Abstract

Unprecedented observations of Hurricane Isabel (2003) at category 5 intensity were collected from 12 to 14 September. This study presents a detailed analysis of the inner-core structure, atmospheric boundary layer, sea surface temperature, and outflow layer of a superintense tropical cyclone using high-resolution in situ flight-level, NCAR GPS dropwindsonde, Doppler radar, and satellite measurements. The analysis of the dropwindsonde and in situ data includes a comprehensive discussion of the uncertainties associated with this observational dataset and provides an estimate of the storm-relative axisymmetric inner-core structure using Barnes objective analysis. An assessment of gradient and thermal wind balance in the inner core is also presented. The axisymmetric data composites presented in this study suggest that Isabel built a reservoir of high moist entropy air by sea-to-air latent heat flux inside the low-level eye that was utilized as an additional energy source to nearly maintain its extreme intensity even after crossing the cool wake of Hurricane Fabian. It is argued here that the combined mean and asymmetric eddy flux of high moist entropy air from the low-level eye into the eyewall represents an additional power source or “turbo boost” to the hurricane heat engine. Recent estimates of the ratio of sea-to-air enthalpy and momentum exchange at high wind speeds are used to suggest that Isabel utilized this extra power to exceed the previously assumed intensity upper bound for the given environmental conditions on all three days. This discrepancy between a priori potential intensity theory and observations may be as high as 35 m s⁻¹ on 13 September.

Abstract

Observations from the Pre-Depression Investigation of Cloud Systems in the Tropics (PREDICT), Genesis and Rapid Intensification Processes (GRIP), and Intensity Forecast Experiment (IFEX) field campaigns are analyzed to investigate the mesoscale processes leading to the tropical cyclogenesis of Hurricane Karl (2010). Research aircraft missions provided Doppler radar, in situ flight level, and dropsonde data documenting the structural changes of the predepression disturbance. Following the pre-Karl wave pouch, variational analyses at the meso-β and meso-α scales suggest that the convective cycle in Karl alternately built the low- and midlevel circulations leading to genesis episodically rather than through a sustained lowering of the convective mass flux from increased stabilization. Convective bursts that erupt in the vorticity-rich environment of the recirculating pouch region enhance the low-level meso-β- and meso-α-scale circulation through vortex stretching. As the convection wanes, the resulting stratiform precipitation strengthens the midlevel circulation through convergence associated with ice microphysical processes, protecting the disturbance from the intrusion of dry environmental air. Once the column saturation fraction returns to a critical value, a subsequent convective burst below the midlevel circulation further enhances the low-level circulation, and the convective cycle repeats. The analyses suggest that the onset of deep convection and associated low-level spinup were closely related to the coupling of the vorticity and moisture fields at low and midlevels. Our interpretation of the observational analysis presented in this study reaffirms a primary role of deep convection in the genesis process and provides a hypothesis for the supporting role of stratiform precipitation and the midlevel vortex.

Abstract

A set of multiscale, nested, idealized numerical simulations of mesoscale convective systems (MCSs) and mesoscale convective vortices (MCVs) was conducted. The purpose of these simulations was to investigate the dependence of MCV development and evolution on background conditions and to explore the relationship between MCVs and larger, moist baroclinic cyclones. In all experiments, no mesoscale convective system (MCS) developed until a larger-scale, moist baroclinic system with surface pressure amplitude of at least 2 hPa was present. The convective system then enhanced the development of the moist baroclinic system by its diabatic production of eddy available potential energy (APE), which led to the enhanced baroclinic conversion of basic-state APE to eddy APE. The most rapid potential vorticity (PV) development occurred in and just behind the leading convective line. The entire system grew upscale with time as the newly created PV rotated cyclonically around a common center as the leading convective line continued to expand outward. Ten hours after the initiation of deep moist convection, the simulated MCV radii, heights of maximum winds, tangential velocity, and shear corresponded reasonably well to their counterparts in BAMEX. The increasing strength of the simulated MCVs with respect to larger values of background CAPE and shear supports the hypothesis that as long as convection is present, CAPE and shear both add to the strength of the MCV.

Abstract

Using brightness temperatures from channels 3 and 4 of the Microwave Sounding Unit (MSU) as approximations to mean-layer temperatures, the geostrophic winds at 50 mb can be computed through a “bottom-up” approach. When this method is applied at high latitudes during austral winter and spring, it is found that accurate descriptions of the seasonal evolution and interannual variability of the lower-stratospheric circumpolar vortex are obtained. Variations in early-spring vortex strength from year to year appear to relate well to variations in the timing of the first large late-winter wavenumber one event in the lower stratosphere. Since wave forcing of the mean flow in the lower stratosphere is known to be weak, the variability in vortex strength may result from variations in wave-induced subsidence through the downward control principle.

