Mountain Wake Research Articles

A comprehensive study of terrain‐disrupted airflow at Hong Kong International Airport – observations and numerical simulations

Complex terrain near the Hong Kong International Airport (HKIA) is likely to bring about low‐level windshear and turbulence to the aircraft, which could result in aviation hazards. In the past, some isolated case studies have been presented on the complicated airflow at the airport region in Hong Kong. This paper presents a more comprehensive discussion of the terrain‐induced airflow disturbances at the HKIA, including extensive mountain wake, transient low‐level jet stream, elongated mountain wake, reverse flow and mountain wave. The performance of a super high‐resolution numerical weather prediction model in the forecasting of these weather phenomena is also discussed. The model turns out to be capable of forecasting most of the terrain‐disrupted airflow, and it may be possible to obtain an earlier indication of the occurrence of low‐level windshear for airports with complex terrain. However, some mesoscale weather phenomena, such as low‐level jet stream, may not be captured by the models, and the associated windshear events would be difficult to forecast. This paper could hopefully be a useful reference to the aviation weather forecasters in the prediction of wind and windshear at airports with similar complex terrain in other parts of the world.

Lagrangian dynamics of the mistral during the HyMeX SOP2

AbstractThe mistral refers to a severe wind blowing over the Gulf of Lions after being channeled in the Rhone valley. It influences the western Mediterranean climate as it brings cold and dry continental air over the warm western Mediterranean, generating intense air‐sea heat exchanges and sea surface cooling, inducing the formation of the western Mediterranean deep water that moves into the Atlantic Ocean. The mistral is frequently observed to extend as far as a few hundred kilometers from the coast, and its fine‐scale dynamics over the sea is still only partially understood as finely resolved observations in time and space are lacking. The boundary layer pressurized balloons (BLPB) developed by the Centre National d'Etudes Spatiales and deployed during HyMeX SOP2 allowed the Lagrangian documentation of the mistral events that occurred between beginning of February to mid‐March 2013. Analyzed in synergy with the AROME‐WMED weather forecast model, all the terms of the Lagrangian formulation of the momentum conservation equation could be quantified showing three different regions: (1) an injection zone where the mistral flow is directed toward the center of the Genoa cyclone due to a strong zonal pressure gradient, enhanced friction, and entrainment in the mountain wake; (2) an ejection zone where the flow is deflected outward of the cyclone due to either the nonnegligible inertia pseudoforce or an inertial oscillation caused by a sudden friction decrease; and (3) a region of geostrophic deceleration due to the weakening of the pressure gradient.

Far-field vortex structures in mountain wakes

Laboratory experiments on scaled mountain wakes have provided the first demonstration that indicates the existence of two distinct mechanisms for the formation of the long-lived vortex structures that have been frequently observed in satellite imagery. In a series of laboratory experiments the well-known von Karman vortex shedding mechanism has been observed at low values of Froude number (corresponding to low speed flow and strong stratification). At high Froude numbers the initial highly turbulent wake evolves into a vortex pattern similar to the far-wake dipole eddies previously observed in laboratory studies of submerged stratified wakes. In satellite imagery vortex structures have been observed to persist for hundreds of kilometers. In the present laboratory experiments far-wake dipole eddies were observed to persist for at least 200 diameters downstream, which corresponds to ~4,000 km when scaled to actual mountains. When generated at multiple sites along a continental mountain chain, these eddy structures could play a significant role in the development and variability of multi-scale weather and climate patterns.

Mesocirculation Associated with Summer Convection over the Central Antilles

Abstract The mesoscale structure of the circulation and convection over central Caribbean Antilles islands in midsummer is analyzed. Afternoon thunderstorms are frequent over islands such as Hispaniola and Jamaica as confluent trade winds circulate over heated mountainous topography. Observational data from a rain gauge network, profiles from aircraft and radiosonde, satellite estimates of rainfall, and mesoscale reanalysis fields are studied with a focus on July 2007. A statistical decomposition of 3-hourly high-resolution satellite rainfall reveals an “island mode” with afternoon convection. Trade winds pass over the mountains of Hispaniola and Jamaica with a Froude number <1, leading to a long meandering wake. The Weather Research and Forecasting model is used to simulate climatic conditions during July 2007. The model correctly locates areas of diurnal rainfall that develop because of island heat fluxes, confluent sea breezes, and mountain wakes.

