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We analyse a tropical cyclone simulated for a realistic ocean-eddy field using the global, nonhydrostatic, fully coupled atmosphere–ocean ICOsahedral Nonhydrostatic (ICON) model. After intensifying rapidly, the tropical cyclone decays following its interaction with a cold wake and subsequently reintensifies as it encounters a subsurface, warm-core eddy. To understand the change in the azimuthal-mean structure and intensity of the tropical cyclone, we invoke a conceptual framework, which recognises the importance of both boundary-layer dynamics and air–sea interactions. Crucially, the framework recognises that the change in the mean radius of updraught at the boundary-layer top is regulated by the expanding outer tangential wind field through boundary-layer dynamics. The decrease in the average equivalent potential temperature of the boundary-layer updraught during the early decay phase is related to an increase in the mean radius of the updraught rather than air–sea interactions. However, later in the decay phase, air–sea interactions contribute to the decrease, which is accompanied by a decrease in the vertical mass flux in the eyewall updraught and, ultimately, a more pronounced spin-down of the tropical cyclone. Air–sea interactions are also important during reintensification, where the tendencies are reversed, that is, the mean radius of the boundary-layer updraught decreases along with an increase in its average equivalent potential temperature and vertical mass flux. The importance of boundary-layer dynamics to the change in the azimuthal-mean structure is underscored by the ability of a steady-state slab boundary-layer model to predict an increasing and, to a lesser extent, decreasing radius of forced ascent for periods of decay and reintensification, respectively. Finally, our simulation highlights the importance of the ocean-eddy field for tropical cyclone intensity forecasts, since the simulated warm-core eddy does not display any sea-surface temperature (SST) signal until it is encountered by the tropical cyclone. © 2021 The Authors. Quarterly Journal of the Royal Meteorological Society published by John Wiley Sons Ltd on behalf of the Royal Meteorological Society.

An idealized, three‐dimensional, numerical simulation of tropical cyclone evolution in a quiescent environment on an f‐plane is used to explore aspects of the cyclone's life cycle in the context of the rotating‐convection paradigm. In the 20‐day simulation, the vortex undergoes a life cycle including a gestation period culminating in genesis, a rapid intensification phase, a mature phase, a transient decay and re‐intensification phase, a second mature phase and a rapid decay phase. During much of the life cycle, the flow evolution is highly asymmetric, although important aspects of it can be understood within an azimuthally averaged framework, central to which are a boundary‐layer control mechanism and a new ventilation diagnostic. The boundary‐layer control mechanism provides an explanation for the gradual expansion of the inner core of the vortex. The ventilation diagnostic characterizes the ability of deep convection within a given radius to evacuate the mass of air ascending out of the boundary layer within that radius. The transient decay and re‐intensification phase is not associated with an eyewall replacement cycle, but rather with a hitherto undescribed process in which the eyewall becomes fragmented as a rainband complex forms beyond it. This process is interpreted as an interplay between the boundary layer and ventilation. The final rapid decay of the vortex results from the ever increasing difficulty of deep convection to ventilate the air exiting the boundary layer. Any unventilated air flows radially outwards in the lower troposphere and leads to spin‐down because of the approximate conservation of mean absolute angular momentum. If found in real cyclones, such transience or final decay might be erroneously attributed to ambient vertical wind shear. The results support the hypothesis that, even in a quiescent environment, isolated tropical cyclone vortices are intrinsically transient and never reach a globally steady state., A three‐dimensional, idealized numerical simulation of tropical cyclone evolution on an f‐plane is used to explore aspects of the cyclone's life cycle in the framework of the rotating‐convection paradigm. In the simulation, which lasts for 20 days, the vortex undergoes a life cycle that includes a gestation period cultimating in genesis, a rapid intensification period, a mature stage followed by a transient decay and re‐intensification stage, a second mature stage and a final rapid decay stage. The results support the hypothesis that, even in a quiescent environment on an f‐plane, isolated tropical cyclone vortices are intrinsically transient and never reach a globally steady state., U.S. Office of Naval Research http://dx.doi.org/10.13039/100000006, German Research Council

17 USC 105 interim-entered record; under review. The article of record as published may be found at http://dx.doi.org/10.1002/qj.4012 Motivated by prior research examining the myth of inertial stability as a radial restoring force in the tropical cyclone boundary layer, we explore factors deter mining the vertical velocity at the top of the linear vortex boundary layer. Possible applications of these findings to mature tropical cyclone vortices are discussed briefly. NSF ONR AGS-1313948 and IAA-1656075 N0001417WX 00336, U.S. Naval Postgraduate School

