competitive Swimmers ◽  
Drag Factor ◽  
Swimming Technique

The aims of this study were to compare drag in swimming children and adults, quantify technique using the technique drag index (TDI), and use the Froude number (Fr) to study whether children or adults reach hull speed at maximal velocity (vmax). Active and passive drag was measured by the perturbation method and a velocity decay method, respectively, including 9 children aged 11.7 ± 0.8 and 13 adults aged 21.4 ± 3.7. The children had significantly lower active (kAD) and passive drag factor (kPD) compared with the adults. TDI (kAD/kPD) could not detect any differences in swimming technique between the two groups, owing to the adults swimming maximally at a higher Fr, increasing the wave drag component, and masking the effect of better technique. The children were found not to reach hull speed atvmax, and their Fr were 0.37 ± 0.01 vs. the adults 0.42 ± 0.01, indicating adults’ larger wave-making component of resistance atvmaxcompared with children. Fr is proposed as an evaluation tool for competitive swimmers.

scholarly journals Investigation of the Diurnal Variation of Marine Boundary Layer Cloud Microphysical Properties at the Azores

2014 ◽  
Vol 27 (23) ◽  
pp. 8827-8835 ◽  
Author(s):  
Xiquan Dong ◽  
Baike Xi ◽  
Peng Wu
Keyword(s):  
Boundary Layer ◽  
Diurnal Variation ◽  
Marine Boundary Layer ◽  
Surface Wind ◽  
Number Concentration ◽  
Diurnal Variations ◽  
Cloud Layer ◽  
Entire Study ◽  
Microphysical Properties ◽  
Seasonal And Diurnal Variations

Abstract A new method has been developed to retrieve the nighttime marine boundary layer (MBL) cloud microphysical properties, which provides a complete 19-month dataset to investigate the diurnal variation of MBL cloud microphysical properties at the Azores. Compared to the corresponding daytime results presented in the authors' previous study over the Azores region, all nighttime monthly means of cloud liquid water path (LWP) exceed their daytime counterparts with an annual-mean LWP of 140 g m−2, which is ~30.9 g m−2 larger than daytime. Because the MBL clouds are primarily driven by convective instabilities caused by cloud-top longwave (LW) radiative cooling, more MBL clouds are well mixed and coupled with the surface during the night; thus, its cloud layer is deeper and its LWP is higher. During the day, the cloud layer is warmed by the absorption of solar radiation and partially offsets the cloud-top LW cooling, which makes the cloud layer thinner with less LWP. The seasonal and diurnal variations of cloud LWC and optical depth basically follow the variation of LWP. There are, however, no significant day–night differences and diurnal variations in cloud-droplet effective radius (re), number concentration (Nd), and corresponding surface measured cloud condensation nuclei (CCN) number concentration (NCCN) (at supersaturation S = 0.2%). Surface NCCN increases from around sunrise (0300–0600 LT) to late afternoon, which strongly correlates with surface wind speed (r = 0.76) from 0300 to 1900 LT. The trend in hourly-mean Nd is consistent with NCCN variation from 0000 to 0900 LT but not for afternoon and evening with an averaged ratio (Nd/NCCN) of 0.35 during the entire study period.

The effects of terrain shape on nonlinear hydrostatic mountain waves

1979 ◽  
Vol 95 (2) ◽  
pp. 241-261 ◽  
Author(s):  
D. K. Lilly ◽  
J. B. Klemp
Keyword(s):  
Strong Influence ◽  
Wave Amplitude ◽  
Solution Method ◽  
Wave Structure ◽  
Wave Drag ◽  
Mountain Range ◽  
Horizontal Scale ◽  
Vertical Wavelength ◽  
Mountain Waves ◽  
Nonlinear Nature

Solutions to Long's equation for a stably stratified incompressible fluid traversing a mountain range are obtained for various terrain shapes and amplitudes when the horizontal scale is large compared to the vertical wavelength. Nonlinear lower and upper (radiative) boundary conditions are utilized and found to have a strong influence on the wave structure at large amplitudes. The results for symmetric and asymmetric mountain profiles reveal that the wave amplitude and wave drag are significantly enhanced for mountains with gentle windward and steep leeward slopes. These results confirm and explain those obtained by Raymond (1972) using a different solution method. Several results obtained by Smith (1977) from perturbation analysis are also confirmed and extended to large amplitudes. The methods are also applied to investigate the nonlinear nature of the singularity predicted by linear theory for flow over a step.

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