Transient Mountain Waves and Their Interaction with Large Scales

2007 ◽  
Vol 64 (7) ◽  
pp. 2378-2400 ◽  
Author(s):  
Chih-Chieh Chen ◽  
Gregory J. Hakim ◽  
Dale R. Durran
Keyword(s):  
Potential Vorticity ◽  
Large Scale ◽  
Wave Drag ◽  
Mountain Waves ◽  
Flow Acceleration ◽  
Spatial Response ◽  
Flow Deceleration ◽  
Large Scale Flow ◽  
The Impact ◽  
Zonal Momentum

Abstract The impact of transient mountain waves on a large-scale flow is examined through idealized numerical simulations of the passage of a time-evolving synoptic-scale jet over an isolated 3D mountain. Both the global momentum budget and the spatial flow response are examined to illustrate the impact of transient mountain waves on the large-scale flow. Additionally, aspects of the spatial response are quantified by potential vorticity inversion. Nearly linear cases exhibit a weak loss of domain-averaged absolute momentum despite the absence of wave breaking. This transient effect occurs because, over the time period of the large-scale flow, the momentum flux through the top boundary does not balance the surface pressure drag. Moreover, an adiabatic spatial redistribution of momentum is observed in these cases, which results in an increase (decrease) of zonally averaged zonal momentum south (north) of the mountain. For highly nonlinear cases, the zonally averaged momentum field shows a region of flow deceleration downstream of the mountain, flanked by broader regions of weak flow acceleration. Cancellation between the accelerating and decelerating regions results in weak fluctuations in the volume-averaged zonal momentum, suggesting that the mountain-induced circulations are primarily redistributing momentum. Potential vorticity anomalies develop in a region of wave breaking near the mountain, and induce local regions of flow acceleration and deceleration that alter the large-scale flow. A “perfect” conventional gravity wave–drag parameterization is implemented on a coarser domain not having a mountain, forced by the momentum flux distribution from the fully nonlinear simulation. This parameterization scheme produces a much weaker spatial response in the momentum field and it fails to produce enough flow deceleration near the 20 m s−1 jet. These results suggest that the potential vorticity sources attributable to the gravity wave–drag parameterization have a controlling effect on the longtime downstream influence of the mountain.

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 Stochastic resonance in a nonlinear model of a rotating, stratified shear flow, with a simple stochastic inertia-gravity wave parameterization

2004 ◽  
Vol 11 (1) ◽  
pp. 127-135 ◽  
Author(s):  
P. D. Williams ◽  
T. W. N. Haine ◽  
P. L. Read
Keyword(s):  
Shear Flow ◽  
Gravity Wave ◽  
Stochastic Resonance ◽  
Potential Vorticity ◽  
Large Scale ◽  
Zonal Wavenumber ◽  
Stratified Shear Flow ◽  
Balanced Flow ◽  
Large Scale Flow ◽  
The Impact

Abstract. We report on a numerical study of the impact of short, fast inertia-gravity waves on the large-scale, slowly-evolving flow with which they co-exist. A nonlinear quasi-geostrophic numerical model of a stratified shear flow is used to simulate, at reasonably high resolution, the evolution of a large-scale mode which grows due to baroclinic instability and equilibrates at finite amplitude. Ageostrophic inertia-gravity modes are filtered out of the model by construction, but their effects on the balanced flow are incorporated using a simple stochastic parameterization of the potential vorticity anomalies which they induce. The model simulates a rotating, two-layer annulus laboratory experiment, in which we recently observed systematic inertia-gravity wave generation by an evolving, large-scale flow. We find that the impact of the small-amplitude stochastic contribution to the potential vorticity tendency, on the model balanced flow, is generally small, as expected. In certain circumstances, however, the parameterized fast waves can exert a dominant influence. In a flow which is baroclinically-unstable to a range of zonal wavenumbers, and in which there is a close match between the growth rates of the multiple modes, the stochastic waves can strongly affect wavenumber selection. This is illustrated by a flow in which the parameterized fast modes dramatically re-partition the probability-density function for equilibrated large-scale zonal wavenumber. In a second case study, the stochastic perturbations are shown to force spontaneous wavenumber transitions in the large-scale flow, which do not occur in their absence. These phenomena are due to a stochastic resonance effect. They add to the evidence that deterministic parameterizations in general circulation models, of subgrid-scale processes such as gravity wave drag, cannot always adequately capture the full details of the nonlinear interaction.