Previous studies have demonstrated a biennial harmonic in both extratropical wave forcing and the mean flow, suggesting a link with the equatorially confined quasi-biennial oscillation (QBO). This study attempts to find a similar signal in the strength of the lower-stratospheric austral circumpolar vortex. It is first found that during the easterly (westerly) phase of the QBO large-amplitude wavenumber one in MSU channel 4, brightness temperature generally occurs earlier (later) in the season than normal. Subsequently, for most years of the study when the QBO is in its easterly (westerly) phase, the circumpolar vortex is observed to be weaker (stronger) than average.

Abstract

Hurricane intensity forecasting has lagged far behind the forecasting of hurricane track. In an effort to improve the understanding of the hurricane intensity dilemma, several attempts have been made to compute an upper bound on the intensity of tropical cyclones. This paper investigates the strides made into determining the maximum intensity of hurricanes. Concentrating on the most recent attempts to understand the maximum intensity problem, the theories of Holland and Emanuel are reviewed with the objective of assessing their validity in real tropical cyclones. Each theory is then tested using both observations and the axisymmetric hurricane numerical models of Ooyama and Emanuel.

It is found that ambient convective instability plays a minor role in the determination of the maximum intensity and that the Emanuel model is the closest to providing a useful calculation of maximum intensity. Several shortcomings are revealed in Emanuel's theory, however, showing the need for more basic research on the axisymmetric and asymmetric dynamics of hurricanes. As an illustration of the importance of asymmetric vorticity dynamics in the determination of a hurricane's maximum intensity it is shown, using Ooyama's hurricane model, that the maximum intensity of a tropical cyclone may be diminished by convectively generated vorticity anomolies excited outside the primary eyewall. The vorticity anomolies are parameterized by adding a concentric ring of vorticity outside the primary eyewall that acts to cut off its supply of angular momentum and moist enthalpy. It is suggested that the generation of vorticity rings (or bands) outside the primary eyewall is a major reason why tropical cyclones fail to attain their maximum intensity even in an otherwise favorable environment.

The upshot of this work points to the need for obtaining a more complete understanding of asymmetric vorticity processes in hurricanes and their coupling to the boundary layer and convection.

Abstract

A new mechanism of vortex intensification by convectively forced vortex Rossby waves was proposed by Montgomery and Kallenbach. As demonstrated by them, the axisymmetrization process is described by vortex Rossby waves that eventually propagate outward before their symmetrization. Montgomery and Kallenbach were able to relate these waves to intensity changes in barotropic hurricane-like vortices. In the present work these ideas are applied to better understand structure change and intensification of hurricanes in a baroclinic setting. The work of Möller and Montgomery, who examined the wave kinematics and wave–mean flow interaction of vortex Rossby waves in a barotropic model, is extended here to three dimensions. The model is based on the asymmetric balance theory of Shapiro and Montgomery. A nonlinear prognostic model is used to examine the effect of convectively generated potential vorticity (PV) disturbances on the evolution of a hurricane-like vortex on an f plane. This investigation generalizes that of Montgomery and Enagonio, who studied tropical cyclogenesis using a quasigeostrophic balance model, to a larger Rossby number. Convection is represented to the extent that the prescribed initial PV anomalies could be convectively forced. As in this formulation gravity waves are excluded, the dynamics of vortex Rossby waves and their interaction with the mean vortex and each other can be focused upon.

Simple relaxation (“axisymmetrization”) experiments with monochromatic azimuthal-wavenumber disturbances show that vortex Rossby waves propagate both radially and vertically. The higher the wavenumber the weaker the vertical propagation of the PV asymmetries and corresponding response of the basic state. Experiments where double-cluster PV anomalies are superimposed complement the cyclogenesis results of Montgomery and Enagonio. The lower-level cyclonic PV anomaly intensifies the vortex while symmetrizing for a wide range of anomaly amplitudes. Depending on the strength of the cluster, however, the upper-level anticyclonic PV anomaly is expelled outward (stronger anomaly), as in Montgomery and Enagonio, or is symmetrized (weaker anomaly) similar to the lower-level positive PV anomaly. When the ongoing process of convection is simulated by adding double-cluster PV anomalies to the PV fields (so-called pulsing), the tropical storm intensifies to hurricane strength whose intensity depends on the location and extent of the anomaly. These results confirm that there exists an alternative means of tropical cyclone intensification to the symmetric mode.

Abstract

A recent study using a discrete three-region nondivergent approximation for the inner-core region of a mature hurricane-like vortex fails to catch wavenumber-2 azimuthal shear instabilities that are observed in experiments and predicted by similar continuous model representations.

With hurricane applications in mind, a generalized version of a piecewise uniform three-region vortex model is presented. The necessary and sufficient criteria for wavenumber-2 and -m instabilities are derived and discussed. The peculiar dynamics of vortex Rossby waves on a discrete circular waveguide elucidate why wavenumber-2 instabilities have been difficult to find in previous analyses and also demonstrate some of the idiosyncrasies of the discrete model. The physical structure of the instabilities is also briefly examined.

