Search

The article of record as published may be found at https://doi.org/10.1175/JAS-D-17-0179.1 This paper revisits the evolution of an idealized tropical cyclone–like vortex forced by a prescribed distribution of diabatic heating in the context of both inviscid and frictional axisymmetric balance dynamics. Prognostic solutions are presented for a range of heating distributions, which, in most cases, are allowed to contract as the vortex contracts and intensifies. Interest is focused on the kinematic structure and evolution of the secondary circulation in physical space and on the development of regions of sym- metric and static instability. The solutions are prolonged beyond the onset of unstable regions by regularizing the Sawyer–Eliassen equation in these regions, but for reasons discussed, the model ultimately breaks down. The intensification rate of the vortex is essentially constant up to the time when regions of instability ensue. This result is in contrast to previous suggestions that the rate should increase as the vortex intensifies because the heating becomes progressively more "efficient" when the local inertial stability increases. The solutions provide a context for reexamining the classical axisymmetric paradigm for tropical cyclone intensification in the light of another widely invoked intensification paradigm by Emanuel, which postulates that the air in the eyewall flows upward and outward along sloping absolute angular momentum (M) surfaces after it exits the frictional boundary layer. The conundrum is that the classical mechanism for spinup requires the air above the boundary layer to move inward while materially conserving M. Insight provided by the balance solutions helps to refine ideas for resolving this conundrum. Office of Naval Research German Research Council National science Foundation NOAA HFIP NASA U.S. Naval Postgraduate School MTM acknowledges the sup- port of NSF Grant AGS-1313948, NOAA HFIP Grant N0017315WR00048, NASA Grant NNG11PK021, ONR Grant N0001417WX00336, and the U.S. Naval Postgraduate School.

The long term behaviour of tropical cyclones in the prototype problem for cyclone intensification on an f -plane is examined using a nonhydrostatic, three-dimensional, numerical model. After reaching a mature intensity, the model storms progressively decay while both the inner core size, characterized by the radius of the eyewall, and size of the outer circulation, measured for example by the radius of gale force winds, progressively increase. This behaviour is explained in terms of a boundary-layer control mechanism in which the expansion of the swirling wind in the lower troposphere leads through boundary-layer dynamics to an increase in the radii of forced eyewall ascent as well as to a reduction in the maximum tangential wind speed in the layer. These changes are accompanied by changes in the radial and vertical distribution of diabatic heating. As long as the aggregate effects of inner-core convection, characterized by the distribution of diabatic heating are able to draw absolute angular momentum surfaces inwards, the outer circulation will continue to expand. The quantitative effects of latitude on the foregoing processes are investigated also. The study provides new insight on the factors controlling the evolution of size and intensity of a tropical cyclone. It provides also a plausible, and arguably simpler, explanation for the expansion of the inner-core of Hurricane Isabel (2003) than that given previously. German Research Council National Science Foundation NASA Naval Postgraduate School NOAA Grant no. SM30-23 (GRC) Grant no. N62909-15-1-N021 (NSF) AGS-1313948 (NSF) HFIP Grant no. N0017315WR00048 (NOAA) NNG11PK021 (NASA)

The authors review an emerging paradigm of tropical cyclone intensification in the context of the prototype intensification problem, which relates to the spinup of a preexisting vortex near tropical storm strength in a quiescent environment. In addition, the authors review briefly what is known about tropical cyclone intensification in the presence of vertical wind shear. The authors go on to examine two recent lines of research that seem to offer very different views to understanding the intensification problem. The first of these proposes a mechanism to explain rapid intensification in terms of surface pressure falls in association with upper-level warming accompanying outbreaks of deep convection. The second line of research explores the relationship between the contraction of the radius of maximum tangential wind and intensification in the classical axisymmetric convective ring model, albeit in an unbalanced framework. The authors challenge a finding of the second line of research that appears to cast doubt on a recently suggested mechanism for the spinup of maximum tangential wind speed in the boundary layer—a feature seen in observations. In doing so, the authors recommend some minimum requirements for a satisfactory explanation of tropical cyclone intensification.

