Vortex Rossby Research Articles

In LIGO’s Sight? Vigorous Coherent Gravitational Waves from Cooled Collapsar Disks

We present the first numerical study of gravitational waves (GWs) from collapsar disks, using state-of-the-art 3D general relativistic magnetohydrodynamic simulations of collapsing stars. These simulations incorporate a fixed Kerr metric for the central black hole (BH) and employ simplified prescriptions for disk cooling. We find that cooled disks with an expected scale height ratio of H/R ≳ 0.1 at ∼10 gravitational radii induce Rossby instability in compact, high-density rings. The trapped Rossby vortices generate vigorous coherent emission regardless of disk magnetization and BH spin. For BH mass of ∼10 M ⊙, the GW spectrum peaks at ∼100 Hz, with some breadth due to various nonaxisymmetric modes. The spectrum shifts toward lower frequencies as the disk viscously spreads and the circularization radius of the infalling gas increases. Weaker-cooled disks with H/R ≳ 0.3 form a low-density extended structure of spiral arms, resulting in a broader, lower-amplitude spectrum. Assuming an optimistic detection threshold with a matched-filter signal-to-noise ratio of 20 and a rate similar to Type Ib/c supernovae, LIGO–Virgo–KAGRA (LVK) could detect ≲1 event annually, suggesting that GW events may already be hidden in observed data. Third-generation GW detectors could detect dozens to hundreds of collapsar disks annually, depending on the cooling strength and the disk formation rate. The GW amplitudes from collapsar disks are ≳100 times higher with a substantially greater event rate than those from core-collapse supernovae, making them potentially the most promising burst-type GW class for LVK and Cosmic Explorer. This highlights the importance of further exploration and modeling of disk-powered GWs, promising insights into collapsing star physics.

On the evidence of helico-spiralling recirculation within coherent cores of eddies using Lagrangian approach

Oceanic eddies exhibit remarkable coherence and longevity compared to other transient features in the surrounding flow. They possess the ability to transport properties over extensive distances while maintaining their material identity intact. The Lagrangian Coherent Structure (LCS) framework has proven effective in capturing these coherent eddies, where they display a solid-body-like rotation. Although various LCS approaches have been employed to investigate different facets of coherent eddies, a comprehensive understanding of their three-dimensional structures and internal dynamics remains elusive. This study aims to advance our comprehension of coherent eddies’ structural characteristics and delve into the precise nature of their internal dynamics by utilizing the Lagrangian Averaged Vorticity Deviation approach. Two eddies, one cyclonic and the other anti-cyclonic, were chosen from a high-resolution simulation carried out in the Bay of Bengal using the Regional Ocean Modeling System (ROMS). The findings unveil that these eddies have three-dimensional coherent cores resembling gently tapered cones that are broader at the surface and gradually narrow towards the bottom. Intriguingly, the dynamically coherent core of these eddies exhibits simultaneous upwelling and downwelling while maintaining their volumes during advection due to persistent material coherence. The three-dimensional trajectories followed by the fluid parcels inside the coherent core are helical. Their two-dimensional horizontal projections show alternating spiral bands of upwelling and downwelling which are the manifestations of Vortex Rossby Waves. These observations lead to a conceptual framework of a three-dimensional helico-spiralling recirculation pattern within the coherent cores of eddies.

Turbulent stress within dead zones and magnetic field dragging induced by Rossby vortices

ABSTRACT By means of three-dimensional resistive-magnetohydrodynamical models, we study the evolution of the so-called dead zones focused on the magnitude of the Reynolds and Maxwell stresses. We consider two different types of static resistivity radial profiles that give rise to an intermediate dead zone or an intermediate active zone. As we are interested in analysing the strength of angular momentum transport in these intermediate regions of the disc, we use as free parameters the radial extent of the intermediate dead (Δridz) or active (Δriact) zones, and the widths of the inner ($H_{b_1}$) and outer ($H_{b_2}$) transitions. We find that regardless of the width or radial extent of the intermediate zones, Rossby wave instability (RWI) develops at these transition boundaries, leading to the emergence of vortices and spiral waves. In the case of an intermediate dead zone when $H_{b_1}\, ,H_{b_2}\le 0.8$, the vortices are almost completely confined to the dead zone. Remarkably, we find that the formation of vortices at the inner transition can drag magnetic field lines into the dead zone stirring up the region that the vortex covers (reaching an α ≈ 10−2 value similar to that of an active zone). Vortices formed in the outer transition only modify the Reynolds stress tensor. Our results can be important to understanding angular momentum transport in poorly ionized regions within the disc due to magnetized vortices within dead zones.

