Low Phase Speed Research Articles

Increased vertical resolution in the stratosphere reveals role of gravity waves after sudden stratospheric warmings

Abstract. Sudden stratospheric warmings (SSWs) have a long-lasting effect within the stratosphere as well as impacts on the underlying troposphere. However, sub-seasonal forecasts of the winter polar stratosphere fail to use their full potential for predictability as they tend to underestimate the magnitude and persistence of these events already within the stratosphere. The origin of this underestimation is unknown. Here, we demonstrate that the associated polar stratospheric cold bias following SSW events in sub-seasonal hindcasts can be halved by increasing vertical model resolution, suggesting a potential sensitivity to gravity wave forcing. While the predictability of the planetary Rossby wave flux into the stratosphere at lead times longer than a week is limited, the existence of a critical layer for gravity waves with a low zonal phase speed caused by the disturbed polar vortex provides predictability to the upper stratosphere. Gravity wave breaking near that critical layer can, therefore, decelerate the zonal flow consistently with anomalous subsidence over the polar cap leading to warmer temperatures in the middle polar stratosphere. Since the spectrum of gravity waves involves vertical wavelengths of less than 4000 m, as estimated by wavelet analysis, a high vertical model resolution is needed to resolve the positive feedback between gravity wave forcing and the state of the polar vortex. Specifically, we find that at a spectral resolution of TCo639 (approximate horizontal grid spacing of 18 km) at least 198 levels are needed to correctly resolve the spectrum of gravity waves in the ECMWF Integrated Forecasting System. Increasing vertical resolution in operational forecasts will help to mitigate stratospheric temperature biases and improve sub-seasonal predictions of the stratospheric polar vortex.

Whistler-regulated Magnetohydrodynamics: Transport Equations for Electron Thermal Conduction in the High-β Intracluster Medium of Galaxy Clusters

Abstract Transport equations for electron thermal energy in the high-β e intracluster medium (ICM) are developed that include scattering from both classical collisions and self-generated whistler waves. The calculation employs an expansion of the kinetic electron equation along the ambient magnetic field in the limit of strong scattering and assumes whistler waves with low phase speeds V w ∼ v te /β e ≪ v te dominate the turbulent spectrum, with v te the electron thermal speed and β e ≫ 1 the ratio of electron thermal to magnetic pressure. We find: (1) temperature-gradient-driven whistlers dominate classical scattering when L c > L/β e , with L c the classical electron mean free path and L the electron temperature scale length, and (2) in the whistler-dominated regime the electron thermal flux is controlled by both advection at V w and a comparable diffusive term. The findings suggest whistlers limit electron heat flux over large regions of the ICM, including locations unstable to isobaric condensation. Consequences include: (1) the Field length decreases, extending the domain of thermal instability to smaller length scales, (2) the heat flux temperature dependence changes from T e 7 / 2 / L to V w nT e ∼ T e 1 / 2 , (3) the magneto-thermal- and heat-flux-driven buoyancy instabilities are impaired or completely inhibited, and (4) sound waves in the ICM propagate greater distances, as inferred from observations. This description of thermal transport can be used in macroscale ICM models.

Impacts of Rossby Wave Packets and Atmospheric Rivers on Meteorological Drought in the Continental United States

AbstractDrought is among the costliest natural disasters and its spatiotemporal scales are larger than those of other water‐related disasters. However, our understanding of drought causative mechanisms is less than complete, inhibiting accurate drought forecasting and early warning. While sea surface temperature teleconnection governs the 10%–60% precipitation variability at intra‐seasonal to multiannual scales, some of the unexplained variability can be understood by internal atmospheric variability, represented by Rossby wave packets (RWPs), and implications of atmospheric rivers (ARs). The amplitude and phase speed of the recurrent RWPs (RRWPs) and the atmospheric river index developed in this study were used to analyze the impacts of RWPs and ARs on droughts detected by three‐dimensional drought clustering algorithms for the continental U.S. during 1951–2015. The occurrence of RRWPs raised the likelihood of the occurrence of more severe drought than did the non‐occurrence of RRWPs. The increase in the occurrence of RRWPs with low phase speed escalated drought severity, area, and duration. Furthermore, the drying ARs affected drought by increasing the severity and area of extreme droughts. We measured the level of contributions of RRWPs and ARs to extreme drought occurrences. The drying ARs contributed more to extreme droughts than did RRWPs. However, the co‐occurrence of ARs and RRWPs contributed more than did each individually. Our findings of the implications of RRWPs and ARs on drought characteristics offer the potential to improve our understanding of drought causative mechanisms that can enhance the proactive drought mitigation measures.