Tropical cyclone formation and evolution at, or near, the Equator is explored using idealized three‐dimensional model simulations, starting from a prescribed, initial, weak counterclockwise rotating vortex in an otherwise quiescent, nonrotating environment. Three simulations are carried out in which the maximum tangential wind speed (5 m surn:x-wiley:qj:media:qj3701:qj3701-math-0001) is specified at an initial radius of 50, 100, or 150 km. After a period of gestation lasting between 30 and 60 hr, the vortices intensify rapidly, the evolution being similar to that for vortices away from the Equator. In particular, the larger the initial vortex size, the longer the gestation period, the larger the maximum intensity attained, and the longer the vortex lifetime. Beyond a few days, the vortices decay as the cyclonic vorticity source provided by the initial vortex is depleted and negative vorticity surrounding the vortex core is drawn inwards by the convectively driven overturning circulation. In these negative vorticity regions, the flow is inertially/centrifugally unstable. The vortex evolution during the mature and decay phases differs from that in simulations away from the Equator, where inertially unstable regions are much more limited in area. Vortex decay in the simulations appears to be related intimately to the development of inertial instability, which is accompanied by an outward‐propagating band of deep convection. The degree to which this band of deep convection is realistic is unknown.

We revisit the theoretical possibility of long-term, sustained tropical cyclone solutions using a state-of-the-art numerical model that incorporates the most recent observational guidance for subgrid scale parameters and airsea exchange coefficients of heat and momentum. Emphasis is placed on the realism of such solutions and the sources of cyclonic relative angular momentum (RAM) that are necessary to replenish that lost by friction at the surface. For simplicity, we confine our attention to strictly axisymmetric numerical experiments.We are able to replicate Hakim’s long-term simulation of a quasi-steady state cyclone in a 1500 km radial domain. The structure of the wind field is found to be somewhat realistic compared to observations, but sustained by unrealistic processes. Artificial sources of cyclonic RAM are quantified and the lateral damping of the anticyclonic wind near the outer boundary is found to make the largest contribution to the source of cyclonic RAM. When the domain size is extended to 9,000 km radius and lateral damping is removed altogether, a quasi-steady vortex emerges, but the structure of this vortex has many unrealistic features. In this solution, the remaining upper-level Rayleigh damping contributes a major portion of the needed source of cyclonic RAM. In a simulation in which the upper-level damping is removed also, the solution is found to be neither quasi-steady nor realistic.These findings call into question the realism of long-term, sustained tropical cyclone simulations, which require a sufficiently large source of cyclonic RAM to facilitate the existence of a quasi-steady state. Keywords: tropical cyclones, steady-state, angular momentum

The article of record as published may be found at http://dx.doi.org/10.1146/annurev-fluid-010816-060022 This article reviews progress in understanding the fluid dynamics and moist thermodynamics of tropical cyclone vortices. The focus is on the dynamics and moist thermodynamics of vortex intensification and structure. We discuss previous ideas on many facets of the subject and articulate also some open questions. The advances reviewed herein provide new insight and tools for interpreting complex vortex-convective phenomenology in simulated and observed tropical cyclones. NSF Office of Naval Research Global German Research Council US Naval Postgraduate School NASA NOAA HFIP SM30-23 AGS-1313948 N0017315WR00048 NNG11PK021 N62909-15-1-N021

The key physical processes responsible for inner-core structural changes and associated fluctuations in the intensification rate for a recent, high-impact western North Pacific tropical cyclone that underwent rapid intensification [Nepartak (2016)] are investigated using a set of convection-permitting ensemble simulations. Fluctuations in the inner-core structure between ringlike and monopole states develop in 60% of simulations. A tangential momentum budget analysis of a single fluctuation reveals that during the ringlike phase, the tangential wind generally intensifies, whereas during the monopole phase, the tangential wind remains mostly constant. In both phases, the mean advection terms spin up the tangential wind in the boundary layer, whereas the eddy advection terms deepen the storm’s cyclonic circulation by spinning up the tangential wind between 1.5 and 4 km. Calculations of the azimuthally averaged, radially integrated vertical mass flux suggest that periods of near-constant tangential wind tendency are accompanied by a weaker eyewall updraft, which is unable to evacuate all the mass converging in the boundary layer. Composite analyses calculated from 18 simulations produce qualitatively similar results to those from the single case, a finding that is also in agreement with some previous observational and modeling studies. Above the boundary layer, the integrated contribution of the eddy term to the tangential wind tendency is over 80% of the contribution from the mean term, irrespective of inner-core structure. Our results strongly indicate that to fully understand the storm’s three-dimensional evolution, the contribution of the eddies must be quantified.