Subvortices within a Numerically Simulated Tornado: The Role of Unstable Vortex Rossby Waves

Abstract Multiple subvortices corresponding to suction vortices in observations are obtained within a simulated tornado for the EF4 tornado case of Funing, China, on 23 June 2016. Within the simulation, the tornado evolves from a one-cell structure with vorticity maximum at its center to a two-cell structure with a ring of vorticity maximum. Five well-defined subvortices develop along the ring. The radial profile of tangential wind across the vorticity ring satisfies the necessary condition of barotropic instability associated with phase-locked, counterpropagating vortex Rossby waves (VRWs) along the ring edges. The phased-locked waves revolve around the parent vortex at a speed less than the maximum azimuthal-mean tangential velocity, agreeing with theoretically predicted VRW phase speed. The radii within which the wave activities are confined are also correctly predicted by the VRW theory where radial group velocity approaches zero. Several other characteristics related to the simulated subvortices agree with VRW theories also. The most unstable azimuthal wavenumber depends on the width and the relative magnitude of vorticity of the vortex ring. Their values estimated from the simulation prior to subvortex formation correctly predict wavenumber 5 as the most unstable. The largest contribution to wave kinetic energy is diagnosed to be from the radial shear of azimuthal wind term, consistent with barotropic instability. Vorticity diagnostics show that vertical vorticity stretching is the primary vorticity source for the intensification and maintenance of the simulated subvortices. Significance Statement Multiple subvortices or suction vortices in tornadoes can produce extreme damage but their cause is not well understood. An intense tornado from China that developed five strong subvortices, along a vorticity ring a distance from the tornado vortex center, was successfully simulated. By examining the propagation and other characteristics of these subvortices and comparing them with theoretical models of vortex Rossby waves (VRWs) that have been studied mostly in the context of typhoons/hurricanes, it is believed that nonlinear growth of unstable VRWs associated with barotropic instability is the primary reason for the development of subvortices within the tornado. The conclusion is further supported by analyses of the primary source of wave growth energy. Vertical vorticity stretching is the main vorticity source for intensifying and maintaining the subvortices at their development and mature stages. The unstable growth of VRWs as the cause of tornado suction vortices has not been analyzed in detail for realistic tornadoes until now.

Diagnosing Tropical Cyclone Intensity Variations from the Surface Wind Field Evolution

Abstract Tropical cyclone (TC) intensity fluctuations remain a challenge for TC forecasters. Occurring through a wide range of processes, such as vortex contraction, eyewall replacements, or emission of vortex Rossby waves, they are inherently multiscale, transient, and asymmetric. In a recent study, estimates of surface wind field inner-core properties from high-resolution satellite observations were spotted as valuable for the improvement of intensity variations statistical predictability. The present study evaluates how the temporal evolution of the vortex structure, at scales ranging from O(1) km to vortexwide, further provides insights on the modulation of intensity. The study is based on a set of seven realistic TC simulations with 1-km grid spacing. The surface wind field structure is studied through an original set of descriptors that characterize the radial profile, the azimuthal asymmetries, and their spectral distribution. While radial gradients evolve concurrently with intensity, the azimuthal variability of the inner core shows a stronger connection with shorter-scale intensity modulation. The increase of high-wavenumber asymmetries distributed around the ring of maximum winds is shown to precede phases of rapid (re)intensification by 5–6 h, while the concentration of asymmetry in wavenumbers 1 and 2 leads to intensity weakening. A machine learning classification finally highlights that the classification of intensification phases (i.e., intensification or weakening) can be improved by at least 11% (thus reaching ∼75%) when accounting for the evolution of the radial wind gradient and the variance distribution among scales in the ring of maximum wind, relative to the sole use of vortex-averaged parameters. Significance Statement The purpose of this study is to relate changes in the surface wind field structure of tropical cyclones to their intensity variations. We design an original set of parameters to characterize the inner- and near-core contraction and asymmetry, and evaluate their connection with TC intensity modulations in a set of realistic high-resolution numerical simulations. The main outcome of our study is that opposite trends in high- and low-wavenumber variance tend to occur prior to intensity changes on short time scales, with more widespread and local-scale asymmetry corresponding to intensification, and vortex-scale polarized asymmetry corresponding to weakening. These diagnoses are shown to improve the classification of intensification and weakening phases. These results advocate for enhancing real-time high-resolution observations of surface wind fields under TCs, and using asymmetry distribution as predictors in statistical forecast models.

