scholarly journals Mechanisms for the Generation of Mesoscale Vorticity Features in Tropical Cyclone Rainbands

2006 ◽  
Vol 134 (10) ◽  
pp. 2649-2669 ◽  
Author(s):  
Charmaine N. Franklin ◽  
Greg J. Holland ◽  
Peter T. May
Keyword(s):  
Tropical Cyclone ◽  
Rossby Waves ◽  
Primary Source ◽  
Vortex Pair ◽  
Cloud Microphysics ◽  
Vortex Center ◽  
Mean Flow ◽  
Fourier Decomposition ◽  
Freezing Level ◽  
The Mean

Abstract A high-resolution tropical cyclone model with explicit cloud microphysics has been used to investigate the dynamics and energetics of tropical cyclone rainbands. Analysis of the vorticity interactions that occur within the simulated rainbands demonstrates that couplets of cyclonic–anticyclonic mesovortices can be produced in outer bands. The primary source of this vorticity is the upward tilting of system-generated horizontal vorticity by diabatic heating gradients. The vertical heating gradient in the stratiform cloud also creates a potential vorticity (PV) dipole that accelerates the tangential flow and develops a midlevel jet. The strength of the jet is enhanced by the vortex pair that is oriented radially across the rainband. The Fourier decomposition of the absolute vorticity field shows two counterpropagating vortex Rossby waves associated with the rainband. The wave located on the inner side of the band transports energy toward the vortex center. The outer wave is made up of high wavenumbers and uses the vorticity gradients generated by the rainband. The results support the hypothesis that the heating profile in the stratiform regions of rainbands generates cyclonic PV across the freezing level, which develops a midlevel jet. This mechanism creates a vorticity gradient that enables the propagation of vortex Rossby waves that could allow the rainbands to interact with the mean flow and potentially influence the evolution of the storm by contributing to the symmetric component of vorticity and the development of secondary eyewalls.

On the mechanism of high-incidence lift generation for steadily translating low-aspect-ratio wings

2017 ◽  
Vol 813 ◽  
pp. 110-126 ◽  
Author(s):  
Adam C. DeVoria ◽  
Kamran Mohseni
Keyword(s):  
Vortex Pair ◽  
Tip Vortex ◽  
Mean Flow ◽  
Flow Topology ◽  
Trailing Edge ◽  
Negative Contribution ◽  
High Incidence ◽  
The Mean ◽  
Stall Angle ◽  
Strong Vorticity

High-incidence lift generation via flow reattachment is studied. Different reattachment mechanisms are distinguished, with dynamic manoeuvres and tip vortex downwash being separate mechanisms. We focus on the latter mechanism, which is strictly available to finite wings, and isolate it by considering steadily translating wings. The tip vortex downwash provides a smoother merging of the flow at the trailing edge, thus assisting in establishing a Kutta condition there. This decreases the strength/amount of vorticity shed from the trailing edge, and in turn maintains an effective bound circulation resulting in continued lift generation at high angles of attack. Just below the static lift-stall angle of attack, strong vorticity is shed at the trailing edge indicating an increasingly intermittent reattachment/detachment of the instantaneous flow at mid-span. Above this incidence, the trailing-edge shear layer increases in strength/size representing a negative contribution to the lift and leads to stall. Lastly, we show that the mean-flow topology is equivalent to a vortex pair regardless of the particular physical flow configuration.

scholarly journals The interaction of free Rossby waves with semi-transparent equatorial waveguide – wave-mean flow interaction

2009 ◽  
Vol 16 (3) ◽  
pp. 381-392 ◽  
Author(s):  
G. M. Reznik ◽  
V. Zeitlin
Keyword(s):  
Rossby Waves ◽  
Landau Equation ◽  
Spatial Modulation ◽  
Equilibrium Solutions ◽  
Mean Flow ◽  
Ginzburg Landau ◽  
Baroclinic Waves ◽  
Ginzburg Landau Equation ◽  
The Mean ◽  
Mean Current