Mesoscale Structure of Trade Wind Convection over Puerto Rico: Composite Observations and Numerical Simulation

We examine the mesoscale structure of the atmospheric boundary layer (ABL), low-level circulation, and trade wind convection over the sub-tropical island of Puerto Rico in mid-summer. Shallow afternoon thunderstorms are frequently seen over the western plains of the island. Observational data include automatic weather station measurements, radiosonde profiles, infrared satellite images, and mesoscale reanalysis data with a focus on the summer of 2006. Satellite microwave radar data (TRMM and CloudSat) indicate that island clouds typically extend just above the −20°C level during afternoon hours with reflectivity values reaching 50 dBz. A singular value decomposition of 3-hourly high resolution satellite rainfall maps reveals an island mode. From this a composite is constructed for a group of ten cases. With a Froude number ≈1 the trade winds pass over the mountains and standing vortices and gravity waves are trapped in the meandering wake. The Weather and Research Forecasting (WRF) model at 1-km resolution with 51 vertical layers is used to simulate the short-lived thunderstorms for two cases: 27 June and 20 July 2006. The model correctly locates the convective cells that develop between 1400 and 1700 LST. The shallow afternoon thunderstorms are triggered by surface heat fluxes, confluent sea breezes and a mountain wake. Recommendations for enhanced observations are given.

Forecasting wind gusts in complex terrain

Wind gusts are calculated in a collection of simulated atmospheric flows in complex terrain. This study focuses on a region in West-Iceland during February to April 2007 which includes several windstorms. The atmospheric data is a subset in a large collection of realtime numerical simulations used for forecasting in Iceland. It is generated at horizontal resolutions of 9 and 3 km, and in two sensitivity tests at 1 km. The gust prediction method is based on turbulence kinetic energy, static stability and wind speed in the atmospheric boundary layer. The gust prediction method is implemented as post-processing. The calculated gust strength is compared with wind gust observations from several automatic weather stations. The estimated gusts are strongly dependent on the quality of the simulated flow and are on average well captured when the mean winds are correctly simulated. Maximum gusts in downslope windstorms are however frequently underestimated. The error is presumably related to an inadequate simulation of the downslope surface winds which are also too weak. The windstorms in the current study appear to be related to gravity wave activity aloft and are better reproduced at higher resolutions than at coarse resolution. There are cases of overestimated gusts on the upstream side of mountains, which may be related to an inadequate simulation of the upstream deceleration of the flow and overestimated surface winds. Gustiness in mountain wakes is frequently too great, which appears to be related to overestimated turbulence in the wakes.

“I Ain't Queer”: Love, Masculinity and History in Brokeback Mountain

Ang Lee's big gay tragic historical love story, Brokeback Mountain, was released internationally in late 2005 and early 2006. Lee's film told the story of two cowboys who fell in love while shepherding on Brokeback Mountain in Wyoming in 1963. Although the film narrated a demonstrably American story, appeals to a socio-historical connection were widespread in Australia. Indeed, Brokeback Mountain's cinematic release in this country coincided with “Australia Day,” a national day of celebration in which beginnings, nationhood, and “settlement” are reaffirmed. In this article, we track the explosion of publicly-audible conversations that took place in Brokeback Mountain's wake in Australia in 2006. On the one hand, we seek to historicize this film; on the other, we also consider the political ways in which it historicized. As the “noise” about Brokeback Mountain became almost impossible to ignore in Australia, we noticed the film tended to be viewed as a cause for celebration. Through an analysis of the politics of historical stories and the gendered politics of emotion, we seek to complicate the notion that this film signified a radical departure from homophobic cinematic and cultural traditions.