An Observational Study of Short‐Cycle Lightning Outbreaks in the Inner Core of Typhoon Hato (2017) Before Landfall

AbstractThis study analyzes microphysical signatures relevant to the short‐cycle lightning activity in the inner core of Typhoon Hato (2017) before landfall in China. Observations reveal that the lightning bursts were accompanied by enhanced inner‐core convection with a behavior cycle of about 3 hr. The coupling between wavenumber‐2 vortex Rossby waves (VRWs) and shear‐forced convective asymmetries resulted in a local updraft enhancement. Furthermore, supercooled liquid water droplets invigorated a striking enhancement of graupel via riming immediately above the freezing level, further enhancing charge separation and lightning generation outside the eyewall. Furthermore, when similar phase‐locking was associated with other propagating VRWs, graupel and updraft volumes were significantly boosted, leading to short‐cycle lightning outbreaks in the inner core.

On Wave Interference in Planet Migration: Dead Zone Torques Modified by Active Zone Forcing

We investigate planetary migration in the dead zone of a protoplanetary disk where there is a set of spiral waves propagating inward due to the turbulence in the active zone and the Rossby wave instability, which occurs at the transition between the dead and active zones. We perform global 3D unstratified magnetohydrodynamical simulations of a gaseous disk with the FARGO3D code, using weak gradients in the static resistivity profiles that trigger the formation of a vortex at the outer edge of the dead zone. We find that once the Rossby vortex develops, spiral waves in the dead zone emerge and interact with embedded, migrating planets by wave interference, which notably changes their migration. The inward migration becomes faster depending on the mass of the planet, due mostly to the constructive (destructive) interference between the outer (inner) spiral arm of the planet and the destruction of the dynamics of the horseshoe region by means of the set of background spiral waves propagating inward. The constructive wave interference produces a more negative Lindblad differential torque, which inevitably leads to an inward migration. Lastly, for massive planets embedded in the dead zone, we find that the spiral waves can create an asymmetric, wider, and deeper gap than in the case of α-disks and can prevent the formation of vortices at the outer edge of the gap. The latter could generate a faster or slower migration compared to the standard type-II migration.

Intensity fluctuations in Hurricane Irma (2017) during a period of rapid intensification

Abstract. This study aims to understand the fluctuations observed in Hurricane Irma (2017), which change the tangential wind speed and the size of the radius of maximum surface wind and therefore affect short-term destructive potential. Intensity fluctuations observed during a period of rapid intensification of Hurricane Irma between 4 and 6 September 2017 are investigated in a detailed modelling study using an ensemble of Met Office Unified Model (MetUM) convection-permitting forecasts. Although weakening and strengthening phases were defined using 10 m wind, structural changes in the storm were observed through the lower troposphere, with the most substantial changes just above the boundary layer (at around 1500 m). Isolated regions of rotating deep convection, coupled with outward propagating vortex Rossby waves, develop during the strengthening phases. Although these isolated convective structures initially contribute to the increase in azimuthally averaged tangential wind through positive radial eddy vorticity fluxes, the continued outward expansion of convection eventually leads to a negative radial eddy vorticity flux, which halts the strengthening of the tangential wind above the boundary layer at the start of the weakening phase. The outward expansion of the azimuthally averaged convection also enhances the outflow above the boundary layer in the eyewall region, as the convection is no longer strong enough to ventilate the mass inflow from the boundary layer in a process similar to one described in a recent idealised study.

