Most atmospheric scientists are familiar with the type of inertia-gravity wave that is trapped along coastlines or the equator, known as a Kelvin wave (e.g., Thomson 1879; Gill 1982, chapter 10; Wang 2003). These waves play a prominent role in tropical dynamics including El Niño–Southern Oscillation (e.g., Battisti 1988). Perhaps less familiar within the atmospheric science community is the work by Lord Kelvin on vortex waves (Thomson 1880), in which he laid out the general strategy for obtaining a wide variety of solutions of waves traveling within infinitely extended, concentrated cylindrical vortices resembling the flow in tornadoes or dust devils some distance away from the lower boundary. These waves are usually referred to as “Kelvin waves” in the fluid physics literature, and they are also known as vortex waves, centrifugal waves, or inertial waves (Lugt 1989). Such waves play an important role in the vortex breakdown phenomenon as well as in the development of multiple-vortex tornadoes (e.g., Lewellen 1993; Rotunno 2013). Examples of these waves within tornadoes are shown in Fig. 1. Here so-called bending waves (to be introduced in section 3) are visible, which are often well pronounced during the rope stage of the tornado (Fig. 1b). The effects of vortex waves are also seen in Fig. 2, where Fig. 2a shows a multivortex structure that results from unstable growth of spiral modes (section 3), and Fig. 2b displays what appears to be a vortex breakdown. This phenomenon often occurs in vortices with nonzero axial velocity and it will be reviewed in section 8.
The need for this review arises from the observation that, despite the ubiquity of Kelvin vortex waves in tornado-like vortices, a detailed introduction to the structure and behavior of these waves appears to be absent from the atmospheric science literature. Although this topic is introduced in some fluid physics texts (e.g., Thomson 1880; Lamb 1932, section 158; Chandrasekhar 1961, chapter VII, especially p. 284; Drazin and Reid 1981, p. 75; Saffman 1992, chapters 11 and 12; Lim and Redekopp 1998; Fritts et al. 1998; Rossi 2000; Batchelor 2002, p. 559; Alekseenko et al. 2007, chapter 4), in a given presentation either only limited, or rather advanced, aspects of vortex waves are discussed. As a consequence, there is a large gap between the knowledge base offered in mainstream texts about atmospheric dynamics (e.g., Gill 1982; Holton and Hakim 2013; Markowski and Richardson 2010) and the advanced peer-reviewed literature on the remarkably rich Kelvin vortex wave dynamics. Thus, the present paper seeks to
narrow the gap between the atmospheric-dynamics literature and the advanced fluid-physics literature by providing a detailed introduction to these waves. Except where the mathematical steps are readily available in mainstream texts, the complete mathematical development is included in this paper;
retrace Kelvin’s original approach, and point to possible applications of his contributions to tornado-like flows, including multiple-vortex development and vortex breakdown.
To achieve this, an almost trivial generalization of Kelvin’s equations is included by allowing for a piecewise constant axial flow, which admits a wide class of unstable solutions not considered in Kelvin’s original work. These solutions have been arrived at previously, but along with a review of these topics, it is demonstrated that the results follow from Kelvin’s approach. In the context of stability analyses, Kelvin’s (slightly modified) approach presented in this review has been superseded by much more advanced analysis techniques, but these are still based on analyses such as those presented here, and will be touched upon in section 8.
The remainder of the paper is structured as follows. In the next section a brief history of vortex waves will be offered and the relevance of these waves will be described, and section 3 offers an overview of the classification of vortex waves. In section 4 the governing equations describing infinitely extended columnar vortices will be presented following Kelvin’s approach. Subsequently, consecutively more refined scenarios are introduced that follow directly from Kelvin’s equations. Starting with the scenario of vanishing base-state axial flow, a cylindrical domain bounded by rigid walls will be considered (section 5) to gain an intuition for the structure and dynamics of these waves. Thereafter the Rankine vortex will be discussed in section 6, and two scenarios that allow for unstable wave growth will be considered in section 7, i.e., a Rankine vortex with upward motion in its core as well as a two-celled vortex with descending motion in its irrotational core, and rising motion outside of it. The unstable waves in the latter scenario provide a rudimentary model for multiple-vortex formation in tornado-like vortices. These ideas will then be applied to vortex breakdown in section 8. Limitations of Kelvin’s approach and its applicability to tornadoes will be addressed in section 9. Finally, section 10 offers concluding remarks and possible directions for future investigations.