On the possibility of dust acoustic waves over sunlit lunar surface

ABSTRACTThe photoelectron sheath and floating fine positively charged dust particles constitute two-component dusty plasma in the sunlit lunar regolith’s vicinity. By including the charge fluctuation into photoelectron–dust dynamics, the lunar exospheric plasma is proposed to support the propagation of long-wavelength dust acoustic (DA) modes. Using the standard approach based on the dynamical equations for continuity, momentum, plasma potential, and dust charging along with Fowler's treatment of photoemission and non-Maxwellian nature of the sheath photoelectrons, the wave dispersion is derived. The dust charge variation modifies the usual DA wave dispersion and excites the ultralow frequency modes that propagate with sufficiently low phase speed. Such ultralow frequency modes are predicted as pronounced for smaller values of dust charge and sheath potential. The DA wave dispersion is also depicted as sensitive to the photoelectrons’ energy distribution within the sheath. The quantitative estimates suggest that the nominal exospheric plasma may exhibit DA waves propagating with frequencies of the order of unity.

Resonant electron–plasmon interactions in drifting electron gas

In this paper, we investigate the resonant electron–plasmon interactions in a drifting electron gas of arbitrary degeneracy. The kinetic-corrected quantum hydrodynamic model is transformed into the effective Schrödinger–Poisson model, and the driven coupled pseudoforce system is obtained via separation of variables from the appropriately linearized system. It is noted that in the low phase–speed kinetic regime, the characteristic particle-like plasmon branch is significantly affected by the correction factor, which is a function of electron number density and temperature. It is shown that the electron current density of drifting electron gas sharply peaks at two distinct drift wavenumbers for a given value of electron density, temperature, plasmon energy, and damping parameter. The Fano-resonance of current density profile confirms the electron–plasmon resonant interaction in the presence of underlying interference effect. The electron drift current density shows fundamentally different resonance effects for plasmon energies with a wavenumber below and above a critical wavenumber. Moreover, an extension to the multistream model is presented, and the total current density of drifting electron gas in the presence of resonant electron–plasmon interactions is obtained. We further investigate the kinetic correction effect on matter-wave energy dispersion of the electron gas. It is also found that the increase in the electron number density leads to an increase in effective mass and consequently a decrease in electron mobility, whereas the increase in electron temperature has the converse effect. The kinetic correction is noted to significantly lower the quasiparticle conduction band minimum. The current model may be further elaborated to investigate the electron beam–plasma interactions.

Super-resolved shear shock focusing in the human head

Shear shocks, which exist in a completely different regime from compressional shocks, were recently observed in the brain. These low phase speed (≈ 2 m/s) high Mach number (≈ 1) waves could be the primary mechanism behind diffuse axonal injury due to a very high local acceleration at the shock front. The extreme nonlinearity of these waves results in unique behaviors that are different from more commonly studied nonlinear compressional waves. Here we show the first observation of super-resolved shear shock wave focusing. Shear shock wave imaging and numerical simulations in a human head phantom over a range of frequencies/amplitudes shows the super-resolution of shock waves in the low strain and high strain-rate regime. These results suggest that even for mild accelerations injuries as small as a grain of rice on the scale of mm2 can be easily created deep inside the brain. Statement of SignificanceThe relationship between brain motion and traumatic brain injury remains poorly understood despite many decades of investigation. We have developed high frame-rate ultrasound imaging techniques combined with motion tracking sequences that can capture a previously unobtainable high strain and high strain-rate regime. This quantitative imaging method has led to the discovery that shear waves can develop into shear shocks. To the best of our knowledge, we are the only group in the world that has observed these shear shocks in soft tissue. In this manuscript we demonstrate that shear waves are focused into destructive shocks deep inside the human head where rate-dependent metrics, such as acceleration and strain-rate, are amplified by an order of magnitude. Furthermore, it is shown that the destructive power of these shear shocks is superresolved into tiny areas about the size of a grain of rice. To achieve these results, we have made technical innovations in the field of ultrasound by designing shock-capturing imaging sequences, and simulations tools that can model shear shocks. There is an overwhelming amount of evidence that shear shock wave physics is a necessary and primary component of brain biomechanics and, we hypothesize, brain injury. Local measurements and simulations of this shock wave behavior, which are absent from current biomechanical models of the brain, may fundamentally change the way we approach the design of protective equipment in transportation, sports, playground safety, falls and our understanding of the extreme biomechanical environment to which our brains can be subjected.