Abstract. Nonlinear interactions of the barotropic Rossby waves propagating across the equator with trapped baroclinic Rossby or Yanai modes and mean zonal flow are studied within the two-layer model of the atmosphere, or the ocean. It is shown that the equatorial waveguide with a mean current acts as a resonator and responds to barotropic waves with certain wavenumbers by making the trapped baroclinic modes grow. At the same time the equatorial waveguide produces the barotropic response which, via nonlinear interaction with the mean equatorial current and with the trapped waves, leads to the saturation of the growing modes. The excited baroclinic waves can reach significant amplitudes depending on the magnitude of the mean current. In the absence of spatial modulation the nonlinear saturation of thus excited waves is described by forced Landau-type equation with one or two attracting equilibrium solutions. In the latter case the spatial modulation of the baroclinic waves is expected to lead to the formation of characteristic domain-wall defects. The evolution of the envelopes of the trapped Rossby waves is governed by driven Ginzburg-Landau equation, while the envelopes of the Yanai waves obey the "first-order" forced Ginzburg-Landau equation. The envelopes of short baroclinic Rossby waves obey the damped-driven nonlinear Schrodinger equation well studied in the literature.

scholarly journals Transitional Flow Dynamics Behind a Micro-Ramp

2019 ◽  
Vol 104 (2-3) ◽  
pp. 533-552
Author(s):  
J. Casacuberta ◽  
K. J. Groot ◽  
Q. Ye ◽  
S. Hickel
Keyword(s):  
Boundary Layer ◽  
Stability Analysis ◽  
Large Scale ◽  
Vortex Pair ◽  
Base Flow ◽  
Transitional Flow ◽  
Mean Flow ◽  
Primary Vortex ◽  
The Mean ◽  
Near Wall

AbstractMicro-ramps are popular passive flow control devices which can delay flow separation by re-energising the lower portion of the boundary layer. We compute the laminar base flow, the instantaneous transitional flow, and the mean flow around a micro-ramp immersed in a quasi-incompressible boundary layer at supercritical roughness Reynolds number. Results of our Direct Numerical Simulations (DNS) are compared with results of BiLocal stability analysis on the DNS base flow and independent tomographic Particle Image Velocimetry (tomo-PIV) experiments. We analyse relevant flow structures developing in the micro-ramp wake and assess their role in the micro-ramp functionality, i.e., in increasing the near-wall momentum. The main flow feature of the base flow is a pair of streamwise counter-rotating vortices induced by the micro-ramp, the so-called primary vortex pair. In the instantaneous transitional flow, the primary vortex pair breaks up into large-scale hairpin vortices, which arise due to linear varicose instability of the base flow, and unsteady secondary vortices develop. Instantaneous vortical structures obtained by DNS and experiments are in good agreement. Matching linear disturbance growth rates from DNS and linear stability analysis are obtained until eight micro-ramp heights downstream of the micro-ramp. For the setup considered in this article, we show that the working principle of the micro-ramp is different from that of classical vortex generators; we find that transitional perturbations are more efficient in increasing the near-wall momentum in the mean flow than the laminar primary vortices in the base flow.

scholarly journals Characterizing the Energetics of Vortex-Scale and Sub-Vortex-Scale Asymmetries during Tropical Cyclone Rapid Intensity Changes

2019 ◽  
Vol 77 (1) ◽  
pp. 315-336
Author(s):  
Saiprasanth Bhalachandran ◽  
Daniel R. Chavas ◽  
Frank D. Marks Jr. ◽  
S. Dubey ◽  
A. Shreevastava ◽  
...  
Keyword(s):  
Tropical Cyclone ◽  
Kinetic Energy ◽  
Mean Flow ◽  
Length Scales ◽  
Asymmetric Structures ◽  
Multiple Length Scales ◽  
Precise Understanding ◽  
Intensity Changes ◽  
Resolution Model ◽  
The Mean

Abstract Our collective understanding of azimuthally asymmetric features within the coherent structure of a tropical cyclone (TC) continues to improve with the availability of more detailed observations and high-resolution model outputs. However, a precise understanding of how these asymmetries impact TC intensity changes is lacking. Prior attempts at investigating the asymmetric impacts follow a mean–eddy partitioning that condenses the effect of all the asymmetries into one term and fails to highlight the differences in the role of asymmetries at different scales. In this study, we present a novel energetics-based approach to analyze the asymmetric impacts at multiple length scales during periods of rapid intensity changes. Using model outputs of TCs under low and high shear, we compute the different energy pathways that enhance/suppress the growth of multiscale asymmetries in the wavenumber (WN) domain. We then compare and contrast the energetics of the mean-flow field (WN 0) with that of the persistent, coherent vortex-scale asymmetric structures (WNs 1 and 2) and the more local, transient, sub-vortex-scale asymmetries (WNs ≥ 3). We find in our case studies that the dominant mechanisms of growth/decay of the asymmetries are the baroclinic conversion from available potential to kinetic energy at individual scales of asymmetries and the transactions of kinetic energy between the asymmetries of various length scales, rather than the barotropic mean–eddy transactions as is typically assumed. Our case study analysis further shows that the growth/decay of asymmetries is largely independent of the mean. Certain aspects of eddy energetics can potentially serve as early-warning indicators of TC rapid intensity changes.