In a recent paper, Lee and Chen (2012, hereafter LC12) presented numerical simulations of symmetric and asymmetric hurricane boundary layer structures in a fully coupled atmosphere wave ocean model and used these simulations to compare aspects of the boundary layer structure against an analysis of observations. One of their main conclusions was that the azimuthally averaged inflow layer tends to misrepresent the overall inflow structure in tropical cyclones, especially the asymmetric structure (p. 3593). Another main conclusion was that the complicated asymmetric three-dimensional boundary layer structures (attributed by them to be) due in part to the air sea and wind wave coupling make it difficult to parameterize the atmosphere wave ocean coupling effects without a fully coupled model (p. 3593). After careful examination of their study, we have a number of questions regarding their methodology, their interpretations (including their interpretations of previous literature), and their conclusions. Specifically, we inquire about aspects of the methodology for defining the dynamical boundary layer depth, the selection of the boundary layer scheme, and we question the conclusions inferred. In addition to the foregoing concerns, inaccuracies in their literature review are noted and inconsistencies between their conclusions and reported results are identified. For many decades, physical processes across the air sea interface and within the atmospheric boundary layer have been known to be essential for the development and maintenance of a tropical cyclone (Ooyama 1969; Emanuel 1986, 1995; Smith et al. 2009; Smith and Montgomery 2010; Bryan and Rotunno 2009; Bryan 2012). However, the boundary layer is the least-observed part of a storm in particular, its turbulence structure. With the advent of the global positioning system (GPS) dropsonde (Hock and Franklin 1999), the mean boundary layer structure has been progressively studied.

The interaction between a tropical cyclone and the ocean is investigated using a minimal three-dimensional tropical cyclone model coupled with a two-layer ocean model. Two representations for entrainment into the ocean mixed layer are compared: one based on the assumption that the velocity scale for entrainment is the surface friction velocity, the other on the assumption that this scale is the magnitude of the mean velocity difference across the base of the mixed layer. It is shown that the magnitude and distribution of the ocean cooling depends strongly on the method for representing entrainment velocity. The method based on the surface friction velocity is more effective in reducing the heat flux from the ocean to the storm in the inner-core region and leads to a greater reduction of the tropical cyclone intensity. With ocean coupling, the surface heat flux is reduced in the inner core, mainly in the rear-right quadrant relative to the track, which is toward the northwest. As a result, the pot...

The minimal three-dimensional tropical cyclone model developed by Zhu et al. is used to explore the role of shallow convection, precipitation-cooled downdrafts, and the vertical transport of momentum by deep convection on the dynamics of tropical cyclone intensification. The model is formulated in σ coordinates and has three vertical levels, one characterizing a shallow boundary layer, and the other two representing the upper and lower troposphere, respectively. It has three options for treating cumulus convection on the subgrid scale and a simple scheme for the explicit release of latent heat on the grid scale. In the model, as in reality, shallow convection transports air with low moist static energy from the lower troposphere to the boundary layer, stabilizing the atmosphere not only to itself, but also to deep convection. Also it moistens and cools the lower troposphere. For realistic parameter values, the stabilization in the vortex core region is the primary effect: it reduces the deep conv...