Multiple compound Tamm waves with Uller–Zenneck wave characteristics guided by dissipative dielectric defect in periodic multilayered isotropic dielectric material

A canonical problem configured in three different arrangements of periodic multilayered isotropic dielectric material layers with a dissipative dielectric defect, to guide multiple compound Tamm waves with Uller–Zenneck wave characteristics, was formulated and solved. The numerical solutions showed multiple Tamm waves guided at a fixed wavelength along the dissipative defect with same polarization state. These waves propagate with different phase speeds, different propagation distances, different field localizations, and different field profiles. The results identified excitation of symmetric and anti-symmetric solutions of p-polarized Tamm waves and p- and s-polarized waveguide modes. The high phase speed solutions ceased to exist beyond a certain limit of dissipative dielectric defect thickness, but the low phase speed solutions were computed for a wide range of dissipative dielectric defect thickness. The Tamm waves were compounded when the thickness of defect is very small and transmuted into waveguide modes for increasing thickness of dissipative dielectric defect because most of the energy is then strongly confined to the dissipative dielectric defect. The excitation of multiple Tamm waves and waveguide modes at a given dissipative dielectric defect thickness and fixed wavelength can be considered for multi-channel optical communication and sensing applications.

Space-Time Folding of the Wake Produced by a Supervelocity Rotating Point Source.

Wave sources moving faster than the waves they emit create a wake whose topological features are directly related to the geometry of the source trajectory. These features can be understood by considering space-time surfaces representing past emitted wave fronts. Specifically, for a supervelocity source moving along a circular path the space-time envelope folds and a cusp appears on the inner part of the wake. As a result, the wake is ultimately contained within two parallel corotating spiraling branches. In this Letter we take advantage of the low phase speed of water waves to study experimentally supervelocity sources moving at velocities up to several time the wave speed. We image in real time their emission patterns and characterize the topological features of their wakes.

First OH Airglow Observation of Mesospheric Gravity Waves Over European Russia Region

AbstractFor the first time, we perform a study of mesospheric gravity waves (GWs) for four different seasons of 1 year in the latitudinal band from 45°N to 75°N using an OH all‐sky airglow imager over Kazan (55.8°N, 49.2°E), Russia, during the period of August 2015 to July 2016. Our observational study fills a huge airglow imaging observation gap in Europe and Russia region. In total, 125 GW events and 28 ripple events were determined by OH airglow images in 98 clear nights. The observed GWs showed a strong preference of propagation toward northeast in all seasons, which was significantly different from airglow imager observations at other latitudes that the propagation directions were seasonal dependent. The middle atmosphere wind field is used to explain the lack of low phase speed GWs since these GWs were falling into the blocking region due to the filtering effects. Deep tropospheric convections derived from the European Centre for Medium‐Range Weather Forecasts reanalysis data are determined near Caucasus Mountains region, which suggests that the convections are the dominant source of the GWs in spring, summer, and autumn seasons. This finding extends our knowledge that convection might also be an important source of GWs in the higher latitudes. In winter the generation mechanism of the GWs are considered to be jet stream systems. In addition, the occurrence frequency of ripple is much lower than other stations. This study provides some constraints on the range of GW parameters in GW parameterization in general circulation models in Europe and Russia region.