Overtransmission of Rossby Waves at a Lower-Layer Critical Latitude in the Two-Layer Model

2020 ◽  
Vol 77 (3) ◽  
pp. 859-870 ◽  
Author(s):  
Matthew T. Gliatto ◽  
Isaac M. Held
Keyword(s):  
Rossby Waves ◽  
Lower Layer ◽  
Scattering Problem ◽  
Lower Troposphere ◽  
Vertical Shear ◽  
Mean Flow ◽  
External Mode ◽  
The Mean ◽  
Critical Latitude ◽  
Pv Gradient

Abstract Rossby waves, propagating from the midlatitudes toward the tropics, are typically absorbed by critical latitudes (CLs) in the upper troposphere. However, these waves typically encounter CLs in the lower troposphere first. We study a two-layer linear scattering problem to examine the effects of lower CLs on these waves. We begin with a review of the simpler barotropic case to orient the reader. We then progress to the baroclinic case using a two-layer quasigeostrophic model in which there is vertical shear in the mean flow on which the waves propagate, and in which the incident wave is assumed to be an external-mode Rossby wave. We use linearized equations and add small damping to remove the critical-latitude singularities. We consider cases in which either there is only one CL, in the lower layer, or there are CLs in both layers, with the lower-layer CL encountered first. If there is only a CL in the lower layer, the wave’s response depends on the sign of the mean potential vorticity gradient at this lower-layer CL: if the PV gradient is positive, then the CL partially absorbs the wave, as in the barotropic case, while for a negative PV gradient, the CL is a wave emitter, and can potentially produce overreflection and/or overtransmission. Our numerical results indicate that overtransmission is by far the dominant response in these cases. When an upper-layer absorbing CL is encountered, following the lower-layer encounter, one can still see the signature of overtransmission at the lower-layer CL.

Signatures of midlatitude heat waves in Rossby wave variability spectra 

2021 ◽  
Author(s):  
Iana Strigunova ◽  
Richard Blender ◽  
Frank Lunkeit ◽  
Nedjeljka Žagar
Keyword(s):  
Rossby Wave ◽  
Large Scale ◽  
Rossby Waves ◽  
Heat Waves ◽  
Probability Distributions ◽  
Physical Space ◽  
Mean Flow ◽  
Single Wave ◽  
Fourier Decomposition ◽  
Latitude Belt

<p>This work aims at identifying extreme circulation conditions such as heat waves in modal space which is defined by eigensolutions of the linearized primitive equations. Here, the Rossby waves are represented in terms of Hough harmonics that are an orthogonal and complete expansion set allowing Rossby wave diagnostics in terms of their total (kinetic and available potential) energies. We expect that this diagnostic provides a more clear picture of the Rossby wave variability spectra compared to the common Fourier decomposition along a latitude belt. </p> <p>The probability distributions of Rossby wave energies are analysed separately for the zonal mean flow, for the planetary and synoptic zonal wavenumbers. The robustness is ensured by considering the four reanalyses ERA5, ERA-Interim, JRA-55 and MERRA. A single wave is characterized by Gaussianity in the complex Hough amplitudes and by a chi-square distribution for the energies. We find that the distributions of the energy anomalies in the wavenumber space are non-Gaussian with almost the same positive skewness in the four reanalyses.  The skewness increases during persistent heat waves for all energy anomaly distributions, in agreement with the recent trend of increased subseasonal variance in large-scale Rossby waves and decreased variance at synoptic scales. The new approach offers a selective filtering to physical space. The reconstructed circulation during heat waves is dominated by large-scale anticyclonic systems in northeastern Europe with zonal wavenumbers 2 and 3, in agreement with previous studies, thereby demonstrating physical meaningfulness of the skewness. </p> <p> </p>

scholarly journals Strong Intraseasonal Variability of Meridional Currents near 5°N in the Eastern Indian Ocean: Characteristics and Causes