scholarly journals The Impact of Mountain Waves on an Idealized Baroclinically Unstable Large-Scale Flow

2018 ◽  
Vol 75 (9) ◽  
pp. 3285-3302 ◽  
Author(s):  
Maximo Q. Menchaca ◽  
Dale R. Durran
Keyword(s):  
Wave Breaking ◽  
Large Scale ◽  
Lower Stratosphere ◽  
Unstable Wave ◽  
Mean Flow ◽  
Mountain Waves ◽  
Upper Troposphere ◽  
Low Level ◽  
Flow Deceleration ◽  
Large Scale Flow

Abstract The feedback of mountain waves and low-level blocking on an idealized baroclinically unstable wave passing over an isolated ridge is examined through numerical simulation. Theoretical analysis implies that the volume-integrated perturbation momentum budget is dominated by mean-flow deceleration, the divergence of vertical fluxes of horizontal momentum, and the Coriolis force acting on the perturbation ageostrophic wind. These do indeed appear as the dominant balances in numerically computed budgets averaged over layers containing 1) wave breaking in the lower stratosphere, 2) flow blocking with wave breaking near the surface, and 3) a region of pronounced horizontally averaged mean-flow deceleration in the upper troposphere where there is no wave breaking. The local impact of wave breaking on the jet in the lower stratosphere is dramatic, with winds in the jet core reduced by almost 50% relative to the no-mountain case. Although it is the layer with the strongest average deceleration, the local patches of decelerated flow are weakest in the upper troposphere. The cross-mountain pressure drag over a 2-km-high ridge greatly exceeds the vertical momentum flux at mountain-top level because of low-level wave breaking, blocking, and lateral flow diversion. These pressure drags and the low-level momentum fluxes are significantly different from corresponding values computed for simulations with steady forcing matching the instantaneous conditions over the mountain in the evolving large-scale flow.

Mountain waves produced by a stratified shear flow with a boundary layer: transition from downstream sheltering to upstream blocking

2020 ◽  
Author(s):  
Francois Lott ◽  
Bruno Deremble ◽  
Clément Soufflet
Keyword(s):  
Boundary Layer ◽  
Reynolds Stress ◽  
Internal Waves ◽  
Large Scale ◽  
Wave Drag ◽  
Mountain Waves ◽  
Reflected Waves ◽  
Stable Case ◽  
Large Scale Flow ◽  
Neutral Case

<p>A non-hydrostatic theory for mountain flow with a boundary layer of constant eddy viscosity is presented. The theory predicts that dissipation impacts the dynamics over a an inner layer which depth δ is predicted by viscous critical level theory. 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 quantitatively. With stratification, internal waves start to control the dynamics and produce a wave drag that can also be predicted. For weak stratification, upward propagating mountain waves and reflected waves interact destructively and low drag states occur, whereas for moderate stability they interact constructively and high drag states are reached. In very stable cases the reflected waves do not affect the drag much.</p><p>The sign and vertical profiles of the Reynolds stress are profoundly affected by stability. In the neutral case and up to the point where internal waves interact constructively, 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 stable case, the opposite occurs, and the large scale flow above the inner layer is decelerated as expected for dissipated mountain waves. These opposed behaviors challenge how mountain form drag and mountain wave drag should be parameterized in large-scale models.</p><p>The structure of the flow around the mountain is also strongly affected by stability: it is characterized by non separated sheltering in the neutral case, 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.</p>

