A series of numerical simulations of tropical cyclones in idealized large-scale environments is performed to examine the effects of vertical wind shear on the structure and intensity of hurricanes. The simulations are performed using the nonhydrostatic Pennsylvania State University‐National Center for Atmospheric Research fifth-generation Mesoscale Model using a 5-km fine mesh and fully explicit representation of moist processes. When large-scale vertical shears are applied to mature tropical cyclones, the storms quickly develop wavenumber one asymmetries with upward motion and rainfall concentrated on the left side of the shear vector looking downshear, in agreement with earlier studies. The asymmetries develop due to the storm’s response to imbalances caused by the shear. The storms in shear weaken with time and eventually reach an approximate steady-state intensity that is well below their theoretical maximum potential intensity. As expected, the magnitude of the weakening increases with increasing shear. All of the storms experience time lags between the imposition of the large-scale shear and the resulting rise in the minimum central pressure. While the lag is at most a few hours when the storm is placed in very strong (15 m s21) shear, storms in weaker shears experience much longer lag times, with th e5ms 21 shear case showing no signs of weakening until more than 36 h after the shear is applied. These lags suggest that the storm intensity is to some degree predictable from observations of largescale shear changes. In all cases both the development of the asymmetries in core structure and the subsequent weakening of the storm occur before any resolvable tilt of the storm’s vertical axis occurs. It is hypothesized that the weakening of the storm occurs via the following sequence of events: First, the shear causes the structure of the eyewall region to become highly asymmetric throughout the depth of the storm. Second, the asymmetries in the upper troposphere, where the storm circulation is weaker, become sufficiently strong that air with high values of potential vorticity and equivalent potential temperature are mixed outward rather than into the eye. This allows the shear to ventilate the eye resulting in a loss of the warm core at upper levels, which causes the central pressure to rise, weakening the entire storm. The maximum potential vorticity becomes concentrated in saturated portions of the eyewall cloud aloft rather than in the eye. Third, the asymmetric features at upper levels are advected by the shear, causing the upper portions of the vortex to tilt approximately downshear. The storm weakens from the top down, reaching an approximate steady-state intensity when the ventilated layer can descend no farther due to the increasing strength and stability of the vortex at lower levels.

Effects of Vertical Wind Shear on the Intensity and Structure of Numerically Simulated Hurricanes

The future impacts of climate change on landfalling tropical cyclones are unclear. Regardless of this uncertainty, flooding by tropical cyclones will increase as a result of accelerated sea-level rise. Under similar rates of rapid sea-level rise during the early Holocene epoch most low-lying sedimentary coastlines were generally much less resilient to storm impacts. Society must learn to live with a rapidly evolving shoreline that is increasingly prone to flooding from tropical cyclones. These impacts can be mitigated partly with adaptive strategies, which include careful stewardship of sediments and reductions in human-induced land subsidence.

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Coastal flooding by tropical cyclones and sea-level rise

The influence of various environmental factors on tropical cyclone intensity is explored using a simple coupled ocean‐atmosphere model. It is first demonstrated that this model is capable of accurately replicating the intensity evolution of storms that move over oceans whose upper thermal structure is not far from monthly mean climatology and that are relatively unaffected by environmental wind shear. A parameterization of the effects of environmental wind shear is then developed and shown to work reasonably well in several cases for which the magnitude of the shear is relatively well known. When used for real-time forecasting guidance, the model is shown to perform better than other existing numerical models while being competitive with statistical methods. In the context of a limited number of case studies, the model is used to explore the sensitivity of storm intensity to its initialization and to a number of environmental factors, including potential intensity, storm track, wind shear, upper-ocean thermal structure, bathymetry, and land surface characteristics. All of these factors are shown to influence storm intensity, with their relative contributions varying greatly in space and time. It is argued that, in most cases, the greatest source of uncertainty in forecasts of storm intensity is uncertainty in forecast values of the environmental wind shear, the presence of which also reduces the inherent predictability of storm intensity.