We revisit the linear boundary-layer approximation that expresses a generalized Ekman balance and use it to clarify a range of interpretations in the previous literature on the tropical cyclone boundary layer. Some of these interpretations relate to the reasons for inflow in the boundary layer and others relate to the presumed effects of inertial stability on boundary-layer dynamics. Inertial stability has been invoked, for example, to explain aspects of boundary-layer behaviour, including the frontogenetic nature of the boundary layer and its relationship to vortex spin-up. Our analysis exposes the fallacy of invoking inertial stability as a resistance to radial inflow in the boundary layer. The analysis shows also that the nonlinear acceleration terms become comparable to the linear Coriolis acceleration terms in relatively narrow vortices that are inertially stable above the boundary layer. Estimates of the nonlinear accelerations using the linear solutions are expected to underestimate the actual contribution in a nonlinear boundary-layer model, cautioning against neglecting the nonlinear terms in diagnostic or prognostic models.

The evolution of an idealized tropical-cyclone-like vortex forced by a prescribed distribution of diabatic heating rate and near-surface frictional force is studied using a recently developed, prognostic, axisymmetric balance model. Starting with a prescribed tangential wind field, a series of calculations is carried out in which the strength of the diabatic heating rate is varied while keeping the formulation for the frictional force the same. When the strength of the heating rate falls below a certain value, the secondary circulation it generates is no longer able to oppose the tendency of the boundary layer to produce a layer of outflow in the lower troposphere, above the boundary layer. In these circumstances, this net outflow advects absolute momentum surfaces outwards above the boundary layer and the tangential velocity there declines. In the balance model, and presumably in reality, intensification of the tangential velocity requires the diabatic heating rate to be strong enough to ventilate the inflow in the frictional boundary layer, thereby averting frictionally induced outflow just above the boundary layer. In reality, the strength and location of diabatic heating will be strongly influenced by the boundary-layer dynamics and thermodynamics, a coupling not present in the current model formulation.

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.

Composites of graphite foam infiltrated with a magnesium chloride phase-change material have been developed as high-temperature thermal energy storage media for concentrated solar power applications. This storage medium provides a high thermal energy storage density, a narrow operating temperature range, and excellent heat transfer characteristics. In this study, experimental investigations were conducted on laboratory-scale prototypes with magnesium chloride/graphite foam composite as the latent heat thermal energy storage system. Prototypes were designed and built to monitor the melt front movement during the charging/discharging tests. A test loop was built to ensure the charging/discharging of the prototypes at temperatures >700 °C. Repeated thermal cycling experiments were carried out on the fabricated prototypes, and the experimental temperature profiles were compared to the predicted results from numerical simulations using COMSOL Multiphysics software. Experimental results were found to be in good agreement with the simulations to validate the thermal models.

Three high-resolution numerical simulations are carried out to investigate the effects of initial vortex size on tropical cyclogenesis and intensification, starting with a relatively weak initial vortex with a maximum tangential wind speed of 5 m s(-1), but located at different radii. In all simulations, there is a progressive organization of convectively induced, cyclonic relative vorticity into a monopole structure. As the initial vortex size is increased, the organization occurs later. In addition, the size of the vorticity monopole, the sizes of the inner and outer core tangential wind circulations and the lifetime intensity of the vortex all become larger. An explanation for this behaviour is offered in terms of a boundary-layer control of the overturning circulation. The findings are consistent with those of previous studies that bypass the genesis stage and start with appreciably stronger initial vortices.

A recent idealized, high-resolution, numerical model simulation of tropical cyclogenesis is compared with a simulation in which the surface drag is set to zero. It is shown that, while spin up occurs in both simulations, the vortex in the one without surface drag takes over twice as long to reach its intensification begin time. When surface friction is not included, the inner core size of the simulated vortex is considerably larger and the subsequent vortex intensity is significantly weaker than in the case with friction. In the absence of surface drag, the convection eventually develops without any systematic organization and lies often outside the radius of azimuthally-averaged maximum tangential winds. The results underscore the crucial role of friction in organizing deep convection in the inner core of the nascent vortex and raise the possibility that the timing of tropical cyclogenesis in numerical models may have an important dependence on the boundary layer parameterization scheme used in the model.