Understanding Midlatitude Jet Variability and Change Using Rossby Wave Chromatography: Poleward-Shifted Jets in Response to External Forcing

Abstract Rossby wave chromatography (RWC) is implemented in a linearized barotropic model as a tool to understand the response of the midlatitude jet to external forcing. Given the background zonal-mean flow and the space–time structure of the baroclinic wave activity source, RWC calculates the space–time structure of the upper-tropospheric eddy momentum fluxes. RWC is used to diagnose and understand the poleward shift of the jet in an idealized GCM using the convergence of the vertical EP flux in the upper troposphere as the wave activity source. The poleward-shifted jet is maintained via a selective “reflecting level” on the poleward flank of jet: for a given wavenumber, low phase speed waves are reflected but high phase speed waves are absorbed at the critical level on the poleward flank of jet. When the zonal-mean zonal wind increases on the poleward flank of the jet, a wider range of poleward-propagating waves encounter a reflecting level instead of a critical level on the poleward flank. The increased wave reflection leads to increased equatorward-propagating waves (and, therefore, poleward momentum flux) across the jet. Increases in wave phase speeds directly oppose the poleward shift because, in addition to the well-recognized effect of phase speed on wave dissipation in the subtropics, increased phase speeds imply more wave dissipation rather than reflection on the poleward flank via the selective reflecting level.

Exact evaluation of the rates of electrostatic decay and scattering off thermal ions for an unmagnetized Maxwellian plasma

Electrostatic decay of Langmuir waves into Langmuir and ion sound waves (L→L′+S) and scattering of Langmuir waves off thermal ions (L+i→L′+i′, also called “nonlinear Landau damping”) are important nonlinear weak-turbulence processes. The rates for these processes depend on the quadratic longitudinal response function α(2) (or, equivalently, the quadratic longitudinal susceptibility χ(2)), which describes the second-order response of a plasma to electrostatic wave fields. Previous calculations of these rates for an unmagnetized Maxwellian plasma have relied upon an approximate form for α(2) that is valid where two of the wave fields are fast (i.e., vϕ=ω/k≫Ve where ω is the angular frequency, k is the wavenumber, and Ve is the electron thermal speed) and one is slow (vϕ≪Ve). Recently, an exact expression was derived for α(2) that is valid for any phase speeds of the three waves in an unmagnetized Maxwellian plasma. Here, this exact α(2) is applied to the calculation of the three-dimensional rates for electrostatic decay and scattering off thermal ions, and the resulting exact rates are compared with the approximate rates. The calculations are performed using previously derived three-dimensional rates for electrostatic decay given in terms of a general α(2), and newly derived three-dimensional rates for scattering off thermal ions; the scattering rate is derived assuming a Maxwellian ion distribution, and both rates are derived assuming arc distributions for the wave spectra. For most space plasma conditions, the approximate rate is found to be accurate to better than 20%; however, for sufficiently low Langmuir phase speeds (vϕ/Ve≈3) appropriate to some spatial domains of the foreshock regions of planetary bow shocks and type II solar radio bursts, the use of the exact rate may be necessary for accurate calculations. The relative rates of electrostatic decay and scattering off thermal ions are calculated for a range of parameters using the exact expressions for the rates; electrostatic decay is found to have the larger growth rate over the whole range of parameters, consistent with previous approximate calculations.

Plasmon polariton deceleration in graphene structures

Surface plasmon-polariton waves with low-phase speed in carbon nanostructures can be utilized for the generation of coherent terahertz radiation through the Cerenkov mechanism, the effect being especially pronounced in bilayer and multilayer graphene. Using the many-body formalism and the tight-binding approach, we derived the dispersion equations of the surface plasmon-polariton waves in graphene. In single-layer graphene, the phase speed is about three to five times smaller than the speed of light in a vacuum. In bilayer graphene, inter-layer electron tunneling suppresses the reduction of the phase speed. Reduction of the phase speed by as much as 300 times is possible in a graphene structure with two spatially expanded monolayers, because inter-layer tunneling is suppressed, and the interlayer distance can be used to tune the plasmon frequency and the phase speed.

On surface-plasmon-polariton waves guided by the interface of a metal and a rugate filter with a sinusoidal refractive-index profile Part II: high-phase-speed solutions

In continuation of a previous paper [J. Opt. Soc. Am. B27, 2218 (2010)], by solving a canonical boundary-value problem we found that the planar interface of a metal and a rugate filter can guide surface-plasmon-polariton (SPP) waves with phase speeds greater than the speed of light in free space. These SPP waves can be either p or s polarized, and can have extremely high phase speeds, but are very loosely bound to the metal/rugate-filter interface. In two practically implementable configurations with both partnering materials of finite thickness, both p- and s-polarized SPP waves with high phase speeds can be excited, provided that the thickness of the rugate filter is much larger than needed for SPP waves with low phase speeds.