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Environmental Control of Tropical Cyclone Intensity

A significant number of tropical cyclones move into the midlatitudes and transform into extratropical cyclones. This process is generally referred to as extratropical transition (ET). During ET a cyclone frequently produces intense rainfall and strong winds and has increased forward motion, so that such systems pose a serious threat to land and maritime activities. Changes in the structure of a system as it evolves from a tropical to an extratropical cyclone during ET necessitate changes in forecast strategies. In this paper a brief climatology of ET is given and the challenges associated with forecasting extratropical transition are described in terms of the forecast variables (track, intensity, surface winds, precipitation) and their impacts (flooding, bush fires, ocean response). The problems associated with the numerical prediction of ET are discussed. A comprehensive review of the current understanding of the processes involved in ET is presented. Classifications of extratropical transition ...

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The extratropical transition of tropical cyclones : forecast challenges, current understanding, and future directions

An objectively defined three-dimensional cyclone phase space is proposed and explored. Cyclone phase is described using the parameters of storm-motion-relative thickness asymmetry (symmetric/nonfrontal versus asymmetric/frontal) and vertical derivative of horizontal height gradient (cold- versus warm-core structure via the thermal wind relationship). A cyclone's life cycle can be analyzed within this phase space, providing substantial insight into the cyclone structural evolution. An objective classification of cyclone phase is possible, unifying the basic structural description of tropical, extratropical, and hybrid cyclones into a continuum. Stereotypical symmetric warm-core (tropical cyclone) and asymmetric cold-core (extratropical cyclone) life cycles are illustrated using 1° Navy Operational Global Atmospheric Prediction System (NOGAPS) operational analyses and 2.5° NCEP–NCAR reanalyses. The transitions between cyclone phases are clearly illustrated within the phase space, including extratro...

https://journals.ametsoc.org/doi/pdf/10.1175/1520-0493(2003)131%3C0585%3AACPSDF%3E2.0.CO%3B2

A Cyclone Phase Space Derived from Thermal Wind and Thermal Asymmetry

The behaviour of initially-barotropic vortices in vertically-sheared environmental flows is investigated. the strength and structure of the vortices used are representative of tropical cyclones. the calculations are performed using a primitive-equation numerical model on an f-plane. It is found that the initial response of the vortex to the vertical shear is to tilt in the plane of the shear. As soon as a tilt is established, the upper- and lower-level centres begin to rotate cyclonically about the mid-level centre. This rotation can be understood in terms of upper- and lower-level potential-vorticity anomalies which are displaced in the horizontal relative to one another. the flow associated with the vertical projection of each anomaly advects the other anomaly, leading to the observed cyclonic rotation. the rotation rate decreases with time, so that the direction of tilt becomes constant, but the magnitude of the tilt continues to increase. We argue that the observed rotation acts to oppose the destructive action of the vertical shear on the vortex, even in the absence of diabatic processes.

The role of the vertical circulation is considered in detail. It is shown that the vertical circulation develops in a manner which is consistent with the model flow remaining balanced. It is found that the mesoscale nature of the vertical circulation leads to a distortion of the axisymmetric vortex. This results in the inner core having a smaller vertical tilt than the outer region. the vertical circulation does not act on a large enough scale to explain why the vortex is not destroyed by the vertical shear.

The behaviour of the vortex is found to depend on various parameters. Results are presented where the vertical shear, the strength and size of the vortex, the Coriolis parameter, and the static stability are varied. With the exception of the vertical shear, altering any of these parameters alters the vertical penetration of a potential-vorticity anomaly. the results show that increasing the penetration depth leads to an increase in the rotation rate of the upper- and lower-level vortex centres about the mid-level centre, and to a reduction in the magnitude of the vertical tilt.

