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Paul D. Reasor and Matthew D. Eastin

Abstract

This paper examines the structure and evolution of a mature tropical cyclone in vertical wind shear (VWS) using airborne Doppler radar observations of Hurricane Guillermo (1997). In Part I, the modulation of eyewall convection via the rotation of vorticity asymmetries through the downshear-left quadrant was documented during rapid intensification. Here, the focus is on the relationship between VWS, vortex tilt, and associated asymmetry within the tropical cyclone core region during two separate observation periods. A method for estimating local VWS and vortex tilt from radar datasets is further developed, and the resulting vertical structure and its evolution are subjected to statistical confidence tests. Guillermo was a highly resilient vortex, evidenced by its small tilt magnitude relative to the horizontal scale of the vortex core. The deep-layer tilt was statistically significant, oriented on average ~60° left of shear. Large-scale vorticity and thermal asymmetries oriented along the tilt direction support a response of Guillermo to shear forcing that is consistent with balanced dynamics. The time-averaged vertical motion asymmetry within the eyewall exhibited maximum ascent values ~40° left of the deep-layer shear, or in this case, right of the deep-layer tilt. The observation-based analysis of Guillermo’s interaction with VWS confirms findings of recent theoretical and numerical studies, and serves as the basis for a more comprehensive investigation of VWS and tropical cyclone intensity change using a recently constructed multistorm database of Doppler radar analyses.

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Paul D. Reasor and Michael T. Montgomery

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.

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Paul D. Reasor, Robert Rogers, and Sylvie Lorsolo

Abstract

Following a recent demonstration of multicase compositing of axisymmetric tropical cyclone (TC) structure derived from airborne Doppler radar measurements, the authors extend the analysis to the asymmetric structure using an unprecedented database from 75 TC flights. In particular, they examine the precipitation and kinematic asymmetry forced by the TC's motion and interaction with vertical wind shear. For the first time they quantify the average magnitude and phase of the three-dimensional shear-relative kinematic asymmetry of observed TCs through a composite approach. The composite analysis confirms principal features of the shear-relative TC asymmetry documented in prior numerical and observational studies (e.g., downshear tilt, downshear-right convective initiation, and a downshear-left precipitation maximum). The statistical significance of the composite shear-relative structure is demonstrated through a stratification of cases by shear magnitude. The impact of storm motion on eyewall convective asymmetry appears to be secondary to the much greater constraint placed by vertical wind shear on the organization of convection, in agreement with prior studies using lightning and precipitation data.

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Paul D. Reasor and Michael T. Montgomery

Abstract

This work examines the applicability of a previously postulated heuristic model for the temporal evolution of the small-amplitude tilt of a tropical cyclone–like vortex under vertical shear forcing for both a dry and cloudy atmosphere. The heuristic model hinges on the existence of a quasi-discrete vortex Rossby wave and its ability to represent the coherent precession and tilt decay of a stable vortex in the free-alignment problem. Linearized numerical solutions for a dry and cloudy vortex confirm the model predictions that an increase in the magnitude of the radial potential vorticity (PV) gradient within the vortex skirt surrounding the core yields a more rapid evolution of a sheared vortex toward the equilibrium, left-of-shear tilt configuration. However, in the moist-neutral limit, in which the effective static stability vanishes in rising and sinking regions, the heuristic model yields a poor approximation to the simulated vortex core evolution, but a left-of-shear tilt of the near-core vortex, radially beyond the heating region, remains the preferred long-time solution. Within the near-core skirt, the PV perturbation generated by vertical shearing exhibits continuous-spectrum-type vortex Rossby waves, features that are not captured by the heuristic model. Nevertheless, the heuristic model continues to predict the rapid vertical alignment and equilibrium, left-of-shear tilt configuration of the simulated near-core vortex in the moist-neutral limit.

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Paul D. Reasor and Michael T. Montgomery

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The vertical alignment of an initially tilted geostrophic vortex is shown here to be captured by linear vortex Rossby wave dynamics when the vortex cores at upper and lower levels overlap. The vortex beta Rossby number, defined as the ratio of nonlinear advection in the potential vorticity equation to linear radial advection, is less than unity in this case. A useful means of characterizing a tilted vortex flow in this parameter regime is through a wave–mean flow decomposition. From this perspective the alignment mechanism is elucidated using a quasigeostrophic model in both its complete and linear equivalent barotropic forms. Attention is focused on basic-state vortices with continuous and monotonically decreasing potential vorticity profiles.