A Mechanism for the Effect of Tropospheric Jet Structure on the Annular Mode–Like Response to Stratospheric Forcing

Abstract For many climate forcings the dominant response of the extratropical circulation is a latitudinal shift of the tropospheric midlatitude jets. The magnitude of this response appears to depend on climatological jet latitude in general circulation models (GCMs): lower-latitude jets exhibit a larger shift. The reason for this latitude dependence is investigated for a particular forcing, heating of the equatorial stratosphere, which shifts the jet poleward. Spinup ensembles with a simplified GCM are used to examine the evolution of the response for five different jet structures. These differ in the latitude of the eddy-driven jet but have similar subtropical zonal winds. It is found that lower-latitude jets exhibit a larger response due to stronger tropospheric eddy–mean flow feedbacks. A dominant feedback responsible for enhancing the poleward shift is an enhanced equatorward refraction of the eddies, resulting in an increased momentum flux, poleward of the low-latitude critical line. The sensitivity of feedback strength to jet structure is associated with differences in the coherence of this behavior across the spectrum of eddy phase speeds. In the configurations used, the higher-latitude jets have a wider range of critical latitude locations. This reduces the coherence of the momentum flux anomalies associated with different phase speeds, with low phase speeds opposing the effect of high phase speeds. This suggests that, for a given subtropical zonal wind strength, the latitude of the eddy-driven jet affects the feedback through its influence on the width of the region of westerly winds and the range of critical latitudes on the equatorward flank of the jet.

Dynamics of the Lower Stratospheric Circulation Response to ENSO

Abstract A robust feature of the observed response to El Niño–Southern Oscillation (ENSO) is an altered circulation in the lower stratosphere. When sea surface temperatures (SSTs) in the tropical Pacific are warmer there is enhanced upwelling and cooling in the tropical lower stratosphere and downwelling and warming in the midlatitudes, while the opposite is true of cooler SSTs. The midlatitude lower stratospheric response to ENSO is larger in the Southern Hemisphere (SH) than in the Northern Hemisphere (NH). In this study the dynamical version of the Canadian Middle Atmosphere Model (CMAM) is used to simulate 25 realizations of the atmospheric response to the 1982/83 El Niño and the 1973/74 La Niña. This version of CMAM is a comprehensive high-top general circulation model that does not include interactive chemistry. The observed lower stratospheric response to ENSO is well reproduced by the simulations, allowing them to be used to investigate the mechanisms involved. Both the observed and simulated responses maximize in December–March and so this study focuses on understanding the mechanisms involved in that season. The response in tropical upwelling is predominantly driven by anomalous transient synoptic-scale wave drag in the SH subtropical lower stratosphere, which is also responsible for the compensating SH midlatitude response. This altered wave drag stems from an altered upward flux of wave activity from the troposphere into the lower stratosphere between 20° and 40°S. The altered flux of wave activity can be divided into two distinct components. In the Pacific, the acceleration of the zonal wind in the subtropics from the warmer tropical SSTs results in a region between the midlatitude and subtropical jets where there is an enhanced source of low phase speed eddies. At other longitudes, an equatorward shift of the midlatitude jet from the extratropical tropospheric response to El Niño results in an enhanced source of waves of higher phase speeds in the subtropics. The altered resolved wave drag is only apparent in the SH and the difference between the two hemispheres can be related to the difference in the climatological jet structures in this season and the projection of the wind anomalies associated with ENSO onto those structures.

Excitation of nonlinear electrostatic waves with phase velocity close to the ion-thermal speed

Recent hybrid Vlasov–Maxwell simulations predicted the presence of a significant level of electrostatic activity, consisting of ion-acoustic waves and of a new branch of waves (called ion-bulk (IBk) waves), at short spatial scalelengths in the tail of the solar-wind turbulent cascade along the direction of the ambient magnetic field. These waves, driven by particle trapping processes, have phase speed comparable to the ion-thermal speed and acoustic dispersion. In this paper, the hybrid Vlasov–Maxwell code redesigned in the electrostatic configuration is used to reproduce the excitation of the IBk waves. An external driver electric field is applied to the plasma ions in order to create a population of trapped particles. This inhibits Landau damping that would otherwise suppress these low phase speed oscillations. The aim of this work is to show that the IBk waves, like ordinary linear modes, can be excited by general type drivers (even of low amplitude) applied to the plasma. Finally, Vlasov–Poisson simulations, with kinetic ions and Boltzmann electrons, are used to point out the limitations of the quasi-neutrality assumption, on which the hybrid model is based.