The evolution of vortices in vertical shear. I: Initially barotropic vortices

The evolution of baroclinic tropical-eye lone-like vortices in environmental vertical shear on an f-plane is investigated Idealized numerical calculations are performed using a primitive-equation model The vertical structure of the initial vortex is varied by changing the strength of the upper-level tangential wind The baroclinic vortices develop a vertical tilt and a cyclonic rotation of the mid-level centre about the surface centre occurs In most of the calculations the upper-level part of the vortex is advected away from the surface centre The motion of the upper-level part of the vortex can be attributed to advection by the flow associated with large-scale asymmetries in the potential vorticity and to the vertically penetrating flow associated with the potential vorticity of the lower portion of the tilted vortex As in the case of initially barotropic vortices the cyclonic rotation is found to depend on the parameters in the Rossby penetration depth The height at which the environmental flow is equal to the Speed of vortex motion is found to be higher for vortices with stronger flow at upper levels This is consistent with observations of tropical cyclones

Significant changes in the potential-vorticity structure of the vortex occur, especially at upper levels The vortex becomes elliptic in shape and may extrude filaments of potential vorticily These filaments thin, and sometimes break away from the main vortex, which as a result becomes more symmetric The separation of the filaments from the main part of the vortex is often accompanied by a decrease in the vortex tilt A: some levels no filaments form and the elliptic vortex becomes more symmetric with time Calculations with a barotropic model show that both the potential-vorticity structure of the initial vortex, and the horizontally sheared flow associated with the vertical projection of the tilted potential-vorticity anomaly, play a role in the changes in the vortex Structure

The development of potential-temperature asymmetries and an adiabatic vertical circulation are shown to be related to the direction of the vortex tilt This occurs even when the direction of tilt changes with height The changes in the low-level static stability associated with the vortex tilt are investigated and the implications for tropical-cyclone intensity change are discussed

The evolution of vortices in vertical shear. III: Baroclinic vortices

The extratropical transitions of Hurricanes Felix and Iris in 1995 are examined and compared. Both systems affected northwest Europe but only Iris developed significantly as an extratropical system. In both cases the hurricane interacts with a preexisting extratropical system over the western Atlantic. The remnants of the exhurricanes can be identified and tracked across the Atlantic as separate low-level potential vorticity (PV) anomalies. The nature of the baroclinic wave involved in the extratropical transition is described from a PV perspective and shown to differ significantly between the two cases. The role of vertical shear in modifying the hurricane structure during the early phase of the transition is investigated. Iris moved into a region of strong shear. The high PV tower of Iris developed a marked downshear tilt. Felix moved into a vertically sheared environment also but the shear was weaker than for Iris and the PV tower of Felix did not tilt much. Iris maintained its warm-core struc...

The Extratropical Transitions of Hurricanes Felix and Iris in 1995

The role of large-scale asymmetries in the evolution of a tropical-eye lone-like vortex in vertical shear on an f-plane is investigated. Idealized numerical calculations using a primitive-equation model are used to illustrate the development of the asymmetries and their role in the evolution of the vortex lilt. The asymmetries develop in the outer region of the vortex, well outside the radius of maximum wind. For the vortex profile used in most of the calculations, the region where the asymmetries develop has anomalously negative potential vorticity (PV) compared with the undisturbed environment, and the flow fields associated with the asymmetries consist of large-scale anticyclonic gyres. The orientation of the gyres changes with height and they are located on opposite sides of the vortex at lower and upper levels. The influence of the asymmetries on the vortex evolution depends on both the structure and location of the asymmetries and on the orientation of the tilted vortex within the asymmetries.



It is hypothesized that the asymmetries arise due to the distortion of the initially symmetric vortex by the horizontally sheared flow associated with the vertical projection of the tilted PV anomaly. Calculations using a barotropic model are presented in support of this hypothesis. The sensitivity of the large-scale asymmetries to the initial vortex structure is investigated. For a vortex profile in which the tangential wind decreases more slowly with radius, the asymmetries form in a region of positive PV anomaly and the associated flow field contains large-scale cyclonic gyres. The implications of the large-scale asymmetries for tropical-cyclone motion and intensity change are discussed.

Sarah C. Jones

Papers

The extratropical transition of tropical cyclones : forecast challenges, current understanding, and future directions

The evolution of vortices in vertical shear. I: Initially barotropic vortices

The evolution of vortices in vertical shear. III: Baroclinic vortices

The Extratropical Transitions of Hurricanes Felix and Iris in 1995

The evolution of vortices in vertical shear. II: Large‐scale asymmetries