Analyses of 1-min data from automatic weather stations (AWSs) in the Gulf of Carpentaria region are used to study southerly nocturnal bores over northeastern Australia. The data cover the time period 1 August to 31 December in the three years 2012-2014. A total of 12 days on which a bore passed over the region of interest were found. Ten of the disturbances occurred between August and mid-October in the presence of synoptic-scale ridging across the continent behind a frontal trough. The other two occurred in early November in the presence of localized ridging behind an inland trough separate from a frontal trough. Six of the bores were followed some hours later by a clear air mass change. Four of the 12 cases are examined in detail wherein interpretations of the AWS data are aided by analysis fields from the European Centre for Medium-range Weather Forecasts (ECMWF). The results indicate that the longevity of the bore is related to the time of day that the cold front interacts with the stable maritime layer to the north of it. The ECMWF analyses are able to capture the frontal troughs as well as the timing of their passage through the AWS network. In the analyses, the troughs are seen as narrow strips of elevated cyclonic vorticity at 950 mb. Vertical cross-sections intersecting the troughs showed structural characteristics of a shallow cold front, but the horizontal resolution of the analyses is inadequate to capture the generation of bores ahead of the fronts.

We use a high resolution numerical simulation of Atlantic Hurricane Earl (2010) to increase our understanding of Earl's intensification in relatively strong vertical shear in the context of a recent paradigm for tropical cyclone intensification. The integrity of the simulation is judged by comparing analyses thereof with those of the unprecedented observational data gathered in Earl. Consistent with the classical view of spin up, the amplification of the tangential wind field above the boundary layer is found to occur as the absolute angular momentumsurfaces are drawn inwards by the aggregate heating of the rotating convective clouds in the interior of the vortex. In addition to this classical pathway, spin up occurs within the inner-core boundary layer, where the maximum tangential winds occur. The latter is another element of the new paradigm. Despite the detrimental influence of the shear on the vortex alignment and on depressing the pseudo-equivalent potential temperature outside the developing eyewall, the combined eddy processes associated with the vortical plume structures in and around the developing eyewall region are shown to contribute to an enhanced overturning circulation and an intensifying storm. These eddy processes are distinctly agradient effects that are not features of the classical spin up mechanism. It remains to be understood how the rotating convective updraughts combine to produce the diagnosed structures of the eddy terms, themselves, and how vortex Rossby waves and other eddies contribute to the alignment of the vortex during intensification.

The article of record as published may be found at https://doi.org/10.1175/JAS-D-19-0051.1 We seek to understand the mechanism of vortex spinup in Emanuel’s 2012 axisymmetric theory for tropical cyclone intensification in physical coordinates, starting from first principles. It is noted that, while spinup of the maximum tangential wind must occur at low levels, within or at the top of the friction layer, this spinup is unconstrained by a radial momentum equation in this layer. Instead, the spinup is controlled by a parameterization of turbulent mixing in the upper-tropospheric outflow layer, which, as is shown, determines indirectly the rate of inward movement of the absolute angular momentum surfaces. Nevertheless, the physics of how upper-tropospheric mixing leads to spinup in or at the top of the friction layer are unclear and, as discussed, may be irrelevant to spinup in the model.

The consequences of regularizing the Sawyer–Eliassen equation to calculate the stream function for the axisymmetric secondary circulation of a tropical cyclone are explored. Regularization is an ad hoc procedure in which the coefficients of the equation are suitably modified to replace negative values of the discriminant by small positive values, thereby ensuring that the equation is globally elliptic. The consequences of the procedure may be understood in terms of the analogue behaviour of a stretched membrane subject to a particular force distribution. Several regularization procedures are assessed by comparing the azimuthally averaged radial flow from a three‐dimensional numerical simulation of a tropical cyclone with that from an axisymmetric balance calculation of the Sawyer–Eliassen equation, forced by diabatic and frictional terms diagnosed from the simulation. The comparison shows that the largest challenge for regularization occurs in regions of inertial instability, especially when the diagnosed forcing overlaps with such regions. In the example shown, the diagnosed balanced flow is sensitive to the particular regularization procedure and none of the procedures examined give a flow that is structurally and quantitatively close to that obtained from the numerical solution in and near the region of regularization. The flow in regions of large vertical shear that are common in the lower part of the boundary layer is less sensitive to the regularization procedure, even though such regions are ones in which there is (frictional) forcing. Nevertheless, there are comparatively large differences between the low‐level inflow in the azimuthally averaged numerical solution and the axisymmetric balance solution. These differences can be attributed to the intrinsic lack of balance in the boundary layer. This finding, together with the issues associated with regularization, is further confirmation that balance dynamics is unable to adequately capture the flow in the boundary layer, contrary to recent claims.