Trapping (sub-)Neptunes similar to TOI-216b at the inner disk rim

Context. The occurrence rate of observed sub-Neptunes has a break at 0.1 au, which is often attributed to a migration trap at the inner rim of protoplanetary disks where a positive co-rotation torque prevents inward migration. Aims. We argue that conditions in inner disk regions are such that sub-Neptunes are likely to open gaps, lose the support of the co-rotation torque as their co-rotation regions become depleted, and the trapping efficiency then becomes uncertain. We study what it takes to trap such gap-opening planets at the inner disk rim. Methods. We performed 2D locally isothermal and non-isothermal hydrodynamic simulations of planet migration. A viscosity transition was introduced in the disk to (i) create a density drop and (ii) mimic the viscosity increase as the planet migrated from a dead zone towards a region with active magneto-rotational instability (MRI). We chose TOI-216b as a Neptune-like upper-limit test case, but we also explored different planetary masses, both on fixed and evolving orbits. Results. For planet-to-star mass ratios q ≃ (4–8) × 10−5, the density drop at the disk rim becomes reshaped due to a gap opening and is often replaced with a small density bump centred on the planet's co-rotation. Trapping is possible only if the bump retains enough gas mass and if the co-rotation region becomes azimuthally asymmetric, with an island of librating streamlines that accumulate a gas overdensity ahead of the planet. The overdensity exerts a positive torque that can counteract the negative torque of spiral arms. Under suitable conditions, the overdensity turns into a Rossby vortex. In our model, efficient trapping depends on the a viscosity and its contrast across the viscosity transition. In order to trap TOI-216b, αDZ = 10−3 in the dead zone requires αMRI ≳ 5 × 10−2 in the MRI-active zone. If αDZ = 5 × 10−4, αMRI ≳ 7.5 × 10−2 is needed. Conclusions. We describe a new regime of a migration trap relevant for massive (sub-)Neptunes that puts valuable constraints on the levels of turbulent stress in the inner part of their natal disks.

The Diabatic Rossby Vortex: Growth Rate, Length Scale, and the Wave–Vortex Transition

Abstract In idealized simulations of moist baroclinic instability on a sphere, the most unstable mode transitions from a periodic wave to an isolated vortex in sufficiently warm climates. The vortex mode is maintained through latent heating and shows the principal characteristics of a diabatic Rossby vortex (DRV) that has been found in a range of different simulations and observations of the current climate. Currently, there is no analytical theory for DRVs or understanding of the wave–vortex transition that has been found in warmer climates. Here, we introduce a minimal moist two-layer quasigeostrophic model with tilted boundaries capable of producing a DRV mode, and we derive growth rates and length scales for this DRV mode. In the limit of a convectively neutral stratification, the length scale of ascent of the DRV is the same as that of a periodic moist baroclinic wave, but the growth rate of the DRV is 54% faster. We explain the isolated structure of the DRV using a simple potential vorticity (PV) argument, and we create a phase diagram for when the most unstable solution is a periodic wave versus a DRV, with the DRV emerging when the moist static stability and meridional PV gradients are weak. Last, we compare the structure of the DRV mode with DRV storms found in reanalysis and with a DRV storm in a warm-climate simulation. Significance Statement Past research has identified a special class of midlatitude storm, dubbed the diabatic Rossby vortex (DRV), which derives its energy from the release of latent heat associated with condensation of water vapor and as such goes beyond the traditional understanding of midlatitude storm formation. DRVs have been implicated in extreme and poorly predicted forms of cyclogenesis along the east coast of the United States and the west coast of Europe with significant damage to property and human life. The purpose of this study is to develop a mathematical theory for the intensification rate and length scale of DRVs to gain a deeper understanding of the dynamics of these storms in current and future climates.

Determining Tropical Cyclone Center and Rainband Size in Geostationary Satellite Imagery

Brightness temperature (TB) observations at an infrared channel (10.3 μm) of the Advanced Baseline Imager (ABI) on board the U. S. 16th Geostationary Operational Environmental Satellite (GOES-16) are used for determining tropical cyclone (TC) center positions and rainband sizes. Firstly, an azimuthal spectral analysis method is employed to obtain an azimuthally symmetric center of a TC. Then, inner and outer rainbands radii, denoted as RIR and ROR, respectively, are estimated based on radial gradients of TB observations at different azimuthal angles. The radius RIR describes the size of the TC inner-core region, and the radius ROR reflects the maximum radial extent of TC rainbands. Compared with the best track centers, the root mean square differences of ABI-determined centers for tropical storms and hurricanes, which totals 108 samples, are 45.35 and 29.06 km, respectively. The larger the average wavenumber-0 amplitude, the smaller the difference between the ABI-determined center and the best track center. The TB-determined RIR is close but not identical to the radius of the outermost closed isobar and usually coincides with the radius where the strongest wavenumber 1 asymmetry is located. The annulus defined by the two circles with radii of ROR and RIR is the asymmetric area of rainbands described by azimuthal wavenumbers 1–3. In general, amplitudes of wavenumber 0 component centered on the ABI-determined center are greater than or equal to those from the best track. For a case of a 60 km distance between the ABI-determined and the best track TC center, the innermost azimuthal waves of wavenumbers 1–3 are nicely distributed along or within the radial distance RIR that is determined based on the ABI-determined TC center. If RIR is determined based on the best track, the azimuthal waves of wavenumbers 1–3 are found at several radial distances that are smaller than RIR. The TC center positions, and rainband size radii are important for many applications, including specification of a bogus vortex for hurricane initialization and verification of propagation mechanism of vortex Rossby waves.