For internal Rossby deformation radii larger than the horizontal scale of the tilted vortex an azimuthal wavenumber 1 quasi mode exists. The quasi mode is characterized by its steady cyclonic propagation, long lifetime, and resistance to differential rotation, behaving much like a discrete vortex Rossby wave. The quasi mode traps disturbance energy causing the vortex to precess, or corotate, and thus prevents alignment. For internal deformation radii smaller than the horizontal vortex scale, the quasi mode disappears into the continuous spectrum of vortex Rossby waves. Alignment then proceeds through the irreversible redistribution of potential vorticity by the sheared vortex Rossby waves. Further decreases in the internal deformation radius result in a decreased dependence of vortex evolution on initial tilt magnitude, consistent with a reduction of the vortex beta Rossby number.

These results are believed to have relevance to the problem of tropical cyclone (TC) genesis. Cyclogenesis initiated through the merger and alignment of low-level convectively generated positive potential vorticity within a weak incipient vortex is captured by quasi-linear dynamics. A potential dynamical barrier to TC development in which the quasi mode frustrates vertical alignment can be identified using the linear alignment theory in this case.

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Paul D. Reasor, Michael T. Montgomery, and Lewis D. Grasso

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A new paradigm for the resiliency of tropical cyclone (TC) vortices in vertical shear flow is presented. To elucidate the basic dynamics, the authors follow previous work and consider initially barotropic vortices on an f plane. It is argued that the diabatically driven secondary circulation of the TC is not directly responsible for maintaining the vertical alignment of the vortex. Rather, an inviscid damping mechanism intrinsic to the dry adiabatic dynamics of the TC vortex suppresses departures from the upright state.

Recent work has demonstrated that tilted quasigeostrophic vortices consisting of a core of positive vorticity surrounded by a skirt of lesser positive vorticity align through projection of the tilt asymmetry onto vortex Rossby waves (VRWs) and their subsequent damping (VRW damping). This work is extended here to the finite Rossby number (Ro) regime characteristic of real TCs. It is shown that the VRW damping mechanism provides a direct means of reducing the tilt of intense cyclonic vortices (Ro > 1) in unidirectional vertical shear. Moreover, intense TC-like, but initially barotropic, vortices are shown to be much more resilient to vertical shearing than previously believed. For initially upright, observationally based TC-like vortices in vertical shear, the existence of a “downshear-left” tilt equilibrium is demonstrated when the VRW damping is nonnegligible.

On the basis of these findings, the axisymmetric component of the diabatically driven secondary circulation is argued to contribute indirectly to vortex resiliency against shear by increasing Ro and enhancing the radial gradient of azimuthal-mean potential vorticity. This, in addition to the reduction of static stability in moist ascent regions, increases the efficiency of the VRW damping mechanism.

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Paul D. Reasor, Matthew D. Eastin, and John F. Gamache

Abstract

The structure and evolution of rapidly intensifying Hurricane Guillermo (1997) is examined using airborne Doppler radar observations. In this first part, the low-azimuthal-wavenumber component of the vortex is presented. Guillermo’s intensification occurred in an environmental flow with 7–8 m s−1 of deep-layer vertical shear. As a consequence of the persistent vertical shear forcing of the vortex, convection was observed primarily in the downshear left quadrant of the storm. The greatest intensification during the ∼6-h Doppler observation period coincided with the formation and cyclonic rotation of several particularly strong convective bursts through the left-of-shear semicircle of the eyewall. Some of the strongest convective bursts were triggered by azimuthally propagating low-wavenumber vorticity asymmetries. Mesoscale budget analyses of axisymmetric angular momentum and relative vorticity within the eyewall are presented to elucidate the mechanisms contributing to Guillermo’s structural evolution during this period. The observations support a developing conceptual model of the rapidly intensifying, vertically sheared hurricane in which shear-forced mesoscale ascent in the downshear eyewall is modulated by internally generated vorticity asymmetries yielding episodes of anomalous intensification.