A minimal 3D numerical model designed for basic studies of tropical cyclone behavior is described. The model is formulated in s coordinates on an f or b plane and has three vertical levels, one characterizing a shallow boundary layer and the other two representing the upper and lower troposphere, respectively. It has three options for treating cumulus convection on the subgrid scale and a simple scheme for the explicit release of latent heat on the grid scale. The subgrid-scale schemes are based on the mass-flux models suggested by Arakawa and Ooyama in the late 1960s, but modified to include the effects of precipitation-cooled downdrafts. They differ from one another in the closure that determines the cloud-base mass flux. One closure is based on the assumption of boundary layer quasi-equilibrium proposed by Raymond and Emanuel. It is shown that a realistic hurricane-like vortex develops from a moderate strength initial vortex, even when the initial environment is slightly stable to deep convection. This is true for all three cumulus schemes as well as in the case where only the explicit release of latent heat is included. In all cases there is a period of gestation during which the boundary layer moisture in the inner core region increases on account of surface moisture fluxes, followed by a period of rapid deepening. Precipitation from the convection scheme dominates the explicit precipitation in the early stages of development, but this situation is reversed as the vortex matures. These findings are similar to those of Baik et al., who used the Betts‐Miller parameterization scheme in an axisymmetric model with 11 levels in the vertical. The most striking difference between the model results using different convection schemes is the length of the gestation period, whereas the maximum intensity attained is similar for the three schemes. The calculations suggest the hypothesis that the period of rapid development in tropical cyclones is accompanied by a change in the character of deep convection in the inner core region from buoyantly driven, predominantly upright convection to slantwise forced moist ascent.

Results of a field experiment carried out in 1991 to gather upper-air data on the morning-glory environment are presented. The data include daily early morning radiosonde soundings from Burketown in north Queensland, Australia, for a 28-day period during the late dry season, together with pressure, wind, temperature, and humidity data from a number of surface stations in the region. During the experiment, 16 morning glories were recorded. On all but one day, radiosonde soundings were carried out in the pre-morning-glory environment. On 7 days, additional soundings were carried out within an hour or two of the passage of a morning glory. Soundings made on days on which morning glories were generated over Cape York Peninsula but failed to reach Burketown are compared with those on days when morning glories were recorded at Burketown. The comparison shows that the depth and strength of the surface-based inversion did not differ significantly and that the stratification of the almost neutral layer above the stable layer was similar on days with and without morning glories. An examination of the wind profiles is unrevealing and leads the authors to reject the hypothesis that the trapping of wave energy is the key factor that determines the longevity of the disturbances. That the leakiness of the wave-guide is not the only factor in the ability of disturbances to cover large distances from their place of origin is consistent with a numerical study by Noonan and Smith, which suggests that the morning-glory bore-wave system is formed and maintained by mesoscale circulations associated with the sea breezes over Cape York Peninsula.

An analytical theory is presented for the motion of an initially symmetric barotropic vortex on a beta-plane at rest, the prototype problem in the theory of tropical cyclone motion. In the case of vortices with parameter values appropriate to tropical cyclones, the theory shows excellent agreement with equivalent numerical model calculations for a period of between one and two days. In particular, the vortex track and the evolution of vortex asymmetries, the so-called beta gyres, are accurately predicted. The calculations provide further insight into dynamics of tropical cyclone motion in general and provide a firmer basis for interpreting the numerical solutions in particular. They are relevant also to the important problem of designing more appropriate “bogus” vortices for the initialization of dynamically based tropical cyclone forecast models.

The dynamics of frontal evolution is examined in terms of the Australian summertime cool change using a two-dimensional numerical model. The model is synthesized from observational data on surface cold fronts obtained during the Australian Cold Fronts Research Program, and the model develops a quasi-steady surface cold front during the 24 hours of integration. The characteristics of this model are compared with those of a kinematic model; it is observed that the features of the two models correspond. The two-dimensional and kinematic models are also compared with a 24-hour prediction of the cold front of February 1983 using the three-dimensional nested-grid model of the Australian Numerical Meteorology Research Center, developed by Gauntlett et al. (1984). Good correlation between these models is detected.