Microphysical Characteristics of the Phase‐Locking VRW‐Induced Asymmetric Convection in the Outer Eyewall of Super Typhoon Lekima (2019)

AbstractMicrophysical signatures relevant to the asymmetric convection in the outer eyewall of Super Typhoon Lekima (2019) before its landfall in China were analyzed from ground‐based radar observations. The results indicate that the microphysical characteristics varied in quadrants. In the upshear‐left outer eyewall, the convection was deepened on the inner edge due to the phase locking between vortex Rossby waves (VRWs), and warm rain was the predominant process contributing to the enhancement of rainfall. In contrast, the strongest precipitation occurred on the outer edge of the upshear‐right outer eyewall. In this region, exuberant riming processes and graupel formation prevailed above the melting layer due to the strongest and more outward tilted updrafts induced by the phase locking between the VRWs. A good deal of graupel melted into raindrops, along with significant accretion processes, produced intense rainfall with larger drop sizes.

On the weakening of northward propagation of intraseasonal oscillations during positive Indian Ocean Dipole events

The northward propagation of intraseasonal oscillations (ISO) is one of the major modes of variability in the tropics during boreal summers, associated with active and break spells of monsoon rainfall over the Indian region. These northward propagations modulate the Indian summer monsoon rainfall. The northward march starts close to the equator over warm waters of the Indian Ocean and continues till the foothills of the Himalayas. In this study, we investigate the influence of Indian Ocean Dipole (IOD) on northward propagations. We show that northward propagations tend to be weaker during positive Indian Ocean Dipole (pIOD) events. The “moisture mode” framework is used to understand the processes responsible for the weakening of northward propagations during pIOD years. Our analyses show that moistening caused by the horizontal advection is the major contributor for the northward propagations during negative IOD (nIOD) years, and its amplitude is much smaller during pIOD years. Further analyses revealed that the reduction in the advection of the background entropy/moisture by zonal wind perturbations during pIOD is primarily responsible for the reduction in the horizontal advection. The mean structure of entropy between 925 and 500 hpa levels remained similar over most of the Asian monsoon region across the contrasting IOD years, and the reason for weaker northward propagations can be attributed to the weaker zonal wind perturbations at intraseasonal timescales. These weaker zonal wind perturbations during ISO events in pIOD years results from weak Rossby Vortex lobes. The weakening of Rossby wave response owing to cooler than average sea surface temperatures in the South-East Equatorial Indian Ocean and warmer than average West Equatorial Indian Ocean is proposed to be the possible reason for the weakening of northward propagations during pIOD years.

Synthesis of Vortex Rossby Waves. Part III: Rossby Waveguides, Vortex Motion, and the Analogy with Midlatitude Cyclones

AbstractVortex Rossby waves (VRWs) affect tropical cyclones’ (TCs’) motion, structure, and intensity. They propagate within annular waveguides defined by a passband between Ω1D, the Doppler-shifted frequency of a one-dimensional VRW, and zero. Wavenumber-1 VRWs cause TC motion directly and have wider waveguides than wavenumbers ≥ 2. VRWs forced with fixed rotation frequency propagate away from the forcing. Initially outward-propagating waves are Doppler shifted to zero at a critical radius, where they are absorbed. Initially inward-propagating waves are Doppler shifted to Ω1D, reflect from a turning point, propagate outward, and are ultimately absorbed at the critical radius. Between the forcing and the turning radii, the VRWs have standing-wave structure; outward from the forcing they are trailing spirals. They carry angular momentum fluxes that act to accelerate the mean flow at the forcing radius and decelerate it at the critical radius. Mean-flow vorticity monopoles are inconsistent with Stokes’s theorem on a spherical Earth, because a contour enclosing the monopole’s antipode would have nonzero circulation but would enclose zero vorticity. The Rossby waveguide paradigm also fits synoptic-scale Rossby waves in a meridionally sheared zonal flow. These waves propagate within a waveguide confined between a poleward turning latitude and an equatorward critical latitude. Forced waves are comma-shaped gyres that resemble observed frontal cyclones, with trailing filaments equatorward of the forcing latitude and standing waves poleward. Even neutral forced Rossby waves converge westerly momentum at the latitude of forcing.