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David A. Schecter, Michael T. Montgomery, and Paul D. Reasor

Abstract

This article presents a new theory for the rate at which a quasigeostrophic vortex realigns, under conservative dynamics, after being tilted by an episode of external vertical shear. The initial tilt is viewed as the excitation of a three-dimensional “vortex Rossby mode.” This mode, that is, the tilt, decays exponentially with time during its early evolution. The decay rate γ is proportional to the potential vorticity gradient at a critical radius, where the fluid rotation is resonant with the mode. The decay rate γ also depends on the internal Rossby deformation radius l R, which is proportional to the stratification strength of the atmospheric or oceanic layer containing the vortex. The change of γ with l R is sensitive to the form of the vortex. For the case of a “Rankine-with-skirt” vortex, the magnitude of γ increases (initially) with increasing l R. On the other hand, for the case of a “Gaussian” vortex, the magnitude of γ decreases with increasing l R. The relevance of this theory to tropical cyclogenesis is discussed.

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Paul D. Reasor, Michael T. Montgomery, and Lance F. Bosart

Abstract

Recent numerical studies of tropical cyclone genesis suggest a new paradigm for how the surface vortex is established based on a highly nonaxisymmetric mechanism involving the interaction of low-level cyclonic circulations generated by deep cumulonimbus convection. A reexamination of mesoscale observations during the genesis of Hurricane Guillermo (1991) confirms the presence of multiple cyclonic circulations. More recently, airborne Doppler radar wind observations during the genesis of Atlantic Hurricane Dolly (1996) also reveal multiple lower-to-middle-tropospheric mesoscale cyclonic circulations during sequential 15–20-min compositing periods.

A particularly well-organized, but initially weak (mean tangential wind of 7 m s−1), low-level cyclonic vortex embedded within the pre-Dolly tropical disturbance is observed coincident with deep, vertically penetrating cumulonimbus convection. The earliest observations of this vortex show the peak circulation near 2-km height with a mean diameter of 30–40 km. The circulation undergoes a slight intensification over a 2-h period, with the maximum tangential winds ultimately peaking below 1-km height. Approximately 18 h after these observations Dolly is classified as a hurricane by the National Hurricane Center.

A synthesis of observations during the early development of Dolly supports a stochastic view of tropical cyclone genesis in which multiple lower-to-middle-tropospheric mesoscale cyclonic circulations are involved in building the surface cyclonic circulation. It is suggested that, in particular, the interaction of low-level circulations generated by a series of deep cumulonimbus convective events, like the one documented here, within an environment of elevated cyclonic vorticity was instrumental to the formation of the Dolly surface vortex.

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Stephen R. Guimond, Mark A. Bourassa, and Paul D. Reasor

Abstract

Despite the fact that latent heating in cloud systems drives many atmospheric circulations, including tropical cyclones, little is known of its magnitude and structure, largely because of inadequate observations. In this work, a reasonably high-resolution (2 km), four-dimensional airborne Doppler radar retrieval of the latent heat of condensation/evaporation is presented for rapidly intensifying Hurricane Guillermo (1997). Several advancements in the basic retrieval algorithm are shown, including 1) analyzing the scheme within the dynamically consistent framework of a numerical model, 2) identifying algorithm sensitivities through the use of ancillary data sources, and 3) developing a precipitation budget storage term parameterization. The determination of the saturation state is shown to be an important part of the algorithm for updrafts of ~5 m s−1 or less.

The uncertainties in the magnitude of the retrieved heating are dominated by errors in the vertical velocity. Using a combination of error propagation and Monte Carlo uncertainty techniques, biases are found to be small, and randomly distributed errors in the heating magnitude are ~16% for updrafts greater than 5 m s−1 and ~156% for updrafts of 1 m s−1. Even though errors in the vertical velocity can lead to large uncertainties in the latent heating field for small updrafts/downdrafts, in an integrated sense the errors are not as drastic.

In Part II, the impact of the retrievals is assessed by inserting the heating into realistic numerical simulations at 2-km resolution and comparing the generated wind structure to the Doppler radar observations of Guillermo.

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