Resonant interactions between a marginally unstable baroclinic wave and one or two pairs of neutral waves in a rotating, two-layer zonal Row model are studied. Analyses based on a multiple scaling technique are carried out for inviscid flow on a beta plane, inviscid flow on an f-plane, and slightly viscous flow on an f-plane. It is assumed that the neutral waves are of smaller order in amplitude than the unstable wave, and the flow domain is taken to be zonally periodic so that zonal wavelengths are discrete. In this and other respects, the theory differs from an earlier study by Loesch (1974a,b) and avoids a number of difficulties inherent in that work. For an inviscid beta plane flow, the evolution of a wave triad is governed by a certain parameter N2N3/U. If this is negative, the neutral waves are unable to grow substantially larger in amplitude than their respective initial amplitudes, despite which they have a disproportionately large effect on the evolution of the unstable wave. If O

A new perspective of the dynamics of a tropical cyclone eye is given in which eye subsidence and the adiabatic warming accompanying it are accounted for directly from the equations of motion. Subsidence is driven by an adverse, axial gradient of perturbation pressure which is associated principally with the decay and/or radial spread of the tangential wind field with height at those levels of the cyclone where the tangential winds are approximately in gradient wind balance. However, this pressure gradient is almost exactly opposed by the buoyancy force field due to adiabatic warming. This corroborates with observational data. The relationship between the present view of eye dynamics and those of Malkus and Kuo and a recent study by Willoughby is discussed in detail.

This paper contains a study of the energy conversions in Pedlosky's solution for finite-amplitude baroclinic wave development for the case of small dissipation. It is shown that the solution contains an implicit, non-physical source of mean kinetic energy owing to the neglect of a certain boundary condition on the developing mean flow. Additional features of the energetics are described which do not depend explicitly on the solution with the relevant boundary condition inserted. However, the possibility of limit-cycle solutions when this boundary condition is satisfied remains an open question.

A recent numerical study of vortex growth in a flow configuration which models the principal characteristics of a tornado cyclone (Smith and Leslie, 1978) is extended to take account of vertical stability. It is shown that for a given strength of convection and rotation (in the model, the driving effect of a ‘supercell’ updraft is simulated by an imposed body force), the intensity of the mature vortex which forms in the presence of a typical vertical gradient of potential temperature is significantly lower than that which forms in an adiabatic atmosphere. We conclude that the effects of vertical stratification on tornadogenesis may often be important and may prevent some vortices, which might otherwise do so, from establishing ground contact.

A numerical study of sea-breeze circulations over Cape York Peninsula in northern Australia is presented. Simulations using a two-dimensional version of the University of Virginia mesoscale model provide insight into the thermally induced circulations over the peninsula during the “dry” season under typical conditions of a prevailing easterly geostrophic flow. Calculations are performed for a wide southern cross section of the peninsula (400 km of land) and a narrow northern section (200 km of land), with terrain appropriate to these cross sections included. A southern section simulation without topography is discussed for comparison. Particular interest is focused on the collision of the east and west coast sea breezes. The possibility is explored that the double sea-breeze convergence may act as a trigger, not only for morning glory cloud line disturbances in the southern Gulf, but also for the so-called North Australian Cloud Line (NACL). The latter is a line of convective cloud which forms fr...

There is a tendency for certain flows in the atmosphere and oceans to concentrate into narrow streams or currents, which preserve their identity over very large distances. The ability of such flows to overcome the diffusive effects normally encountered in laboratory jet flows is usually explained in terms of the entrainment into the jet of relative vorticity, with a sign which helps to maintain the shear. A simple model which one would expect to exhibit this properly was described by Long who examined the flow of a two-dimensional, viscous jet on a beta-plane, and obtained the first term of an asymptotic series solution for the flow in a region far upstream from some origin. This term is essentially the solution of the linearized jet equations, which represent a balance between Coriolis and viscous forces with the pressure gradient. Unfortunately, Long did not calculate further terms in his expansion and his solution did not therefore contain the effects of advection (or entrainment) of fluid. In...

This paper examines air parcel trajectories in the two-dimensional model for a cold front by Reeder and Smith (1987). These are found to be in close agreement with trajectories deduced from analyses of summertime 'cool changes' in southeastern Australia, adding further support to the applicability of the numerical model to this kind of cold front. The favorable comparison points also to the dynamical consistency of the conceptual model for the cool change, which has evolved from the analysis of data from observational experiments.