The impact of mountain wakes on the drag exerted on downstream mountains

AbstractThe pressure drag exerted on two identical high idealised mountain ridges aligned perpendicular to the mean flow, with the second mountain lying directly downstream of the first, is investigated using a numerical model. The drag is measured for a variety of different distances between the two peaks, mountain heights and lengths and incident flows. Each two‐mountain simulation has a companion single mountain simulation, with an identical mountain and identical upstream flow, with which to compare the drag. The maximum modelled drag in the two‐mountain simulations is close to two times the drag in the single mountain case, while the minimum two‐mountain drag is similar to that exerted on the single mountain. This change in drag is a result of the wake behind the first mountain reducing the speed of the flow impacting on the second mountain. A parametrization of this effect is suggested which combines the distance between the two mountain peaks, a measure of the velocity deficit in the wake of the first mountain and the wake decay‐scale, a measure of the length‐scale over which the flow perturbation generated by the first mountain decays downwind. Copyright © 2008 Royal Meteorological Society and Crown Copyright 2008, published by John Wiley & Sons, Ltd.

Jets and Orography: Idealized Experiments with Tip Jets and Lighthill Blocking

Abstract This paper describes qualitative features of the generation of jetlike concentrated circulations, wakes, and blocks by simple mountainlike orography, both from idealized laboratory experiments and shallow-water numerical simulations on a sphere. The experiments are unstratified with barotropic lee Rossby waves, and jets induced by mountain orography. A persistent pattern of lee jet formation and lee cyclogenesis owes its origins to arrested topographic Rossby waves above the mountain and potential vorticity (PV) advection through them. The wake jet occurs on the equatorward, eastern flank of the topography. A strong upstream blocking of the westerly flow occurs in a Lighthill mode of long Rossby wave propagation, which depends on βa2/U, the ratio of Rossby wave speed based on the scale of the mountain, to zonal advection speed, U (β is the meridional potential vorticity gradient, f is the Coriolis frequency, and a is the diameter of the mountain). Mountains wider (north–south) than the east–west length scale of stationary Rossby waves will tend to block the oncoming westerly flow. These blocks are essentially β plumes, which are illustrated by their linear Green function. For large βa2/U, upwind blocking is strong; the mountain wake can be unstable, filling the fluid with transient Rossby waves as in the numerical simulations of Polvani et al. For small values, βa2/U ≪ 1 classic lee Rossby waves with large wavelength compared to the mountain diameter are the dominant process. The mountain height, δh, relative to the mean fluid depth, H, affects these transitions as well. Simple lee Rossby waves occur only for such small heights, δh/h ≪ aβ/f, that the f/h contours are not greatly distorted by the mountain. Nongeostrophic dynamics are seen in inertial waves generated by geostrophic shear, and ducted by it, and also in a texture of finescale, inadvertent convection. Weakly damped circulations induced in a shallow-water numerical model on a sphere by a lone mountain in an initially simple westerly wind are also described. Here, with βa2/U ∼1, potential vorticity stirring and transient Rossby waves dominate, and drive zonal flow acceleration. Low-latitude critical layers, when present, exert strong control on the high-latitude waves, and with no restorative damping of the mean zonal flow, they migrate poleward toward the source of waves. While these experiments with homogeneous fluid are very simplified, the baroclinic atmosphere and ocean have many tall or equivalent barotropic eddy structures owing to the barotropization process of geostrophic turbulence.

Mesoscale dynamics, structure and predictability of a severe Adriatic bora case

The dynamics, structure and temporal evolution of a severe Adriatic bora, which occurred during 14 and 15 November 2004 was inspected. Numerical simulation of the investigated episode was performed by the mesoscale model MM5. The model was validated against the radiosonde data and the wind data from one automatic meteorological station and three ultrasonic anemometers. Two anemometers where located in the region extremely favorable for the bora occurrence (Senj and Vratnik Pass), while the third one was placed in the mainland (Zagreb-Horvatovac). The model reproduced well the onset and the strength of the investigated bora, as well as the establishment of bora-induced potential vorticity (PV) banners. On the other hand, surface turbulent kinetic energy (TKE) was poorly predicted. Inspection of gap wind characteristics indicated the absence of strong dissipation in the flow through a mountain pass, which gives rise to a horizontally elongated jet. Appearance of wave breaking in the lee of a mountain peak leads to the creation of a mountain wake. Shear lines between individual jets and wakes created in this way are then responsible for the generation of PV banners. Also, the ability of the model to predict hourly wind gusts was validated using a recently developed method.