Roles of Barotropic Instability across the Moat in Inner Eyewall Decay and Outer Eyewall Intensification: Essential Dynamics

AbstractIntense tropical cyclones (TCs) often experience secondary eyewall formations and the ensuing eyewall replacement cycles. Better understanding of the underlying dynamics is crucial to make improvements to the TC intensity and structure forecasting. Radar imagery of some double-eyewall TCs and a real-case simulation study indicated that the barotropic instability (BI) across the moat (aka type-2 BI) may play a role in inner eyewall decay. A three-dimensional numerical study accompanying this paper pointed out that type-2 BI is able to withdraw the inner eyewall absolute angular momentum (AAM) and increase the outer eyewall AAM through the eddy radial transport of eddy AAM. This paper explores the reason why the eddy radial transport of eddy AAM is intrinsically nonzero. Linear and nonlinear shallow water experiments are performed and they produce expected evolutions under type-2 BI. It will be shown that only nonlinear experiments have changes in AAM over the inner and outer eyewalls, and the changes solely originate from the eddy radial transport of eddy AAM. This result highlights the importance of nonlinearity of type-2 BI. Based on the distribution of vorticity perturbations and the balanced-waves arguments, it will be demonstrated that the nonzero eddy radial transport of eddy AAM is an essential outcome from the intrinsic interaction between the mutually growing vortex Rossby waves across the moat under type-2 BI. The analyses of the most unstable mode support the findings and will further attribute the inner eyewall decay and outer eyewall intensification to the divergence and convergence of the eddy angular momentum flux, respectively.

Representation by two climate models of the dynamical and diabatic processes involved in the development of an explosively deepening cyclone during NAWDEX

Abstract. The dynamical and microphysical properties of a well-observed cyclone from the North Atlantic Waveguide and Downstream Impact Experiment (NAWDEX), called the Stalactite cyclone and corresponding to intensive observation period 6, is examined using two atmospheric components (ARPEGE-Climat 6.3 and LMDZ6A) of the global climate models CNRM-CM6-1 and IPSL-CM6A, respectively. The hindcasts are performed in “weather forecast mode”, run at approximately 150–200 km (low resolution, LR) and approximately 50 km (high resolution, HR) grid spacings, and initialised during the initiation stage of the cyclone. Cyclogenesis results from the merging of two relative vorticity maxima at low levels: one associated with a diabatic Rossby vortex (DRV) and the other initiated by baroclinic interaction with a pre-existing upper-level potential vorticity (PV) cut-off. All hindcasts produce (to some extent) a DRV. However, the second vorticity maximum is almost absent in LR hindcasts because of an underestimated upper-level PV cut-off. The evolution of the cyclone is examined via the quasi-geostrophic ω equation which separates the diabatic heating component from the dynamical one. In contrast to some previous studies, there is no change in the relative importance of diabatic heating with increased resolution. The analysis shows that LMDZ6A produces stronger diabatic heating compared to ARPEGE-Climat 6.3. Hindcasts initialised during the mature stage of the cyclone are compared with airborne remote-sensing measurements. There is an underestimation of the ice water content in the model compared to the one retrieved from radar-lidar measurements. Consistent with the increased heating rate in LMDZ6A compared to ARPEGE-Climat 6.3, the sum of liquid and ice water contents is higher in LMDZ6A than ARPEGE-Climat 6.3 and, in that sense, LMDZ6A is closer to the observations. However, LMDZ6A strongly overestimates the fraction of super-cooled liquid compared to the observations by a factor of approximately 50.