2017 ◽  
Vol 47 (5) ◽  
pp. 979-998 ◽  
Author(s):  
Gengxin Chen ◽  
Weiqing Han ◽  
Yuanlong Li ◽  
Michael J. McPhaden ◽  
Ju Chen ◽  
...  
Keyword(s):  
Indian Ocean ◽  
Rossby Waves ◽  
Intraseasonal Variability ◽  
Upper Ocean ◽  
Mean Flow ◽  
Eastern Boundary ◽  
Typical Period ◽  
The Indian Ocean ◽  
The Mean

AbstractThis paper reports on strong, intraseasonal, upper-ocean meridional currents observed in the Indian Ocean between the Bay of Bengal (BOB) and the equator and elucidates the underlying physical processes responsible for them. In situ measurements from a subsurface mooring at 5°N, 90.5°E reveal strong intraseasonal variability of the meridional current with an amplitude of ~0.4 m s−1 and a typical period of 30–50 days in the upper 150 m, which by far exceeds the magnitudes of the mean flow and seasonal cycle. Such prominent intraseasonal variability is, however, not seen in zonal current at the same location. Further analysis suggests that the observed intraseasonal flows are closely associated with westward-propagating eddylike sea surface height anomalies (SSHAs) along 5°N. The eddylike SSHAs are largely manifestations of symmetric Rossby waves, which result primarily from intraseasonal wind stress forcing in the equatorial waveguide and reflection of the equatorial Kelvin waves at the eastern boundary. Since the wave signals are generally symmetric about the equator, similar variability is also seen at 5°S but with weaker intensity because of the inclined coastline at the eastern boundary. The Rossby waves propagate westward, causing pronounced intraseasonal SSHA and meridional current in the upper ocean across the entire southern BOB between 84° and 94°E. They greatly weaken in the western Indian Basin, but zonal currents near the equator remain relatively strong.

The propagation of atmospheric Rossby gravity waves in latitudinally sheared zonal flows

1977 ◽  
Vol 287 (1340) ◽  
pp. 115-143 ◽  
Keyword(s):  
Gravity Waves ◽  
Rossby Waves ◽  
Zonal Flow ◽  
Flow Speed ◽  
Wave Number ◽  
Mean Flow ◽  
Zonal Flows ◽  
Planetary Scale ◽  
The Mean ◽  
Critical Latitude

Using the B-plane approximation we formulate the equations which govern small perturbations in a rotating atmosphere and describe a wide class of possible wave motions, in the presence of a background zonal flow, ranging from ‘moderately high’ frequency acoustic-gravity-inertial waves to ‘low’ frequency planetary-scale (Rossby) waves. The discussion concentrates mainly on the propagation properties of Rossby waves in various types of latitudinally sheared zonal flows which occur at different heights and seasons in the earth’s atmosphere. However, it is first shown that gravity waves in a latitudinally sheared zonal flow exhibit critical latitude behaviour where the ‘intrinsic ’ wave frequency matches the Brunt-Vaisala frequency (in contrast to the case of gravity waves in a vertically sheared flow where a critical layer exists where the horizontal wave phase speed equals the flow speed) and that the wave behaviour near such a latitude is similar to that of Rossby waves in the vicinity of their critical latitudes which occur where the ‘intrinsic’ wave frequency approaches zero. In the absence of zonal flow in the atmosphere the geometry of the planetary wave dispersion equation (which is described by a highly elongated ellipsoid in wave-number vector space) implies that energy propagates almost parallel to the /--planes. This feature may provide a reason why there seems to be so little coupling between planetary scale motions in the lower and upper atmosphere. Planetary waves can be made to propagate eastward, as well as westward, if they are evanescent in the vertical direction. The W.K.B. approximation, which provides an approximate description of wave propagation in slowly varying zonal wind shears, shows that the distortion of the wave-number surface caused by the zonal flow controls the dependence of the wave amplitude on the zonal flow speed. In particular it follows that Rossby waves propagating into regions of strengthening westerlies are intensified in amplitude whereas those waves propagating into strengthening easterlies are diminished in amplitude. A classification of the various types of ray trajectories that arise in zonal flow profiles occurring in the Earth’s atmosphere, such as jet-like variations of westerly or easterly zonal flow or a belt of westerlies bounded by a belt of easterlies, is given, and provides the conditions giving rise to such phenomena as critical latitude behaviour and wave trapping. In a westerly flow there is a tendency for the combined effects on wave propagation of jet-like variations of B and zonal flow speed to counteract each other, whereas in an easterly flow such variations tend to reinforce each other. An examination of the reflexion and refraction of Rossby waves at a sharp jump in the zonal flow speed shows that under certain conditions wave amplification, or over-reflexion, can arise with the implication that the reflected wave can extract energy from the background streaming motion. On the other hand the wave behaviour near critical latitudes, which can be described in terms of a discontinuous jump in the ‘wave invariant’, shows that such latitudes can act as either wave absorbers (in which case the mean flow is accelerated there) or wave emitters (in which case the mean flow is decelerated there).