Dynamics of the MAP IOP 15 severe Mistral event: Observations and high‐resolution numerical simulations

AbstractThis paper investigates the fundamental processes involved in a severe Mistral event that occurred during the Mesoscale Alpine Program (from 6 to 9 November 1999). The Mistral refers to a violent north/north‐westerly wind blowing in south‐eastern France from the Rhône valley to the French Riviera. The study is based on measurements from radiosoundings launched from Lyon and Nîmes and from two UHF wind profilers located near Marseille and Toulon allowing a good description of the flow in the complex terrain formed by the southwestern Alps. Observational results are compared with RAMS non‐hydrostatic numerical simulations performed with 27 km, 9 km and 3 km nested grids. The numerical simulations capture the flow complexity both upstream of the Alps and in the coastal area affected by the Mistral. They correctly reproduce horizontal wind speeds and directions, vertical velocities, virtual potential temperature and relative humidity documented by the observational network. The simulations are used to point out the main dynamical processes generating the Mistral. It is found that flow splitting around the Alps and around the isolated peaks bordering the south‐eastern part of the Rhône valley (Mont Ventoux 1909 m, Massif du Lubéron 1425 m) induces the low‐level jet observed near Marseille that lasts for 36 hours. The high‐resolution simulation indicates that the transient low‐level jet lasting for only 9 hours observed at Toulon is due to a gravity wave breaking over local topography (the Sainte Baume 1147 m) where hydraulic jumps are involved. A mountain wake with two opposite‐sign potential‐vorticity banners is generated. The mesoscale wake explains the westward progression of the large‐scale Alpine wake. Copyright © 2006 Royal Meteorological Society.

An Observational Study of the Mesoscale Mistral Dynamics

We investigate the mesoscale dynamics of the mistral through the wind profiler observations of the MAP (autumn 1999) and ESCOMPTE (summer 2001) field campaigns. We show that the mistral wind field can dramatically change on a time scale less than 3 hours. Transitions from a deep to a shallow mistral are often observed at any season when the lower layers are stable. The variability, mainly attributed in summer to the mistral/land–sea breeze interactions on a 10-km scale, is highlighted by observations from the wind profiler network set up during ESCOMPTE. The interpretations of the dynamical mistral structure are performed through comparisons with existing basic theories. The linear theory of R. B. Smith [Advances in Geophysics, Vol. 31, 1989, Academic Press, 1–41] and the shallow water theory [Schar, C. and Smith, R. B.: 1993a, J. Atmos. Sci. 50, 1373–1400] give some complementary explanations for the deep-to-shallow transition especially for the MAP mistral event. The wave breaking process induces a low-level jet (LLJ) downstream of the Alps that degenerates into a mountain wake, which in turn provokes the cessation of the mistral downstream of the Alps. Both theories indicate that the flow splits around the Alps and results in a persistent LLJ at the exit of the Rhone valley. The LLJ is strengthened by the channelling effect of the Rhone valley that is more efficient for north-easterly than northerly upstream winds despite the north–south valley axis. Summer moderate and weak mistral episodes are influenced by land–sea breezes and convection over land that induce a very complex interaction that cannot be accurately described by the previous theories.

Vorticity and Potential Vorticity in Mountain Wakes

A wake is traditionally defined as the region of nearly stagnant flow downstream of a body in a uniform stream. In a stratified fluid, the motions and density surfaces downstream of an obstacle become primarily horizontal; the vertical component of the vorticity associated with the wake, coexisting with the stable vertical density stratification, implies that there is potential vorticity (PV) in the wake. Recent work has demonstrated that dissipation aloft, associated with a breaking mountain wave over an isolated peak, produces a dipole in PV downstream; the dipolar vertical vorticity of the wake is associated with the PV dipole. Although one may infer the existence of vorticity downstream, the PV argument is silent on the question, Where does the wake vorticity come from? To answer this question, a weakly nonlinear model for PV production and wake formation in the case of a small-amplitude mountain has been analyzed, and numerical simulations pertaining to the strongly nonlinear large-amplitude case have been carried out. The simple model indicates that even with dissipation in the system, the vertical vorticity of the wake arises through the tilting of baroclinically generated horizontal vorticity by the dissipating mountain wave. This analysis shows that there need not be any direct effect of friction in the vorticity equation to produce the vorticity of the wake; dissipation (due to friction and/or heating) enters indirectly through its effect on the tilting term. Analysis of numerical simulations of the large-amplitude case shows that the conclusions from the weakly nonlinear model regarding the source of wake vorticity continue to hold in the strongly nonlinear regime.