Observed Near-Surface Wind Structure in the Inner Core of Typhoon Goni (2015)

AbstractThe very strong Typhoon Goni passed over the Yaeyama Islands in southwestern Japan during the rapid intensification stage on August 23, 2015. Surface data collected by the dense network of weather stations as well as Doppler radar observations over the islands revealed a finescale structure in the inner core of the typhoon near the surface.Goni had a clear eye surrounded by a square-shaped eyewall with intense convection. The surface observations revealed that several vortices with a diameter of ~7–10 km accompanied by a pressure deficit were present inside the eye. From the Doppler velocity field, mesovortices approximately 10 km in diameter were found at the apexes of the square-shaped eyewall. These mesovortices and the inner rainbands emanating outward from the apexes of the polygonal eyewall generally exhibited features typical of vortex Rossby waves. The mesovortices were accompanied by a pressure deficit at the surface and enhanced surface winds. The data also indicated the first observational evidence of near-surface mixing between the eye and eyewall through the mesovortices, that is, the transport of high equivalent potential temperature in the eye toward the eyewall.The radar data revealed that many radar-reflectivity filaments that had a pleated shape with lengths of a few kilometers extended perpendicularly from the inner edge of the eyewall at low levels. The filaments associated with wind perturbations at low levels caused significant wind gusts accompanied by sudden pressure drops and shifts in wind direction at the surface.

Quasi‐Periodic Intensification of Convective Asymmetries in the Outer Eyewall of Typhoon Lekima (2019)

AbstractTyphoon Lekima (2019) possessed a double‐eyewall structure before making landfall in eastern China, with its outer eyewall showing quasi‐periodic convective intensification. Several physical mechanisms that may cause eyewall convection asymmetries were examined. The upshear occurrence of the strongest convection could not be explained by either the typhoon's motion or the effect of descending inflow from outer rainbands. Radar reflectivity analysis showed that phase locking occurred between the wavenumber‐2 vortex Rossby waves (VRWs) propagating radially outward from the inner eyewall and the azimuthally propagating wavenumber‐1 VRWs on the inner edge of the outer eyewall. Additional phase locking further arose between the aforementioned wavenumber‐1 VRWs and the azimuthally propagating wavenumber‐2 VRWs on the outer edge of the outer eyewall. These two phase‐locking processes led to the pronounced quasi‐periodic intensification of the convective asymmetry in the western semicircle of Lekima's outer eyewall.

Dynamical signatures of Rossby vortices in cavity-hosting disks

Context. Planets are formed amidst young circumstellar disks of gas and dust. The latter is traced by thermal radiation, where strong asymmetric clumps have been observed in a handful of cases. These dust traps could be key to understanding the early stages of planet formation, when solids grow from micron-size to planetesimals. Aims. Vortices are among the few known asymmetric dust trapping scenarios. The present work aims to predict their characteristics in a complementary observable. Namely, line-of-sight velocities are well suited to trace the presence of a vortex. Moreover, the dynamics of disks is subject to recent developments. Methods. Two-dimensional hydro simulations were performed in which a vortex forms at the edge of a gas-depleted region. We derived idealized line-of-sight velocity maps, varying disk temperature and orientation relative to the observer. The signal of interest, as a small perturbation to the dominant axisymmetric component in velocity, may be isolated in observational data using a proxy for the dominant quasi-Keplerian velocity. We propose that the velocity curve on the observational major axis be such a proxy. Results. Applying our method to the disk around HD 142527 as a study case, we predict that line-of-sight velocities are barely detectable by currently available facilities, depending on disk temperature. We show that corresponding spirals patterns can also be detected with similar spectral resolutions, which will help to test against alternative explanations.

Polygonal Eyewall Asymmetries During the Rapid Intensification of Hurricane Michael (2018)

AbstractPolygonal eyewall asymmetries of Hurricane Michael (2018) during rapid intensification (RI) are analyzed from ground‐based single Doppler radar. Here, we present the first observational evidence of the evolving wind field of a polygonal eyewall during RI to Category 5 intensity by deducing the axisymmetric and asymmetric winds at 5‐min intervals. Spectral time decomposition of the retrieved tangential wind structure shows quantitative evidence of low (1–4) azimuthal wavenumbers with propagation speeds that are consistent with linear wave theory on a radial vorticity gradient, suggesting the presence of rapidly evolving vortex Rossby waves. Dual‐Doppler winds from the NOAA P‐3 Hurricane Hunter airborne radar provide further evidence of the three‐dimensional vortex structure that supports growth of asymmetries during RI. Both reflectivity and tangential wind fields show polygonal structure and propagate at similar speeds, suggesting a close coupling of the dynamics and the convective organization during the intensification.