Experimental Study of Large-Scale Flow Structures in an Aneurysm

2018 ◽  
Author(s):  
Paulo Yu ◽  
Vibhav Durgesh
Keyword(s):  
Large Scale ◽  
Flow Structures ◽  
Pump System ◽  
Vortical Structures ◽  
Abnormal Growth ◽  
Image Velocimetry ◽  
Large Scale Flow ◽  
The Impact ◽  
Inflow Conditions ◽  
Aneurysm Model

An aneurysm is an abnormal growth in the wall of a weakened blood vessel, and can often be fatal upon rupture. Studies have shown that aneurysm shape and hemodynamics, in conjunction with other parameters, play an important role in growth and rupture. The objective of this study was to investigate the impact of varying inflow conditions on flow structures in an aneurysm. An idealized rigid sidewall aneurysm model was prepared and the Womersley number (α) and Reynolds number (Re) values were varied from 2 to 5 and 50 to 250, respectively. A ViVitro Labs pump system was used for inflow control and Particle Image Velocimetry was used for conducting velocity measurements. The results showed that the primary vortex path varied with an increase in α, while an increase in Re was correlated to the vortex strength and formation of secondary vortical structures. The evolution and decay of vortical structures were also observed to be dependent on α and Re.

scholarly journals Combined measurement of velocity and temperature in liquid metal convection

2019 ◽  
Vol 876 ◽  
pp. 1108-1128 ◽  
Author(s):  
Till Zürner ◽  
Felix Schindler ◽  
Tobias Vogt ◽  
Sven Eckert ◽  
Jörg Schumacher
Keyword(s):  
Reynolds Number ◽  
Liquid Metal ◽  
Large Scale ◽  
Scale Up ◽  
Convection Cell ◽  
Small Scale ◽  
Convection Flow ◽  
Large Scale Circulation ◽  
Large Scale Flow ◽  
The Impact

Combined measurements of velocity components and temperature in a turbulent Rayleigh–Bénard convection flow at a low Prandtl number of $Pr=0.029$ and Rayleigh numbers of $10^{6}\leqslant Ra\leqslant 6\times 10^{7}$ are conducted in a series of experiments with durations of more than a thousand free-fall time units. Multiple crossing ultrasound beam lines and an array of thermocouples at mid-height allow for a detailed analysis and characterization of the complex three-dimensional dynamics of the single large-scale circulation roll in a cylindrical convection cell of unit aspect ratio which is filled with the liquid metal alloy GaInSn. We measure the internal temporal correlations of the complex large-scale flow and distinguish between short-term oscillations associated with a sloshing motion in the mid-plane as well as varying orientation angles of the velocity close to the top/bottom plates and the slow azimuthal drift of the mean orientation of the roll as a whole that proceeds on a time scale up to a hundred times slower. The coherent large-scale circulation drives a vigorous turbulence in the whole cell that is quantified by direct Reynolds number measurements at different locations in the cell. The velocity increment statistics in the bulk of the cell displays characteristic properties of intermittent small-scale fluid turbulence. We also show that the impact of the symmetry-breaking large-scale flow persists to small-scale velocity fluctuations thus preventing the establishment of fully isotropic turbulence in the cell centre. Reynolds number amplitudes depend sensitively on beam-line position in the cell such that different definitions have to be compared. The global momentum and heat transfer scalings with Rayleigh number are found to agree with those of direct numerical simulations and other laboratory experiments.

scholarly journals Nitrification of the lowermost stratosphere during the exceptionally cold Arctic winter 2015/16

2019 ◽  
Author(s):  
Marleen Braun ◽  
Jens-Uwe Grooß ◽  
Wolfgang Woiwode ◽  
Sören Johansson ◽  
Michael Höpfner ◽  
...  
Keyword(s):  
Cross Sections ◽  
Large Scale ◽  
Potential Temperature ◽  
Lagrangian Model ◽  
The Arctic ◽  
Small Scale ◽  
Mountain Waves ◽  
Mixing Ratios ◽  
Fine Structures ◽  
The Impact