Abstract

In this paper, the first of two parts, the dynamics of linearized perturbations to hurricane-like vortices are studied. Unlike previous studies, which are essentially two-dimensional or assume that the perturbations are quasi-balanced, the perturbations are fully three-dimensional and nonhydrostatic. The vortices used as basic states are also three-dimensional (though axisymmetric), with wind fields modeled closely after observations of hurricanes and tropical storms, and are initially in hydrostatic and gradient wind balance.

The equations of motion, computational methods for solving them, and methods for generating the basic-state hurricane-like vortices are presented. In particular, three basic states are studied: a vortex modeled after an intense (category 3) hurricane, a moderate (category 1) hurricane, and a weak tropical storm. The stability of each vortex is considered. The category 3 vortex is found to be rather unstable, with its fastest growing mode occurring for azimuthal wavenumber three with an e-folding time of approximately 1 h. The category 1 vortex is less unstable, as its most unstable mode occurs for wavenumber two with an e-folding time of 5 h. In both cases, these unstable modes are found to be close analogs of their strictly two-dimensional counterparts, and essentially barotropic in nature.

The tropical storm–like vortex is found to be stable for all azimuthal wavenumbers. For this vortex, the evolution of purely thermal, unbalanced perturbations in the vortex environment are studied; such disturbances might be the result of asymmetric bursts of convection in the vicinity of the vortex, which are typical for developing storms. The evolution of these perturbations goes through two phases. First, there is substantial gravity wave radiation and rapid adjustment to quasi-gradient wind balance. In the second phase, the quasi-balanced perturbations are axisymmetrized by the shear of the basic-state vortex, and cause localized accelerations of the symmetric vortex via eddy momentum and heat fluxes. The full response of the symmetric vortex and comparisons to fully nonlinear simulations are the topics of the second part.

Abstract

This work investigates the problem of tropical cyclogenesis in three dimensions. In particular, the authors examine the interaction of small-scale convective disturbances with a larger-scale vortex circulation in a nonlinear quasigeostrophic balance model. Convective forcing is parameterized by its estimated net effect on the potential vorticity (PV) field. Idealized numerical experiments show that vortex intensification proceeds by ingestion of like-sign potential vorticity anomalies into the parent vortex and expulsion of opposite-sign potential vorticity anomalies during the axisymmetrization process. For the finite-amplitude forcing considered here, the weakly nonlinear vortex Rossby wave mean-flow predictions for the magnitude and location of the spinup are in good agreement with the model results. Vortex development is analyzed using Lagrangian trajectories, Eliassen–Palm flux vectors, and the Lorenz energy cycle.

Using numerical estimates of the magnitude of PV injection based on previous observational and theoretical work, the authors obtain spinup to a 15 m s⁻¹ cyclone on realistic timescales. Simulation of a midlevel vortex with peripheral convection shows that axisymmetrization results in the spinup of a surface cyclone. The axisymmetrization mechanism demonstrates the development of a warm-core vortex. The relative contribution from eddy-heat and eddy-momentum fluxes to the warm core structure of the cyclone is investigated.

The vortex spinup obtained shows greater than linear dependence on the forcing amplitude, indicating the existence of a nonlinear feedback mechanism associated with the vortex Rossby waves.

Building on recent work by several authors, this work further clarifies the significance of the axisymmetrization process for the problem of tropical cyclogenesis. The theory is shown to be consistent with published observations of tropical cyclogenesis. Further observational and modeling tests of the theory, specific to the dynamics examined here, are proposed.

Abstract

In this study the balanced evolution of small but finite as well as large-amplitude asymmetries in a rapidly rotating hurricane-like vortex is investigated. In particular, the wave kinematics and wave–mean flow interaction of vortex Rossby waves in a barotropic nonlinear asymmetric balance (AB) model are examined. By diagnosing the evolution of different asymmetric initial potential vorticity (PV) disturbances and their effect on the symmetric vortex, recent linear and quasi-linear predictions are verified and the proposed AB model is shown to be a viable balance model for azimuthal wavenumbers >1. For disturbance amplitudes that are 40% of the basic-state PV at the radius of maximum wind, a discrete normal mode propagating cyclonically around the vortex is excited as a by-product of the process by which energy is transferred from the asymmetries into the basic state (axisymmetrization). In addition we are able to show that even a strong disturbance axisymmetrizes in a circular flow and is able to intensify the basic state. Side-by-side comparison with some experiments from a primitive equation model show good agreement for both weak and strong asymmetric disturbances. The results raise intriguing questions about the dynamical role of discrete and continuous spectrum vortex Rossby waves in the moist convective dynamics of the hurricane. The application of the results to hurricane intensification will be addressed.