Extremely low frequency electromagnetic energy in the air

The present paper reveals that the air contains electromagnetic energy of extremely low frequency, low amplitude as well as of a low phase speed. The energy is of great interest because of its impact on certain biological processes. It is created by the interaction of two well-known phenomena. The rotation of the earth generates 24 h periods currents in the magnetosphere, known as the Birkeland currents. The currents generate transverse electromagnetic waves (EM waves) propagating parallel to the geomagnetic field lines. Furthermore, the air and the earth crust contain electrons caused by the global electric circuit. The electric field vectors of the EM waves exert a force on these electrons, causing them to oscillate and thus generate currents of extremely low frequency both in the air and in the earth crust. A theoretical model of the system is presented and measurement techniques are described. Measurements have been performed during a six year period. The results of the performed measurements verified the theoretical model. Impact on biological processes is discussed.

Modeling columnar thin films as platforms for surface–plasmonic–polaritonic optical sensing

Via exploitation of surface plasmon polaritons (SPPs), columnar thin films (CTFs) are attractive potential platforms for optical sensing as their relative permittivity dyadic and porosity can be tailored to order. Nanoscale model parameters of a CTF were determined from its measured relative permittivity dyadic, after inverting the Bruggeman homogenization formalism. These model parameters were then used to determine the relative permittivity dyadic of a fluid-infiltrated CTF. Two boundary-value problems were next solved: the first relating to SPP-wave propagation guided by the planar interface of a semi-infinitely thick metal and a semi-infinitely thick CTF, and the second to the plane-wave response of the planar interface of a finitely thick metallic layer and a CTF in a modified Kretschmann configuration. Numerical studies revealed that SPP waves propagate at a lower phase speed and with a shorter propagation length, if the fluid has a larger refractive index. Furthermore, the angle of incidence required to excite an SPP wave in a modified Kretschmann configuration increases as the refractive index of the fluid increases.

Estimation of Gravity Wave Momentum Flux and Phase Speeds from Quasi-Lagrangian Stratospheric Balloon Flights. Part I: Theory and Simulations

A methodology for estimating gravity wave characteristics from quasi-Lagrangian observations provided by long-duration, superpressure balloon flights in the stratosphere is reviewed. Wavelet analysis techniques are used to detect gravity wave packets in observations of pressure, temperature, and horizontal velocity. An emphasis is put on the estimation of gravity wave momentum fluxes and intrinsic phase speeds, which are generally poorly known on global scales in the atmosphere. The methodology is validated using Monte Carlo simulations of time series that mimic the balloon measurements, including the uncertainties associated with each of the meteorological parameters. While the azimuths of the wave propagation direction are accurately retrieved, the momentum fluxes are generally slightly underestimated, especially when wave packets overlap in the time–frequency domain, or for short-period waves. A proxy is derived to estimate by how much momentum fluxes are reduced by the analysis. Retrievals of intrinsic phase speeds are less accurate, especially for low phase speed waves. A companion paper (Part II) implements the methodology to observations gathered during the Vorcore campaign that took place in Antarctica between September 2005 and February 2006.

A numerical study of upward‐propagating gravity waves in two different MJO phases

Convectively generated atmospheric gravity waves that propagate into the equatorial stratosphere were investigated using a cloud resolving model. Numerical simulations were conducted to study wave generation during convectively inactive and active phases in a cycle of the Madden‐Julian oscillation, which occurred during an observational campaign conducted in Sumatra. Greater convective activity during the active phase caused stronger disturbances in the troposphere at relatively low phase speeds less than about 20m/s than the inactive phase. However, the contrast was weaker at higher phase speeds. Furthermore, the vertical structure of convective forcing in the inactive phase was more suitable to excite waves with wavelength longer than twice the depth of convection. Owing to the combination of these factors, upward gravity wave propagation was significantly enhanced during the inactive phase. This study demonstrates the necessity to study convection spectrally to investigate possible links between intraseasonal variabilities in the troposphere and the middle atmosphere.