2. A brief history of Kelvin vortex waves
Around the mid-1800s, Sir William Thomson, who was granted the title Lord Kelvin in 1892, pursued the idea of describing the previously discovered atoms in terms of microscopic knotted vortex rings, called “vortex atoms” (Thomson 1867; Fabre et al. 2006). The medium in which these vortex rings were thought to exist was the hypothesized all-pervading, homogeneous perfect fluid known as the aether. This idea was fueled by Hermann von Helmholtz’s discovery of the laws of vortex motion (von Helmholtz 1858), specifically his second law, which implies that these vortex rings could persist forever in perfect homogeneous (and thus barotropic) fluids. Falconer (2019) offers a detailed summary of Kelvin’s vortex atom theory. Kelvin was particularly interested in the vibrational modes of these vortex rings, postulating that different vibrational modes could account for the different atomic spectra that had previously been discovered. As a first step, Kelvin formulated the equations describing wave motions within an infinitely long, cylindrical vortex. This effort led to the paper entitled “Vibrations of a columnar vortex” (Thomson 1880), which is the basis of this review. Although the idea of vortex atoms did not survive past the 1890s, it does bear an intriguing resemblance to string theory, and Kelvin vortex waves still do play an important role in fundamental physics, e.g., in the dynamics of quantum vortices in superfluids (e.g., Fonda et al. 2014).
Kelvin’s work entered the field of aeronautical engineering following the discovery that lift-generating devices produce a pair of trailing vortices, which pose a hazard to aircraft that encounter these vortices, which consequently reduce airspace capacity (Hallock and Holzäpfel 2018). The stability of the wake vortices is directly related to the unstable growth of bending Kelvin modes (the different Kelvin modes will be introduced in the next section). This instability is known as “cooperative instability” and is sometimes visibly manifest as a contortion of aircraft contrails and the formation of contrail lobes (Lewellen and Lewellen 2001; Wu et al. 2006, p. 499; Schultz and Hancock 2016).
In the late 1950s, another phenomenon was discovered by aerodynamicists, i.e., the vortex breakdown. Peckham and Atkinson (1957) are generally credited for first documenting this phenomenon during the investigation of lift-generating vortices produced at the leading edge of ogival delta wings. At large angles of attack, the observation included a disintegration of the vortex structure, which appeared to “bell out before disappearing—as though the core was becoming more diffuse” (Peckham and Atkinson 1957, p. 5), indicating turbulence and an undesirable drop of lift. (The reader may skip ahead to Fig. 31 for an example of such a breakdown in a tornado.)1 On the other hand, vortex breakdown was found to have beneficial effects in some applications such as stabilizing the flame in combustion chambers of gas turbines (e.g., Escudier 1988; Spencer and McGuirk 2008). Vortex breakdown has thus garnered considerable interest in the engineering and fluid physics community, and soon after its official discovery, Squire (1960) and Benjamin (1962) explained the vortex breakdown phenomenon as an axisymmetric analog of the hydraulic jump, which is related to the inability of wave perturbations, in this case axisymmetric Kelvin vortex waves, to propagate upstream (Squire 1960; Benjamin 1962). Among other vortex profiles, these studies also considered the Rankine vortex, and the vortex breakdown criterion discovered in these studies directly follows from Kelvin’s analysis. Vortex breakdown and its relation to Kelvin vortex waves will be reviewed in section 8.