Abstract. The Arctic winter 2015/16 was characterized by exceptionally cold stratospheric temperatures, favouring the formation of polar stratospheric clouds (PSCs) from mid-December until the end of February down to low stratospheric altitudes. Observations by GLORIA (Gimballed Limb Observer for Radiance Imaging of the Atmosphere) on HALO (High Altitude and LOng range research aircraft) during the PGS (POLSTRACC/GW-LCYLCE II/SALSA) campaign from December 2015 to March 2016 allow an investigation of the influence of denitrification on the lowermost stratosphere (LMS) with a high spatial resolution. For the first time vertical cross-sections of nitric acid (HNO3) along the flight track and tracer-tracer correlations derived from the GLORIA observations document detailed pictures of wide-spread nitrification of the Arctic LMS during the course of an entire winter. GLORIA observations show large-scale structures and local fine structures with strongly enhanced absolute HNO3 volume mixing ratios reaching up to 11 ppbv at altitudes of 11 km in January and nitrified filaments persisting until the middle of March. Narrow streaks of enhanced HNO3, observed in mid-January, are interpreted as regions recently nitrified by sublimating HNO3-containing particles. Overall, a nitrification of the LMS between 5.0 ppbv and 7.0 ppbv at potential temperature levels between 350 and 380 K is estimated. This extent of nitrification has never been observed before in the Arctic lowermost stratosphere. The GLORIA observations are compared with CLaMS (Chemical Lagrangian Model of the Stratosphere) simulations. The fundamental structures observed by GLORIA are well reproduced, but differences in the fine structures are diagnosed. Further, CLaMS predominantly underestimates the spatial extent of maximum HNO3 mixing ratios derived from the GLORIA observations as well as the enhancement at lower altitudes. Sensitivity simulations with CLaMS including (i) enhanced sedimentation rates in case of ice supersaturation (to resemble ice nucleation on NAT), (ii) a global temperature offset, (iii) modified growth rates (to resemble aspherical particles with larger surfaces) and (iv) temperature fluctuations (to resemble the impact of small-scale mountain waves) mostly improve the agreement with the GLORIA observations. The sensitivity simulations suggest that details of particle microphysics play a significant role for simulated LMS nitrification in January, while air subsidence, transport and mixing become increasingly important towards the end of the winter.

scholarly journals On the Time Evolution of Limited-Area Ensemble Variance: Case Studies with the Convection-Permitting Ensemble COSMO-E

2018 ◽  
Vol 76 (1) ◽  
pp. 11-26 ◽  
Author(s):  
Christina Klasa ◽  
Marco Arpagaus ◽  
André Walser ◽  
Heini Wernli
Keyword(s):  
Case Studies ◽  
Time Evolution ◽  
Large Scale ◽  
Horizontal Wind ◽  
Limited Area ◽  
Convective Activity ◽  
Moist Convection ◽  
Domain Boundaries ◽  
Large Scale Flow ◽  
The Impact

Abstract Dynamical processes determining the time evolution of difference kinetic energy (DKE) in a limited-area domain are investigated with the convection-permitting ensemble model COSMO-E for a forecasting period of 4 days. DKE is quantified by means of ensemble variance of the irrotational and nondivergent horizontal wind. For three case studies characterized by contrasting predictability levels of precipitation, it is shown that DKE of the irrotational wind strongly increases during periods of solar-forced moist convective activity and decreases when the latter ceases. The response of DKE of the nondivergent wind is also clearly related to the convective activity, but delayed by a few hours, pointing to interactions between both wind components. Apart from the impact of moist convection, DKE of the nondivergent wind is primarily governed by large-scale advection, imposed at the lateral domain boundaries of the limited-area ensemble. This forcing may also sustain or increase DKE of the irrotational wind when moist convection is absent. Consequently, the large-scale flow and diurnal solar forcing, associated with higher spatiotemporal predictability, determines the overall evolution of the limited-area ensemble variance of the horizontal wind, which increases in the presence of moist convective activity or strong synoptic-scale forcing, and stagnates or decreases otherwise, rendering forecasts of convection-permitting ensembles valuable beyond the very short forecast range.