scholarly journals Effect of Fluid Viscoelasticity on Turbulence and Large-Scale Vortices behind Wall-Mounted Plates

2014 ◽  
Vol 6 ◽  
pp. 823138
Author(s):  
Takahiro Tsukahara ◽  
Masaaki Tanabe ◽  
Yasuo Kawaguchi
Keyword(s):  
Viscoelastic Fluid ◽  
Large Scale ◽  
Vortex Pair ◽  
Mean Flow ◽  
Turbulent Structures ◽  
Frictional Drag ◽  
Local Skin ◽  
Smooth Channel ◽  
The Mean ◽  
Free Area

Direct numerical simulations of turbulent viscoelastic fluid flows in a channel with wall-mounted plates were performed to investigate the influence of viscoelasticity on turbulent structures and the mean flow around the plate. The constitutive equation follows the Giesekus model, valid for polymer or surfactant solutions, which are generally capable of reducing the turbulent frictional drag in a smooth channel. We found that turbulent eddies just behind the plates in viscoelastic fluid decreased in number and in magnitude, but their size increased. Three pairs of organized longitudinal vortices were observed downstream of the plates in both Newtonian and viscoelastic fluids: two vortex pairs were behind the plates and the other one with the longest length was in a plate-free area. In the viscoelastic fluid, the latter vortex pair in the plate-free area was maintained and reached the downstream rib, but its swirling strength was weakened and the local skin-friction drag near the vortex was much weaker than those in the Newtonian flow. The mean flow and small spanwise eddies were influenced by the additional fluid force due to the viscoelasticity and, moreover, the spanwise component of the fluid elastic force may also play a role in the suppression of fluid vortical motions behind the plates.

scholarly journals The Interaction of a Baroclinic Mean Flow with Long Rossby Waves

2005 ◽  
Vol 35 (5) ◽  
pp. 865-879 ◽  
Author(s):  
A. Colin de Verdière ◽  
R. Tailleux
Keyword(s):  
Doppler Shift ◽  
Rossby Waves ◽  
Numerical Solutions ◽  
Phase Speed ◽  
Standard Theory ◽  
Buoyancy Frequency ◽  
Implicit Assumption ◽  
Constant Sign ◽  
Mean Flow ◽  
The Mean

Abstract The effect of a baroclinic mean flow on long oceanic Rossby waves is studied using a combination of analytical and numerical solutions of the eigenvalue problem. The effect is summarized by the value of the nondimensional numberwhen the mean flow shear keeps a constant sign throughout the water column. Because previous studies have shown that no interaction occurs if the mean flow has the shape of the first unperturbed mode (the non–Doppler shift effect), an implicit assumption in the application of the present work to the real ocean is that the relative projections of the mean flow on the second and higher modes remain approximately constant. Because R2 is large at low latitudes between 10° and 30° (the southern branches of subtropical gyres or the regions of surface westward shear), the phase speed of the first mode is very slightly decreased from the no mean flow standard theory case. Between 30° and 40° latitudes (the northern branches of the subtropical gyres or the regions of surface eastward shear), R2 is O(10) and the westward phase speed can increase significantly (up to a factor of 2). At still higher latitudes when R2 is O(1) a critical transition occurs below which no discrete Rossby waves are found that might explain the absence of observations of zonal propagations at latitudes higher than 50°. Our case studies, chosen to represent the top-trapped and constant-sign shear profiles of observed mean flows, all show the importance of three main effects on the value of the first baroclinic mode Rossby wave speed: 1) the meridional gradient of the quantity N2/f (where N is the buoyancy frequency) rather than that of the potential vorticity fN2; 2) the curvature of the mean flow in the vertical direction, which appears particularly important to predict the sign of the phase speed correction to the no-mean-flow standard theory case: increase (decrease) of the westward phase speed when the surface-intensified mean flow is eastward (westward); and 3) a weighted vertical average of the mean flow velocity, acting as a Doppler-shift term, which is small in general but important to determine the precise value of the phase speed.

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