Aside from the application to engineering problems, the roles of Kelvin vortex waves have been studied in the general context of transition to turbulence in rotating flows (e.g., Hopfinger et al. 1982). Importantly, these experiments revealed the existence of solitary-like vortex waves (Maxworthy et al. 1985). In the atmospheric science literature, Kelvin’s work does not appear to be mentioned frequently. One exception is Fultz (1959), who studied oscillations in convective motion. He observed vortex waves in his rotating tank experiments and pointed to a textbook by Bjerknes et al. (1933), where vortex waves are introduced. Apparently, however, Bjerknes et al. (1933) had been unaware of Kelvin’s treatment, prompting Fultz to note that “its significance in meteorological dynamics had to be rediscovered.” Kelvin vortex waves feature prominently in the studies by Andreassen et al. (1998), Fritts et al. (1998) and Fritts and Alexander (2003), which are concerned with turbulence generation as a result of shear instability as well as in breaking internal gravity waves (Fritts and Alexander 2003). These authors refer to vortex waves as “twist waves” (Arendt et al. 1997). Kieu (2016) extended Kelvin’s solutions with the goal of describing waves in the inner core region of tropical cyclones.
In the field of tornado research, centrifugal waves attracted some attention in the 1970s and 1980s when research using tornado vortex chambers flourished (Ward 1972; Church et al. 1977; Rotunno 1979; Snow 1982; Church and Snow 1993). However, the focus of these analyses was the determination of how the flow parameters, most notably the swirl ratio (Davies-Jones 1973), led to different tornado structures including vortex breakdown and multiple vortices. The importance of centrifugal waves is mentioned in these studies in the context of vortex breakdown (Church et al. 1977; Rotunno 1979; Snow 1982; Fiedler and Rotunno 1986), but the structure and dynamics of these waves are not analyzed further. Nolan and Farrell (1999) analyzed these waves in detail, and they observed downward propagating axisymmetric waves in their numerically simulated tornado-like vortex. Their analysis suggests that these waves are less likely to be observed in flows with low swirl ratios, consistent with such flows being supercritical (section 8), but their linear analysis did not fully explain the behavior of these waves. More recently, Nolan (2012) revisited this phenomenon by studying the linear instability of fully nonlinear flows.
Because the vortex chamber experiments reproduced the observed transition of a single tornado vortex into multiple vortices, investigations into the stability of the tornado followed (e.g., Rotunno 1978; Walko and Gall 1984; Nolan 2012). This instability is a manifestation of unstable centrifugal waves, and will be discussed in section 7. With the exception of Nolan’s analyses (Nolan and Farrell 1999; Nolan 2012), these centrifugal waves apparently have not played important roles in interpreting tornadic flows since the 1980s, and Snow (1982) at the end of his review on tornado dynamics, added that “features that should be investigated…include the nature of the wavelike features often seen on the walls of condensation funnels” (Snow 1982, p. 963). It is fair to say that his recommendation has not materialized, and consideration of centrifugal waves in the context of tornadoes appears to be scarce. Aside from Nolan and Farrell (1999), some examples include Lugt (1989) and Trapp (2000), who discuss vortex breakdown in tornadoes and mesocyclones, respectively, and in this context briefly mention centrifugal waves; Bluestein et al. (2003) observed oscillations of a tornado’s intensity and as a possible explanation, centrifugal waves were invoked; in their textbook, Markowski and Richardson (2010) introduce the stability criterion for centrifugal oscillations, which correspond to an axisymmetric Kelvin mode.
This brief overview reveals that, while the relevance of vortex waves in tornadoes is generally acknowledged, the association with Kelvin’s work is not widely appreciated, and the structure and behavior of these waves has attracted somewhat limited attention in the atmospheric science community. To start the discussion of vortex wave dynamics, the basic types and naming conventions of these waves are introduced in the next section. The main distinction between the different wave types is the azimuthal wavenumber m, which determines whether a wave is manifest as, e.g., axisymmetric or spiral perturbation. The azimuthal wavenumber is dimensionless and must be an integer to guarantee continuity of the wave perturbation. The vortex column serves as waveguide, and for infinitely extended vortex columns, each wave type (determined by m) may generally attain an arbitrary axial wavenumber k.