scholarly journals An Objective Identification and Climatology of Upper-Tropospheric Jets near Atlantic Tropical Cyclones

2020 ◽  
Vol 148 (7) ◽  
pp. 3015-3036
Author(s):  
Levi P. Cowan ◽  
Robert E. Hart
Keyword(s):  
Tropical Cyclones ◽  
Potential Vorticity ◽  
Clustering Analysis ◽  
Large Scale ◽  
Reanalysis Data ◽  
Atlantic Basin ◽  
Jet Behavior ◽  
Consistent Feature ◽  
Large Scale Flow ◽  
Westerly Jets

Abstract An objective algorithm is developed for identifying jets in 200-hPa flow and applied to reanalysis data within 2000 km of Atlantic tropical cyclones (TCs) during 1979–2015. The resulting set of 16 512 jets is analyzed both qualitatively and quantitatively to describe the climatology of TC–jet configurations and jet behavior near TCs. Jets occur most commonly poleward of TCs within the 500–1000-km annulus, where TC outflow amplifies the background potential vorticity gradient. A rigorous clustering analysis is performed, resulting in statistically distinct clusters of jet traces that correspond to common configurations of large-scale flow near Atlantic TCs. The speed structure of westerly jets poleward of TCs is found to vary with location in the Atlantic basin, but acceleration of jets downstream of their closest approach to the TC due to interaction with the TC’s diabatic outflow is a consistent feature of these structures. In addition to the climatology developed here, this objectively constructed dataset of upper-tropospheric jets opens unique avenues for exploring TC–environment interactions and utilizing jets to quantitatively describe large-scale flow.

scholarly journals Binary Interaction between Typhoons Fengshen (2002) and Fungwong (2002) Based on the Potential Vorticity Diagnosis

2008 ◽  
Vol 136 (12) ◽  
pp. 4593-4611 ◽  
Author(s):  
Chung-Chuan Yang ◽  
Chun-Chieh Wu ◽  
Kun-Hsuan Chou ◽  
Chia-Ying Lee
Keyword(s):  
Potential Vorticity ◽  
Large Scale ◽  
Early Period ◽  
Subtropical High ◽  
Binary Interaction ◽  
Monsoon Circulation ◽  
Good Representation ◽  
Steering Flow ◽  
Large Scale Flow ◽  
Storm Characteristics

Abstract A cyclonic loop was observed in the track of Typhoon Fungwong (2002) when it was about 765 n mi from Supertyphoon Fengshen (2002). It is shown that Fungwong’s special path is associated with the circulation of Fengshen, and such an association is regarded as an indication of binary interaction. In this paper, the binary interaction between Fengshen and Fungwong is studied based on the potential vorticity diagnosis. The impacts of large-scale flow fields on their motions are also investigated. Furthermore, the sensitivity of the storm characteristics to the binary interaction is demonstrated by the mesoscale numerical model simulations with different sizes and intensities for the initial bogused storms. Results of the study show that before Fungwong and Fengshen interacted with each other, their motions were governed by the large-scale environmental flow, that is, mainly associated with the subtropical high. During this binary interaction, Fungwong’s looping is partly attributed to Fengshen’s steering flow. This pattern shows up first as a case of one-way interaction in the early period, and then develops into a mutual interaction during the later stages. The numerical experiments show the sensitivity of the storm size and intensity to the binary interaction, implicating that a good representation of the initial storm vortex is important for the prediction of binary storms. Further analyses also indicate the influence of the monsoon trough and subtropical high systems on the binary interaction. These results provide some new insights into the motions of nearby typhoons embedded in the monsoon circulation.

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