The Wake of St. Vincent

Abstract The island of St. Vincent and the other Windward Islands in the southeastern Caribbean were chosen as a field site for the study of weak mountain wakes. By the authors’ definition, a “weak wake” forms when the potential vorticity generated by a mountain is not strong enough to advect itself into eddies; rather, it is simply advected downstream by the ambient flow. GOES-8 and Landsat sunglint images unambiguously revealed that the mountainous Windward Islands have remarkably long straight wakes. The length of St. Vincent’s wake exceeds 300 km although its width is only 20 km. Near the islands, the wake structures reflect the details of the island topography. These wakes do not exhibit any obvious diurnal effect. Boat surveys in the lee of St. Vincent confirmed the existence of features seen in the images: the sharp wake boundary, the small valley-induced jet embedded in the near wake, and the absence of any reverse flow. Aircraft surveys gave evidence of descent over the island and showed that the...

Comparison between the orographic response of the ECMWF model and the PYREX 1990 data

Recent observations and analyses of the atmospheric flow in the vicinity of the Pyrénées made during PYREX are compared with numerical experiments carried out with the T213L31 European Centre for Medium-range Weather Forecasts (ECMWF) global model. This study shows that the model simulates a certain proportion of the measured pressure drag, but always underestimates it. In the current model, this deficit is not adequately made up by any of the parametrized subgrid-scale processes. When the flow is along the range, the pressure-drag value depends on the synoptic background flow structure. It is close to zero when the flow impinging on the range is baroclinic. It is not zero when the atmosphere is more barotropic (e.g. in the absence of any fronts) and the model misses important lift processes. When the flow is normal to the ridge, the model response to the mountain forcing is essentially ageostrophic. In this case comparison between the vertical profiles of the model stresses and the measured profile suggests that further drag should be applied in the model at low level. This is supported by comparisons of isentropic diagnostics, from the ECMWF model and the French mesoscale model, Peridot. These diagnostics show that the ECMWF model underestimates irreversible low-level processes, such as the flow separation on the mountain flanks and the associated downstream wake, which can be related to the drag. They also show that low-level parametrized drags have a realistic impact on the model simulation of the mountain wake. This analysis supports the current developments at the ECMWF of a new subgrid-scale orographic drag scheme which parametrizes the low-level impact of mesoscale mountains.

Comparison between the orographic response of the ECMWF model and the pyrex 1990 data

AbstractRecent observations and analyses of the atmospheric flow in the vicinity of the Pyrénées made during PYREX are compared with numerical experiments carried out with the T213L31 European Centre for Medium‐range Weather Forecasts (ECMWF) global model. This study shows that the model simulates a certain proportion of the measured pressure drag, but always underestimates it. In the current model, this deficit is not adequately made up by any of the parametrized subgrid‐scale processes. When the flow is along the range, the pressure‐drag value depends on the synoptic background flow structure. It is close to zero when the flow impinging on the range is baroclinic. It is not zero when the atmosphere is more barotropic (e.g. in the absence of any fronts) and the model misses important lift processes. When the flow is normal to the ridge, the model response to the mountain forcing is essentially ageostrophic. In this case comparison between the vertical profiles of the model stresses and the measured profile suggests that further drag should be applied in the model at low level. This is supported by comparisons of isentropic diagnostics, from the ECMWF model and the French mesoscale model, Peridot. These diagnostics show that the ECMWF model underestimates irreversible low‐level processes, such as the flow separation on the mountain flanks and the associated downstream wake, which can be related to the drag. They also show that low‐level parametrized drags have a realistic impact on the model simulation of the mountain wake. This analysis supports the current developments at the ECMWF of a new subgrid‐scale orographic drag scheme which parametrizes the low‐level impact of mesoscale mountains.