3. Classification of Kelvin vortex waves
a. Axisymmetric modes, m = 0
If m = 0, the perturbation is axisymmetric, resulting in a widening and narrowing of the vortex (e.g., in a Rankine vortex the radius of maximum winds extends and contracts). The axial wavenumber must be nonzero for axisymmetric waves to exist. A commonly used name of this mode goes back to Lord Rayleigh (Strutt 1902, p. 444). When discussing photographs of a liquid jet undergoing axisymmetric oscillations before breaking up into drops (as happens to a water stream emanating from a faucet), he noted: “…I have often been embarrassed for want of an appropriate word to describe the condition in question. But a few days ago, during a biological discussion, I found that there is a recognised, if not very pleasant, word. The cylindrical jet may be said to become varicose, and varicosity goes on increasing with time, until eventually it leads to absolute disruption.” Chandrasekhar (1961, p. 515), after citing this passage from Rayleigh, adds: “In recent times, ‘sausage instability’ has been used to describe the same condition; but this is also not a very ‘pleasant’ description, and varicose instability would seem preferable.” The designations “sausage” or “sausaging” as well as “varicose” modes have been adopted widely to describe axisymmetric vortex waves, though in this paper these waves will be referred to mainly as axisymmetric waves. This wave can only propagate in the axial direction and it plays an important role in the onset of vortex breakdown (section 8).
b. Spiral modes, |m| > 0
Waves with azimuthal wavenumbers |m| ≥ 1 are referred to as helical or spiral modes. If |m| = 1, the waves displace the vortex axis and are called “bending” modes. For all other choices of m the vortex axis remains centered. An example of how an azimuthal wave perturbation is related to the vortex structure is shown in Fig. 3, demonstrating that a wavenumber of two corresponds to an elliptic deformation of the vortex core. The sign of the azimuthal wavenumber determines the handedness of the spiral. As shown in Fig. 4, m > 0 implies left-handedness and m < 0 implies right-handedness.2
If the slope of the phase lines in the (θ, z) plane wrapping around the cylindrical vortex is taken to be dh/dθ, where h is the local height of the phase line and θ is the azimuthal angle, this slope is identical to the pitch of the wave, given by −m/k (Fabre et al. 2006). Defined this way, pitch is proportional to the distance by which the helix advances during one revolution (i.e., to the axial wavelength of the helix). Saffman (1992) and Alekseenko et al. (2007) use slightly different definitions of pitch, which only apply to |m| = 1 but are proportional to each other as well as to the definition used here. It follows that, e.g., left-handed spirals have a negative pitch and axisymmetric waves have zero pitch. For a given axial (vertical) wavenumber, the pitch becomes larger in magnitude as the number of spirals winding around the vortex (i.e., m) is increased.
An azimuthal wavenumber of m = 2 implies a double helix or double spiral structure. Waves with k = 0 and m ≠ 0 are sometimes called “fluted” modes (Maxworthy 1988), which are 2D waves that only propagate in azimuthal direction. Hopfinger et al. (1982), Hopfinger and Browand (1982), and Maxworthy et al. (1985) coined the term “kink wave” for helical solitary waves (m = ±1) they observed in their experiments. These waves resembled the soliton solution by Hasimoto (1972).3
The azimuthal angular phase speed of spiral modes is given by ω/m, where ω is the wave frequency. Assuming that the angular velocity of the vortex Ω is positive, implying cyclonic rotation, then if the azimuthal phase speed is larger than the angular velocity of the vortex, the waves are said to be “cograde,” meaning these waves propagate downstream relative to the motion within the vortex. If the angular speed of the wave is less than Ω, but the waves still move in the same sense as the azimuthal vortex flow, the wave is said to be “retrogade.” These waves propagate upstream relative to the vortex flow, but are advected downstream. Finally, the waves are “countergrade” if they move in the opposite sense than the vortex, now also for a stationary observer (Fabre et al. 2006). From these basic definitions and conventions the rich structure of vortex waves is already becoming apparent. The next section introduces the governing equations describing the dynamics of these waves.
4. Governing equations
One goal of this paper is to reintroduce Kelvin’s original mathematical treatment, which is summarized in Fig. 5. Starting with the linearized, inviscid, incompressible equations of motion in cylindrical coordinates, the existence of a base state flow is assumed, and the equations are linearized about this base state. Subsequently, a normal-mode solution is inserted, and the resulting simplified equation set is solved for the radial and azimuthal flow components. One then specifies the base state flow for a given vortex configuration, uses the mass continuity equation, and arrives at a linear ordinary differential equation (ODE) whose solution governs the radial wave structure. Application of the boundary conditions finally leads to the complete flow field as well as the dispersion relation, from which the wave frequencies and speeds, as well as the existence and growth rates of unstable modes, may be inferred.
a. Linearization of the momentum and mass-continuity equations
b. Equations for the radial and azimuthal velocity
c. Conditions at the free boundary
1) Kinematic boundary condition
2) Dynamic boundary condition
5. Solid-body rotation in a bounded domain
a. General solution and boundary condition
b. Axisymmetric modes in a bounded domain
Restoring force and propagation mechanism of the axisymmetric mode
To understand why the phase speed depends on the axial wavenumber, consider the variation of the width of the vortex (e.g., as measured by the radial displacement of a concentric material surface relative to the base state), which serves as a measure of vertical vorticity. The narrower the vortex, the larger the vertical vorticity due to angular-momentum conservation. As shown in Fig. 9, which depicts an upward propagating wave, the regions of horizontal convergence near the vortex axis are shifted upward by a quarter wavelength relative to the axial vorticity maxima, implying that the vorticity extrema are propagating upward via vortex stretching and compression (see also Shapiro 2001a; Fabre et al. 2006). The regions of horizontal divergence and convergence are associated with meridional vorticity, which arises from tilting of the axial vorticity into and out of the meridional plane by the axial gradients of azimuthal velocity. The periodic vertical motion induced by the meridional vorticity stretches/compresses the axial vorticity. This is associated with a twisting and untwisting of the vortex lines (Melander and Hussain 1994; Arendt et al. 1997). The longer the wave, the larger are the pressure perturbations that accompany the vertical vorticity extrema [see, e.g., Markowski and Richardson (2010, p. 27) for the relationship between vorticity and pressure] or alternatively, the larger the induced velocity magnitudes (e.g., Dahl 2020) associated with the meridional vorticity extrema. As a consequence, as the vertical wavelength increases there are stronger vertical pressure-gradient accelerations and w′ gradients, resulting in more vigorous stretching/compression of vertical vorticity and hence faster wave motion.
c. Spiral modes in a bounded domain
Propagation mechanism of the spiral mode
To gain an understanding of the propagation of these modes, the velocity and the pressure fields of the fundamental spiral mode (m = 1) are shown in Fig. 13. The fields are displayed on an unrolled cylindrical surface, i.e., in the (θ, z) plane and for a small axial wavenumber. Like in the axisymmetric case, positive axial vorticity perturbations are associated with a negative pressure perturbation, and negative axial vorticity perturbations are associated with a positive pressure perturbation, so p′ ~ −ζ′, where ζ′ is the axial vorticity perturbation (Fabre et al. 2006). It is apparent that there is axial stretching a quarter wavelength above the perturbation vorticity maxima for the cograde (upward propagating) mode (Fig. 13a). For the retrograde mode, (Fig. 13b), the maximum stretching occurs below the perturbation vorticity maxima. Similarly to the axisymmetric mode, these waves thus propagate due to stretching and compression of axial perturbation vorticity.
To summarize this section, each of the azimuthal modes (i.e., axisymmetric and spiral modes) have an infinite number of radial modes, which are characterized by an increasing number of circulation cells in the radial direction with increasing order of the radial mode. The perturbation flow of the fundamental mode has only one circulation cell in radial direction. Moreover, it was highlighted that the longwave limit of the axisymmetric fundamental mode moves the fastest. A better approximation of a tornado-like flow is a vortex not bounded by cylindrical walls, which will be considered next.
6. Rankine vortex
a. General solution and boundary conditions
For the Rankine vortex, one combines the solution for solid-body rotation of the previous section with an irrotational outer region. As mentioned in section 4c, the inner solution is designated by the suffix “1” and the outer solution is designated by the suffix “2.” For now, it is assumed that W1 = W2 = W, such that g1 = g2.